Thermal decomposition pathways and rates for dimethylaluminum hydride

Calculations have been carried out for the thermal decomposition of dimethy laluminum hydride (DMAlH), For each decomposition pathway, the stationary p oint geometries and harmonic frequencies were characterized using complete active space self-consistent field (CASSCF)/derivative methods and the corr elation consistent polarized valence double-zeta; (cc-pVDZ) basis set. Accu rate 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 c c-pVDZ, cc-pVTZ, and cc-pVQZ basis sets at the Moller-Plesset second-order perturbation theory (MP2) level. The geometries, energetics, and harmonic f requencies were used to obtain rate constants using conventional transition state theory. It was found that the lowest energy pathway leads to CH3AlCH 2 + H-2 with a barrier of 71.1 kcal/mol, which is below the first product r esulting from direct bond breaking (CH3Al + CH3 at 82.2 kcal/mol). Decompos ition of DMAlH dimer was also considered. The rate-limiting step here is el imination of Hz 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 rol e in Al chemical vapor deposition.