Mechanistic and kinetic approach on methyl isocyanate (CH3NCO) with OH and Cl.

Methyl isocyanate (CH3NCO) is an important agricultural and industrial precursor and emission from wildfires. In this paper, we study thermodynamic, kinetic and mechanistic approach of CH3NCO with OH radical and Cl atom using M06-2X level in combination with 6-311++G(d,p) basis set, and corrected by relatively accurate the single point CCSD(T) method. The rate constants of the title reactions are calculated in the temperature range of 200 and 400 K. The reactions exhibit a positive temperature depending on rate constants and fitted to the three-parameter Arrhenius expressions: kOH = 2.76 × 10−19 T2.65 exp[−486/T] and kCl = 3.51 × 10−18 T2.28 exp[95/T] with Wigner's and kOH = 4.95 × 10−19 T2.57 exp[−504/T] and kCl = 1.20 × 10−17 T2.11 exp[63/T] with Eckart's tunnelling corrections between 200 and 400 K. The calculated rate constants for the CH3NCO + OH/Cl reaction are in very good agreement with the previously reported experimental values. Thermochemical data (ΔH°298 K and ΔG°298 K) illustrate the feasibility and spontaneity of the title reactions. The atmospheric degradation mechanism of CH3NCO is discussed. The OH/Cl-driven tropospheric lifetimes and global warming potential (GWP) for CH3NCO were calculated. It was concluded that CH3NCO existed in the Earth's atmosphere for a few days and its GWP is not significantly large. The ozone formation potential of CH3NCO was also reported in this work. [ABSTRACT FROM AUTHOR]