Unraveling Environmental Effects on Hydrogen-Bonded Complexes:  Matrix Effects on the Structures and Proton-Stretching Frequencies of Hydrogen−Halide Complexes with Ammonia and Trimethylamine

Anharmonicity and matrix effects play important roles in determining the proton-stretching frequencies in hydrogen-bonded complexes of HCl and HBr with NH 3 and N(CH 3 ) 3 . These effects have been investigated through ab initio calculations carried out at MP2/aug‘-cc-pVDZ for complexes with HCl and at MP2/6-31+G(d,p) for complexes with HBr. The potential surfaces of these complexes are very anharmonic, since the region surrounding the global minimum may be very broad and relatively flat, or a second region of the surface, displaced from the global minimum, can be accessed in either the ground ( v = 0) or the first excited ( v = 1) state of the proton-stretching mode. As a result, two-dimensional anharmonic frequencies, particularly for the proton-stretching vibration, can be dramatically different from the corresponding harmonic frequencies. Moreover, the zero-point energy contribution to binding enthalpies based on harmonic vibrational frequencies can be significantly overestimated in some complexes. To model the effects of matrices on the structures and spectra of these complexes, potential surfaces have been generated in the presence of external electric fields applied along the hydrogen-bonding X−H−N direction. These fields preferentially stabilize more polar hydrogen-bonded structures. The changes in anharmonic frequencies computed from these surfaces depend on the strength of the field and the nature of the equilibrium structure at zero field. Comparisons between computed frequencies for these complexes and experimental frequencies obtained in Ar and N 2 matrices provide insight into the dependence of proton-stretching frequencies on the environment. It is now possible to understand the apparently disparate effects of Ar and N 2 matrices on the spectra of closely related complexes.