The rotational spectrum and molecular structure of the benzene–hydrogen chloride complex

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Abstract
The microwave spectrum of the weakly bound complex benzene–HCl was studied in the gas phase using Fourier transform microwave spectroscopy carried out in a Fabry–Perot cavity with a pulsed supersonic nozzle as the molecular source. Several R-branch a-dipole transitions have been observed for benzene–H 35Cl, benzene–D 35Cl, benzene–H 37Cl, and benzene(d6)–H 35Cl. The spectrum was characteristic of a symmetric top, indicating that the time averaged displacement of the H and Cl atoms from the benzene C6 axis is zero. Deuterium substitution of HCl demonstrated that the acidic proton lies between the Cl atom and the benzene ring. The chlorine nuclear quadrupole coupling constant χClaa, was measured for all four isotopic species and is interpreted in terms of a projection of the chlorine quadrupole coupling constant in free HCl, averaged over two degenerate vibrational ground state bending modes involving the angles between the benzene C6 axis and the HCl bond axis. The spectroscopic constants for benzene–HCl are: aNumbers in parentheses represent one standard deviation in the fit.