Biogenic volatile organic compounds (VOCs) react with Cl and OH radicals and the resulting radicals combine with oxygen to form peroxy radicals RO2. Organic peroxy radicals can then react with NO to form NO2, a precursor of tropospheric ozone. The work presented here explored the initial reaction between Cl and thujone, a VOC emitted by Great Basin sagebrush. The rate constant for the reaction of thujone + Cl at 296 K was measured with the method of relative rates with FTIR for detection of reactants. LEDs were used to photolyze Cl2 to generate Cl in the reaction cell. Thujone was also photolyzed by the LEDs and therefore the relative rates model was revised to account for this photolysis. With toluene as the reference compound, the rate constant for thujone + Cl at 296 K is 2.62 ± 1.90 × 10-12 molecules-1 s-1, giving an atmospheric lifetime of 0.5--2.6 minutes for thujone. Cline et al. showed that the rate of the self-reaction of HO2CH2CH2O2 (β-HEP) increases in the presence of water vapor. This enhancement has a strong temperature dependence with a greater enhancement observed at colder temperatures. The observed rate enhancement has been attributed to the formation of a β-HEP--H2O complex. In this work, the equilibrium constant for the formation of the β-HEP--H2O complex was calculated by ab initio calculations. Given the energy available at room temperature, the complex will populate three local minimum geometries and β-HEP will populate two local minimum geometries. The partition function for each of these geometries was calculated and used to calculate the equilibrium constant for complex formation as a function of temperature. Based on these computational results, the observed temperature dependence for the rate enhancement can be attributed to the strong temperature dependence for the rate constant of the reaction of β-HEP--H2O + β-HEP rather than the temperature dependence of complex formation.



College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry



Date Submitted


Document Type





biogenic volatile organic compounds, thujone, Cl atom, relative rates, peroxy radical-water complex, HOCH2CH2O2, β-HEP, water vapor