Je. Delbene et Mjt. Jordan, A COMPARATIVE-STUDY OF ANHARMONICITY AND MATRIX EFFECTS ON THE COMPLEXES XH-NH3, X=F, CI, AND BR, The Journal of chemical physics, 108(8), 1998, pp. 3205-3212
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.