A b i n i t i o study of the electronic structure and hyperfine coupling in simple hydrocarbon radicals. I. Test of the calculation method on methyl and vinyl

Abstract

Nonempirical calculations of the energy, proton, and carbon−13 hyperfine splittings for methyl (ĊH₃) and vinyl radicals (H₂C=ĊH) in their equilibrium geometry are presented. The spin−restricted SCF method and first−order double−perturbation theory including all spin−adapted monoexcited states with three unpaired electrons have been used. The basis set consists of Gaussian−type orbitals contracted in a double−zeta form. The orbital exponents of the hydrogen functions have been simultaneously varied using a scaling procedure in order to simulate a minimum Slater hydrogenoid orbital. The sensitivity of SCF energies and hyperfine splittings to variations in values of the orbital exponent ζ_H have been investigated. The lowest total energy is obtained for ζ_H=1.15. Although calculated hydrogen hyperfine splittings increase with ζ_H, neither the zeroth−order nor the first−order spin density, computed from canonical MO’s or quasilocalized equivalent MO’s can be correlated to ζ³_H. A structural analysis of the contributions of the various excited states reveals that the intrabond CH→CH* excitation yields about 90% of the first−order spin density at the proton in each case. This quantity depends most critically on the nuclear effective charge of the hydrogen atom through the value of the CH* antibonding MO at the nucleus. For vinyl, the splittings computed using the unoptimized exponents (ζ_H=1.0) are a_Hα=+7.52 G; a_Hcis =+37.56 G; a_Htrans=+23.20 G, and a_Ċ=+123.52 G; a_Cα=−4.53 G.