AN AB-INITIO STUDY OF ANHARMONICITY AND MATRIX EFFECTS ON THE HYDROGEN-BONDED BRH-NH3 COMPLEX

Ab initio calculations have been carried out to investigate the struct ure 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 constructe d, and a model two-dimensional nuclear vibrational problem has been so lved to obtain the fundamental dimer stretching and proton stretching frequencies. These vibrational modes are strongly coupled anharmonic m odes. The anharmonic proton stretching frequencies are lower than the harmonic by 1100 and 900 cm(-1) respectively at the two levels of theo ry and are in better agreement with the experimental frequency. The ar gon matrix alters the potential energy surface by preferentially stabi lizing the more polar proton-shared structure over the structure with a traditional hydrogen bond.