Carbonyl stretch of CH⋯O hydrogen-bonded methyl acetate in supercritical trifluoromethane.

Infrared spectroscopy in the gas phase was used to study the formation reaction of the CH⋯O hydrogen bonding complex involving the CH group of trifluoromethane, as a hydrogen donor, and the carbonyl group of methyl acetate, as a hydrogen acceptor, under different (T, p) conditions. The hydrogen-bonded carbonyl stretch of the molecular pair was monitored in dilute mixtures of methyl acetate in trifluoromethane at near-critical temperatures, from gas- to liquid-like densities. In the gas region, it was possible to discriminate the carbonyl signal of the hydrogen-bonded complex from that of the free ester and have access to their relative concentration. The equilibrium constant of the hydrogen bonding reaction and the standard enthalpy and entropy changes in the process were determined using the spectroscopic data. CH⋯O bonding was favored by lowering temperature or pressurizing FCH in the mixture, remaining essentially no free carbonyl groups about the critical density. The carbonyl band of the hydrogen-bonded pair appeared as a single symmetric peak up to liquid-like densities, suggesting that the 1:1 methyl acetate-trifluoromethane complex has the most abundant stoichiometry. Spectral features as frequency shift and bandwidth of the hydrogen-bonded carbonyl were studied as a function of temperature and solvent-density. A bathochromic (red) vibrational shift was registered for the bound carbonyl band against density, with a sudden change in behavior in the near-critical region, while the width of this band remains mostly unresponsive.