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

Hyperfine structures of n-alkene cations (C2H4+, C3H6+). c-alkane cations ( C3H6+, C5H10+, C6H12+), and benzene cation (C6H6+), have been studied by us ing B3LYP and MP2 methods based on the B3LYP optimized grometries, and the isotropic proton hyperfine coupling constants (a(H)) are predicted. A 6-311 G(d,p) basis was used. For C2H4+, the B3LYP torsion angle value of 28 degre es and MP2//B3LYP a(H) value of -3.2 G are the best ones reported to date. For n-C3H6+, the B3LYP calculations predict a planar geometry (no twisting around the C=C bond) and the calculated a(H) values are in agreement with e xperiment. For c-C3H6+, and C6H6+, the a(H) values predicted by the B3LYP// B3LYP and MP2//B3LYP calculations are in good agreement with the ESR experi ments. For c-C5H10+, 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 (2)A state of C-2 conformation is fav ored. For c-C6H12+, the B3LYP calculations predict that the ground state is (2)A(g) of the C-2h chair conformation, to which the ESR spectrum at 140 K is assigned, and suggest that the previously reported C-s structure does n ot exit. (C) 2001 Elsevier Science B.V. All rights reserved.