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

Abstract

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 νCC band at 926 cm−1, νHCl the band at 2677 cm−1, and the νCN 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 νHCl bands, which differed, and were ultimately found to shift systematically across various condensed-phase media. A plot of medium-induced shifts for the νHCl 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.