Gas Phase Reaction of Isocyanic Acid: Kinetics, Mechanisms, and Formation of Isopropyl Aminocarbonyl.

Isocyanic acid, HNCO, mainly emitted by combustion processes, is doubted to be detrimental to human health if its concentration surpasses ∼1 ppbv. Very little information has been found regarding the HNCO loss in the gas phase. This study aims to close this knowledge gap by performing a theoretical kinetic study on the reaction of HNCO with the propargyl radical. The potential energy surface of the HNCO + C 3 H 3 reaction was characterized utilizing high-level CCSD(T)/CBS(TQ5)//B3LYP/6-311++G(3df,2p) quantum-chemical approaches, followed by TST and RRKM/ME kinetic computations. The obtained results reveal that the reaction can proceed via H-abstraction, leading to the C 3 H 4 + NCO bimolecular products with energy barriers of 23-25 kcal/mol, and/or addition, resulting in C 4 H 4 NO intermediates with 23-26 kcal/mol barrier heights. The C 4 H 4 NO adducts when formed can decompose to products and/or return to HNCO + C 3 H 3 in which the reverse decompositions are found to be dominant with a branching ratio that accounts for nearly 100% at 300 K and 760 Torr. The calculated P -independent rate coefficients indicate that at low temperatures, the H-abstraction channels are insignificant. However, at high temperatures ( T > 1500 K), the H-abstraction path leading to H 3 CCCH + NCO prevails with a branching ratio of ∼50-53% in the descending 1800-1500 K temperature range at 760 Torr, while the H-abstraction leading to H 2 CCCH 2 + NCO is favorable at T > 1800 K, with the yield reaching above 50% at 760 Torr. In contrast to the H-abstraction rate constants, the calculated values for the additions and the C 4 H 4 NO decompositions show a positive pressure dependence. Both the total rate constants for the reactions HNCO + C 3 H 3 → products and C 4 H 4 NO → products, which are, respectively, k _total_bimo ( T ) = 3.53 × 10 -23 T 3.27 exp[(-21.35 ± 0.06 kcal/mol)/RT] cm 3 molecule -1 s -1 and k _total_uni ( T ) = 1.13 × 10 25 T -4.02 exp[(-11.77 ± 0.16 kcal/mol)/RT] s -1 , increase with the increasing temperature in the 300-2000 K range at 760 Torr. The rate constant of HNCO + C 3 H 3 → products is about 8 orders of magnitude smaller than the value of HCHO + C 3 H 3 → products, showing that HCHO is more reactive toward the C 3 H 3 free radicals than HNCO. The computed heats of formation for several species agree well with the available literature data with the deviation less than 1.0 kcal/mol, indicating that the methods used in this study are extremely reliable. With the given results, it is vigorously suggested that the predicted rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling HNCO-related systems under atmospheric and combustion conditions.