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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head profile="http://gmpg.org/xfn/1"> <title>Computational Organic Chemistry » Schleyer</title> <meta name="google-site-verification" content="g1Myv4tUVAmqRbwZeBi7IPuSZpP64RWjVJ6itIoouCo"> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <style type="text/css" media="screen">@import url( /blog/wp-content/themes/comporg/style.css);</style> <link rel="stylesheet" id="wp-block-library-css" href="/blog/wp-includes/css/dist/block-library/style.min.css?ver=5.6.1" type="text/css" media="all"> <script type="text/javascript">  </script> </head> <body> <div id="header"> <div id="header_img"></div> </div> <div id="link_section"> <div style="float:left"> <a href="/blog/about">About this Blog</a> | <a href="/">Book Homepage</a> | <a href="http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471713422.html">Purchase the Book</a> </div> </div> <div id="after_links"></div> <div id="content"> <div id="main"> <h2 class="post-title">Archive for the 'Schleyer' Category</h2> <div class="box"> <h2><a href="/blog/archives/3573" rel="bookmark" title="Permanent Link: Review of planar hypercoordinate atoms">Review of planar hypercoordinate atoms</a></h2> <div class="post-content"> <p>Yang, Ganz, Chen, Wang, and Schleyer have published a very interesting and comprehensive review of planar hypercoordinate compounds, with a particular emphasis on planar tetracoordinate carbon compounds.<a href="#ptC1"><sup>1</sup></a> A good deal of this review covers computational results.</p> <p>There are two major motifs for constructing planar tetracoordinate carbon compounds. The first involves some structural constraints that hold (or force) the carbon into planarity. A fascinating example is <b>1</b> computed by Rasmussen and Radom in 1999.<a href="#ptC2"><sup>2</sup></a> This molecule taxed their computational resources, and as was probably quite typical for that time, there is no supplementary materials. But since this compound has high symmetry (<i>D<sub>2h</sub></i>) I reoptimized its structure at ω-B97X-D/6-311+G(d) and computed its frequencies in just a few hours. This structure is shown in Figure 1. However, it should be noted that at this computational level, <b>1</b> possesses a single imaginary frequency corresponding to breaking the planarity of the central carbon atom. Rasmussen and Radom computed the structure of <b>1</b> at MP2/6-31G(d) with numerical frequencies all being positive. They also note that the B3LYP/6-311+G(3df,2p) structure also has a single imaginary frequency.</p> <p>A second approach toward planar tetracoordinate carbon compounds is electronic: having π-acceptor ligands to stabilize the p-lone pair on carbon and σ-donating ligands to help supply sufficient electrons to cover the four bonds. Perhaps the premier simple example of this is the dication <b>2</b>¸ whose ω-B97X-D/6-311+G(d,p) structure is also shown in Figure 1.</p> <p>The review covers heteroatom planar hypercoordinate species as well. It also provides brief coverage of some synthesized examples.</p> <table align="center" border="0" cellspacing="0" cellpadding="4"> <tr align="center" valign="bottom"> <td> <p><img src="/blog/wp-content/ptC1Img.png"></p> </td> <td> <p><img src="/blog/wp-content/ptC2Img.png"></p> </td> </tr> <tr align="center" valign="bottom"> <td> <p></p> <div class="jmol" id="ptC1"> <a onclick="return false"><br> <img src="/blog/wp-content/ptC1.jpg" onclick="insertJmol('ptC1',300,300,'ptC1.xyz')"></a> </div> <p><b>1</b></p> </td> <td> <p></p> <div class="jmol" id="ptC2"> <a onclick="return false"><br> <img src="/blog/wp-content/ptC2.jpg" onclick="insertJmol('ptC2',260,260,'ptC2.xyz')"></a> </div> <p><b>2</b></p> </td> </tr> </table> <p align="center"><b>Figure 1</b>. Optimized structures of <b>1</b> and <b>2</b>.</p> <h3>References</h3> <p><a name="ptC1"></a></p> <p>(1) Yang, L.-M.; Ganz, E.; Chen, Z.; Wang, Z.-X.; Schleyer, P. v. R. "Four Decades of the Chemistry of Planar Hypercoordinate Compounds," <i>Angew. Chem. Int. Ed.</i> <b>2015</b>, <i>54</i>, 9468-9501, DOI: <a href="http://dx.doi.org/10.1002/anie.201410407">10.1002/anie.201410407</a>.</p> <p><a name="ptC2"></a></p> <p>(2) Rasmussen, D. R.; Radom, L. "Planar-Tetracoordinate Carbon in a Neutral Saturated Hydrocarbon: Theoretical Design and Characterization," <i>Angew. Chem. Int. Ed.</i> <b>1999</b>, <i style="mso-bidi-font-style:normal">38</i>, 2875-2878, DOI: <a href="http://dx.doi.org/10.1002/(SICI)1521-3773(19991004)38:19%3c2875::AID-ANIE2875%3e3.0.CO;2-D">10.1002/(SICI)1521-3773(19991004)38:19<2875::AID-ANIE2875>3.0.CO;2-D</a>.</p> <h3>InChIs</h3> <p><b>1</b>: InChI=1S/C23H24/c1-7-11-3-15-9-2-10-17-5-13-8(1)14-6-18(10)22-16(9)4-12(7)20(14,22)23(22)19(11,13)21(15,17)23/h7-18H,1-6H2<br>InChIKey=LMDPKFRIIOUORN-UHFFFAOYSA-N</p> <p><b>2</b>: InChI=1S/C5H4/c1-2-5(1)3-4-5/h1-4H/q+2<br>InChIKey=UGGTXIMRHSZRSQ-UHFFFAOYSA-N</p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">26 Aug 2015</span> <span class="comments"><a href="/blog/archives/3573#comments">6 Comments</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/3446" rel="bookmark" title="Permanent Link: Two review articles for the general audience">Two review articles for the general audience</a></h2> <div class="post-content"> <p>In trying to clean up my in-box of articles for potential posts, I write here about two articles for a more general audience, authored by two of the major leaders in computational organic chemistry.</p> <p>Ken Houk offers an overview of how computational simulation is a partner with experiment and theory in aiding and guiding our understanding of organic chemistry.<a href="#houkSchleyer1"><sup>1</sup></a> The article is written for the non-specialist, really even more for the non-scientist. Ken describes how computations have helped understand relatively simple reactions like pericyclic reactions, that then get more subtle when torquoselection is considered, to metal-catalysis, to designed protein catalysts. If you are ever faced with discussing just what you do as a computational chemist at a cocktail party, this article is a great resource of how to explain our science to the interested lay audience.</p> <p>Paul Schleyer adds a tutorial on transition state aromaticity.<a href="#houkSchleyer2"><sup>2</sup></a> The authors discusses a variety of aromaticity measures (energetics, geometry, magnetic properties) that can be employed to analyze the nature of transition states, in addition to ground state molecules. This article provides a very clear description of the methods and a few examples. It is written for a more specialized audience than Houk’s article, but is nonetheless completely accessible to any chemist, even those with no computational background.</p> <h3>References</h3> <p><a name="houkSchleyer1"></a></p> <p>(1) Houk, K. N.; Liu, P. "Using Computational Chemistry to Understand & Discover Chemical Reactions," <i>Daedalus</i> <b>2014</b>, <i>143</i>, 49-66, DOI: <a href="http://dx.doi.org/10.1162/DAED_a_00305">10.1162/DAED_a_00305</a>.</p> <p><a name="houkSchleyer2"></a></p> <p>(2) Schleyer, P. v. R.; Wu, J. I.; Cossio, F. P.; Fernandez, I. "Aromaticity in transition structures," <i>Chem. Soc. Rev.</i> <b>2014</b>, <i>43</i>, 4909-4921, DOI: <a href="http://dx.doi.org/10.1039/C4CS00012A">10.1039/C4CS00012A</a>.</p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/authors/houk" rel="category tag">Houk</a> &<a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">22 Dec 2014</span> <span class="comments"><a href="/blog/archives/3446#respond">No Comments</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/3403" rel="bookmark" title="Permanent Link: Paul Schleyer: In Memorium">Paul Schleyer: In Memorium</a></h2> <div class="post-content"> <p>Professor Paul von Ragué Schleyer passed away November 21, 2014. Paul was a major force in physical organic and computational organic chemistry. I followed his career closely for the entirety of my own career; my doctoral studies with Andrew Streitwieser involved the analysis of the nature of the C-Li bond and we were in constant communication with Schleyer. Paul’s work on aromaticity greatly informed my thinking and my studies in this area.