Gas-phase infrared spectroscopy of ClHCl−

The ν3 band of the hydrogen bichloride anion in a D∞h structure has been detected by infrared diode laser spectroscopy. The anion was produced in a discharge of a CHCl3, H2, and He mixture. Analysis of the observed spectra gave the fundamental frequencies; ν3(35ClH35Cl−)=722.8965(2) and ν3(37ClH35Cl−)=722.9589(2) cm−1 with one standard deviation in parentheses. The anomalous isotope shift was explained by an interaction between the ν3 and 2ν1 states in 37ClH35Cl−. From the rotational constant obtained, the Cl–Cl internuclear distance in the ground state was calculated to be r0=3.146 76(5) for 35ClH35Cl− and r0=3.146 64(6) Å for 37ClH35Cl−.