Predictive theory for hydrogen atom-hydrocarbon radical association kinetics

Area of Science:

• Physical Chemistry
• Chemical Kinetics
• Computational Chemistry

Background:

• Predicting the kinetics of hydrogen atom associations with hydrocarbon radicals is crucial for understanding combustion and atmospheric chemistry.
• Accurate theoretical methods are needed to model these complex reactions.

Purpose of the Study:

• To develop and apply a reliable computational procedure for predicting the kinetics of hydrogen atom association reactions with hydrocarbon radicals.
• To provide new kinetic predictions for several important reactions.

Main Methods:

• Utilized Complete Active Space Second-Order Perturbation Theory (CASPT2) with a cc-pvdz basis set to calculate orientation-dependent interaction energies.
• Employed Variable Reaction Coordinate Transition State Theory (VRCTST).
• Incorporated one-dimensional corrections from CAS+1+2/aug-cc-pvtz calculations and a dynamical correction factor of 0.9.

Main Results:

• The corrected CASPT2 approach achieved results within 10% of full CAS+1+2/aug-cc-pvtz calculations for several reactions (H + CH3, H + C2H5, H + C2H3, H + C2H).
• New predictions were made for H + iso-C3H7, H + tert-C4H9, H + C6H5, and H + C10H7 reactions.
• Excellent agreement was found between theoretical predictions and experimental data for H + CH3 and H + C2H3 reactions.
• For saturated alkyl radicals, each additional CH3 group approximately doubles the steric factor.
• Rate coefficients for unsaturated radicals (H + C6H5, H + C10H7) were similar to that of H + C2H3.

Conclusions:

• The developed computational method provides accurate predictions for hydrogen atom association kinetics with hydrocarbon radicals.
• The findings offer valuable insights into the factors governing these reactions, including steric effects in saturated and unsaturated systems.
• The study contributes to a better understanding of chemical reaction mechanisms relevant to combustion and atmospheric processes.