</p> <p>I interviewed Paul in his office at the University of Georgia for the first edition of my book <i>Computational Organic Chemistry</i>. This interview was reprinted in the second edition without any changes. In honor of Paul, I am posting this interview here, so that our community can remember this important, inspirational figure.</p> <p> </p> <table align="center" border="0"> <tr> <td colspan="3"> <hr> </td> </tr> <tr> <td> </td> <td> <h2>Interview: Professor Paul von Ragué Schleyer</h2> <h3>Interviewed March 28, 2006</h3> <p>Professor Paul Schleyer is the Graham Perdue Professor of Chemistry at the University of Georgia, where he has been for the past 8 years. Prior to that, he was a professor at the University at Erlangen (co-director of the Organic Institute) and the founding director of its Computer Chemistry Center. Schleyer began his academic career at Princeton University.</p> <p>Professor Schleyer’s involvement in computational chemistry dates back to the 1960s, when his group was performing MM and semi-empirical computations as an adjunct to his predominantly experimental research program. This situation dramatically changed when Professor John Pople invited Schleyer to visit Carnegie-Mellon University in 1969 as the NSF Center of Excellence Lecturer. From discussions with Dr. Pople, it became clear to Schleyer that “ab initio methods could look at controversial subjects like the nonclassical carbocations. I became hooked on it!” The collaboration between Pople and Schleyer that originated from that visit lasted well over 20 years, and covered such topics as substituent effects, unusual structures that Schleyer terms “rule-breaking”, and organolithium chemistry. This collaboration started while Schleyer was at Princeton but continued after his move to Erlangen, where Pople came to visit many times. The collaboration was certainly of peers. “It would be unfair to say that the ideas came from me, but it’s clear that the projects we worked on would not have been chosen by Pople. Pople added a great deal of insight and he would advise me on what was computationally possible,” Schleyer recalls of this fruitful relationship.</p> <p>Schleyer quickly became enamored with the power of ab initio computations to tackle interesting organic problems. His enthusiasm for computational chemistry eventually led to his decision to move to Erlangen – they offered unlimited (24/7) computer time, while Princeton’s counteroffer was just 2 hours of computer time per week. He left Erlangen in 1998 due to enforced retirement. However, his adjunct status at the University of Georgia allowed for a smooth transition back to the United States, where he now enjoys a very productive collaborative relationship with Professor Fritz Schaefer.</p> <p>Perhaps the problem that best represents how Schleyer exploits the power of ab initio computational chemistry is the question of how to define and measure aromaticity. Schleyer’s interest in the concept of aromaticity spans his entire career. He was drawn to this problem because of the pervasive nature of aromaticity across organic chemistry. Schleyer describes his motivation: “Aromaticity is a central theme of organic chemistry. It is re-examined by each generation of chemists. Changing technology permits that re-examination to occur.” His direct involvement came about by Kutzelnigg’s development of a computer code to calculate chemical shifts. Schleyer began use of this program in the 1980s and applied it first to structural problems. His group “discovered in this manner many experimental structures that were incorrect.”</p> <p>To assess aromaticity, Schleyer first computed the lithium chemical shifts in complexes formed between lithium cation and the hydrocarbon of interest. The lithium cation would typically reside above the aromatic ring and its chemical shift would be affected by the magnetic field of the ring. While this met with some success, Schleyer was frustrated by the fact that lithium was often not positioned especially near the ring, let alone in the center of the ring. This led to the development of nucleus-independent chemical shift (NICS), where the virtual chemical shift can be computed at any point in space. Schleyer advocated using the geometric center of the ring, then later a point 1 Å above the ring center.</p> <p>Over time, Schleyer came to refine the use of NICS, advocating an examination of NICS values on a grid of points. His most recent paper posits using just the component of the chemical shift tensor perpendicular to the ring evaluated at the center of the ring. This evolution reflects Schleyer’s continuing pursuit of a simple measure of aromaticity. “Our endeavor from the beginning was to select one NICS point that we could say characterizes the compound,” Schleyer says. “The problem is that chemists want a number which they can associate with a phenomenon rather than a picture. The problem with NICS was that it was not soundly based conceptually from the beginning because cyclic electron delocalization-induced ring current was not expressed solely perpendicular to the ring. It’s only <i>that</i> component which is related to aromaticity.”</p> <p>The majority of our discussion revolved around the definition of aromaticity. Schleyer argues that “aromaticity can be defined perfectly well. It is the manifestation of cyclic electron delocalization which is expressed in various ways. The problem with aromaticity comes in its quantitative definition. How big is the aromaticity of a particular molecule? We can answer this using some properties. One of my objectives is to see whether these various quantities are related to one another. That, I think, is still an open question.”</p> <p>Schleyer further detailed this thought, “The difficulty in writing about aromaticity is that it is encrusted by two centuries of tradition, which you cannot avoid. You have to stress the interplay of the phenomena. Energetic properties are most important, but you need to keep in mind that aromaticity is only 5% of the total energy. But if you want to get as close to the phenomenon as possible, then one has to go to the property most closely related, which is magnetic properties.” This is why he focuses upon the use of NICS as an aromaticity measure. He is quite confident in his new NICS measure employing the perpendicular component of the chemical shift tensor. “This new criteria is very satisfactory,” he says. “Most people who propose alternative measures do not do the careful step of evaluating them against some basic standard. We evaluate against aromatic stabilization energies.”</p> <p>Schleyer notes that his evaluation of the aromatic stabilization energy of benzene is larger than many other estimates. This results from the fact that, in his opinion, “all traditional equations for its determination use tainted molecules. Cyclohexene is tainted by hyperconjugation of about 10 kcal mol<sup>-1</sup>. Even cyclohexane is very tainted, in this case by 1,3-interactions.” An analogous complaint can be made about the methods Schleyer himself employs: NICS is evaluated at some arbitrary point or arbitrary set of points, the block-diagonalized “cyclohexatriene” molecule is a <i>gedanken</i> molecule. When pressed on what then to use as a reference that is not ‘tainted’, Schleyer made this trenchant comment: “What we are trying to measure is virtual. Aromaticity, like almost all concepts in organic chemistry, is virtual. They’re not measurable. You can’t measure atomic charges within a molecule. Hyperconjugation, electronegativity, everything is in this sort of virtual category. Chemists live in a virtual world. But science moves to higher degrees of refinement.” Despite its inherent ‘virtual’ nature, “Aromaticity has this 200 year history. Chemists are interested in the unusual stability and reactivity associated with aromatic molecules. The term survives, and remains an enormously fruitful area of research.”