EXPERIMENTAL AND COMPUTATIONAL STUDIES OF 4-COORDINATE ALUMINUM - THE REACTION OF ALUMINATES AND ACIDS

The gas-phase reactions of trimethylaluminates with a variety of acids are considered from both an experimental and computational perspective. The experimental work involves product and kinetic studies of ten aluminates, [(CH3)3AlX]- (X = H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, and OCH3). Both X and methyl cleavage pathways are observed in these reactions. X cleavage occurs with several aluminates (X = H, NH2, OH, and PH2), in contrast to the thermochemical predictions of ab initio computations performed at the MP2/6-31++G(d,p) level. These indicate that methyl cleavage is favored in the reaction of nine aluminates (OCH3 was not computed) and HCl. Kinetic studies show a wide range of reactivities for these aluminates. Ab initio calculations also give estimates of the X- affinities for (CH3)3Al and the Cl- and CH3- affinities of (CH3)2AlX. Analogous semiempirical computations of these aluminates using MNDO, AM1, and PM3 techniques give unreliable results compared to the ab initio computations and have no utility even for qualitative estimates. Computational studies of the reaction paths for [(CH3)3AlX]- (for X = F and OH) with HCl as well as intrinsic reaction coordinate calculations for [(CH3)3AlF]- + HCl leading from the transition state to products have also been carried out. These computations not only describe the transition states, but properly predict the cleavage results for X and methyl cleavage. The intrinsic reaction coordinate calculations give a qualitative interpretation of the reaction dynamics of [(CH3)3AlF]- plus HCl.