Infrared Spectrum and Stability of the H2O-HO Complex: Experiment and Theory

Infrared action spectroscopy is utilized to characterize the gas-phase, hydrogen-bonded H2O-HO complex, a primary interaction in the hydration of the hydroxyl radical. The OH radical stretch of the H2O-HO complex is identified at 3490 cm(-1), shifted 78 cm(-1) to lower frequency of the OH monomer transition. The stability of the complex, D-0 <= 5.14 kcal mol(-1), is derived from the highest observed OH product channel in the associated product state distribution. The assignment is supported by high level ab initio calculations of the spectral shift of the binary complex from free OH and its dissociation energy, D-e(CBS-infinity) = 5.6 kcal mol(-1). A second weaker feature, appearing 15 cm(-1) to lower frequency at 3475 cm(-1). is attributed to a hot band, the OH radical stretch originating from an out-of-plane H2O bending state, based on two-dimensional calculations of frequencies and strengths of transitions involving the coupled vibrational modes.