TRIMETHYLAMINE ALANE AND ITS DISSOCIATION PRODUCTS

Ab initio quantum mechanical studies employing basis sets approaching the quality of triple-zeta plus double-polarization on Al and N and si ngle-polarization on C and H are reported for AlH3N(CH3)(3) (TMAA), N( CH3)(3) (TMA), and AlH3. The self-consistent-field method was employed for all basis sets; the single- and double-excitation configuration i nteraction and comparable coupled-cluster methods were utilized with t he double-zeta plus polarization function basis set. The dissociation energy of AlH3-N(CH3)(3) is predicted to be 31 kcal/mol after correcti ng for zero-point vibrational energy. With the exception of the brans methyl hydrogens, the AlH3 and TMA moieties do not undergo large chang es upon formation of TMAA. The hyperconjugative effect on the trans hy drogens is half that deduced experimentally, but is verified to be rea l. Rotations of methyl groups and alane groups influence the hyperconj ugative effect. This work corrects earlier assignments of the Al-N str etching vibrational mode. TMAA structures examined include the minimum ; one, two and three rotated methyls; and rotated alane. The minimum s tructure and a singly rotated methyl TMA structure were also studied. Equilibrium geometries, energies, harmonic vibrational frequencies, in frared intensities, atomic charges,:vertical ionization potentials, di pole moments, and hyperconjugative effects are reported.