A COMPARATIVE-STUDY OF ANHARMONICITY AND MATRIX EFFECTS ON THE COMPLEXES XH-NH3, X=F, CI, AND BR

Ab initio calculations have been performed to investigate the structur es and infrared spectra of the complexes FH:NH3, ClH:NH3, and BrH:NH3, and the effects of the presence of inert gas atoms on structures and spectra. Two-dimensional MP2/6-31+G(d,p) potential energy surfaces wer e constructed for the complexes XH:NH3, and model two-dimensional Schr odinger equations were solved for the proton stretching and dimer (hea vy-atom) stretching modes. Although all complexes have equilibrium str uctures characterized by traditional hydrogen bonds, their infrared sp ectra differ significantly. In FH:NH3 both the ground (v = 0) and firs t excited state for the proton stretching mode (v = 1) are confined to the potential well describing the equilibrium structure. In this case the harmonic approximation is appropriate, and matrix effects are uni mportant. In ClH:NH3 the v = 1 proton stretching vibration accesses th e more polar, proton-shared region of the potential surface. Here the harmonic treatment leads to a significant overestimation of the experi mental proton-stretching frequency. Significant improvement results fr om an anharmonic treatment, which shows some coupling between proton a nd dimer modes. The presence of rare gas atoms preferentially stabiliz es the proton-shared region of the surface, lowering the energy of the v = 1 state. Further improvement results if the MP2/aug'-cc-pVDZ pote ntial energy surface is used. In BrH:NH3 the proton-shared region of t he potential surface is accessible in both v = 0 and v = 1 vibrational states, and an anharmonic treatment is required to obtain reasonable agreement with experiment. In BrH:NH3 proton stretching and dimer stre tching modes are highly coupled, and rare gas atoms have structural an d spectral effects. (C) 1998 American Institute of Physics.