Study of spin density distributions in the radical ions of two non-alternant hydrocarbons by unrestricted Hartree-Fock method: Effect of introducing the variation of β and the inclusion of non-neighbour resonance integrals on the spin density values

Abstract

UHFAA (unrestricted Hartree-Fock incorporating the annihilation of the quartet spin component) calculations of the spin densities have been made for the cation and anion radicals of two non-alternant hydrocarbons, acepleiadylene (I) and acenaphth [1, 2 (a)] acenaphthylene (II). Effect of varying β (variable β approximation) and the inclusion of non-neighbour resonance integrals on spin density values in these systems have been studied. It has been found that with these modified UHFAA procedures the spin densities obtained are not very different from those obtained by the usual UHFAA procedure. It has been found that in these systems the McConnell relation is as good as Colpa-Bolton relation or Giacometti-Nordio-Pavan relation. If the McConnell relation is to be used a higher value for the proportionality constant (Q) is indicated for the cation radical than for the anion radical in these systems. This result is contrary to the theoretical predictions of Higuchi. For the cation radical of (I) it also appears that with the McConnell relation not only a higher Q value is required, but that this must also vary from a position in a five-membered ring to that in a seven-membered ring. This variation is more marked in the cation radical of (I) than in the anion radical of (I).