Mechanistic and kinetic insights of the formation of allene and propyne from the C3H3 reaction with water

ContextAllene (H2C = C = CH2) and propyne (CH3-C equivalent to CH) are important compounds in the combustion chemistry. They can be created from the reaction of proparyl radicals with water. In this study, therefore, a computational study into the C3H3 + H2O potential energy landscape has been carefully conducted. The computed results indicate that the reaction paths forming the products (allene: CH2CCH2 + (OH)-O-center dot) and (propyne: HCCCH3 +(OH)-O- center dot) prevail under the 300-2000 K temperature range, where the latter is much more predominant compared to the former. However, these two products are not easily formed under ambient conditions due to the high energy barriers. In the 300 - 2000 K temperature range, the branching ratio for the propyne + (OH)-O-center dot product declines from 100 to 86%, whereas the allene + (OH)-O-center dot product shows an increase, reaching 14% at 2000 K. The overall bimolecular rate constant of the title reaction can be presented by the modified Arrhenius expression of k(total) = 1.94 x 10(-12) T-0.14 exp[(-30.55 kcal.mol(-1))/RT] cm(3) molecule(-1) s(-1). The total rate constant at the ambient conditions in this work, 2.37 x 10(-34) cm(3) molecule(-1) s(-1), was found to be over five orders of magnitude lower than the total rate constant of the C3H3 + NH3 reaction, 7.98 x 10(-29) cm(3) molecule(-1) s(-1), calculated by Hue et al. (Int. J. Chem. Kinet. 2020, 4(2), 84-91). The results in this study contribute to elucidating the mechanism of allene and propylene formation from the C3H3 + H2O reaction, and they can be used for modeling C3H3-related systems under atmospheric and combustion conditions. Methods All the geometric structures of the C3H3 + H2O system were optimized by the B3LYP method in conjunction with the 6-311 + + G(3df,2p) basis set. Single-point energies of these species were calculated at the CCSD(T)/6-311 + + G(3df,2p) level of theory. The CCSD(T)/CBS level has also been used to compute single-point energies for the two major reaction channels (C3H3 + H2O -> allene + (OH)-O-center dot and C3H3 + H2O -> propyne +(OH)-O- center dot). Rate constants and branching ratios of the key reaction channels were calculated in the 300-2000 K temperature interval using the Chemrate software based on the transition state theory (TST) with Eckart tunneling corrections.