</p> <p>His interest in the annulenes is a natural extension of the quest for understanding aromaticity. Schleyer was particularly drawn to [18]-annulene because it can express the same <i>D<sub>6h</sub></i> symmetry as does benzene. His computed chemical shifts for the <i>D<sub>6h</sub></i> structure differed significantly from the experimental values, indicating that the structure was clearly wrong. “It was an amazing computational exercise,” Schleyer mused, “because practically every level you used to optimize the geometry gave a different structure. MP2 overshot the aromaticity, HF and B3LYP undershot it. Empirically, we had to find a level that worked. This was not very intellectually satisfying but was a pragmatic solution.” Schleyer expected a lot of flak from crystallographers about this result, but in fact none occurred. He hopes that the x-ray structure will be re-done at some point.</p> <p>Reflecting on the progress of computational chemistry, Schleyer recalls that “physical organic chemists were actually antagonistic toward computational chemistry at the beginning. One of my friends said that he thought I had gone mad. In addition, most theoreticians disdained me as a black-box user.” In those early years as a computational chemist, Schleyer felt disenfranchised from the physical organic chemistry community. Only slowly has he felt accepted back into this camp. “Physical organic chemists have adopted computational chemistry; perhaps, I hope to think, due to my example demonstrating what can be done. If you can show people that you can compute chemical properties, like chemical shifts to an accuracy that is useful, computed structures that are better than experiment, then they get the word sooner or later that maybe you’d better do some calculations.” In fact, Schleyer considers this to be his greatest contribution to science – demonstrating by his own example the importance of computational chemistry towards solving relevant chemical problems. He cites his role in helping to establish the <i>Journal of Computational Chemistry</i> in both giving name to the discipline and stature to its practitioners.</p> <p>Schleyer looks to the future of computational chemistry residing in the breadth of the periodic table. “Computational work has concentrated on one element, namely carbon,” Schleyer says. “The rest of the periodic table is waiting to be explored.” On the other hand, he is dismayed by the state of research at universities. In his opinion, “the function of universities is to do pure research, <i>not</i> to do applied research. Pure research will not be carried out at any other location.” Schleyer sums up his position this way – “Pure research is like putting money in the bank. Applied research is taking the money out.” According to this motto, Schleyer’s account is very much in the black.</p> <h4>Reprinted from <i>Computational Organic Chemistry</i>, Steven M. Bachrach, <b>2014</b>, Wiley:Hoboken. </h4> </td> <td> </td> </tr> <tr> <td colspan="3"> <hr> </td> </tr> </table>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">02 Dec 2014</span> <span class="comments"><a href="/blog/archives/3403#comments">2 Comments</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/3213" rel="bookmark" title="Permanent Link: A pentacoordinate carbon">A pentacoordinate carbon</a></h2> <div class="post-content"> <p>Trying to get carbon to bond in unnatural ways seems to be a passion for many organic chemists! Schleyer has been interested in unusual carbon structures for decades and he and Schaefer now report a molecule with a pentacoordinate carbon bound to five other carbon atoms. Their proposed target is pentamethylmethane cation C(CH<sub>3</sub>)<sub>5</sub><sup>+</sup> <b>1</b>.<a href="#CMe5"><sup>1</sup></a> The optimized geometry of <b>1</b>, which has <i>C<sub>3h</sub></i> symmetry, at MP2/cc-pVTZ is shown in Figure 1. The bonds from the central carbon to the equatorial carbon are a rather long 1.612 Å, but the bonds to the axial carbon are even longer, namely 1.736 Å. Bader analysis shows five bond critical points, each connecting the central carbon to one of the methyl carbons. Wiberg bond index and MO analysis suggests that the central carbon is tetravalent, with a 2-electron-3-center bond involving the central and axial carbons.</p> <table align="center" border="0" cellspacing="0" cellpadding="4"> <tr align="center"> <td colspan="2"> <p></p> <div class="jmol" id="CMe5"> <a onclick="return false"><br> <img src="/blog/wp-content/CMe5.jpg" onclick="insertJmol('CMe5',300,300,'CMe5.xyz')"></a> </div> <p><b>1</b></p> </td> </tr> <tr align="center"> <td> <p></p> <div class="jmol" id="CMe5ts1"> <a onclick="return false"><br> <img src="/blog/wp-content/CMe5ts1.jpg" onclick="insertJmol('CMe5ts1',300,300,'CMe5ts1.xyz')"></a> </div> <p><b>TS1</b></p> </td> <td> <p></p> <div class="jmol" id="CMe5ts2"> <a onclick="return false"><br> <img src="/blog/wp-content/CMe5ts2.jpg" onclick="insertJmol('CMe5ts2',300,300,'CMe5ts2.xyz')"></a> </div> <p><b>TS2</b></p> </td> </tr> </table> <p align="center"><b>Figure 1</b>. MP2/cc-pVTZ optimized geometries of <b>1</b> and dissociation transition states.</p> <p>So while <b>1</b> is a local energy minimum, it sits in a very shallow well. One computed dissociation path, which passes through <b>TS1</b> (Figure 1) on its way to 2-methyl-butyl cation and methane has a barrier of only 1.65 kcal mol<sup>-1</sup> (CCSD(T)/CBS + ZPE). A second dissociation pathway goes through <b>TS2</b> to t-butyl cation and ethane with a barrier of only 1.34 kcal mol<sup>-1</sup>. Worse still is that the free energy estimates suggest “spontaneous dissociation … through both pathways”.</p> <p>Undoubtedly, this will not be the last word on trying to torture a poor carbon atom.</p> <h3>References</h3> <p><a name="CMe5"></a></p> <p>(1) McKee, W. C.; Agarwal, J.; Schaefer, H. F.; Schleyer, P. v. R. "Covalent Hypercoordination: Can Carbon Bind Five Methyl Ligands?," <i>Angew. Chem. Int. Ed.</i> <b>2014</b>, <i>53</i>, 7875-7878, DOI: <a href="http://dx.doi.org/10.1002/anie.201403314">10.1002/anie.201403314</a>.</p> <h3>InChIs</h3> <p><b>1</b>: InChI=1S/C6H15/c1-6(2,3,4)5/h1-5H3/q+1<br>InChIKey=GGCBGJZCTGZYFV-UHFFFAOYSA-N</p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/authors/schaefer" rel="category tag">Schaefer</a> &<a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">25 Aug 2014</span> <span class="comments"><a href="/blog/archives/3213#comments">1 Comment</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/2774" rel="bookmark" title="Permanent Link: The x-ray structure of norbornyl cation">The x-ray structure of norbornyl cation</a></h2> <div class="post-content"> <p>A long sought-after data point critical to the non-classical cation story has finally been obtained. The elusive x-ray crystal structure of a norbornyl cation was finally solved.<a href="#norbonylXray1"><sup>1</sup></a> The [C<sub>7</sub>H<sub>11</sub>]<sup>+</sup>[Al<sub>2</sub>Br<sub>7</sub>]<sup>–</sup> salt was crystallized in CH<sub>2</sub>Br<sub>2</sub> at low temperature (40 K). This low temperature was needed to prohibit rotation of the norbornyl cation within the crystal (the cation is near spherical and so subject to relatively easy rotation within the crystal matrix) and hydride scrambling among the three carbons (C<sub>1</sub>, C<sub>2</sub>, and C<sub>6</sub>) involved in the non-classical cation structure.</p> <p>The authors report a number of different structures, all very similar, depending on slight differences in the crystals used. However, the important features are consistent with all of the structures. The cation is definitely of the non-classical type (see Figure 1) with the basal C<sub>1</sub>-C<sub>2</sub> bond length of 1.39 Å similar that in benzene and long non-classical C<sub>1</sub>-C<sub>6</sub> and C<sub>2</sub>-C<sub>6</sub> distances of 1.80 Å. These distances match very well with the MP2(FC)/def2-QZVPP optimized distances of 1.393 and 1.825 Å, respectively.</p> <table align="center" border="0"> <tr align="center"> <td> <p></p> <div class="jmol" id="nbXray"> <a onclick="return false"><br> <img src="/blog/wp-content/norbornylXray.png" onclick="insertJmol('nbXray',220,220,'norbornylXray.xyz')"><br> </a> </div> </td> </tr> </table> <p align="center"><b>Figure 1.</b> X-ray structure of norbornyl cation.