HYDROGEN BONDED $OH-C_{2}H_{2}$ REACTANT COMPLEX CHARACTERIZED BY INFARED ACTION SPECTROSCOPY

Abstract

Author Institution: Department of Chemistry, University of PennsylvaniaThe entrance channel to the $OH + C_{2}H_{2} \rightarrow HOCHCH$ reaction has been characterized by infrared spectroscopy of a binary hydrogen-bonded complex between the chemically reactive partners. Infrared action spectra of the $OH-C_{2}H_{2}$ reactant complex have been recorded using an optical parametric oscillator operating in the OH overtone region near $1.4 {\mu}m$ and the asymmetric acetylenic fundamental region near $3.0 {\mu}m$. The $OH (v = 1$ or 0) fragments from vibrational predissociation are detected by laser-induced fluorescence. The pure OH overtone band of $OH-C_{2}H_{2}$ is observed at $6885.6 cm^{-1}$ (band origin), shifted $85.7 cm^{-1}$ to lower energy of the OH monomer transition. The pure OH overtone band exhibits rotationally resolved structure that is characteristic of an A-type transition of a near-prolate asymmetric top. The spectrum also shows interesting gaps between the P, Q and R branches, indicating that the orbital and spin angular momentum of the unpaired electron of OH is unquenched. The P and R line positions have been used to determine values of $\frac{1}{2}(B + C)$ for the upper and lower vibrational states, and yield a center of mass separation between the two subunits of $3.34(3) {\AA}in$ both vibrational states. The spectroscopic data, taken together with the results of ab initio calculations and previous work on the HF-acetylene and HCl-acetylene complexes, show that the OH-acetylene complex is T-shaped, with a hydrogen bond formed between the H atom of OH and the $\pi$ system of the C-C bond. The infrared spectrum in the asymmetric stretch region of acetylene is centered at $3281 cm^{-1}$, with a much smaller spectral red shift of $14 cm^{-1}$, but exhibits more complicated band structure with multiple Q-branches arising from a B-type transition