INFRARED SPECTRA OF MATRIX ISOLATED ALKALI METAL METABORATES

The infrared spectra of MBO2 molecules (M = Li, Na, K, Rb and Cs) isolated in a matrix of solid argon or krypton at liquid hydrogen temperature are reported. Above 500/cm, three absorptions are found which exhibit small monotonic change with increasing mass of the alkali metal. These absorptions, consisting of B10-B11 doublets correspond to modes involving essentially the BO2 portion of the molecules, the measured frequencies being very similar to those of the BO2 group in B2O3 or HBO2. The spectra below 500/cm consist of at most two absorptions which are characterized by negligible B10-B11 shifts and large decreases in frequency with increasing mass of the alkali metal. These absorptions are attributed to vibrational modes involving principally the motion of the metal atoms. The total number of observed fundamentals satisfies the selection rules for a C(s) molecular symmetry. A force constant analysis of the data assuming an MOBO arrangement of atoms suggests an M-O-B bond angle in the neighborhood of 90 deg., a value consistent with the results of electron diffraction studies. (Author)