Estimation of electron transfer parameters from AM1 calculations

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Quantum Chemistry

Background:

• Electron-transfer processes are fundamental in chemical and biological systems.
• Accurate estimation of electron-transfer parameters is crucial for understanding reaction mechanisms.
• Marcus theory provides a framework for describing electron transfer, involving reorganization energy and electronic coupling.

Purpose of the Study:

• To evaluate the utility of AM1 calculations for estimating electron-transfer parameters lambda'(v) and H(ab).
• To compare AM1-derived parameters with experimentally determined values for bis(hydrazine) radical cations.
• To identify limitations and necessary adjustments for AM1 calculations in specific molecular systems.

Main Methods:

• Utilizing AM1 (Austin Model 1) semi-empirical calculations.
• Calculating lambda'(v) using enthalpies of involved species (eq 1) and Koopmans' theorem.
• Estimating H(ab) (electronic coupling) using orbital separations at transition state and relaxed ground state geometries.
• Comparing computational results with experimental optical measurements.

Main Results:

• Koopmans' estimate for lambda'(v) is inaccurate.
• AM1-based enthalpy calculations for lambda'(v) are good for alkyl substituents but overestimate by 33-59% for aryl substituents.
• Koopmans' estimate for H(ab) requires adjustment for twist angles.
• Symmetry breaking in saturated bridge compounds leads to underestimated H(ab) values by Koopmans' estimate.

Conclusions:

• AM1 calculations show limited accuracy for electron-transfer parameters in charge-localized bis(hydrazine) radical cations.
• Enthalpy-based lambda'(v) estimations are more reliable than Koopmans' estimates.
• Adjustments for molecular geometry and twist angles are necessary for accurate H(ab) calculations.
• Further refinement of computational methods is needed for precise electron-transfer parameter prediction.