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

Abstract

Raman spectral depolarization ratios (ρ = I_X(ZY)Z/I_X(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 C_2v 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 p_max; 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.