Institut de Química Computacional and Departament de Química, Universitat de Girona, Girona, Catalonia, Spain.
J Phys Chem A. 2011 Nov 17;115(45):13104-13. doi: 10.1021/jp205152n. Epub 2011 Sep 21.
The concepts of conjugation and hyperconjugation play an important role to provide an explanation for several fundamental phenomena observed in organic chemistry. Because these effects cannot be directly measured experimentally, their assessment became a primary concern for chemists from the very beginning. In general, the stabilization produced by both phenomena has been studied by means of isodesmic reactions and energy based analysis such as the energy decomposition analysis. In recent years, electronic delocalization measures have been successfully applied to elucidate the nature of chemical bonding and the aromatic character of all kind of molecules. Because conjugation and hyperconjugation stabilizations are strongly linked to the concept of electron delocalization, this paper will give an account of both effects from the point of view of electronic delocalization measures calculated within the framework of the quantum theory of atoms in molecules. In particular, we focus our attention in the controversial case of the stabilization by conjugation in 1,3-butadiyne and 1,3-butadiene. Unexpectedly, theoretical calculations based on the scheme proposed by Kistiakowsky to quantify the extent of stabilization due to conjugation predicted that the conjugation of 1,3-butadiyne was zero. Subsequent energetic analyses contradicted this observation. These studies pointed out the presence of hyperconjugation stabilization in the hydrogenated product of 1,3-butadiyne and 1,3-butadiene that were used as reference systems in the Kistiakowsky's scheme. Consequently, the extra stabilization of 1-butyne due to hyperconjugation hides the stabilization by conjugation of 1,3-butadiyne. Our results based on electron delocalization measures confirm both the presence of conjugation in 1,3-butadiene and 1,3-butadiyne and hyperconjugation stabilization in their respective hydrogenated products, 1-butene and 1-butyne.
共轭和超共轭的概念在解释有机化学中观察到的几个基本现象方面起着重要作用。由于这些效应不能直接通过实验测量，因此它们的评估从一开始就成为化学家关注的主要问题。一般来说，通过等电子反应和基于能量的分析（如能量分解分析）来研究这两种现象产生的稳定性。近年来，电子离域度量已成功应用于阐明化学键的本质和各种分子的芳香性质。由于共轭和超共轭稳定化与电子离域的概念密切相关，本文将从分子内原子量子理论计算的电子离域度量的角度来描述这两种效应。特别是，我们将注意力集中在 1,3-丁二炔和 1,3-丁二烯中共轭稳定化的有争议的情况下。出人意料的是，基于 Kistiakowsky 方案的理论计算来量化由于共轭而导致的稳定化程度，预测 1,3-丁二炔的共轭为零。随后的能量分析与这一观察结果相矛盾。这些研究指出，在 1,3-丁二炔和 1,3-丁二烯的氢化产物中存在超共轭稳定化，这些氢化产物被用作 Kistiakowsky 方案中的参考体系。因此，由于超共轭而导致的 1-丁炔的额外稳定化掩盖了 1,3-丁二炔的共轭稳定化。我们基于电子离域度量的结果证实了 1,3-丁二烯和 1,3-丁二炔中存在共轭，以及它们各自的氢化产物 1-丁烯和 1-丁炔中的超共轭稳定化。
