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 angstrom. 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 (epsilon = 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. (C) 2008 Elsevier B.V. All rights reserved.