HYDROGEN-BONDED FORMS OF METHANOL - IR-SPECTRA AND AB-INITIO CALCULATIONS

The IR spectra of methanol as a dilute solution in CCl4 and in the vap our 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 h ydrogen bonding equilibria present. The integrated absorption coeffici ent of the monomeric O-H stretching mode is calculated as (2.157 +/- 0 .025) x 10(4) m mol(-1) and the proportion of the components associate d with the three principal bands and a fourth weaker band estimated. S even possible components were considered which were monomer, closed cy clic and open chain dimers, trimers and tetramers. Ab initio calculati ons were carried out on these components using six basis sets up to th e 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 b e a transition state with the open dimer forming a stable minimum ener gy form. In the case of the trimer and tetramer the hydrogen-bonding e nergy 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 RH FG-31G(d) and RHF 6-31++G(d,p) levels. The effect of basis set superpo sition errors are found to be relatively small. The experimental and t heoretical results are consistent with an equilibrium involving monome r, open dimer, cyclic trimer and cyclic tetramer.