Infrared and ab Initio Study of the Chloride−Ammonia Anion Complex

The infrared spectrum of the Cl-−NH3 anion complex has been recorded in the 2900−3450 cm-1 range by monitoring the infrared-induced production of Cl- anion fragments in a tandem mass spectrometer. The spectrum features an intense band centered at 3140 cm-1, which is shifted by 196 cm-1 to lower frequency with respect to the ammonia ν1 band (3336.2 cm-1). Two much weaker peaks are observed at 3270 and 3340 cm-1. Ab initio calculations performed at the MP2(full)/aug-cc-pVTZ level indicate that the complex possesses a C s proton-bound minimum-energy structure, and yield vibrational frequencies and intensities that accord with the infrared data. Barriers, calculated at the MP4SDTQ(full)/aug-cc-pVTZ//MP2(full)/aug-cc-pVTZ level, for ammonia inversion and turnstile exchange of the bonded proton, were determined to be 5.72 and 1.66 kcal/mol, respectively.