Vibrational Spectroscopy of N₃^- in the Gas and Condensed Phase

Azido-derivatized amino acids are potentially useful, positionally resolved spectroscopic probes for studying the structural dynamics of proteins and macromolecules in solution. To this end, a computational model for the vibrational modes of N₃^- based on accurate electronic structure calculations and a reproducing kernel Hilbert space representation of the potential energy surface for the internal degrees of freedom is developed. Fully dimensional quantum bound state calculations yield the antisymmetric stretch vibration at 1974 cm^-1 compared with 1986 cm^-1 from experiment. This mode shifts by 64 cm^-1 (from the frequency distribution) and 74 cm^-1 (from the IR line shape) to the blue, respectively, compared with 61 cm^-1 from experiment for N₃^- in water. The decay time of the frequency fluctuation correlation function is 1.1 ps, which is in good agreement with experiment (1.2-1.3 ps) and the full width at half maximum of the asymmetric stretch in solution is 18.5 cm^-1 compared with 25.2 cm^-1 from experiment. A computationally more efficient analysis based on instantaneous normal modes is shown to provide comparable, albeit somewhat less quantitative results compared to solving the three-dimensional Schrödinger equation for the fundamental vibrations.