Direct Evaluation of the Hyperconjugative Interactions in 1,1,1-Trihaloethane (CH3CX3, X = F, Cl, and Br)

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• 作者：Zhenhua Chen ; Cl茅mence Corminboeuf ; Yirong Mo
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：August 7, 2014
• 年：2014
• 卷：118
• 期：31
• 页码：5743-5747
• 全文大小：248K
• ISSN：1520-5215

文摘

Following the computational strategy proposed by Mulliken in 1939 ( J. Chem. Phys.pan class="NLM_x"> pan>1939pan class="NLM_x">, pan>7pan class="NLM_x"> (pan>5), 339鈭?52), when the concept of hyperconjugation was coined, we evaluated the hyperconjugative stabilization energy in 1,1,1-trihaloethane using the block-localized wave function (BLW) method. The BLW method is the simplest and most efficient variant of ab initio valence bond (VB) theory and can derive the strictly electron-localized state wave function self-consistently. The latter serves as a reference for the quantification of the electron delocalization effect in terms of the resonance theory. Computations show that the overall hyperconjugative interactions in 1,1,1-trihaloethane, dominated by 蟽_CH 鈫?蟽_CX^{鈥?/sup> with minor contribution from 蟽_CX 鈫?蟽_CH鈥?/sup>, ranges from 9.59 to 7.25 kcal/mol in the staggered structures and decreases in the order Br > Cl > F. This is in accord with the 1H NMR spectra of CH₃CX₃. Notably, the hyperconjugation effect accounts for 35鈥?0% of the rotation barriers in these molecules, which are dominated by the conventional steric repulsion. This is consistent with the recent findings with 1,2-difluoroethane (Freitaspan class="NLM_x">, pan>B眉hlpan class="NLM_x">, and pan>O鈥橦agan. Chem. Comm.pan class="NLM_x"> pan>2012pan class="NLM_x">, pan>48pan class="NLM_x">, pan>2433鈭?435) that the variation of 1J_CF with the FCCF torsional angle cannot be well explained by the hyperconjugation model.}