MICROWAVE STUDY OF THE CVD PRECURSOR TRIMETHYLAMINE ALANE, (CH3)(3)NALH(3) - ROTATIONAL SPECTRA, AL-27 AND N-14 NUCLEAR-QUADRUPOLE COUPLING-CONSTANTS, AND MOLECULAR-STRUCTURE

The rotational spectra of six isotopomers of trimethylamine-alane have been observed by pulsed beam Fourier transform microwave spectroscopy. Hyperfine analyses of the symmetric top isotopomers, (CH3)(3)NAlH3, (CH3)(3)NAlD3, and (CH3)(3)(NAlH3)-N-15, determined the spectroscopic constants, B-0, D-J, D-JK, eQq(Al-27), and eQq(N-14). For (CH3)(3)NAlH3, B-0 = 2745.113(1) MHz, D-J = 643(24) Hz, D-JK = 1372(250) Hz, eQq(Al-27) = 25.032(6) MHz, and eQq(N-14) = -3.777(5) MHz. Spectral assignments have also been made for the asymmetric top isotopomers, [(CH3)(2)(CH3)-C-13]NAlH3, (CH3)(3)NAlH2D, and(CH3)(3)NAlHD2. Substitution structural parameters of r(NC) 1.487(2) Angstrom and theta(AlNC) = 109.9(2)degrees are obtained from the microwave moments of inertia. With the assumption of r(AlN) = 2.063(7) Angstrom from an electron diffraction structure reported previously (Almenningen, A.; Gundersen, G.; Haugen, T.; Haaland, A. Acta Chem. Scand. 1972, 26, 3928) microwave substitution coordinates for the aluminum hydrogens give r(HAl) = 1.585(2) Angstrom and theta(HAlN) = 98.1(6)degrees. The structure of (CH3)(3)NAlH3 is compared to the related complexes, H3AlNH3, H3NBH3, and (CH3)(3)NBH3. In the same series, bond orders calculated from measured values of eQq(Al-27) and eQq(B-11) are compared to Al-N and B-N dative bond dissociation energies.