Hyperfine structure of some hydrocarbon radical cations: a B3LYP and MP2 study

Abstract

Hyperfine structures of n-alkene cations (C 2H 4 +, C 3H 6 +), c-alkane cations (C 3H 6 +, C 5H 10 +, C 6H 12 +), and benzene cation (C 6H 6 +), have been studied by using B3LYP and MP2 methods based on the B3LYP optimized grometries, and the isotropic proton hyperfine coupling constants ( a(H)) are predicted. A 6-311G(d,p) basis was used. For C 2H 4 +, the B3LYP torsion angle value of 28° and MP2//B3LYP a(H) value of −3.2 G are the best ones reported to date. For n-C 3H 6 +, the B3LYP calculations predict a planar geometry (no twisting around the CC bond) and the calculated a(H) values are in agreement with experiment. For c-C 3H 6 +, and C 6H 6 +, the a(H) values predicted by the B3LYP//B3LYP and MP2//B3LYP calculations are in good agreement with the ESR experiments. For c-C 5H 10 +, the B3LYP calculations predict a twisted C 2 structure and a bent C s structure (the previous MP2 calculations failed to predict the twisted structure), and the hyperfine structure calculations imply that assignment of the ESR spectra to the 2A state of C 2 conformation is favored. For c-C 6H 12 +, the B3LYP calculations predict that the ground state is 2A g of the C 2 h chair conformation, to which the ESR spectrum at 140 K is assigned, and suggest that the previously reported C s structure does not exit.