Ab initio calculations using the 6-311G**, cc-pVDZ, aug-cc-pVDZ, and (valence) double-ζ pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, predict that degenerate homolytic substitution by silyl radical at the silicon atom in disilane can proceed by mechanisms which involve both backside and frontside attack at silicon. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/augcc-pVDZ), energy barriers (∆E q ) of 52.7 and 58.2 kJ mol -1 are calculated for the backside and frontside reactions, respectively. Similar results are obtained at the CCSD(T)/DZP// MP2/DZP level of theory for reactions involving germanium and tin with values of ∆E q of 65.2 kJ mol -1 (backside) and 76.7 kJ mol -1 (frontside) for reactions of germyl radical with digermane and 58.5 kJ mol -1 (backside) and 59.1 kJ mol -1 (frontside) for reactions of stannyl radical with distannane. CCSD(T)/DZP//MP2/DZP calculations involving the analogous nondegenerate reactions of disilane, digermane, and distannane, as well as reactions involving silylgermane, silylstannane, and germylstannane, predict that while homolytic substitution at silicon and germanium is expected to favor the backside mechanism, reactions involving free-radical attack at tin are predicted to be less discriminate; indeed, in many cases, the frontside mechanism is calculated to be preferred for reactions involving tin. CCSD-(T)/DZP//MP2/DZP calculated energy barriers range from 39.4 kJ mol -1 for the reaction of silyl radical with distannane by the frontside mechanism to 104.5 kJ mol -1 for the analogous frontside reaction involving stannyl radical and disilane. Except for reactions involving attack at the tin atom in methylstannane, we were unable to locate transition states for frontside attack at correlated levels of theory for reactions involving methyl radical. The mechanistic implications of these computational data are discussed.

Generation of the 5-(2'-deoxyuridinyl)methyl radical (6) was reexamined. Trapping by 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl confirms that 6 is generated. However, trapping by methoxyamine reveals that the respective carbocation (10) is also produced. Examining the effects of these traps on products in DNA reveals that the carbocation and not 6 yields interstrand cross-links. Cross-link formation from the carbocation is consistent with DFT calculations that predict that addition of the former at the N1 position of dA is essentially barrierless.

Ab initio calculations using 6-311G**, cc-pVDZ, aug-cc-pVDZ, and a (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional (B3LYO) calculations predict that homolytic substitution reactions of the methyl radical at the silicon atom in disilane 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 (delta E) of 47.4 and 48.6 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 (delta E) of between 46.5 and 67.3, and 41.0 and 73.3 kJ mol-1 for the backside and frontside mechanisms, respectively. These data suggest that homolytic substitution reactions of methyl radical at silicon, germanium, and tin can proceed via either homolytic substitution mechanism.