</p> <h3>References</h3> <p><a name="norbonylXray1"></a></p> <p>(1) Scholz, F.; Himmel, D.; Heinemann, F. W.; Schleyer, P. v. R.; Meyer, K.; Krossing, I. "Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation," <i>Science</i> <b>2013</b>, <i>341</i>, 62-64, DOI: <a href="http://dx.doi.org/10.1126/science.1238849">10.1126/science.1238849</a>.</p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/molecules/non-classical" rel="category tag">non-classical</a> &<a href="/blog/archives/category/molecules/norbornyl-cation" rel="category tag">norbornyl cation</a> &<a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">16 Sep 2013</span> <span class="comments"><a href="/blog/archives/2774#respond">No Comments</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/2759" rel="bookmark" title="Permanent Link: Nonamethylcyclopentyl cation">Nonamethylcyclopentyl cation</a></h2> <div class="post-content"> <p>The nine methyl groups of nonamethylcyclopentyl cation <b>1</b> all interconvert with a barrier of 7 kcal mol<sup>-1</sup>. However, at low temperature only partial scrambling occurs: there are two sets of methyl groups, one containing five groups and the other containing four methyl groups. The barrier for this scrambling is only 2.5 kcal mol<sup>-1</sup>. While this behavior was found more than 20 years ago, Tantillo and Schleyer<a href="#Me9CPr1"><sup>1</sup></a> only now have offered a complete explanation.</p> <p align="center"><img src="/blog/wp-content/Me9CPimg1.png"><br><b>1</b></p> <p>The ground state structure of <b>1</b> is shown in Figure 1 and has <i>C<sub>1</sub></i> symmetry. The two pseudo-axial methyl groups adjacent to the cationic center show evidence of hyperconjugation: long C-C bonds and Me-C-C<sup>+</sup> angles of 100°.</p> <p>The transition state <b>TS1</b>¸also in Figure 1, is of <i>C<sub>s</sub></i> symmetry. This transition state leads to interchange of the pseudo-axial methyls, and interchange of the pseudo-equatorial methyls, but no exchange between the members of these two groups. The M06-2x/6-31+G(d,p) and mPW1PW91/6-31+G(d,p) estimate of this barrier is 1.5 and 2.5 kcal mol<sup>-1</sup>, respectively. This agrees well with the experiment.</p> <table align="center" border="0" cellspacing="0" cellpadding="3"> <tr align="center" valign="middle"> <td colspan="2"> <p></p> <div class="jmol" id="Me9CP1"> <a onclick="return false"><br> <img src="/blog/wp-content/Me9CP1.jpg" onclick="insertJmol('Me9CP1',300,300,'Me9CP1.xyz')"><br> </a> </div> <p><b>1</b></p> </td> </tr> <tr align="center" valign="middle"> <td> <p></p> <div class="jmol" id="Me9CPTS1"> <a onclick="return false"><br> <img src="/blog/wp-content/Me9CPTS1.jpg" onclick="insertJmol('Me9CPTS1',300,300,'Me9CPTS1.xyz')"><br> </a> </div> <p><b>TS1</b></p> </td> <td> <p></p> <div class="jmol" id="Me9CPTS2"> <a onclick="return false"><br> <img src="/blog/wp-content/Me9CPTS2.jpg" onclick="insertJmol('Me9CPTS2',300,300,'Me9CPTS2.xyz')"><br> </a> </div> <p><b>TS2</b></p> </td> </tr> </table> <p align="center"><b>Figure 1</b>. B3LYP/6-3+G(d,p) optimized geometries.</p> <p>A second transition state <b>TS2</b> was found and it corresponds with a twisting motion that interconverts an axial methyl with an equatorial methyl. This TS has <i>C<sub>s</sub></i> symmetry (shown in Figure 1) and the eclipsing interaction give rise to a larger barrier: 7.3 (M06-2x/6-31+G(d,p)) and 6.7 kcal mol<sup>-1</sup> (mPW1PW91/6-31+G(d,p)). So twisting through <b>TS2</b> and scrambling through <b>TS1</b> allows for complete exchange of all 9 methyl groups.</p> <p>An interesting point also made by these authors is that these three structures represent the continuum of cationic structure: a classical (localized) cation in <b>TS2</b>, a bridged structure in <b>TS1</b> and hyperconjugated cation in <b>1</b>.</p> <h3>References</h3> <p><a name="Me9CPr1"></a></p> <p>(1) Tantillo, D. J.; Schleyer, P. v. R. “Nonamethylcyclopentyl Cation Rearrangement Mysteries Solved,” <i>Org. Lett.</i> <b>2013</b>, <i>15</i>, 1725-1727, DOI: <a href="http://dx.doi.org/10.1021/ol4005189">10.1021/ol4005189</a>.</p> <h3>InChIs</h3> <p><b>1</b>: InChI=1S/C14H27/c1-10-11(2,3)13(6,7)14(8,9)12(10,4)5/h1-9H3/q+1<br>InChIKey=WUGVCUSQGLXERW-UHFFFAOYSA-N</p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/molecules/non-classical" rel="category tag">non-classical</a> &<a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">23 Jul 2013</span> <span class="comments"><a href="/blog/archives/2759#comments">4 Comments</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/2749" rel="bookmark" title="Permanent Link: Triplet state aromaticity">Triplet state aromaticity</a></h2> <div class="post-content"> <p>One of the most widely recognized principles within organic chemistry is Hückel’s rule: an aromatic compound possesses 4<i>n</i>+2 π-electrons while an antiaromatic compound possesses 4<i>n</i> π-electrons. Much less well known is Baird’s rule:<a href="#tripletArom1"><sup>1</sup></a> the first excited triplet state will be aromatic if it has 4<i>n</i> π-electrons and antiaromatic if it has 4<i>n</i>+2 π-electrons.<a href="#tripletArom2"><sup>2</sup></a></p> <p>Schleyer used a number of standard methods for assessing aromatic character of a series of excited state triplets, including NICS values and geometric parameters.<a href="#tripletArom3"><sup>3</sup></a> However, Schleyer has long been a proponent of an energetic assessment of aromaticity and it is only now in this recent paper<a href="#tripletArom4"><sup>4</sup></a> that he and co-workers examine the stabilization energy of excited triplet states. The isomerization<br> stabilization energy (ISE)<a href="#tripletArom5"><sup>5</sup></a> compares an aromatic (or antiaromatic) compound against a non-aromatic reference, one that typically is made by appending an exo-methylene group to the ring. So, to assess the ISE of the T<sub>1</sub> state of benzene, Reaction 1 is used. (Note that the inherent assumption here is that the stabilization energy of benzene is essentially identical to that of toluene.) At B3LYP/6-311++G(d,p) the energy of Reaction 1 is +13.5 kcal mol<sup>-1</sup>. This reaction should be corrected for non-conservation of <i>s</i>-cis and <i>s</i>-trans conformers by adding on the energy of Reaction 2, which is +3.4 kcal mol<sup>-1</sup>. So, the ISE of triplet benzene is +16.9 kcal mol<sup>-1</sup>, indicating that it is <i>antiaromatic</i>. In contrast, the ISE for triplet cyclooctatetraene is -15.6 kcal mol<sup>-1</sup>, and when corrected its ISE value is -24.7 kcal mol<sup>-1</sup>, indicating <i>aromatic</i> character. These are completely consistent with Baird’s rule. Schleyer also presents an excellent correlation between the computed ISE values for the triplet state of 9 monocyclic polyenes and their NICS(1)<sub>zz</sub> values.</p> <table align="center" border="0" cellspacing="2" cellpadding="6"> <tr align="center" valign="middle"> <td> <p><img src="/blog/wp-content/tripletAromRx1.png"></p> </td> <td> <p>Reaction 1</p> </td> </tr> <tr align="center" valign="middle"> <td> <p><img src="/blog/wp-content/tripletAromRx2.png"></p> </td> <td> <p>Reaction 2</p> </td> </tr> </table> <p>I want to thank Henrik Ottosson for bringing this paper to my attention and for his excellent seminar on the subject of Baird’s rule on his recent visit to Trinity University.</p> <h3>References</h3> <p><a name="tripletArom1"></a></p> <p>(1) Baird, N. C. "Quantum organic photochemistry. II. Resonance and aromaticity in<br> the lowest 3ππ* state of cyclic hydrocarbons," <i>J. Am. Chem. Soc.</i> <b>1972</b>, <i>94</i>, 4941-4948, DOI: <a href="http://dx.doi.org/10.1021/ja00769a025">10.1021/ja00769a025</a>.</p> <p>(2) Ottosson, H. "Organic photochemistry: Exciting excited-state aromaticity," <i>Nat Chem</i> <b>2012</b>, <i>4</i>, 969-971, DOI: <a href="http://dx.doi.org/10.1038/nchem.1518">10.1038/nchem.1518</a>.</p> <p>(3) Gogonea, V.; Schleyer, P. v. R.; Schreiner, P. R. "Consequences of Triplet Aromaticity in 4<i>n</i>π-Electron Annulenes: Calculation of Magnetic Shieldings for Open-Shell Species," <i>Angew. Chem. Int. Ed.</i> <b>1998</b>, <i>37</i>, 1945-1948, DOI: <a href="http://dx.doi.org/10.1002/(SICI)1521-3773(19980803)37:13/14%3c1945::AID-ANIE1945%3e3.0.CO;2-E">10.1002/(SICI)1521-3773(19980803)37:13/14<1945::AID-ANIE1945>3.0.CO;2-E</a>.