INFRARED SPECTRA OF MATRIX ISOLATED ALKALI METAL METABORATES

The infrared spectra of MBO2 molecules (M = Lin, 6Li, Na, K, Rb, and Cs) isolated in a matrix of solid argon or krypton at liquid hydrogen temperature are reported. Above 500 cm-1, three absorptions are found which exhibit small monotonic change with increasing mass of the alkali metal. These absorptions, consisting of 10B-11B doublets correspond to modes involving essentially the BO2 portion of the molceules, the measured frequencies being very similar to those of the BO2 group in B2O3 or HBO2. The spectra below 500 cm-1 consist of at most two absorptions which are characterized by negligible 10B-11B 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 Cs molecular symmetry. A force constant analysis of the data assuming an MOBO arrangement of atoms suggests an MOB bond angle in the neighborhood of 90°, a value consistent with the results of electron diffraction studies. © 1969.