Structural properties of CH3CN–SO2 in the gas phase and condensed-phase media via density functional theory and infrared spectroscopy

Structural properties of the CH3CN–SO2 complex were studied in the gas phase via density functional theory, as well as in argon matrices, nitrogen matrices, and solid-state thin films via low-temperature infrared spectroscopy. Gas-phase structures for two nearly isoenergetic conformations were obtained from B3PW91/aug-cc-pVTZ calculations, and these results indicate that the complex is quite weak in the gas phase, with a long N–S distance of 3.02 Å. Several vibrational bands of the complex were observed in nitrogen and argon matrices. For the principle isotopomer, the SO2 asymmetric stretch was observed at 1345 cm−1 in solid nitrogen and 1342 cm−1 in argon. The SO2 symmetric stretch was observed at 1156 cm−1 in nitrogen, and 1148 cm−1 in argon. Also, the SO2 bend was observed at 527 cm−1 in nitrogen, and 520 cm−1 (tentative assignment) in sold argon. In “matrix-free” spectra of solid CH3CN/SO2 thin films, the SO2 asymmetric stretch is the only SO2-localized mode appreciably shifted relative to the corresponding matrix frequencies, by about 20 cm−1 to the red, but the band is essentially coincident that observed in a low temperature, bulk sample of pure SO2. Calculated frequencies (B3PW91/aug-cc-pVTZ) for the gas-phase CH3CN–SO2 complex show some larger discrepancies, but not to the degree that would imply any significant medium-induced structural changes. The N–S distance potential was also mapped in the gas phase via several density functional methods, and also for dielectric media (ε = 1.5–80) via PCM/B3LYP/6–311+G(2fd,p) calculations. The computational results are quite consistent with the structural implications of the experiments, and collectively, these data indicate that condensed-phase media induce at most only minimal changes in the structural properties of CH3CN–SO2.