Knowledge Bank

The microwave spectrum of benzene-HCl was studied in the gas phase using a pulsed Fourier-transform microwave spectrometer employing a Fabry Perot Cavity and a pulser supersonic nozzle as the molecular source. The $1\to 2,2\to 3,3\to 4$ and $4\to 5$ R branches were observed for benzene $H35Cl$ and $D35Cl$. The $3\to 4$ and $4\to 5$ R branches were observed for benzene $H37Cl$ and benzene $D37Cl$. The microwave spectrum was characteristic of a symmetric top with the time averaged displacement of the H and Cl atoms from the $C6$ axis being zero. Deuterium substitution of HCl demonstrated that the acidic proton lies between Cl and the benzene ring. $\chi aaCl$ was measured for all four isotopes and is interpreted in terms of vibrationally averaging $\chi oCl$ in HCl over two degenerate bending modes involving the angle between the $C6$ axis and the HCl bond. The microwave spectrum, structure and interpretation of spectroscopic constants will be discussed.