AB-INITIO STUDY OF SOME FREE-RADICAL HOMOLYTIC SUBSTITUTION-REACTIONSAT SILICON, GERMANIUM AND TIN

Ab initio molecular orbital calculations using pseudopotential (DZP) a nd all-electron (6-311G*) basis sets, with (MP2, QCISD) and without ( SCF) the inclusion of electron correlation, predict that hydrogen atom s and methyl radicals undergo homolytic substitution at the heteroatom in silane, germane and stannane without the involvement of hypervalen t 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 at om, 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 rea ctions involving attack of a methyl radical at methylsilane, methylger mane and methylstannane. QCISD/DZP//MP2/DZP barriers of 86-106 kJ mol( -1) are predicted for the formation of intermediates (3) which are con strained by barriers of only 2-3 kJ mol(-1). When zero-point vibration al energy correction is included, the structures (3) are predicted to behave much more like transition states than intermediates.