Freitas Matheus P, Rittner Roberto
Departamento de Química, Universidade Federal de Lavras, C.P. 3037, 37200-000 Lavras, MG, Brazil.
J Phys Chem A. 2007 Aug 2;111(30):7233-6. doi: 10.1021/jp0728196. Epub 2007 Jul 6.
The stabilities of the gauche and anti conformations of butane, 1,2-dicyanoethane (DCE), and 1,2-dinitroethane (DNE) have been investigated through theoretical calculations. The gauche effect-the tendency of keeping close vicinal electronegative substituents (thetaX-C-C-X approximately 60 degrees ) in an ethane fragment-is expected to drive the conformational equilibrium of DCE and DNE toward the gauche conformation. It was found that, for butane, where the gauche effect is supposed to be poor/null, the hyperconjugation effect contributes mostly to the anti stabilization in opposition to the traditional sense that the methyl groups repel each other, and this should govern its conformational equilibrium. For DCE the equilibrium was shifted to the anti conformer, essentially due to a gauche repulsion, while for DNE, despite the higher electronic delocalization energies, a predominance of the gauche conformer was obtained, and this was attributed mainly to the attractive dipolar interaction between the two nitro groups. A full orbital energy analysis was performed using the natural bond orbital approach, which showed that bond bending and anti-C-H/C-X* hyperconjugation models, usually applied to explain the origin of the gauche effect in fluorinated derivatives, are not adequate to completely explain the conformational behavior of the titled compounds.
通过理论计算研究了丁烷、1,2 - 二氰基乙烷（DCE）和1,2 - 二硝基乙烷（DNE）的邻位交叉构象和反式构象的稳定性。邻位交叉效应——乙烷片段中保持相邻电负性取代基紧密排列（θX - C - C - X约为60°）的趋势——预计会使DCE和DNE的构象平衡向邻位交叉构象移动。研究发现，对于丁烷，邻位交叉效应被认为较弱/不存在，超共轭效应主要有助于反式构象的稳定，这与传统观点中甲基相互排斥相反，且这应决定其构象平衡。对于DCE，平衡向反式构象移动，主要是由于邻位交叉排斥作用；而对于DNE，尽管电子离域能较高，但邻位交叉构象占主导，这主要归因于两个硝基之间有吸引力的偶极相互作用。使用自然键轨道方法进行了完整的轨道能量分析，结果表明，通常用于解释含氟衍生物中邻位交叉效应起源的键弯曲和反式C - H/C - X*超共轭模型，不足以完全解释标题化合物的构象行为。
