Effects of modification of molecular structure on Raman spectral depolarization ratios: aqueous betaine (CH3)3NCH2CO2*29D2O versus (CH3)4NBr*29D2O

Replacement of a CH in (CH3)4N+ by a CO2- group raises rho from 0.012 to 0.034 for symmetric N-C stretching and from ca. 0.026 to 0.031 for all-in-phase methyl C-H stretching. rho for antisymmetric C-H stretching and CH3 bending remains in the 0.73-0.75 range. No significant effect on rho attributable to the failure of solvation sheath symmetry to match the symmetry of the solute molecule is seen either near rho=0 or near p=3/4. The drastic change in charge distribution on going from cation to zwitterion, too, does not cause a major change near rho=0. Corroborative evidence of weakness of interaction between most surrounding D2O molecules and the Me3N+ portion of the zwitterion, derived from the O-D stretching region of the Raman spectrum, and implying the absence of symmetry in the solvation sheath, is discussed in Appendix A.