Thermal Decomposition Pathways and Rates for Dimethylaluminum Hydride

Abstract

Calculations have been carried out for the thermal decomposition of dimethylaluminum hydride (DMAlH). For each decomposition pathway, the stationary point geometries and harmonic frequencies were characterized using complete active space self-consistent field (CASSCF)/derivative methods and the correlation consistent polarized valence double-ζ (cc-pVDZ) basis set. Accurate energetics were obtained by combining the coupled cluster singles and doubles with perturbational estimate of triples [CCSD(T)] results using the cc-pVTZ basis set with an extrapolation to the basis set limit using the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets at the Moller-Plesset second-order perturbation theory (MP2) level. The geometries, energetics, and harmonic frequencies were used to obtain rate constants using conventional transition state theory. It was found that the lowest energy pathway leads to CH 3 AlCH 2 + H 2 with a barrier of 71.1 kcal/mol, which is below the first product resulting from direct bond breaking (CH 3 Al + CH 3 at 82.2 kcal/mol). Decomposition of DMAlH dimer was also considered. The rate-limiting step here is elimination of H 2 from the DMAlH dimer, and the best estimate of the barrier for this process is 80 kcal/mol [from CCSD(T) calculations with the cc-pVDZ basis set]. This barrier is too large for this pathway to play a major role in Al chemical vapor deposition.

Reactions of SiCl2 and SiHCl with H and Cl Atoms

“…The CCSD(T) method used here is a single reference based method, but this method has been shown to deal with all but the most severe near-degeneracy effects. In a recent paper, we showed that for the decomposition of dimethylaluminum hydride CCSD(T) gave results in good agreement with the multireference internally contracted CI method …”

Carbene Complexes of Nonmetals

“…Thermochemistry of the methylene-benzene reaction sum of ∆H f ( 1 CH 2 ) and ∆H f (benzene) by 37 kJ/mol, it was concluded that X must be a methylene-benzene complex. However, ∆H f values for CH 2 N 2 have been computed (285 and 281 kJ/mol) [107][108] that exceed Goodman's. On the product side, the isomerization of norcaradiene to give cycloheptatriene (k 298 ഠ 10 7 s Ϫ1 ) [109] may not be complete on the time scale of the calorimetric experiment.…”