Further Studies of Depolarization Ratios in the Raman Spectrum of Aqueous Formate Ion

Abstract

Raman spectral depolarization ratios (ρ = IX(ZY)Z/IX(YY)Z) have been determined for solutions of H12C16O2Na, D12C16O2Na, H13C16O2Na, and H13C18O2Na in D2O. The maximum value of ρ, 3/4, required for all vibrations in which some molecular symmetry is lost, is observed throughout only for the out-of-plane bending band. For CH in-plane bending ρ, in solution in D2O, ranges from 0.55 for H12C16O2- to 0.62 for H13C18O2-; for H12C16O2- in H2O p is lower still, viz. 0.48. For the carboxylate stretching band in the region 1593 cm-1 to 1533 cm-1 ρ is in the range 0.50 to 0.58. For an HCO2- ion of C2v symmetry both these vibrations are of species b1, i.e., non-totall symmetric. The fact that ρ for both is well below 0.75 shows that the formate ion in water actually has no plane of symmetry perpendicular to the ionic plane. Coriolis interaction between CH in-plane bending and the nearby (second) carboxylate stretching motion is not a major cause of the low ρ for the former; the principal interaction involved is one between two vibrations of identical symmetry species, which for H12C16O2- are close to accidental resonance. For CD in-plane bending in DCO2- ρ is much higher, and close to pmax; a similar difference in p has now been found between CH and CD in-plane bending in (liquid) formic acid HCO2H and DCO2H. Over the concentration range HCO2Na-4.5aq. to HCO2Na-15aq. concentration effects on the Raman and the infrared spectrum are generally very small; however, a concentration-dependent satellite band at 1622 cm-1 is taken to indicate the presence of some cation-anion pairs in intimate contact (of the order of 13% for HCO2Na-4.5D2O). ρ is almost the same for the satellite band and the main band (at 1593 cm-1), however. By contrast, solvent isotope effects on ρ are sometimes relatively large. The spectrum of the solvent was therefore studied, too.