Abinitiostudy of the electronic structure and hyperfine coupling in simple hydrocarbon radicals. I. Test of the calculation method on methyl and vinyl

摘要

Nonempirical calculations of the energy, proton, and carbonminus;13 hyperfine splittings for methyl (Cdot;H3) and vinyl radicals (H2C=Cdot;H) in their equilibrium geometry are presented. The spinminus;restricted SCF method and firstminus;order doubleminus;perturbation theory including all spinminus;adapted monoexcited states with three unpaired electrons have been used. The basis set consists of Gaussianminus;type orbitals contracted in a doubleminus;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 zgr;Hhave been investigated. The lowest total energy is obtained for zgr;H=1.15. Although calculated hydrogen hyperfine splittings increase with zgr;H, neither the zerothminus;order nor the firstminus;order spin density, computed from canonical MOrsquo;s or quasilocalized equivalent MOrsquo;s can be correlated to zgr;3H. A structural analysis of the contributions of the various excited states reveals that the intrabond CHrarr;CHast; excitation yields about 90percnt; of the firstminus;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 CHast; antibonding MO at the nucleus. For vinyl, the splittings computed using the unoptimized exponents (zgr;H=1.0) areaHagr;=+7.52 G;aHcis=+37.56 G;aHtrans=+23.20 G, andaCdot;=+123.52 G;aCagr;=minus;4.53 G.