Polysilane and related radical rearrangements: an ab initio study of (1,2)-silyl, germyl and stannyl translocations in radicals derived from trisilanes and related species

Abstract

Ab initio molecular orbital calculations using a (valence) double-ζ pseudopotential basis set (DZP) with (MP2, QCISD) and without (SCF) the inclusion of electron correlation predict that the transition states (5, 7) involved in homolytic (1,2)-translocation reactions of silyl (SiH₃), germyl (GeH₃) and stannyl (SnH₃) groups between silicon and other group (IV) centres proceed via homolytic substitution mechanisms involving frontside attack at the heteroatom undergoing translocation. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), an energy barrier (ΔE^‡) of 135.9 kJ mol⁻¹ is calculated for the translocation of SiH₃ between silicon centres; this value is 143.8 kJ mol⁻¹ at the CCSD(T)/DZP//MP2/DZP level. Similar results are obtained at the CCSD(T)/DZP//MP2/DZP level of theory for reactions involving germanium and tin with values of ΔE^‡ of 146.5 and 129.1 kJ mol⁻¹ respectively for the rearrangements of trigermapropyl and tristannapropyl radicals respectively. These data strongly suggest that homolytic (1,2)-translocation reactions are unlikely to be involved in the free-radical degradation of polysilanes, polygermanes and polystannanes. CCSD(T)/DZP//MP2/DZP calculated energy barriers associated with mixed systems range from 108.1 kJ mol⁻¹ for the (1,2)-translocation of SnH₃ from tin to silicon, to 181.0 kJ mol⁻¹ for the similar migration of SiH₃ from silicon to tin. The mechanistic implications of these observations are discussed.