Infrared spectrum of CH₃CN-HCl in solid neon, and modeling matrix effects in CH₃CN-HCl and H₃N-HCl

Several IR bands for two isotopic forms of the CH₃CN-HCl complex have been measured in neon matrices at 6 K. Assigned bands for the primary isotopomer include the ν_CC band at 926 cm^-1, ν_HCl the band at 2677 cm^-1, and the ν_CN band, which occurs as a doublet at 2308/2279 cm^-1. The observed frequencies are reasonably consistent with previous observations in solid argon and/or nitrogen, with the exception of ν_HCl bands, which differed, and were ultimately found to shift systematically across various condensed-phase media. A plot of medium-induced shifts for the ν_HCl band vs. polarizability of the host substances is essentially continuous; except for the N₂-matrix frequency, for which the shift is considerably larger than that expected on this basis. In an effort to clarify the physical underpinnings of these shifts, and their structural implications, a computational study was undertaken, which examined CH₃CN-HCl as well as H₃N-HCl; a system for which analogous frequency shifts have been observed and characterized previously. Beyond equilibrium structure and frequency calculations, which predict that the ammonia complex is stronger and undergoes a greater degree of structural change in bulk, condensed-phase media, the N-Cl potentials of both complexes were mapped in the gas-phase and in bulk, dielectric media. The predicted medium effects on these potentials are consistent with observations. The CH₃CN-HCl curves shift slightly in response to the medium, consistent with a subtle enhancement of the hydrogen bond, and relatively small frequency shifts. The effects of the dielectric media on H₃N-HCl are more substantial, and indicate a significant degree of proton transfer, even for low-dielectric environments, which is consistent with the extreme spectral shifts noted previously for this system.