</p> <p>(4) Zhu, J.; An, K.; Schleyer, P. v. R. "Evaluation of Triplet Aromaticity by the<br> Isomerization Stabilization Energy," <i>Org. Lett.</i> <b>2013</b>, <i>15</i>, 2442-2445, DOI: <a href="http://dx.doi.org/10.1021/ol400908z">10.1021/ol400908z</a>.</p> <p>(5) Schleyer, P. v. R.; Puhlhofer, F. "Recommendations for the Evaluation of Aromatic Stabilization Energies," <i>Org. Lett.</i> <b>2002</b>, <i>4</i>, 2873-2876, DOI: <a href="http://dx.doi.org/10.1021/ol0261332">10.1021/ol0261332</a>.</p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/aromaticity" rel="category tag">Aromaticity</a> &<a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">16 Jul 2013</span> <span class="comments"><a href="/blog/archives/2749#respond">No Comments</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/2304" rel="bookmark" title="Permanent Link: Aromatic TS for a non-pericyclic reaction">Aromatic TS for a non-pericyclic reaction</a></h2> <div class="post-content"> <p>The activation energy for the 5-endo-dig reaction of the anion <b>1</b> is anomalously low compared to its 4-endo-dig and 6-endo-dig analogues. Furthermore, the TS is quite early, earlier than might be expected based on the Hammond Postulate. Alabugin and Schleyer have examined this reaction and found some interesting results.<a href="#5endodig1"><sup>1</sup></a></p> <p align="center"><img src="/blog/wp-content/5endoI1.png"></p> <p>First, NICS(0) values for a series of related intermolecular anionic attack at alkynes show some interesting trends (Table 1). Two of the transition states look like they might be aromatic: the TSs for the 3-exo-dig and the 5-endo-dig reaction have NICS(0) values that are quite negative. However, given the geometry of these TSs, particularly the close proximity of the σ bonds to the ring center, one might be concerned about contamination of these orbitals. So, NICS(0)<sub>MOzz</sub> computations, which look at the tensor component perpendicular to the ring using just the π-MOs, shows that the 3-exo-dig is likely non-aromatic (NICS(0)<sub>MOzz</sub> is near zero), the TS for the 4-endo-dig reaction is antiaromatic (NICS(0)<sub>MOzz</sub> very positive) and the TS for the 5-endo-dig reaction is aromatic (NICS(0)<sub>MOzz</sub> is very negative. So this last reaction is the first example of an aromatic transition that is <i>not</i> for a pericyclic reaction!</p> <p align="center"><b>Table 1</b>. NICS(0) and NICS(0)<sub>MOzz</sub> for the TS of some anionic alkyne cyclizations.</p> <table align="center" border="0" cellspacing="0" cellpadding="4"> <tr align="center" valign="middle"> <td> <p> </p> </td> <td> <p>NICS(0)</p> </td> <td> <p>NICS(0)<sub>MOzz</sub><sub></sub></p> </td> </tr> <tr align="center" valign="middle"> <td> <p><img src="/blog/wp-content/5endoI2.png"><br>3-exo-dig</p> </td> <td> <p>-19.3</p> </td> <td> <p>-1.6</p> </td> </tr> <tr align="center" valign="middle"> <td> <p><img src="/blog/wp-content/5endoI3.png"><br>4-endo-dig</p> </td> <td> <p>1.8</p> </td> <td> <p>23.9</p> </td> </tr> <tr align="center" valign="middle"> <td> <p><img src="/blog/wp-content/5endoI4.png"><br>5-endo-dig (<b>1</b>)</p> </td> <td> <p>-15.2</p> </td> <td> <p>-20.5</p> </td> </tr> </table> <p>These authors argue that the reaction of <b>1</b> is an “aborted” sigmatropic shift. A normal pericyclic reaction is a single step with a single (concerted) transition state. An interrupted sigmatropic shift has an intermediate that lies higher in energy than the reactants, such as in the Bergman cyclization of an enediyne. The aborted sigmatropic shift has an intermediate that lies lower in energy than the reactants, such as in the cyclization of <b>1</b>.</p> <h3>References</h3> <p><a name="5endodig1"></a></p> <p>(1) Gilmore, K.; Manoharan, M.; Wu, J. I. C.; Schleyer, P. v. R.; Alabugin, I. V. "Aromatic Transition States in Nonpericyclic Reactions: Anionic 5-Endo Cyclizations Are Aborted Sigmatropic Shifts," <i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 10584–10594, DOI: <a href="http://dx.doi.org/10.1021/ja303341b">10.1021/ja303341b</a></p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/aromaticity" rel="category tag">Aromaticity</a> &<a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">24 Jul 2012</span> <span class="comments"><a href="/blog/archives/2304#comments">5 Comments</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/1757" rel="bookmark" title="Permanent Link: Electrophilic aromatic substitution is really addition-elimination">Electrophilic aromatic substitution is really addition-elimination</a></h2> <div class="post-content"> <p>We have all learned about aromatic substitution as proceeding via the following mechanism</p> <p align="center"><img src="/blog/wp-content/aromSubimg1.gif"></p> <p>(Worse yet – many of us have taught this for years!) Well, Galabov, Zou, Schaefer and Schleyer pour a whole lot of cold water on this notion in their recent <i>Angewandte</i> article.<a href="#AromSub"><sup>1</sup></a> Modeling the reaction of benzene with Br<sub>2</sub> and using B3LYP/6-311+G(2d,2p) for both the gas phase and PCM simulating a CCl<sub>4</sub> solvent, attempts to locate this standard intermediate led instead to a concerted substitution transition state <b>TS1</b> (see Figure 1).</p> <table align="center" border="0" cellspacing="0" cellpadding="0"> <tr align="center"> <td> <p></p> <div class="jmol" id="aromSubTS1"> <a onclick="return false"><br> <img src="/blog/wp-content/aromSubTS1.jpg" onclick="insertJmol('aromSubTS1',225,225,'aromSubTS1.xyz')"><br> </a> </div> <p><b>TS1</b></p> </td> </tr> </table> <p align="center"><b>Figure 1</b>. PCM/B3LYP/6-311+G(2d,2p) optimized transitin state along the concerted pathway</p> <p>However, this is not the lowest energy pathway for substitution. Rather and addition-elimination pathway is kinetically preferred. In the first step Br<sub>2</sub> adds in either a 1,2 or 1,4 fashion to form an intermediate. The lower energy path is the 1,4 addition, leading to <b>P3</b>. This intermediate then undergoes a syn,anti-isomerization to give <b>P5</b>. The last step is the elimination of HBr from <b>P5</b> to give the product, bromobenzene. This mechanism is shown in Scheme 2 and the critical points are shown in Figure 3.</p> <p align="center"><b>Scheme 1</b></p> <p align="center"><img src="/blog/wp-content/aromSubimg2.gif"></p> <table align="center" border="0" cellspacing="0" cellpadding="4"> <tr align="center"> <td> <p></p> <div class="jmol" id="aromSubTS3"> <a onclick="return false"><br> <img src="/blog/wp-content/aromSubTS3.jpg" onclick="insertJmol('aromSubTS3',225,225,'aromSubTS3.xyz')"><br> </a> </div> <p><b>TS3</b></p> </td> <td> <p></p> <div class="jmol" id="aromSubP3"> <a onclick="return false"><br> <img src="/blog/wp-content/aromSubP3.jpg" onclick="insertJmol('aromSubP3',225,225,'aromSubP3.xyz')"><br> </a> </div> <p><b>P3</b></p> </td> </tr> <tr align="center"> <td> <p></p> <div class="jmol" id="aromSubTS6"> <a onclick="return false"><br> <img src="/blog/wp-content/aromSubTS6.jpg" onclick="insertJmol('aromSubTS6',225,225,'aromSubTS6.xyz')"><br> </a> </div> <p><b>TS6</b></p> </td> <td> <p></p> <div class="jmol" id="aromSubP5"> <a onclick="return false"><br> <img src="/blog/wp-content/aromSubP5.jpg" onclick="insertJmol('aromSubP5',225,225,'aromSubP5.xyz')"><br> </a> </div> <p><b>P5</b></p> </td> </tr> <tr align="center"> <td> <p></p> <div class="jmol" id="aromSubTS9"> <a onclick="return false"><br> <img src="/blog/wp-content/aromSubTS9.jpg" onclick="insertJmol('aromSubTS9',225,225,'aromSubTS9.xyz')"><br> </a> </div> <p><b>TS9</b></p> </td> <td> <p> </p> </td> </tr> </table> <p align="center"><b>Figure 2</b>. PCM/B3LYP/6-311+G(2d,2p) optimized critical points along the addition-elimination pathway</p> <p>The barrier for the concerted substitution process through <b>TS1 </b>is 41.8 kcal mol<sup>-1</sup> (in CCl<sub>4</sub>) while the highest barrier for the addition-elimination process is through <b>TS3</b> of 39.4 kcal mol<sup>-1</sup>.