Internal rotation for predicting conformational population of 1,2-difluorethane and 1,2-dichloroethane

- Contribution of internal rotation to Gibbs free energy estimated using the quantum pendulum model.
- Theoretical prediction of conformational population of 1,2-difluorethane and 1,2-dichloroethane.
- The predicted populations are in excellent agreement with experimental gas phase data available.
- QPM model account for low vibrational frequency modes effect on thermodynamic calculation.
- Caution is needed when the RR-HO approach has to be used in conformational analysis studies.

The contribution of internal rotation to the thermal correction of Gibbs free energy (ΔG) is estimated using the quantum pendulum model (QPM) to solve the characteristic Schrödinger equation. The procedure is applied to theoretical prediction of conformational population of 1,2-difluorethane (1,2-DFE) and 1,2-dichloroethane (1,2-DCE) molecules. The predicted population for the anti form was 37% and 75%, for 1,2-DFE and 1,2-DCE respectively, in excellent agreement with experimental gas phase data available, 37 ± 5% and 78 ± 5%. These results provide great support to the use of the QPM model to account for the low vibrational frequency modes effect on the calculation of thermodynamic properties.

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