Quantum Thermochemistry: Multistructural Method with Torsional Anharmonicity Based on a Coupled Torsional Potential

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Chemical Thermodynamics

Background:

• Accurate calculation of partition functions and thermodynamic functions is crucial for understanding chemical systems.
• Existing multistructural methods with torsional anharmonicity (MS-T) have limitations in accuracy, particularly concerning torsional coupling.

Purpose of the Study:

• To develop and validate a new approximation for calculating partition functions and thermodynamic functions using the MS-T method.
• To improve the accuracy of the MS-T approximation by incorporating local torsional coupling effects.

Main Methods:

• Introduced a new MS-T approximation based on a reference potential with torsional barriers from calculations including local torsional coupling.
• Validated the method by comparing it to a fully coupled classical rotational-torsional partition function evaluated via numerical phase space integration.
• Included quantum effects using the harmonic approximation, with potential upgrades to the quasiharmonic approximation.

Main Results:

• The new coupled-potential MS-T approximation demonstrated improved accuracy in the classical limit compared to the original uncoupled-potential MS-T.
• Calculations were performed on various molecules (ethanol, butanol, hexane, heptane, and their isomers), a radical, and reaction transition states.
• The enhanced method showed improved agreement with experimental thermodynamic functions for 1-butanol, hexane, isohexane, and heptane.

Conclusions:

• The developed coupled-potential MS-T approximation provides a more accurate approach for calculating thermodynamic functions.
• This advancement is particularly beneficial for systems where torsional anharmonicity and coupling are significant.
• The method offers better agreement with experimental data, enhancing its utility in chemical research.