ATR FT-IR studies of supercritical methanol

To measure the Fourier transform infrared spectra of supercritical methanol at 523 K and pressures up to 30 MPa, the application of attenuated total reflectance optical cell is reported. Together with previously published transmittance infrared spectra of pure supercritical methanol at low pressures (up to 5 MPa), present studies covered the entire region from gas-like to liquid-like densities for this simplest organic compound capable of exhibiting hydrogen bonding. At 8 MPa, the position of methanol OH stretching vibration corresponded to the hydrogen-bonded dimer formation. At higher pressures, the methanol OH peak maxima were located between dimer and trimer or tetramer infrared absorption peaks. The shift of the hydrogen-bonding equilibrium with pressure toward higher aggregates was also indicated by the absorptivity changes in the region of OH stretching vibration.