</p> <p>Now a bit of saving grace is that in polar solvents, acidic solvents and/or with Lewis acid catalysts, the intermediate of the standard textbook mechanism may be competitive. </p> <p>Textbook authors – please be aware!</p> <h3>References</h3> <p><a name="AromSub"></a></p> <p>(1) Kong, J.; Galabov, B.; Koleva, G.; Zou, J.-J.; Schaefer, H. F.; Schleyer, P. v. R., "The Inherent Competition between Addition and Substitution Reactions of Br<sub>2</sub> with Benzene and Arenes," <i>Angew. Chem. Int. Ed.</i> <b>2011</b>, <i>50</i>, 6809-6813, DOI: <a href="http://dx.doi.org/10.1002/anie.201101852">10.1002/anie.201101852</a></p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/reactions/electrophilic-aromatic-substitution" rel="category tag">electrophilic aromatic substitution</a> &<a href="/blog/archives/category/authors/schaefer" rel="category tag">Schaefer</a> &<a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">27 Sep 2011</span> <span class="comments"><a href="/blog/archives/1757#comments">4 Comments</a></span> </p> </div> <div class="box"> <h2><a href="/blog/archives/1631" rel="bookmark" title="Permanent Link: 1-Adamantyl cation – Predicting its NMR spectra">1-Adamantyl cation – Predicting its NMR spectra</a></h2> <div class="post-content"> <p>What is required in order to compute <i>very</i> accurate NMR chemical shifts? Harding, Gauss and Schleyer take on the interesting spectrum of 1-adamantyl cation to try to discern the important factors in computing its <sup>13</sup>C and <sup>1</sup>H chemical shifts.<a href="#adamCat1"><sup>1</sup></a></p> <p align="center"><img src="/blog/wp-content/adamantylCat.gif"><br><b>1</b></p> <p>To start, the chemical shifts of 1-adamtyl cation were computed at B3LYP/def2-QZVPP and<br> MP2/qz2p//MP2/cc-pVTZ. The root means square error (compared to experiment) for the carbon chemical shifts is large: 12.76 for B3LYP and 6.69 for MP2. The proton shifts are predicted much more accurately with an RMS error of 0.27 and 0.19 ppm, respectively.</p> <p>The authors speculate that the underlying cause of the poor prediction is the geometry of the molecule. The structure of <b>1</b> was optimized at HF/cc-pVTZ, MP2/cc-pVTZ and CCSD(T)/pVTZ and then the chemical shifts were computed using MP2/tzp with each optimized geometry. The RMS error of the <sup>12</sup>C chemical shifts are HF/cc-pVTZ: 9.55, MP2/cc-pVTZ: 5.62, and CCSD(T)/pVTZ: 5.06. Similar relationship is seen in the proton chemical shifts. Thus, a better geometry does seem to matter. The CCSD(T)/pVTZ optimized structure of <b>1</b> is shown in Figure 1.</p> <table align="center" border="0" cellspacing="0" cellpadding="0"> <tr align="center"> <td> <p></p> <div class="jmol" id="adamCat"> <a onclick="return false"><br> <img src="/blog/wp-content/adamantylCation.jpg" onclick="insertJmol('adamCat',320,320,'adamantylCation.xyz')"><br> </a> </div> <p><b>1</b></p> </td> </tr> </table> <p align="center"><b>Figure 1.</b> CCSD(T)/pVTZ optimized structure of <b>1</b>.</p> <p>Unfortunately, the computed chemical shifts at CCSD(T)/qz2p//CCSD(T)/cc-pVTZ are still in error; the RMS is 4.78ppm for the carbon shifts and 0.26ppm for the proton shifts. Including a correction for the zero-point vibrational effects and adjusting to a temperature of 193 K to match the experiment does reduce the error; now the RMS for the carbon shifts is 3.85 ppm, with the maximum error of 6 ppm for C3. The RMS for the proton chemical shifts is 0.21ppm.</p> <p>The remaining error they attribute to basis set incompleteness in the NMR computation, a low level treatment of the zero-point vibrational effects (which were computed at HF/tz2p), neglect of the solvent, and use of a reference in the experiment that was not dissolved in the same media as the adamantyl cation.</p> <p>So, to answer our opening question – it appears that a very good geometry and treatment of vibrational effects is critical to accurate NMR shift computation of this intriguing molecule. Let the<br> computational chemist beware!</p> <h3>References</h3> <p><a name="adamCat1"></a></p> <p>(1) Harding, M. E.; Gauss, J.; Schleyer, P. v. R., "Why Benchmark-Quality Computations Are Needed To Reproduce 1-Adamantyl Cation NMR Chemical Shifts Accurately," <i>J. Phys. Chem. A</i>, <b>2011</b>, <i>115</i>, 2340-2344, DOI: <a href="http://dx.doi.org/10.1021/jp1103356">10.1021/jp1103356</a></p> <h3>InChI</h3> <p><b>1</b>: InChI=1/C10H15/c1-7-2-9-4-8(1)5-10(3-7)6-9/h7-9H,1-6H2/q+1<br>InChIKey=HNHINQSSKCACRU-UHFFFAOYAC</p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/molecules/adamantane" rel="category tag">adamantane</a> &<a href="/blog/archives/category/nmr" rel="category tag">NMR</a> &<a href="/blog/archives/category/authors/schleyer" rel="category tag">Schleyer</a></span> <span class="user">Steven Bachrach</span> <span class="date">18 Jul 2011</span> <span class="comments"><a href="/blog/archives/1631#comments">4 Comments</a></span> </p> </div> <p align="center"><a href="/blog/archives/category/authors/schleyer/page/2">Next Page »</a></p> </div> <div id="sidebar"> <ul> <li class="box"> <h2> Categories </h2> <ul> <li class="cat-item cat-item-25"> <a href="/blog/archives/category/acidity">Acidity</a> (12) </li> <li class="cat-item cat-item-3"> <a href="/blog/archives/category/aromaticity">Aromaticity</a> (91) </li> <li class="cat-item cat-item-53 current-cat-parent current-cat-ancestor"> <a href="/blog/archives/category/authors">Authors</a> (153) <ul class="children"> <li class="cat-item cat-item-42"> <a href="/blog/archives/category/authors/borden">Borden</a> (12) </li> <li class="cat-item cat-item-12"> <a href="/blog/archives/category/authors/cramer">Cramer</a> (11) </li> <li class="cat-item cat-item-83"> <a href="/blog/archives/category/authors/grimme">Grimme</a> (17) </li> <li class="cat-item cat-item-9"> <a href="/blog/archives/category/authors/houk">Houk</a> (40) </li> <li class="cat-item cat-item-29"> <a href="/blog/archives/category/authors/jorgensen">Jorgensen</a> (3) </li> <li class="cat-item cat-item-16"> <a href="/blog/archives/category/authors/kass">Kass</a> (9) </li> <li class="cat-item cat-item-30"> <a href="/blog/archives/category/authors/schaefer">Schaefer</a> (13) </li> <li class="cat-item cat-item-17 current-cat"> <a aria-current="page" href="/blog/archives/category/authors/schleyer">Schleyer</a> (24) </li> <li class="cat-item cat-item-73"> <a href="/blog/archives/category/authors/schreiner">Schreiner</a> (29) </li> <li class="cat-item cat-item-6"> <a href="/blog/archives/category/authors/singleton">Singleton</a> (11) </li> <li class="cat-item cat-item-18"> <a href="/blog/archives/category/authors/truhlar">Truhlar</a> (8) </li> </ul> </li> <li class="cat-item cat-item-15"> <a href="/blog/archives/category/bond-dissociation-energy">Bond Dissociation Energy</a> (6) </li> <li class="cat-item cat-item-81"> <a href="/blog/archives/category/bsse">BSSE</a> (1) </li> <li class="cat-item cat-item-88"> <a href="/blog/archives/category/cyclophane">cyclophane</a> (0) </li> <li class="cat-item cat-item-4"> <a href="/blog/archives/category/dynamics">Dynamics</a> (35) </li> <li class="cat-item cat-item-57"> <a href="/blog/archives/category/e-publishing">E-publishing</a> (7) </li> <li class="cat-item cat-item-65"> <a href="/blog/archives/category/enzyme">Enzyme</a> (4) </li> <li class="cat-item cat-item-95"> <a href="/blog/archives/category/fep">FEP</a> (1) </li> <li class="cat-item cat-item-86"> <a href="/blog/archives/category/host-guest">host-guest</a> (6) </li> <li class="cat-item cat-item-84"> <a href="/blog/archives/category/hydrogen-bond">Hydrogen bond</a> (5) </li> <li class="cat-item cat-item-91"> <a href="/blog/archives/category/ion-pairs">Ion Pairs</a> (1) </li> <li class="cat-item cat-item-74"> <a href="/blog/archives/category/isotope-effects">Isotope Effects</a> (5) </li> <li class="cat-item cat-item-67"> <a href="/blog/archives/category/keto-enol-tautomerization">Keto-enol tautomerization</a> (3) </li> <li class="cat-item cat-item-54"> <a href="/blog/archives/category/molecules">Molecules</a> (100) <ul class="children"> <li class="cat-item cat-item-48"> <a href="/blog/archives/category/molecules/adamantane">adamantane</a> (3) </li> <li class="cat-item cat-item-26"> <a href="/blog/archives/category/molecules/amino-acids">amino