Communication: Gas phase vibrational spectroscopy of the azide-water complex

The vibrational spectra of the azide-water complex, N₃ ^-(H₂O), and its fully deuterated isotopologue are studied using infrared photodissociation (IRPD) spectroscopy (800-3800 cm^-1) and high-level ab initio computations. The IRPD spectrum of the H₂-tagged complex exhibits four fundamental transitions at 3705, 3084, 2003, and 1660 cm^-1, which are assigned to the free OH stretching, the hydrogen-bonded O-H stretching, the antisymmetric N₃ stretching, and the water bending mode, respectively. The IRPD spectrum is consistent with a planar, singly hydrogen-bonded structure according to an MP2 and CCSD(T) anharmonic analysis via generalized second-order vibrational perturbation theory. The red-shift of the hydrogen-bonded OH stretching fundamental of 623 cm^-1 associated with this structure is computed within 6 cm^-1 (or 1%) and is used to estimate the proton affinity of azide (1410 kJ mol^-1). Born-Oppenheimer molecular dynamics simulations show that large amplitude motions are responsible for the observed band broadening at cryogenic temperature. Temperature-dependent (6-300 K) IR multiphoton dissociation spectra of the untagged complex are also presented and discussed in the context of spectral diffusion observed in the condensed phase.