Conformational study of erythritol and threitol in the gas state by density functional theory calculations.

Density functional theory calculations using the B3LYP functional and the 6-311++G(d,p) basis set were carried out on the isolated molecules of erythritol and L-threitol. For the meso isomer, a relatively large number of conformers have to be considered to describe the gas state structure. The lowest energy conformer is characterized by the establishment of a strong intramolecular H-bond between the two terminal hydroxyl groups, giving rise to a seven-membered ring and two additional weaker H-bonds between vicinal OH groups. In the case of L-threitol, two conformers are predominant in the gas state, and both are stabilized by the formation of a cyclic system of four intramolecular hydrogen bonds involving all OH groups. The conformational stability in both diastereomers is discussed in terms of the electronic energy and of the Gibbs energy. The weighted mean enthalpy of both diastereomers in the gas state at 298.15 K was obtained from the thermodynamic data and Boltzmann populations of the low-energy conformers.