acids</a> (13) </li> <li class="cat-item cat-item-19"> <a href="/blog/archives/category/molecules/annulenes">annulenes</a> (8) </li> <li class="cat-item cat-item-27"> <a href="/blog/archives/category/molecules/benzynes">benzynes</a> (4) </li> <li class="cat-item cat-item-46"> <a href="/blog/archives/category/molecules/biphenyl">biphenyl</a> (1) </li> <li class="cat-item cat-item-70"> <a href="/blog/archives/category/molecules/calixarenes">calixarenes</a> (1) </li> <li class="cat-item cat-item-33"> <a href="/blog/archives/category/molecules/carbenes">carbenes</a> (13) </li> <li class="cat-item cat-item-72"> <a href="/blog/archives/category/molecules/cyclobutadiene">cyclobutadiene</a> (4) </li> <li class="cat-item cat-item-62"> <a href="/blog/archives/category/molecules/dendralenes">dendralenes</a> (1) </li> <li class="cat-item cat-item-66"> <a href="/blog/archives/category/molecules/dewar-benzene">Dewar benzene</a> (1) </li> <li class="cat-item cat-item-39"> <a href="/blog/archives/category/molecules/diradicals">diradicals</a> (8) </li> <li class="cat-item cat-item-59"> <a href="/blog/archives/category/molecules/ephedrine">ephedrine</a> (1) </li> <li class="cat-item cat-item-37"> <a href="/blog/archives/category/molecules/ethyl-cation">ethyl cation</a> (2) </li> <li class="cat-item cat-item-90"> <a href="/blog/archives/category/molecules/fullerene">fullerene</a> (6) </li> <li class="cat-item cat-item-51"> <a href="/blog/archives/category/molecules/fulvalenes">fulvalenes</a> (1) </li> <li class="cat-item cat-item-21"> <a href="/blog/archives/category/molecules/hexacyclinol">hexacyclinol</a> (2) </li> <li class="cat-item cat-item-78"> <a href="/blog/archives/category/molecules/nanohoops">nanohoops</a> (4) </li> <li class="cat-item cat-item-41"> <a href="/blog/archives/category/molecules/non-classical">non-classical</a> (4) </li> <li class="cat-item cat-item-34"> <a href="/blog/archives/category/molecules/norbornyl-cation">norbornyl cation</a> (2) </li> <li class="cat-item cat-item-49"> <a href="/blog/archives/category/molecules/nucleic-acids">nucleic acids</a> (4) </li> <li class="cat-item cat-item-36"> <a href="/blog/archives/category/molecules/oximes">oximes</a> (1) </li> <li class="cat-item cat-item-75"> <a href="/blog/archives/category/molecules/phenyloxenium">phenyloxenium</a> (1) </li> <li class="cat-item cat-item-8"> <a href="/blog/archives/category/molecules/polycyclic-aromatics">polycyclic aromatics</a> (7) </li> <li class="cat-item cat-item-50"> <a href="/blog/archives/category/molecules/propellane">propellane</a> (2) </li> <li class="cat-item cat-item-79"> <a href="/blog/archives/category/molecules/stilbene">stilbene</a> (1) </li> <li class="cat-item cat-item-80"> <a href="/blog/archives/category/molecules/sugars">sugars</a> (5) </li> <li class="cat-item cat-item-85"> <a href="/blog/archives/category/molecules/terpenes">terpenes</a> (2) </li> <li class="cat-item cat-item-89"> <a href="/blog/archives/category/molecules/twistane">twistane</a> (1) </li> </ul> </li> <li class="cat-item cat-item-22"> <a href="/blog/archives/category/nmr">NMR</a> (40) </li> <li class="cat-item cat-item-31"> <a href="/blog/archives/category/optical-rotation">Optical Rotation</a> (16) </li> <li class="cat-item cat-item-28"> <a href="/blog/archives/category/qm-method">QM Method</a> (96) <ul class="children"> <li class="cat-item cat-item-20"> <a href="/blog/archives/category/qm-method/caspt2">CASPT2</a> (1) </li> <li class="cat-item cat-item-7"> <a href="/blog/archives/category/qm-method/dft">DFT</a> (71) </li> <li class="cat-item cat-item-45"> <a href="/blog/archives/category/qm-method/focal-point">focal point</a> (7) </li> <li class="cat-item cat-item-14"> <a href="/blog/archives/category/qm-method/g3">G3</a> (3) </li> <li class="cat-item cat-item-60"> <a href="/blog/archives/category/qm-method/mp">MP</a> (11) </li> </ul> </li> <li class="cat-item cat-item-56"> <a href="/blog/archives/category/reactions">Reactions</a> (83) <ul class="children"> <li class="cat-item cat-item-13"> <a href="/blog/archives/category/reactions/12-addition">1,2-addition</a> (1) </li> <li class="cat-item cat-item-35"> <a href="/blog/archives/category/reactions/aldol">aldol</a> (4) </li> <li class="cat-item cat-item-32"> <a href="/blog/archives/category/reactions/bergman-cyclization">Bergman cyclization</a> (6) </li> <li class="cat-item cat-item-44"> <a href="/blog/archives/category/reactions/claisen-rearrangement">Claisen rearrangement</a> (2) </li> <li class="cat-item cat-item-10"> <a href="/blog/archives/category/reactions/cope-rearrangement">Cope Rearrangement</a> (5) </li> <li class="cat-item cat-item-69"> <a href="/blog/archives/category/reactions/cycloadditions">cycloadditions</a> (12) </li> <li class="cat-item cat-item-23"> <a href="/blog/archives/category/reactions/diels-alder">Diels-Alder</a> (26) </li> <li class="cat-item cat-item-47"> <a href="/blog/archives/category/reactions/electrocyclization">electrocyclization</a> (11) </li> <li class="cat-item cat-item-76"> <a href="/blog/archives/category/reactions/electrophilic-aromatic-substitution">electrophilic aromatic substitution</a> (1) </li> <li class="cat-item cat-item-5"> <a href="/blog/archives/category/reactions/ene-reaction">ene reaction</a> (1) </li> <li class="cat-item cat-item-52"> <a href="/blog/archives/category/reactions/hajos-parrish-reaction">Hajos-Parrish Reaction</a> (1) </li> <li class="cat-item cat-item-61"> <a href="/blog/archives/category/reactions/mannich">Mannich</a> (2) </li> <li class="cat-item cat-item-64"> <a href="/blog/archives/category/reactions/michael-addition">Michael addition</a> (5) </li> <li class="cat-item cat-item-40"> <a href="/blog/archives/category/reactions/ozonolysis">ozonolysis</a> (1) </li> <li class="cat-item cat-item-43"> <a href="/blog/archives/category/reactions/proton-transfer">proton transfer</a> (1) </li> <li class="cat-item cat-item-38"> <a href="/blog/archives/category/reactions/pseudopericyclic">pseudopericyclic</a> (4) </li> <li class="cat-item cat-item-63"> <a href="/blog/archives/category/reactions/strecker">Strecker</a> (1) </li> <li class="cat-item cat-item-24"> <a href="/blog/archives/category/reactions/substitution">Substitution</a> (6) </li> <li class="cat-item cat-item-93"> <a href="/blog/archives/category/reactions/wittig">Wittig</a> (1) </li> </ul> </li> <li class="cat-item cat-item-87"> <a href="/blog/archives/category/second-edition">Second Edition</a> (3) </li> <li class="cat-item cat-item-11"> <a href="/blog/archives/category/solvation">Solvation</a> (17) </li> <li class="cat-item cat-item-77"> <a href="/blog/archives/category/stereochemistry">Stereochemistry</a> (2) </li> <li class="cat-item cat-item-68"> <a href="/blog/archives/category/stereoinduction">stereoinduction</a> (4) </li> <li class="cat-item cat-item-71"> <a href="/blog/archives/category/tunneling">Tunneling</a> (26) </li> <li class="cat-item cat-item-1"> <a href="/blog/archives/category/uncategorized">Uncategorized</a> (57) </li> <li class="cat-item cat-item-82"> <a href="/blog/archives/category/vibrational-frequencies">vibrational frequencies</a> (3) </li> </ul> </li> <li class="box"> <h2> Monthly </h2> <ul> <li><a href="/blog/archives/date/2019/06">June 2019</a></li> <li><a href="/blog/archives/date/2019/04">April 2019</a></li> <li><a href="/blog/archives/date/2019/03">March 2019</a></li> <li><a href="/blog/archives/date/2019/02">February 2019</a></li> <li><a href="/blog/archives/date/2019/01">January 2019</a></li> <li><a href="/blog/archives/date/2018/12">December 2018</a></li> <li><a href="/blog/archives/date/2018/11">November 2018</a></li> <li><a href="/blog/archives/date/2018/10">October 2018</a></li> <li><a href="/blog/archives/date/2018/09">September 2018</a></li> <li><a href="/blog/archives/date/2018/08">August 2018</a></li> <li><a href="/blog/archives/date/2018/07">July 2018</a></li> <li><a href="/blog/archives/date/2018/06">June 2018</a></li> <li><a href="/blog/archives/date/2018/05">May 2018</a></li> <li><a href="/blog/archives/date/2018/04">April 2018</a></li> <li><a href="/blog/archives/date/2018/03">March 2018</a></li> <li><a href="/blog/archives/date/2018/02">February 2018</a></li> <li><a