Vibrational and rotational relaxation times of solvated molecular ions

Infrared pump-probe and infrared polarization spectroscopy have been used to measure the vibrational relaxation times (T₁) of the antisymmetric stretching mode and the reorientation times (T_R) for N ₃ ^-, NCS^-, and NCO^- in D₂O and/or mettianol. For N₃ ^-, experiments were also conducted in H₂O and hexamethyl-phosphamide (HPMA) solutions. The rapid vibrational relaxation and slow reorientation observed demonstrate strong coupling between the ions and the solvents. Longer vibrational relaxation and shorter reorientation times measured for NCS^- reveal weaker solvent interactions that may be due to the importance of the charge distribution and the form of the normal coordinate. A comparison of the T₁ and T _R times in different solvents permits a determination of the relative interaction strengths for the solvents investigated. The relatively weaker coupling of N₃ ^- in the aprotic solvent HMPA demonstrates the importance of hydrogen bonding in strong solvent interactions in ionic solutions. The experimental results are compared with recent molecular dynamics simulations of ionic solutions.