Theoretical study of the ethylene radical cation: geometry and hyperfine structure

Abstract

The geometry and the hyperfine structure in the ethylene radical cation have been studied by means of a number of high-level post-SCF ab initio methods, including quadratic CI with single and double substitutions and perturbative triple corrections, QCISD(T), coupled cluster with single and double substitutions and perturbative triple corrections, CCSD(T), and individually selected multi-reference CI with B K correction, (MRD-CI)/B K. A value of 20.2° has been computed for the torsion angle in this cation with the QCISD(T)/6-311G(d, p) method, which compares favourably with the experimentally reported angle of ∼25°. The computed potential barrier to inversion through the planar conformation, as obtained at the QCISD(T)/6-311++G(2df, p)//QCISD(T)/6-311G(d, p) level (1.24 kJ/mol), is smaller than the experimental value of ∼3 kJ/mol, but an order of magnitude better than in previous work. The hyperfine coupling constants of the protons and of 13 C are calculated by the (MRD-CI)/B K method with a relatively large basis set, yielding somewhat larger absolute values than the experimental couplings (−4.0 G for the protons and +6.0 G for 13 C , as compared with −2.4 and +4.0 G, respectively). For both the geometrical parameters and the hyperfine couplings, the results of the present study are in considerably better agreement with experiment than previously reported theoretical results.