href="/blog/archives/date/2018/01">January 2018</a></li> <li><a href="/blog/archives/date/2017/12">December 2017</a></li> <li><a href="/blog/archives/date/2017/11">November 2017</a></li> <li><a href="/blog/archives/date/2017/10">October 2017</a></li> <li><a href="/blog/archives/date/2017/09">September 2017</a></li> <li><a href="/blog/archives/date/2017/08">August 2017</a></li> <li><a href="/blog/archives/date/2017/07">July 2017</a></li> <li><a href="/blog/archives/date/2017/06">June 2017</a></li> <li><a href="/blog/archives/date/2017/05">May 2017</a></li> <li><a href="/blog/archives/date/2017/04">April 2017</a></li> <li><a href="/blog/archives/date/2017/03">March 2017</a></li> <li><a href="/blog/archives/date/2017/02">February 2017</a></li> <li><a href="/blog/archives/date/2017/01">January 2017</a></li> <li><a href="/blog/archives/date/2016/12">December 2016</a></li> <li><a href="/blog/archives/date/2016/11">November 2016</a></li> <li><a href="/blog/archives/date/2016/10">October 2016</a></li> <li><a href="/blog/archives/date/2016/09">September 2016</a></li> <li><a href="/blog/archives/date/2016/08">August 2016</a></li> <li><a href="/blog/archives/date/2016/07">July 2016</a></li> <li><a href="/blog/archives/date/2016/06">June 2016</a></li> <li><a href="/blog/archives/date/2016/05">May 2016</a></li> <li><a href="/blog/archives/date/2016/04">April 2016</a></li> <li><a href="/blog/archives/date/2016/03">March 2016</a></li> <li><a href="/blog/archives/date/2016/02">February 2016</a></li> <li><a href="/blog/archives/date/2016/01">January 2016</a></li> <li><a href="/blog/archives/date/2015/12">December 2015</a></li> <li><a href="/blog/archives/date/2015/11">November 2015</a></li> <li><a href="/blog/archives/date/2015/10">October 2015</a></li> <li><a href="/blog/archives/date/2015/09">September 2015</a></li> <li><a href="/blog/archives/date/2015/08">August 2015</a></li> <li><a href="/blog/archives/date/2015/07">July 2015</a></li> <li><a href="/blog/archives/date/2015/06">June 2015</a></li> <li><a href="/blog/archives/date/2015/05">May 2015</a></li> <li><a href="/blog/archives/date/2015/04">April 2015</a></li> <li><a href="/blog/archives/date/2015/03">March 2015</a></li> <li><a href="/blog/archives/date/2015/02">February 2015</a></li> <li><a href="/blog/archives/date/2015/01">January 2015</a></li> <li><a href="/blog/archives/date/2014/12">December 2014</a></li> <li><a href="/blog/archives/date/2014/11">November 2014</a></li> <li><a href="/blog/archives/date/2014/10">October 2014</a></li> <li><a href="/blog/archives/date/2014/09">September 2014</a></li> <li><a href="/blog/archives/date/2014/08">August 2014</a></li> <li><a href="/blog/archives/date/2014/07">July 2014</a></li> <li><a href="/blog/archives/date/2014/06">June 2014</a></li> <li><a href="/blog/archives/date/2014/05">May 2014</a></li> <li><a href="/blog/archives/date/2014/04">April 2014</a></li> <li><a href="/blog/archives/date/2014/03">March 2014</a></li> <li><a href="/blog/archives/date/2014/02">February 2014</a></li> <li><a href="/blog/archives/date/2014/01">January 2014</a></li> <li><a href="/blog/archives/date/2013/12">December 2013</a></li> <li><a href="/blog/archives/date/2013/11">November 2013</a></li> <li><a href="/blog/archives/date/2013/10">October 2013</a></li> <li><a href="/blog/archives/date/2013/09">September 2013</a></li> <li><a href="/blog/archives/date/2013/08">August 2013</a></li> <li><a href="/blog/archives/date/2013/07">July 2013</a></li> <li><a href="/blog/archives/date/2013/06">June 2013</a></li> <li><a href="/blog/archives/date/2013/05">May 2013</a></li> <li><a href="/blog/archives/date/2013/04">April 2013</a></li> <li><a href="/blog/archives/date/2013/03">March 2013</a></li> <li><a href="/blog/archives/date/2013/02">February 2013</a></li> <li><a href="/blog/archives/date/2013/01">January 2013</a></li> <li><a href="/blog/archives/date/2012/12">December 2012</a></li> <li><a href="/blog/archives/date/2012/11">November 2012</a></li> <li><a href="/blog/archives/date/2012/10">October 2012</a></li> <li><a href="/blog/archives/date/2012/09">September 2012</a></li> <li><a href="/blog/archives/date/2012/08">August 2012</a></li> <li><a href="/blog/archives/date/2012/07">July 2012</a></li> <li><a href="/blog/archives/date/2012/06">June 2012</a></li> <li><a href="/blog/archives/date/2012/05">May 2012</a></li> <li><a href="/blog/archives/date/2012/04">April 2012</a></li> <li><a href="/blog/archives/date/2012/03">March 2012</a></li> <li><a href="/blog/archives/date/2012/02">February 2012</a></li> <li><a href="/blog/archives/date/2012/01">January 2012</a></li> <li><a href="/blog/archives/date/2011/12">December 2011</a></li> <li><a href="/blog/archives/date/2011/11">November 2011</a></li> <li><a href="/blog/archives/date/2011/10">October 2011</a></li> <li><a href="/blog/archives/date/2011/09">September 2011</a></li> <li><a href="/blog/archives/date/2011/08">August 2011</a></li> <li><a href="/blog/archives/date/2011/07">July 2011</a></li> <li><a href="/blog/archives/date/2011/06">June 2011</a></li> <li><a href="/blog/archives/date/2011/05">May 2011</a></li> <li><a href="/blog/archives/date/2011/04">April 2011</a></li> <li><a href="/blog/archives/date/2011/03">March 2011</a></li> <li><a href="/blog/archives/date/2011/02">February 2011</a></li> <li><a href="/blog/archives/date/2011/01">January 2011</a></li> <li><a href="/blog/archives/date/2010/12">December 2010</a></li> <li><a href="/blog/archives/date/2010/11">November 2010</a></li> <li><a href="/blog/archives/date/2010/10">October 2010</a></li> <li><a href="/blog/archives/date/2010/09">September 2010</a></li> <li><a href="/blog/archives/date/2010/08">August 2010</a></li> <li><a href="/blog/archives/date/2010/07">July 2010</a></li> <li><a href="/blog/archives/date/2010/06">June 2010</a></li> <li><a href="/blog/archives/date/2010/05">May 2010</a></li> <li><a href="/blog/archives/date/2010/04">April 2010</a></li> <li><a href="/blog/archives/date/2010/03">March 2010</a></li> <li><a href="/blog/archives/date/2010/02">February 2010</a></li> <li><a href="/blog/archives/date/2010/01">January 2010</a></li> <li><a href="/blog/archives/date/2009/12">December 2009</a></li> <li><a href="/blog/archives/date/2009/11">November 2009</a></li> <li><a href="/blog/archives/date/2009/10">October 2009</a></li> <li><a href="/blog/archives/date/2009/09">September 2009</a></li> <li><a href="/blog/archives/date/2009/08">August 2009</a></li> <li><a href="/blog/archives/date/2009/07">July 2009</a></li> <li><a href="/blog/archives/date/2009/06">June 2009</a></li> <li><a href="/blog/archives/date/2009/05">May 2009</a></li> <li><a href="/blog/archives/date/2009/04">April 2009</a></li> <li><a href="/blog/archives/date/2009/03">March 2009</a></li> <li><a href="/blog/archives/date/2009/02">February 2009</a></li> <li><a href="/blog/archives/date/2009/01">January 2009</a></li> <li><a href="/blog/archives/date/2008/12">December 2008</a></li> <li><a href="/blog/archives/date/2008/11">November 2008</a></li> <li><a href="/blog/archives/date/2008/10">October 2008</a></li> <li><a href="/blog/archives/date/2008/09">September 2008</a></li> <li><a href="/blog/archives/date/2008/08">August 2008</a></li> <li><a href="/blog/archives/date/2008/07">July 2008</a></li> <li><a href="/blog/archives/date/2008/06">June 2008</a></li> <li><a href="/blog/archives/date/2008/05">May 2008</a></li> <li><a href="/blog/archives/date/2008/04">April 2008</a></li> <li><a href="/blog/archives/date/2008/03">March 2008</a></li> <li><a href="/blog/archives/date/2008/02">February 2008</a></li> <li><a href="/blog/archives/date/2008/01">January 2008</a></li> <li><a href="/blog/archives/date/2007/12">December 2007</a></li> <li><a href="/blog/archives/date/2007/11">November 2007</a></li> <li><a href="/blog/archives/date/2007/10">October 2007</a></li> <li><a href="/blog/archives/date/2007/09">September 2007</a></li> <li><a href="/blog/archives/date/2007/08">August 2007</a></li> <li><a href="/blog/archives/date/2007/07">July 2007</a></li> </ul> </li> </ul> <a rel="license" href="https://creativecommons.org/licenses/by-nd/3.0/"> <img alt="Creative Commons License" style="border-width:0" 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