Rate constant for the reaction C2H5 + HBr → C2H6 + Br

Area of Science:

• Physical Chemistry
• Chemical Kinetics
• Computational Chemistry

Background:

• The kinetics of the title reaction have been a subject of debate, particularly its negative activation energy.
• Understanding reaction mechanisms is crucial for predicting chemical behavior under various conditions.

Purpose of the Study:

• To analyze the kinetics of the title reaction using RRKM theory.
• To investigate the controversial negative activation energy phenomenon.
• To explore the influence of vibrational anharmonicity on reaction kinetics and thermochemistry.

Main Methods:

• Application of RRKM theory to analyze reaction kinetics.
• Characterization of stationary points using coupled cluster theory with basis set extrapolation to the complete basis set limit.
• Quantitative exploration of vibrational anharmonicity effects.

Main Results:

• A shallow minimum was located along the reaction coordinate, with a small energy barrier to dissociation.
• The transition state was found to be tight relative to the adduct.
• Adjusting the adduct binding energy by approximately 4 kJ mol⁻¹ brought computed rate constants into agreement with most experimental data, including new room-temperature results.

Conclusions:

• RRKM theory successfully explains the observed kinetics, including the negative activation energy, when accounting for adduct binding energy and vibrational anharmonicity.
• There are indications that the activation energy may transition from negative to positive at temperatures above those experimentally studied.
• The findings reconcile theoretical calculations with experimental observations for this reaction system.