Ab-initio Harmonic Analysis of Large-amplitude Motions in Ethanol Dimers

Abstract

At relatively low pressures, dimerization of ethanol yields three different structures trans-gauche, trans-trans and gauche-gauche, each of them with different minimum energy conformations. The energy differences among the stable structures are relatively low. All of them may be present in the same sample. Dimerization shifts the whole spectrum to higher frequencies. The six new intermolecular modes push up the remaining vibrational modes which become constrained by the presence of the second molecule. The normal modes involving the hydrogen bonded atoms show the largest vibrational shifts. Five of the additional modes lies below 100 cm⁻¹ and confer some non-rigidity to the dimer. The harmonic fundamental of the hydrogen bond stretching is located at 173.6, 155.3 and 189.4 cm⁻¹ for the different conformers of the trans-gauche structure, i.e., quite below the OH torsion thaf located at 305.7 cm⁻¹ in the molecule and moves up to 700 cm⁻¹ in the dimers. For the trans-trans and gauche-gauche, this transition lies at 164.8 and 188.7 cm⁻¹ respectively. The pattern observed between 150 and 190 cm⁻¹ may be assigned to this stretching.