IR-UV Spectroscopic Studies of OH and CN Radical Complexes

Infrared action spectroscopy is used to identify the OH-HONO2 complex, an intermediate proposed to be important in the reaction of OH with HONO2. Two features are observed in the OH stretching region: a rotationally structured band corresponding to the OH radical stretch and a broadened feature assigned to the OH stretch of HONO2. Assignments are based on vibrational frequencies, analysis of rotational structure, and comparison with ab initio calculations. Nascent OH product state distributions give a binding energy of ≤5.3 kcal mol-1. Infrared action spectroscopy is also used to examine the H2O-HO complex, a primary interaction in the hydration of OH. A rotationally structured band is assigned to the OH radical stretch of H2O-HO. The stability of the complex, ≤5.14 kcal mol-1, is derived from the nascent OH product state distribution. The assignment is supported by ab initio predictions of the spectral shift and dissociation energy. A second feature to lower frequency is attributed to a hot band from an H2O bending state based on theoretical modeling. IR-UV double resonance spectroscopy is used to characterize hindered rotor states in the ground electronic state of CN-Ne and CN-Ar. Infrared spectra exhibit perturbations due to Coriolis coupling: a deperturbation analysis gives rotational constants and coupling strengths. The energetic ordering and spacings of the hindered rotor states provide a probe of the anisotropic intermolecular potential, which is compared with ab initio calculations. The CN monomer is nearly free rotor-like within both complexes. A similar approach yields the infrared spectrum of H2-CN, which exhibits rotational structure consistent with ortho-H2-CN in a linear C≡N-H-H configuration. Lastly, laser-induced fluorescence and IR-UV fluorescence depletion studies are used to characterize the lowest intermolecular levels of CN-Ar correlating with CN B 2Σ+ + Ar. Fluorescence depletion spectra confirm that specific features originate from a common ground state. The observed energy level pattern and intensity profile reflect the change in configuration from a weakly anisotropic potential about linear N≡C-Ar in the ground state to linear C≡N-Ar in the excited electronic state.