Spectra and structure of gallium compounds. II. Microwave, infrared, and Raman spectra, structure, and vibrational assignment of trimethylaminegallane

Abstract

The microwave spectra of (CH3)3 14N 69GaH3, (CH3)3 14N 71GaH3, (CH3)3 15N 69GaH3, (CH3)3 15N 71GaH3, (CH3)3 14N 69GaD3 and (CH3)3 14N 71GaD3 were recorded from 26.5 to 40.0 GHz resulting in the first gas phase structural determination for a gallane adduct. An rs distance of 2.111±0.002 Å was obtained for the Ga–N bond. With reasonable assumptions for the structural parameters for the methyl moiety and the Ga–H distance the following molecular parameters were determined: r(N–C)=1.47 Å, uGaNC=110.0°, and uNGaH=102.0°. The barrier to internal rotation of the GaD3 moiety was calculated from the relative intensity of the vibrational satellites to be 1.2 kcal/mole. The infrared (200–4000 cm−1) and Raman spectra (50–3500 cm−1) of (CH3)3 NGaH3, (CH3)3 NGaD3, and (CH3)3 15NGaH3 have been recorded for the solid state at low temperature. The spectra have been interpreted in detail on the basis of C3v molecular symmetry. The splitting of the degenerate E modes in the spectra of the solids indicates that the previously determined crystal structure where the hydrogen positions were not determined is not correct for the complete molecule. A normal coordinate calculation has been carried out by utilizing a modified valence force field to calculate the frequencies and the potential energy distribution. The Ga–N stretch was found to be extensively mixed with the NC3 symmetric deformation for the (CH3)3 NGaH3 and for the d3 molecule additional mixing was found with the GaD3 symmetric deformation. The Ga–N force constant was found to have a value of 2.44 mdyn/Å which is considerably larger than the value of the Ga–P force constant (2.0 mdyn/Å) in (CH3)3 PGaH3. These results are compared to similar quantities in several corresponding Group IIIA–VA compounds.