Kinetic Study of OH Radicals’ Reactions with Gamma-Heptalactone in the Atmosphere

A comprehensive potential energy surface (PES) of the reaction between OH radicals and gamma-heptalactone has been mapped out at the CCSD(T)/6-311++G(d,p)//M06-2X/6-311++G(d,p) level of theory, in order to explore the detailed reaction mechanisms of OH radicals with gamma-heptalactone. The rate coefficients and branching ratios were predicted using transition state theory (TST), combined with tunneling effects. At room temperature and a pressure of 1 atm, the calculated rate coefficient is 6.37 × 10–12 cm3 molecule–1 s–1, which is in line with the experimentally measured value at the same temperature. The relationship of rate coefficients with temperature can be expressed by a three-parameter Arrhenius equation: k (cm3 molecule–1 s–1) = 5.79 × 10–23 × T3.6 × exp(1464.4/T) at temperatures in the range of 260–420 K. The PES and kinetics indicate that the major and most favorable pathway is H-atom abstraction from the C5 position, adjacent to the heterocyclic oxygen atom, yielding radical intermediate IM3. In the atmosphere, the subsequent reactions of IM3 will finally result in the formation of succinic anhydride, peroxy n-butyryl nitrate (PnBN), and peroxy propionyl nitrate (PPN) as the dominant products, which is in excellent accord with the experimental findings.