Ab initio calculations using the 6-311G(d,p), cc-pVDZ, aug-cc-pVDZ, and (valence) double-ζ pseudopotential (DZP) basis sets, with (MP2, ROMP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation, and density functional (BHandHLYP) calculations predict that homolytic substitution reactions of acetyl radicals at the silicon atoms in dimethylsilane can proceed via both backside and frontside attack mechanisms. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), energy barriers (ΔE
⧧) of 110.4 and 107.5 kJ mol−1 are calculated for the backside and frontside reactions, respectively. Similar results are obtained for reactions involving germanium and tin with energy barriers (ΔE
⧧) of 97.6−191.7 and 100.8−171.3 kJ mol−1 for the backside and frontside mechanisms, respectively. These data suggest that both homolytic substitution mechanisms are feasible for homolytic reactions of acetyl radicals at silicon, germanium, and tin. BHandHLYP calculations provide geometries and energy barriers for backside and frontside transition states in good agreement with those obtained by traditional ab initio techniques.