Anharmonic Rovibrational Partition Functions for Fluxional Species at High Temperatures via Monte Carlo Phase Space...

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Physical Chemistry

Background:

• Accurate computation of molecular partition functions is crucial for understanding chemical thermodynamics and kinetics.
• Classical treatments of rovibrational properties often neglect anharmonicity and complex vibrational modes, limiting their applicability.
• Previous methods struggled to efficiently sample complex potential energy surfaces with multiple minima and low-frequency motions.

Purpose of the Study:

• To compute full-dimensional, anharmonic, classical rovibrational partition functions for 22 diverse molecules and radicals.
• To demonstrate the efficacy of Monte Carlo phase space integration (MCPSI) using curvilinear coordinates for complex molecular systems.
• To analyze trends in anharmonicity corrections and their dependence on molecular structure and temperature.

Main Methods:

• Application of Monte Carlo phase space integration (MCPSI) in curvilinear coordinates (stretch, bend, torsion).
• Computation of rovibrational partition functions for 22 small- to medium-sized molecules and radicals.
• Analysis of systems featuring multiple minima and low-frequency nonlocal motions, including up to 21 coupled degrees of freedom.

Main Results:

• Successful computation of classical rovibrational partition functions for a wide range of molecules, including fluxional species.
• Demonstration of MCPSI's capability to efficiently sample complex potential energy surfaces.
• Identification of consistent trends in anharmonicity corrections with temperature and degrees of freedom, accounting for coupling and torsional effects.

Conclusions:

• The curvilinear coordinate MCPSI method is a robust tool for treating complex rovibrational structures in molecules.
• Rovibrational anharmonicities show predictable trends with temperature and molecular complexity when coupling and torsional effects are considered.
• Complex vibrational structures, multiple large-amplitude modes, and multiple minima lead to larger anharmonicity corrections.