Hydrogen-bonded forms of ethanol—IR spectra and abinitio computations

Abstract

The IR spectra of ethanol as a dilute solution in CCl₄ has been measured between 3100 and 4000 cm^-1. The integrated absorption coefficient of the monomeric O–H stretching mode is calculated as (2.183±0.019)×10⁴ m mol^-1 and the proportion of the components associated with the three principal bands and a fourth weaker band estimated. Components considered were monomer, open dimer, cyclic trimers (two forms) and cyclic tetramers (four forms). Abinitio calculations were carried out using basis sets up to the restricted Hartree Fock 6-311++G(d,p) level. Relevant calculated IR wavenumber and intensity values, O–H···O bond lengths, dipole moments and hydrogen bonding energies are reported and discussed. Models of the cyclic trimers of methanol and ethanol created from a number of possible chair and boat forms are optimised by abinitio calculations and lead to new proposed structures. These comprise high energy structures in which the alkyl groups are on the same side of a six-membered ring (with approximate C₃ symmetry) which are compared with previously described low energy forms in which alkyl groups are on opposite sides (with approximate C₁ symmetry). Selected spectroscopic and thermodynamic properties together with calculated structures are determined for both forms using three basis sets and the results are reported at the highest basis set used {RHF/6-31G(d,p)}. Hydrogen bonding energies are reported for methanol and ethanol trimers as 65.9 and 65.7 kJ mol^-1 for the C₃ form and 69.0 and 68.7 kJ mol^-1 for the C₁ form. These values are susceptible to choice of basis set within the restricted Hartree Fock method, to the energy terms included and to basis set superposition errors. From the four possible structures for the cyclic tetramers, that with S₄ symmetry is found to be the most favoured. The experimental and theoretical results are consistent with an equilibrium involving monomer, open dimer, cyclic trimer and cyclic tetramer.