Effects of modification of molecular structure on Raman spectral depolarization ratios: aqueous betaine (CH3)(3)NCH2CO2 center dot 29D(2)O versus (CH3)(4)NBr center dot 29D(2)O

摘要

Replacement of a CH in (CH3)(4)N+ 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 rho = 3/4. The drastic change in charge distribution on going from cation to zwitterion, too, does not cause a major change near p = 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. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 36]