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 i ntense band centered at 3140 cm(-1), which is shifted by 196 cm(-1) to lowe r frequency with respect to the ammonia nu(1) band (3336.2 cm(-1)). Two muc h 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 pos sesses a C-s proton-bound minimum-energy structure, and yield vibrational f requencies and intensities that accord with the infrared data. Barriers, ca lculated at the MP4SDTQ(full)/aug-cc-pVTZ//MP2(full)/aug-cc-pVTZ level, for ammonia inversion and turnstile exchange of the bonded proton, were determ ined to be 5.72 and 1.66 kcal/mol, respectively.