UC Irvine

This thesis is focused on exploring different variables that affect the kinetics of the reactions between CH2OO and several carbonyls. By using flash photolysis transient absorption spectroscopy and complementary ab-initio calculations, the bimolecular rate constants and thermochemical parameters such as activation energies and the enthalpic barriers for the reactions are discussed. We initially measured bimolecular rate constants for the reactions of CH2OO with acetone (Ac), the α-diketones biacetyl (BiAc) and acetylpropionyl (AcPr), and the β-diketones acetylacetone (AcAc) and 3,3,-dimethyl-2,4-pentanedione (dMAcAc) at 295 K. The bimolecular rate constant for the CH2OO + acetone reaction was measured to be (4.1±0.4)×10–13 cm3 s–1, which is consistent with previous measurements from other groups. The reactions of CH2OO with AcAc was found to be almost twice as fast as the reaction with dMAcAc, with rate constants of (6.6±0.7)×10–13 cm3 s–1 and (3.5±0.8)×10–13 cm3 s–1, respectively. The α-diketones, BiAc and AcPr, reacted significantly faster with rate constants of (1.45±0.18)×10–11 cm3 s–1 and (1.29±0.15)×10–11 cm3 s–1. The increased reactivity of the α-diketones was rationalized using concepts from frontier molecular orbital (FMO) theory, where an inverse relationship between the log of the rate constant and the difference in energy between the interacting orbitals of the CI and carbonyl was found. With an improved experimental setup and a new glass-jacketed flow cell, we were able to measure the temperature dependence of the reactions between CH2OO and Ac, BiAc, and AcAc. In the range of 275–335 K, the reactions of the ketones with CH2OO had different sensitivities to temperature in the following order with listed activation energies: AcAc < BiAc < Ac (Ea /R values of –1830±170 K, –1260±170 K, and –460±180 K, respectively). The different functionalities of the enolone structure of AcAc was also investigated computationally to reveal the cycloaddition reaction at the carbonyl (C=O) and alkene (C=C) site were competitive with nearly identical free energies of activation. The negative temperature dependence for the reaction of Ac and BiAc with CH2OO suggests that the Criegee intermediate could be a viable atmospheric sink for some carbonyls at night under cold conditions. The temperature-dependent kinetics of the reactions between CH2OO and the hydroxyketones, hydroxyacetone (AcOH) and 4-hydroxy-2-butanone (4H2B). The temperature-dependent bimolecular rate constants were reported: kAcOH(T) = (4.3±1.7)×10–15exp[(1630±120)/T] and k4H2B(T) = (3.5±2.6)×10–15exp[(1700±200)/T]. The bifunctionality of the hydroxyketones were explored computationally, and the 1,2-addition reaction at the OH site was found to have a negligible contribution to the rate constants. The relative reactivity of the hydroxyketones was understood using the FMO theory approach, where the hydroxyalkyl substituents were found to be neither electron donating groups (EDG) or electron withdrawing groups (EWG). The CH2OO + hydroxyketone reactions are likely too slow to be of significance in the atmosphere, except at very low temperatures. The final chapter discusses ongoing work that is focused on developing structure activity relationships (SARs) based on calculated orbital energies that are involved in the cycloaddition reactions between CIs and their reacting partners. For the reaction between CH2OO and a test set of carbonyls, a SAR was based on the carbonyl ?* orbital energies which are involved in the dominant orbital interaction with the npc-po orbital on CH2OO. The following equation was developed from the SAR to predict the rate constants for the reaction of COCl2 and CO(CN)2 with CH2OO at room temperature, ln⁡k=(-1.53±0.14) E_(*)-(29.91±0.29). The effect of the substituents on the CI was then investigated by plotting experimental rate constants for the reaction between CIs and SO2 as a function of the npc-po orbital energies of the CIs. A second SAR was developed, and equation was used to predict the rate constants of the reactions of ClCHOO and CH3OCHOO with SO2 at room temperature: ln⁡k=(1.66±0.51) E_(n_(pc-po) )-(12.03±3.39).