Accurate entropy calculation for large flexible hydrocarbons using a multi-structural 2-dimensional torsion method.

Area of Science:

• Thermodynamics
• Combustion Chemistry
• Computational Chemistry

Background:

• Accurate entropy prediction is crucial for combustion kinetic modeling.
• Conformational torsional anharmonicity significantly impacts entropy calculations.
• The multi-structural torsion (MS-T) method, while accurate, is computationally prohibitive for large molecules.

Purpose of the Study:

• To develop a computationally efficient method for predicting entropy in large flexible molecules.
• To accurately account for conformational torsional anharmonicity.
• To reduce the computational cost associated with traditional MS-T methods.

Main Methods:

• Proposed a multi-structural 2-dimensional torsion (MS-2DT) method utilizing minimally coupled torsions.
• Generated conformer space using 2D coupled torsion combinations (size CN2·9).
• Calculated standard entropy and heat capacity for C6-C8 alkanes (5-7 torsions) from 200-2000 K.

Main Results:

• MS-2DT calculations showed excellent agreement with the benchmark MS-T method.
• Observed small deviations: -0.19 ± 0.15 cal mol-1 K-1 for entropy and -0.10 ± 0.21 cal mol-1 K-1 for heat capacity.
• Demonstrated improved accuracy for n-decane (9 torsions) compared to conventional methods.

Conclusions:

• The MS-2DT method offers an affordable and accurate solution for treating conformational torsional anharmonicity in large alkanes.
• This approach enhances the feasibility of entropy prediction in complex molecular systems.
• The MS-2DT method is a valuable tool for advancing combustion kinetic modeling.