Center for Computational Chemistry and Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States.
J Am Chem Soc. 2013 Sep 4;135(35):13008-14. doi: 10.1021/ja403934s. Epub 2013 Aug 22.
The "alkane branching effect" denotes the fact that simple alkanes with more highly branched carbon skeletons, for example, isobutane and neopentane, are more stable than their normal isomers, for example, n-butane and n-pentane. Although n-alkanes have no branches, the "kinks" (or "protobranches") in their chains (defined as the composite of 1,3-alkyl-alkyl interactions-including methine, methylene, and methyl groups as alkyl entities-present in most linear, cyclic, and branched alkanes, but not methane or ethane) also are associated with lower energies. Branching and protobranching stabilization energies are evaluated by isodesmic comparisons of protobranched alkanes with ethane. Accurate ab initio characterization of branching and protobranching stability requires post-self-consistent field (SCF) treatments, which account for medium range (∼1.5-3.0 Å) electron correlation. Localized molecular orbital second-order Møller-Plesset (LMO-MP2) partitioning of the correlation energies of simple alkanes into localized contributions indicates that correlation effects between electrons in 1,3-alkyl groups are largely responsible for the enhanced correlation energies and general stabilities of branched and protobranched alkanes.
“烷烃支化效应”表示，具有更多高度支化碳骨架的简单烷烃，例如异丁烷和新戊烷，比其正常异构体（例如正丁烷和正戊烷）更稳定。尽管正构烷烃没有支链，但它们链中的“拐点”（或“原支链”）（定义为大多数线性、环状和支化烷烃中存在的 1,3-烷基-烷基相互作用的组合，包括甲撑、亚甲基和甲基作为烷基实体，但不包括甲烷或乙烷）也与较低的能量相关。通过与乙烷的等电子比较来评估原支化烷烃的支化和原支链稳定化能。准确的从头算支化和原支链稳定性需要后自洽场（SCF）处理，以考虑中程（约 1.5-3.0 Å）电子相关。简单烷烃的相关能量的局域分子轨道二阶 Møller-Plesset（LMO-MP2）分区表明，1,3-烷基中电子之间的相关效应主要负责支化和原支化烷烃的增强相关能量和一般稳定性。
