Effects of anharmonicity on nonadiabatic electron transfer: a model

Area of Science:

• Physical Chemistry
• Quantum Mechanics
• Chemical Physics

Background:

• Nonadiabatic electron transfer is fundamental in chemical and biological processes.
• The influence of molecular vibrations (intramolecular modes) on electron transfer is significant.
• Anharmonicity in vibrational modes can alter reaction dynamics compared to simpler harmonic models.

Purpose of the Study:

• To investigate the effect of anharmonicity in intramolecular modes on electron transfer.
• To analyze how transition probability depends on exoergicity in a model system.
• To compare the behavior of anharmonic (Morse) and harmonic oscillators in electron transfer.

Main Methods:

• Developing theoretical expressions for the Franck-Condon factor using the Gauss hypergeometric function for Morse potentials.
• Comparing first-order perturbation theory results for transition probabilities.
• Performing quantum dynamical simulations using wave-packet propagation.
• Analyzing transition probabilities with high-frequency quantum modes and low-frequency medium modes.

Main Results:

• Significant deviations from the harmonic approximation occur for anharmonic modes at high exoergicity.
• The Franck-Condon factor for Morse potentials can be expressed using the Gauss hypergeometric function.
• Anharmonic effects are substantial even for moderately anharmonic modes.
• A second quantum mode can negate anharmonic effects at small energy changes.
• Anharmonic modes show a flatter dependence on exoergicity in the inverted regime.

Conclusions:

• Anharmonicity is a critical factor in accurately modeling nonadiabatic electron transfer.
• The harmonic approximation is insufficient for describing electron transfer dynamics under certain conditions, particularly with significant exoergicity.
• Theoretical models incorporating anharmonic potentials provide a more realistic description of electron transfer processes.