Rotationally resolved infrared spectrum of the Cl−–H2 anion complex

The mid-infrared spectrum of the 37Cl−–H2 anion complex has been measured over the 3990–4050 cm−1 range (H–H stretch region) using infrared vibrational predissociation spectroscopy. The spectrum features a well resolved Σ–Σ transition red shifted by 156 cm−1 from the free H2 molecule stretch. Analysis of the P and R branch line positions using a linear molecule energy level expression yields ν0=4004.77±0.08 cm−1, B″=0.853±0.002 cm−1, D″=(9.3±1.0)×10−5cm−1, B′=0.919±0.002 cm−1, and D′=(9.0±1.0)×10−5 cm−1. The Cl−–H2 complex appears to have a linear equilibrium structure, with a vibrationally averaged separation of 3.19 A between the Cl− and the H2 center-of-mass. Vibrational excitation of the H–H stretch induces a 0.12 A contraction in the intermolecular bond.