We performed a comparative spectroscopic FTIR and computational study of the vibrational OH stretching frequencies in liquid ethanol-ethanol (I) and liquid ethanol-N-methylimidazole dimers (II). The latter system mimics the hydrogen bond formation between serine and histidine residues, which is the incipient step in the enzymatic activity of the catalytic triads. Complex (I) was studied as a reference system. The infrared spectra revealed the presence of the OH fundamental stretching transitions at 3339 and 3271 cm(-1) for complexes (I) and (II), respectively. This red shift of 68 cm(-1) indicates that the hydrogen bond between ethanol and N methylimidazole exists and it is favoured over the one occurring in ethanol dimers. It is shown that vibrations exhibit anharmonicity. (C) 2004 Elsevier B.V. All rights reserved.