Fourier Transform Microwave Spectroscopy of Cyanides and Isocyanides of Al, Ga, and In

Microwave spectra of the three Group 13 metal cyanide pairs AlNC/AlCN, GaNC/GaCN, and InNC/InCN have been measured in the frequency range 6-25 GHz using a pulsed jet cavity Fourier transform microwave spectrometer. The molecules were prepared by ablating the metal with a pulsed Nd:YAG laser and allowing the vapor to react with cyanogen or acetonitrile contained in the Ar backing gas of the jet. The spectra are those of linear molecules. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants have been precisely evaluated. Molecular geometries determined from the rotational constants using a linear model have revealed unexpectedly short NC bond lengths for the isocyanides, MNC, and are interpreted in terms of a broadened potential well for the bending mode of these molecules. This interpretation is confirmed with ab initio calculations. The centrifugal distortion constants are consistent with this picture. A new formulation of the rigid bender model has produced plausible values for bond lengths and bending amplitudes for the isocyanides. The 14N quadrupole coupling constants are discussed in terms of the approximate bending amplitudes. A description of the bonding in the molecules has been obtained using Townes-Dailey theory.NRC publication: Ye