Theoretical Studies on Structures and Electronic State of Alkyl-Substituted Ethyl Cations

This paper describes the gas phase structural details for a series of alkyl-substituted ethyl cations 1-11 by mainly natural bond orbital (NBO) analysis. For the simplest ethyl cation, two kinds of optimized structures (nonclassical 1 and classical 2) were found to have minima on the potential surface, and the nonclassical 1 was more stable than the corresponding classical 2 by 1.19 kcal/moL at Moller-Plesset (MP) level calculations in the gas phase. The calculated values of Wiberg's bond index revealed that the central ethylic C-1-C-2 bonds of the nonclassical cations 1, 6, 7, and 10 have 1.10-1.46 bond multiplicities. The compliance constant (C-ii) values indicated that the cation center C-1 of nonclassical 2-butyl cation 6 (0.209 angstrom/mdyn) acquires stronger hyperconjugative stabilization from C-2-R-3 (C-H) bond than that of the nonclassical ethyl cation 1 (0.388 angstrom/mdyn). Moreover, the gas phase stabilities for the alkyl-substituted ethyl cations 1-11 were also estimated by the hydride affinities based on isodesmic equations.