An ab initio study of anharmonicity and field effects in hydrogen-bonded complexes of the deuterated analogues of HCl and HBr with NH₃ and N(CH₃)₃

One- and two-dimensional nuclear Schrödinger equations have been solved on MP2/aug′-cc-pVDZ potential energy surfaces generated for ClH:NH₃ and ClH:N(CH₃)₃ and on MP2/6-31+G(d,p) surfaces generated for BrH:NH₃ and BrH:N(CH₃)₃ to investigate deuterium substituent effects on the expectation values of X-N and X-H distances and on anharmonic dimer- and proton-stretching frequencies. These studies have been carried out on all isotopomers in the presence of electric fields of varying strengths. Deuteration of HX or ammonia or trimethylamine has only minor effects on expectation values and dimer-stretching frequencies. While deuteration of the nitrogen base also has only a minor effect on proton-stretching frequencies, deuteration of the hydrogen halide has a major effect, as expected. X-D stretching frequencies are always lower than the corresponding X-H frequencies at all field strengths, although the ratio ν(D)/ν(H) for corresponding pairs of isotopomers may be less than, equal to, or greater than the harmonic ratio of 0.71. Structural and vibrational spectral changes as a function of field strength are similar for a given complex and each of its isotopomers. The agreement between computed proton-stretching frequencies and experimental frequencies supports the validity of this approach for modeling matrix effects on the structures and vibrational spectra of hydrogen-bonded complexes. The computed results provide insight into the nature of the hydrogen bonds that stabilize these complexes in low-temperature matrixes.