Measurement and Calculation of the Rate Constant for the Reaction of Isopropyl Isocyanate with Hydroxyl Radical

The rate Constant for the gas-phase reaction of hydroxyl radical with isopropyl isocyanate (IIC) has been measured, relative to toluene, in the T = 287-321 K range at atmospheric pressure in air. Ultraviolet photolysis of methyl nitrite served as the source of hydroxyl radical. The experimental Arrhenius expression obtained for this reaction is 7.09 x 10(-13) x exp vertical bar(307 +/- 263)/T vertical bar cm(3) molecule(-1) s(-1), where the indicated error is one least-squares standard deviation and does not include the uncertainty in the rate constant for toluene. Multilevel ab initio calculations, performed at the MP-SAC2 level, indicate that hydrogen atom transfer from the tertiary carbon atom occurs without an energy barrier, in agreement with the experimentally observed slight negative temperature dependence of the rate constant. The calculations indicate that hydrogen-bonded complexes of IIC and OH form in the reactant entrance channels along the minimum-energy paths. Use of conventional transition state theory and MP-SAC2 scaled transition state relative energies, with an asymmetric Eckart tunneling model for transfer of a hydrogen atom from a methyl group, yields a theoretical rate constant of 1.81 x 10(-18) x T-2 x exp(744/T) cm(3) molecule(-1) s(-1) in the T = 200-350 K range, From the rate constant data, an estimated upper limit for the tropospheric lifetime of IIC is determined as 6.0 days at 15 degrees C, based on the global weighted-average OH concentration of 9.4 x 10(5) molecules cm(-3) determined by Prinn et al. (Science 2001, 292, 1882-1888). (C) 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 187-197, 2009