Stoyanov Evgenii S
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences , Novosibirsk 630090, Russia.
Department of Natural Sciences, National Research University-Novosibirsk State University , Novosibirsk 630090, Russia.
J Phys Chem A. 2017 Dec 21;121(50):9638-9644. doi: 10.1021/acs.jpca.7b10068. Epub 2017 Dec 8.
Based on the experimentally established mechanism of hyperconjugative stabilization of the simplest saturated carbocations [Stoyanov, E. S.; et al. PCCP, 2017, 19, 7270], the infrared spectra of t-alkyl and methyl-cyclo-pentyl carbocations were interpreted. This approach allows us to extract new information about the electronic state of (CH)CR cations with R = H, CH, CH, CH, and CH(CH), namely, the electron density distribution over the (CH)C group and the positive charge dispersion on the H atoms of this group. Thus, donation of the electron density to the empty 2p orbital of the sp C atom occurs not only from one C-H bond oriented parallel to the 2p orbital but also equally from all other C-H and C-C bonds of the molecular group involved in hyperconjugation. This mechanism preserved the isoelectronic nature of this group toward the corresponding groups of the neutral alkanes. Hyperconjugation and polarization are closely linked in stabilization of carbocations: the strengthening of one effect weakens the second and vice versa without changing the efficiency of scattering of the positive charge in the carbocation. In the condensed phase, carbocations are additionally stabilized by the bulk effect and hydrogen bonding with the environment: increasing H-bonding strength increased hyperconjugation and decreased polarization. The contribution of all the effects on the stabilization of carbocations was evaluated.
基于通过实验确定的最简单饱和碳正离子的超共轭稳定化机制[斯托亚诺夫，E.S.；等人。《物理化学化学物理》，2017年，19卷，7270页]，对叔烷基和甲基环戊基碳正离子的红外光谱进行了解释。这种方法使我们能够提取有关R = H、CH、CH、CH和CH(CH)的(CH)CR阳离子电子态的新信息，即(CH)C基团上的电子密度分布以及该基团氢原子上的正电荷分散情况。因此，电子密度不仅从一个与2p轨道平行的C-H键向sp² C原子的空2p轨道转移，而且同样从参与超共轭的分子基团的所有其他C-H和C-C键转移。这种机制保持了该基团与相应中性烷烃基团的等电子性质。在碳正离子的稳定化过程中，超共轭和极化紧密相连：一种效应的增强会削弱另一种效应，反之亦然，而不会改变碳正离子中正电荷的分散效率。在凝聚相中，碳正离子通过体积效应和与环境的氢键作用而进一步稳定：氢键强度的增加会增强超共轭并降低极化。评估了所有这些效应对碳正离子稳定化的贡献。
