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 usi ng a pulsed jet cavity Fourier transform microwave spectrometer. The molecu les were prepared by ablating the metal with a pulsed Nd:YAG laser and allo wing the vapor to react with cyanogen or acetonitrile contained in the Ar b acking gas of the jet. The spectra are those of linear molecules. Rotationa l constants. centrifugal distortion constants. and nuclear quadrupole coupl ing constants have been precisely evaluated. Molecular geometries determine d from the rotational constants using a linear model have revealed unexpect edly short NC bond lengths for the isocyanides, MNC, and are interpreted in terms of a broadened potential well for the bending mode of these molecule s. This interpretation is confirmed with ab initio calculations. The centri fugal distortion constants are consistent with this picture. A new formulat ion of the rigid bender model has produced plausible values for bond length s and bending amplitudes for the isocyanides. The N-14 quadrupole coupling constants are discussed in terms of the approximate bending amplitudes. A d escription of the bonding in the molecules has been obtained using Townes-D ailey theory. (C) 2001 Academic Press.