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.