Direct infrared observation of hydrogen chloride anions in solid argon

To facilitate direct spectroscopic observation of hydrogen chloride anions (HCl−), electron bombardment of CH3Cl diluted in excess Ar during matrix deposition was used to generate this anion. Subsequent characterization were performed by IR spectroscopy and quantum chemical calculations. Moreover the band intensity of HCl− decays slowly when the matrix sample is maintained in the dark for a prolonged time. High-level ab inito calculation suggested that HCl− is only weakly bound. Atom-in-molecule charge analysis indicated that both atoms of HCl− are negatively charged and the Cl atom is hypervalent.