Vibrational energy relaxation and reorientation of azide ion pairs in DMSO

Vibrational energy relaxation (VER) and reorientation times have been measured by ultrafast polarization infrared spectroscopy for the antisymmetric stretching ν3 band of free azide ion and NaN3, MgN3+, and CaN3+ ion pairs in dimethyl sulfoxide solutions. The ν3 absorption bands are shifted to higher frequency in the ion pairs relative to the unassociated, dissolved anion, but VER times of the ion pairs are similar, all in the range of 11–18 ps. The reorientation times of the ion pairs are longer (>20 ps) than for free azide (6 ± 2 ps), as expected based on their structures. Ab initio calculations were performed to help interpret the experimental results.