Absolute rate calculations. Proton transfers in solution

Area of Science:

• Chemical kinetics
• Theoretical chemistry
• Computational chemistry

Background:

• Proton-transfer reactions are fundamental in chemistry and biology.
• Accurate theoretical prediction of these rates is challenging.
• Existing models often require empirical parameters.

Purpose of the Study:

• To develop and validate a parameter-free theoretical method for calculating proton-transfer reaction rates.
• To elucidate the key factors influencing proton-transfer reactivity.
• To assess the accuracy of the intersecting-state model with semiclassical tunneling correction (ISM/scTST).

Main Methods:

• Utilized the intersecting-state model combined with transition-state theory and semiclassical tunneling correction (ISM/scTST).
• Calculations were based on fundamental properties: hydrogen-bond energies, reaction energies, electrophilicity indices, bond lengths, and vibrational frequencies.
• No adjustable parameters were employed in the ISM/scTST calculations.

Main Results:

• ISM/scTST calculations achieved rates within one order of magnitude of experimental values at room temperature.
• The method demonstrated accuracy across a wide range of proton-transfer systems, including deprotonations of nitroalkanes, ketones, HCN, carboxylic acids, and excited naphthols.
• Calculated temperature dependencies and kinetic isotope effects showed good agreement with experimental data.

Conclusions:

• ISM/scTST is an efficient and accurate parameter-free method for predicting proton-transfer rates.
• The study highlights the significant roles of reaction energy, electrophilicity, structural parameters, hydrogen bonding, tunneling, and solvent effects in acid-base reactivity.
• The computational efficiency enables broad application and accessibility via the internet.