Ab initio study of some free-radical homolytic substitution reactions at silicon, germanium and tin

Ab initio molecular orbital calculations using pseudopotential (DZP) and all-electron (6-311G**) basis sets, with (MP2, QCISD) and without (SCF) the inclusion of electron correlation, predict that hydrogen atoms and methyl radicals undergo homolytic substitution at the heteroatom in silane, germane and stannane without the involvement of hypervalent intermediates. At the QCISD/DZP level of theory, energy barriers of between 69 and 76 kJ mol(-1) are predicted for attack by a hydrogen atom, while QCISD/DZP//MP2/DZP calculations predict barriers of between 95 and 106 kJ mol(-1) for attack by a methyl radical, with barriers of 56-69 kJ mol(-1) for the reverse reactions. When electron correlation is included (MP2), hypervalent intermediates (3) are predicted in reactions involving attack of a methyl radical at methylsilane, methylgermane and methylstannane. QCISD/DZP//MP2/DZP barriers of 86-106 kJ mol(-1) are predicted for the formation of intermediates (3) which are constrained by barriers of only 2-3 kJ mol(-1). When zero-point vibrational energy correction is included, the structures (3) are predicted to behave much more like transition states than intermediates.