Parafiniuk Monika, Mitoraj Mariusz P
Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, R.Ingardena 3, 30-060, Krakow, Poland.
J Mol Model. 2014 Jun;20(6):2272. doi: 10.1007/s00894-014-2272-y. Epub 2014 May 27.
The internal rotation in ammonia borane (AB) was studied on the basis of natural orbitals for chemical valence (NOCV) and eigenvectors for Pauli repulsion (NOPR). We found that the total hyperconjugation stabilization (ca. 5 kcal mol(-1)), based on the charge transfer from the occupied σ (B-H) orbitals into the empty σ*(N-H), slightly favors the staggered conformation over the eclipsed one; however, the barrier to internal rotation in ammonia borane can be understood predominantly in a 'classical' way, as originating from the steric (Pauli) repulsion contributions (of the kinetic origin) that act solely between N-H and B-H bonds. Repulsion between the lone pair of ammonia and the adjacent B-H bonds was found to be dominant in absolute terms; however, it does not determine the rotational barrier. Similar conclusions on the role of CH↔HC repulsion appeared to be valid for isoelectronic ethane.
基于化学价自然轨道（NOCV）和泡利排斥本征向量（NOPR）对氨硼烷（AB）中的内旋转进行了研究。我们发现，基于从占据的σ（B - H）轨道到空的σ*（N - H）轨道的电荷转移，总的超共轭稳定作用（约5千卡摩尔⁻¹）略微有利于交错构象而非重叠构象；然而，氨硼烷中内旋转的势垒主要可以用一种“经典”方式来理解，即源于仅在N - H和B - H键之间起作用的空间（泡利）排斥贡献（动力学起源）。发现氨的孤对电子与相邻B - H键之间的排斥在绝对值上占主导；然而，它并不决定旋转势垒。关于CH↔HC排斥作用的类似结论似乎对等电子体乙烷也有效。
