Theoretical calculation of enthalpy of formation of multiconformational molecules: 1,2-ethanediol, propanediols, and glycerol

Highlights: • Conformer correction to the enthalpy of formation can worsen the agreement with experiment. • Anharmonicity can play a significant role in predicting accurate enthalpies of formation. • Experimental gas-phase enthalpy of formation of glycerol needs revision. The gas-phase enthalpies of formation of four molecules with high flexibility, which leads to the existence of a large number of low-energy conformers, were calculated with the G4 method to see whether the lowest energy conformer is sufficient to achieve high accuracy in the computed values. The calculated values were in good agreement with the experiment, whereas adding the correction for conformer distribution makes the agreement worse. The reason for this effect is a large anharmonicity of low-frequency torsional motions, which is ignored in the calculation of ZPVE and thermal enthalpy. It was shown that the approximate correction for anharmonicity estimated using a free rotor model is of very similar magnitude compared with the conformer correction but has the opposite sign, and thus almost fully compensates for it. Therefore, the common practice of adding only the conformer correction is not without problems.