No abstract

The reactants, clusters, transition structures, and products for thevarious channels for the thermal decomposition of SiHkCl,, were optimized at the HF/6-31G(d) and MP2/6-31G(d,p) levels. The electron correlation contributions were calculated at the MP4/6-3 lG(d,p), MP4/6-31+G(Zdf,p), MP2/6-31 ++G(3df,3pd), and G-2 levels of theory. In the decomposition of SiH4, SiH2 + H2 is favored over SiH3 + H. For SiH3C1, the SiHCl + H2 channel is preferred over the SiH2 + HCl channel, with fragmentation into SiHzCl+ H and SiH3 + C1 lying significantly higher. The barriers for SiHzClz decomposition are Sic12 + H2 -SiHCl + HCl < SiHCl2 + H < SiHzCl+ C1< SiH2 + C12. For SiHC13, decomposition into Sic12 + HCl is favored over Sic13 + H, with the SiHCl2 + C1 and SiHCl + Cl2 channels lying substantially higher. In tetrachlorosilane, Sic14 -Sic13 + C1 is favored over Sic14 -Sic12 + Cl2.

The 1,3-dipolar cycloaddition (DC) reactions of ethylene with nitrile ylide (CNC), nitrile imine (CNN), nitrile oxide (CNO), diazomethane (NNC), azine (NNN), and nitrous oxide (NNO) in the gas phase were examined using the density functional theory and CCSD(T) calculations. All of the structures, including the precursor complexes and the transition structures, were completely optimized at the B3LYP/6-31G level with single-point energies evaluated at CCSD(T)/6-311G. The theoretical results suggest that the activation energies for the DC reactions of nitrile-type molecules (CNC, CNN, and CNO) are small (5.1-11 kcal/mol) and these reactions are very exothermic (-77 to -46 kcal/mol). In contrast, the DC reactions of NNC, NNN, and NNO are less exothermic (-39 to -6.0 kcal/mol) and have larger activation barriers (13-29 kcal/mol). Moreover, this work shows that the configuration mixing (CM) model based on Pross and Shaik's theory can successfully predict the relative ordering of the activation energy and reaction enthalpies of DC reactions. Combining our theoretical calculations and the CM model, the following conclusion emerges: a 16-electron 1,3-dipole reactant with more electropositive substituents at the terminal positions will possess a smaller singlet-triplet splitting. This will facilitate cycloaddition with the dipolarophile and will result in a larger exothermicity.

Optimized geometries and vibrational frequencies for SiH,Cl, were calculated at the MP2/6-3 lG(d,p) level. Energy differences were computed at the MP4/6-3 1 +G(2df,p) level (all structures) and the G-2 level (structures containing no more than two chlorines). The heats of formation of SiH,Cl, were estimated using the following isodesmic and isogyric reactions: [ (4 -n)/4]SiH4 + ( 4 4 ) Sic14 -, SiHhnCl,, SiH, + SiHhCl, + SiHWnC1, + SiH4, 3/4Si + '/4H2 + SiH3Cl-Sic1 + '/2H + 3/4SiH4, '/2Si + '/zH2 + SiH+nCl, + SiHz-,Cl+ H + '/2SiH4, and l/4Si + '/2H + SiH&,Cl, -SiH3&1, + '/4H2 + l/qSiH4. The calculated heats of formation (kcal/mol; 298 K, 1 atm) are as follows: