Mechanism and kinetics of the reaction of atomic hydrogen with allene.

Abstract

Context: In the present work, mechanism and kinetics of the H + allene reaction have been carefully conducted. The computed results reveal that the abstraction mechanism can lead to the formation of propargyl radical (C₃H₃), an important precursor for the formation of aromatic hydrocarbons, with the energy barrier of about 10.3 kcal/mol. In contrast, the addition mechanism can easily overcome the energy barriers of only 2.1 and 3.8 kcal/mol to form two adducts IS1 (CH₃CCH₂, 2-propenyl) and IS2 (CH₂CHCH₂, allyl), respectively. These two adducts can then decomepose to various bimolecular products such as (C₂H₂ + CH₃) and (H + propyne). Kinetic analysis shows IS1 dominates product formation at T ≤ 600 K (yield 41-70%), while the (H + propyne) channel becomes predominant above 900 K (branching ratio 50-70%). The calculated rate constants for the abstraction channel are consistent with literature values, and the overall rate constants agree well with experimental data from Whytock, Brown, Michael, and Bentz. These results highlight the reliability of the computational approach and provide essential parameters for modeling C₃H₅-related systems.

Methods: All species involved in the H + allene reaction were optimized using the DFT/M06-2X method with the aug-cc-pVTZ basis set. Single-point energies were calculated at the CCSD(T) level and extrapolated to the complete basis set (CBS) limit using aug-cc-pVTZ, aug-cc-pVQZ, and aug-cc-pV5Z. Rate constants were computed using transition state theory (TST) with the ChemRate program for the abstraction pathway, and RRKM/master equation calculations with the MESMER software for the addition-dissociation network. All quantum chemical calculations were performed using the Gaussian software package.