Quantum and anharmonic effects in non-adiabatic transition state theory

Area of Science:

• Chemical Kinetics
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Non-adiabatic transition rates are difficult to quantify at intermediate temperatures due to competing quantum and anharmonic effects.
• Existing models often simplify these complex interactions, limiting their predictive accuracy.

Purpose of the Study:

• To investigate the combined influence of quantum effects and seam anharmonicity on non-adiabatic transition rates.
• To develop and test new theoretical methodologies for describing these rates, particularly for reactions involving complex potential energy surfaces.

Main Methods:

• Derivation of quantized 1-D motion across the seam using Lagrangian formalism within the weak coupling limit.
• Development of a vibrationally adiabatic (VA) approach for high-frequency quantum modes, incorporating wavefunction overlap.
• Application and testing of these methods using the N2O ↔ N2 + O(3P) reaction, with high-level electronic structure calculations.

Main Results:

• Seam anharmonicity significantly impacts the rate constant, showing a factor of ~20 at 2000 K for the N2O reaction.
• Quantum effects, including N-N vibration coupled with seam anharmonicity via the VA approach, are crucial at lower temperatures.
• A 1-D approximation to non-adiabatic instanton theory was used to assess the validity limits of linear models at low temperatures.

Conclusions:

• The study highlights the critical need to verify assumptions like harmonic behavior, classical motion, and linear terms in statistical theories for non-adiabatic reactions.
• The developed methodologies provide a more accurate framework for predicting non-adiabatic transition rates, especially where quantum and anharmonic effects are significant.
• Case-by-case validation of theoretical models is recommended for reliable quantitative descriptions of complex chemical reactions.