The site-specific kinetics of OH radicals with esters

This work uses laser flash photolysis coupled with laser induced fluorescence to experimentally determine the overall and site-specific kinetics of a series of formates. The total kinetics for OH and OD + methyl formate and its deuterated isotopomers (CH3OC(O)H, CH3OC(O)D, CD3OC(O)H and CD3OC(O)D) have been determined between 298 and 573 K and are outlined in Chapter 3. The kinetics of OH + methyl formate determined in this work are in excellent agreement with literature studies at 298 K and bridge the gap between the work of Le Calve et al. (253 – 372 K) and Lam et al. (876 – 1371 K). The KIE indicates similar branching ratios from both the methyl and formate reaction sites, which are determined to be coupled, with a coupling factor of 2.38 at 298 K.
The experimental site-specific kinetics of OH + methyl formate (CH3OC(O)H) and CH3OC(O)D are outlined in Chapter 4. The rates of thermal decomposition of the CH3OC(O) radical formed following formate abstraction have also been determined, and decomposition is demonstrated to dominate over oxygen addition above ~ 423 K. The rate coePicients for CH3OC(O) + O2 and CH2OC(O)H + O2 are investigated in the low-pressure regime between 8 and 84 Torr, and 298 and 573 K. Experiments indicate the ether-centred radicals react with oxygen faster than carbonyl-centred radicals, in agreement with literature trends. The site-specific kinetics for hydrogen abstraction reactions of the OH radical with methyl formate are determined from Stern-Volmer intercepts, utilising the decomposition of CH3OC(O) to determine the branching ratio for methyl abstraction. The site-specific kinetics are also determined from the internal isomerisation of thermalised RO2 radicals, which form a carbon-centred radical (QOOH) that promptly decomposes to regenerate OH. There is good agreement between the OH yields determined from both methods. The experimental site-specific kinetics are also compared with MESMER calculated rate coePicients determined by Dr Robin Shannon.
Chapter 5 outlines the overall and site-specific kinetics of isopropyl formate. This work demonstrates that the current temperature-dependent kinetics of OH radicals with isopropyl formate by Zhang et al. likely underestimate the overall rate coefficient due to interference from OH regeneration. At 298 K, the site-specific kinetics for formate abstraction have been determined in good agreement with Pimentel et al. Above 400 K, decomposition of the (CH3)2CHOC(O) radical prevents OH regeneration following formate abstraction. Thanks to ab initio calculations at the M062X/6-31+G** level of theory, OH regeneration above 400 K is attributed to tertiary abstraction, providing the site-specific kinetics for tertiary abstraction by OH radicals between 400 and 573 K. The ab initio calculations also indicate the acceleration in the overall rate coefficient below room temperature is due to tertiary abstraction, which has a negative energy barrier. Above approximately 400 K, abstraction at the tertiary site is experimentally determined to decrease, coinciding with an increase in the rate coefficient, largely attributed to increasing formate abstraction.
The experimental site-specific kinetics of methyl formate (Chapters 3 and 4), isopropyl formate (Chapter 5), ethyl formate (determined by Dr Lavinia Onel, Appendix E), tertbutyl formate (Appendix E) and n-butyl formate (Appendix E) are combined in Chapter 6 and discussed in terms of wider trends in ester reactivity. The coupling between reaction sites in methyl formate is also demonstrated for the other formates and higher esters. Experimental site-specific kinetics are implemented into an updated structure activity relationship (SAR). The overall and site-specific kinetics of n-butyl formate are reserved from SAR development and used to test the updated SAR. The updated SAR improves upon the current SAR prediction of overall and site-specific ester kinetics. Consequently, the atmospheric chemistry of esters can be better represented within atmospheric chemical models. However, this work also highlights issues with the current SAR approach, namely the inability to predict hydrogen-bonding interactions, which influence reactivity in the esters and other oxygenated molecules. Alternative SAR approaches and suggestions for future work are proposed.