Vibration-spectral studies of carboxylate ions. Part III. Sodium formate, HCO2Na and DCO2Na; Raman-spectral depolarisation ratios in aqueous solution, and band splitting in the solid-state infrared spectrum

Abstract

Accurate measurement of the depolarisation ratios showed that maximum depolarisation (6//₇) is observed only for the out-of-plane bending vibration. The (band area) depolarisation ratio for the CD in-plane bending band of DCO₂^–, 0·80, is slightly less, and that of the CH in-plane bending band of HCO₂^–, 0·64, considerably less than the maximum. In HCO₂^– CH in-plane bending steals polarisation from the CO stretching band at 1354 cm.^–1. These findings are in agreement with C_s(i.e., planarity) but not C_2v symmetry for the formate ion. The splitting of the corresponding band in the solid-state infrared spectrum (at 1366 cm.^–1) is only slight for pure HCO₂Na but is very pronounced for HCO₂Na present as an impurity in the host crystal of DCO₂Na. No such splitting is permitted for a formate ion of C_2v symmetry in a crystal of space-group C_2h⁶.