Two Faces of the Two-Phase Thermodynamic Model.

Area of Science:

• Thermodynamics
• Computational Chemistry
• Physical Chemistry

Background:

• Quantum corrections like the two-phase thermodynamic method (2PT) are crucial for accurate thermodynamic properties.
• Previous calculations of 2PT heat capacities in the literature show inconsistencies and significant errors, particularly for organic liquids.

Purpose of the Study:

• To reanalyze the performance of quantum corrections on heat capacities of organic solvents.
• To identify and rectify flaws in existing 2PT heat capacity calculations.
• To introduce and evaluate the concept of anharmonic correction (AC) for improved accuracy.

Main Methods:

• Reanalysis of quantum-corrected heat capacities using experimental data for common organic solvents.
• Identification of errors in vibrational density of states calculations and isobaric/isochoric heat capacity conversions.
• Introduction and calculation of anharmonic correction (AC) as the deviation from the harmonic oscillator model.

Main Results:

• Previous 2PT heat capacity calculations exhibited up to 45% error due to incorrect vibrational density of states and heat capacity conversions.
• Anharmonic correction (AC) values range from +30 to 40 J/(mol K) for water and -8 to -10 J/(mol K) for hydrocarbons/halocarbons.
• Unrealistically large AC for alcohols and amines suggests OPLS force field deficiencies.

Conclusions:

• Existing quantum correction methods for heat capacities in organic solvents contain substantial errors.
• Anharmonic correction (AC) is a vital component for accurate thermodynamic property calculations.
• Further investigation into force field accuracy is needed, especially for molecules with hydrogen bonding.