Hydrogen-bonded forms of methanol IR spectra and ab initiocalculations

Abstract

The IR spectra of methanol as a dilute solution in CCl4 and in the vapour phase have been measured between 2500 and 4000 cm-1 and between 1000 and 1100 cm-1 in order to better understand the nature of the hydrogen bonding equilibria present. The integrated absorption coefficient of the monomeric O–H stretching mode is calculated as (2.157±0.025)×104 m mol-1 and the proportion of the components associated with the three principal bands and a fourth weaker band estimated. Seven possible components were considered which were monomer, closed cyclic and open chain dimers, trimers and tetramers. Abinitio calculations were carried out on these components using six basis sets up to the restricted Hartree Fock 6-311++G(3df,3pd) level. Relevant calculated infrared wavenumber and intensity values, O–H···O bond lengths and hydrogen bonding energies are reported. The cyclic dimer is shown to be a transition state with the open dimer forming a stable minimum energy form. In the case of the trimer and tetramer the hydrogen-bonding energy is calculated to be, respectively, 12 and 32 kJ mol-1 greater in the cyclic form than in the open form with good agreement at the RHF6-31G(d) and RHF 6-31++G(d,p) levels. The effect of basis set superposition errors are found to be relatively small. The experimental and theoretical results are consistent with an equilibrium involving monomer, open dimer, cyclic trimer and cyclic tetramer.