Vibrational spectroscopy and dynamics of the hydrazoic and thiocyanic acids in protic and aprotic solvents

Azide (N$_3^-$) and thiocyanate (NCS$^-$) in solvents are benchmark systems for studying fast vibrational energy relaxation (VER) and solvent-solute interactions. To investigate the effects of solute charge as well as solvent-isotope effects on solute vibrational spectra and dynamics, infrared pump-probe studies have been carried out to determine VER times for CN and antisymmetric NNN stretching bands XNCS and XN3 (X=H, D), in protic and aprotic solvents to compare with the well studied N$_3^-$ and NCS$^-$ anions. The study suggests that protonating the anions may be viewed as a limiting case of hydrogen bonding. Results from extending the measurements to other solvents demonstrate that the trend in VER relaxation times with solvent is similar for HN$_3$ and N$_3^-$, but there is less of a systematic frequency shift for HN$_3$ .