AB-INITIO MOLECULAR-ORBITAL CALCULATIONS ON THE Q-E SCHEME FOR PREDICTING REACTIVITY IN FREE-RADICAL COPOLYMERIZATION

Ab initio molecular orbital calculations have been performed on the re actions of unsaturated monomers with both hydrogen and methyl radicals . These calculations have utilized several basis sets, namely STO-3G, SV3-21G, SV Dunning and triple zeta valence. The correlation between t he experimental Q and the energy of formation of the product radical w as examined, whilst the electronegativity of the monomer was correlate d with the e parameter. In addition, a Q-e-e-type scheme was analysed to correct for the simplification in the standard Q-e scheme that the monomer and the radical are identical with respect to their charge di stributions. In this instance experimental e parameters were correlate d with the average electronegativity of monomer and radical. This work shows that correlations exist between these parameters as other simpl er studies (based on Huckel calculations) have reported. However, whil e the correlations with both e and the e-e average parameter were pre dictive, the relationship between Q and a general reactivity of the ra dical proved only qualitative. A statistical analysis of the data reve aled a significant polar influence on this relationship; by taking thi s into account we were able to improve the correlation. We also rescal ed the Q-e scheme as suggested by other authors and correlated the res ults of our calculations with the new numbers. This work emphasizes th e difficulties in correlation analysis, which requires the separation and parameterization of polar, steric and general reactivity effects. It also stresses the requirement for accurate experimental data.