AB-INITIO MOLECULAR-ORBITAL CALCULATIONS ON THE TRANSITION-STATE FOR THE ADDITION OF A METHYL RADICAL TO VINYL MONOMERS

Ab initio molecular orbital calculations have been performed on the tr ansition state of the addition of methyl radical to twelve vinyl monom ers using the SV 3-21G basis set. A linear relationship has been found between the calculated energies of activation and previously calculat ed energies of reaction. This supports the assumptions of an Evans-Pol anyi type rule in previous work which attempted to correlate reactivit y with calculated energies of reaction. The activation energies obtain ed for methyl addition to butadiene and styrene were calculated to be negative. This is caused by errors introduced by a number of sources, viz. basis set superposition error, spin contamination and zero point energy. These errors are discussed. Previous authors have reported rea sonable agreement between calculated activation energies at SV3-21G an d experimental values for methyl addition to ethylene, this work sugge sts that this agreement was coincidental and results from the fortuito s cancellation of errors. The nature of the transition state for these radical addition reactions is discussed and the limitations of the SV 3-21G basis set are highlighted. The theoretical prediction of activat ion energies for radical addition reactions would require much larger calculations, beyond the computational means of most research laborato ries.