Disproportionation of dimethylalane on aluminum surfaces. Part II. Quantum chemistry studies

A proposed surface disproportionation growth mechanism of dimethylaluminum hydride (DMAH) on aluminum surfaces has been investigated with quantum chemistry methods and found to be thermodynamically consistent. Plane-wave pseudopotential calculations of optimized adsorbate structures, heats of reaction, and activation barriers have been employed to construct a quantitative growth model. Results support a mechanism involving the decomposition of DMAH to dimethylaluminum, monomethylaluminum, and methyl fragments in the growth process. Rate simulations of the growth mechanism agree well with experiments and are able to reproduce the low apparent activation energies observed experimentally, while still including elementary reactions with activation barriers as high as 20 kcal/mol. Additionally, simulations provide insight into experimental observations via the prediction of a dominant surface methyl coverage under growth conditions. Where experiment and calculations overlap, a general accuracy of approximately 5 kcal/mol is observed.