An ab initio and DFT study of homolytic substitution reactions of acyl radicals at sulfur, selenium, and tellurium

Ab initio calculations using the 6-311G**, cc-pVDZ, aug-cc-pVDZ, and (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, ROMP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation, and density functional (BHandHLYP and B3LYP) calculations predict that homolytic substitution reactions of acetyl radicals at the sulfur, selenium and tellurium atoms in dimethyl sulfide, dimethyl selenide, and dimethyl telluride adopt an almost collinear arrangement of attacking and leaving radicals at the chalcogen atom. Energy barriers (Delta E(double dagger)) for these reactions range from 76.5 (attack at S, BHandHLYP/6-311G**) to 35.5 kJ mol(-1) (attack at Te, BHandHLYP/DZP). While the calculated energy barriers for the forward and reverse energy barriers for substitution of acetyl radical at the sulfur atom are comparable, the reverse reactions are favoured by 3-14 kJ mol(-1) for attack at selenium and by 20-25 kJ mol(-1) for attack at tellurium.