EXPERIMENTAL AND COMPUTATIONAL STUDIES OF 4-COORDINATE ALUMINUM - THEREACTION OF ALUMINATES AND ACIDS

The gas-phase reactions of trimethylaluminates with a variety of acids are considered from both an experimental and computational perspectiv e. The experimental work involves product and kinetic studies of ten a luminates, [(CH3)3AlX]- (X = H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, an d OCH3). Both X and methyl cleavage pathways are observed in these rea ctions. X cleavage occurs with several aluminates (X = H, NH2, OH, and PH2), in contrast to the thermochemical predictions of ab initio comp utations performed at the MP2/6-31++G(d,p) level. These indicate that methyl cleavage is favored in the reaction of nine aluminates (OCH3 wa s not computed) and HCl. Kinetic studies show a wide range of reactivi ties 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 usi ng MNDO, AM1, and PM3 techniques give unreliable results compared to t he ab initio computations and have no utility even for qualitative est imates. Computational studies of the reaction paths for [(CH3)3AlX]- ( for X = F and OH) with HCl as well as intrinsic reaction coordinate ca lculations for [(CH3)3AlF]- + HCl leading from the transition state to products have also been carried out. These computations not only desc ribe the transition states, but properly predict the cleavage results for X and methyl cleavage. The intrinsic reaction coordinate calculati ons give a qualitative interpretation of the reaction dynamics of [(CH 3)3AlF]- plus HCl.