Structure of ammonia trimethylalane (Me3Al-NH3): Microwave spectroscopy, X-ray powder diffraction, and ab initio calculations

The structural parameters of the ground-state geometry of Me3Al-NH3 calcula ted by various ab initio methods (HF, B3LYP, and MP2) are presented. For th e two isotopomers Me3Al-(NH3)-N-14 and Me3Al-(NH3)-N-15, the rotational tra nsitions J = 1 <-- 0 and J = 2 <-- 1 were investigated by Fourier transform microwave spectroscopy (4-12 GHz). All transitions showed a complicated hy perfine structure consisting of a large number of lines, so that only parti al assignment of the experimental data was possible. The best fit was achie ved for the J = 1 <-- 0 transition of the N-15-marked sample (Me3Al-(NH3)-N -15), for which 17 of 42 observed components could be assigned by assuming a symmetric top with one quadrupole nucleus (Al-27) and three internal-meth yl group rotors. The combination of microwave spectroscopy and the calculat ed geometry of Me3Al-NH3 at the MP2(fc)/6-311G(2d,2p) level resulted in an Al-N bond length of 2.066(1) Angstrom as the best estimate for the experime ntal value. These results are compared with those of the well-known isomer H3Al-NMe3 (Warner, H. E.; et al. J. Phys. Chem. 1994, 98, 12215. Atwood, J. L.; et al. J. Am. Chem. Sec. 1991, 113, 8183. Almenningen, A.; et al. Acta Chem. Scand. 1972, 26, 3928. March, M. B. C.; et al. J. Phys. Chem. 1995, 9,; 195). The solid-state structure of Me3Al-NH3 was solved from X-ray powd er diffraction data. The compound crystallizes in the orthorhombic space gr oup Ama2 with four molecules per unit cell. There are significant differenc es between the structure of ammonia trimethylalane in the gas phase and in the solid state. The main differences could be understood on the basis of O nsager's theory using SCRF calculations (B3LYP/6-311++G(2d,p)) (Foresman, J . F.; Frisch, AE. Exploring Chemistry with Electronic Structure Methods, 2n d ed.; Gaussian,:Inc.: Pittsburgh, PA, 1996. Onsager, L,. J. Am. Chem. Sec. 1936, 58, 1486).