An ab initio study of anharmonicity and matrix e ects on the hydrogen - bonded BrH : NH 3 complex

Ab initio calculations have been carried out to investigate the structure and infrared spectrum of BrH:NH3, and to resolve the discrepancies found between its computed harmonic and experimental spectrum. MP2/6-31+G(d, p) and MP2/aug -cc-pVDZ potential surfaces have been constructed, and a model two-dimensional nuclear vibrational problem has been solved to obtain the fundamental dimer stretching and proton stretching frequencies. These vibrational modes are strongly coupled anharmonic modes. The anharmonic proton stretching frequencies are lower than the harmonic by 1100 and 900 cm1 respectively at the two levels of theory and are in better agreement with the experimental frequency. The argon matrix alters the potential energy surface by preferentially stabilizing the more polar proton-shared structure over the structure with a traditional hydrogen bond.