Relaxation processes in aqueous solution: A Raman spectral study of hydrogen bonded interactions

Vibrational relaxation and rotational reorientation times for the nitrate ion in aqueous solutions of group I and group II metal nitrates have been measured at room temperature using polarised Raman spectra. The vibrational relaxation occurs by energy transfer to hydrogen-bound water molecules. In dilute solution all vibrational relaxation times approach a common value of 1.4 ps. At higher concentration there is a marked cation dependence with relaxation being more rapid in the presence of highly polarizing cations. It is shown that hydrogen bonding to only one water molecule is necessary to describe the relaxation process. The rotational reorientation times of the nitrate ion reflect constraints imposed by both hydrogen bonding to water and coulombic interaction with the cationic species. The low concentration limited values of τOR are different for different electrolytes. Also in contrast to vibrational relaxation (which shows linear dependence on cation concentration) the variation of rotational reorientation with concentration is non-linear with the curvature reflecting different contributions to the constraining field