This project has three goals: 1) Experimentally determine the branching ratio for abstraction of hydrogen atoms (H) versus deuterium atoms (D) from mono-deuterated methoxy radicals (CH2DO*) in their reaction with molecular oxygen (O2) as a function of temperature; 2) Experimentally determine absolute rate constants for CH3O* and CD3O* reacting with O2 as a function of temperature; 3) Compute rate constants for methoxy + O2 reactions, including tunneling and variational effects, and extend calculations to larger radicals. Two experimental approaches will be used: Fourier Transform Infrared (FTIR) spectroscopy in the reaction chamber of collaborators at the National Center for Atmospheric Research (NCAR) will be used to determine the branching ratio for production of deuterated versus normal formaladhyde in the CH2DO* + O2 reaction. Rate constant ratios for reaction of CH3O* and CD3O* radical with O2 (kO2) and NO2 (kNO2) will also be determined at NCAR from product yields. Experiments at NCAR will be carried out by one of the Principal Investigator's (PI) graduate students under the direct supervision of senior scientists at NCAR. Direct measurements of kNO2 at the PI's lab (SUNY-ESF) will be carried out using laser flash photolysis to generate radicals and laser-induced fluorescence for time-resolved detection. The combination of kNO2/kO2 from NCAR with kNO2 from SUNY-ESF will enable determination of kO2(T) for both CH3O* and CD3O*. Direct measurements of kO2 at SUNY-ESF will validate the combined results. An understanding of the isotope effects in these reactions will be achieved via high-level quantum calculations coupled to cutting-edge algorithms for statistical rate theory and multi-dimensional tunneling calculations. Calculations will be extended to larger alkoxy radicals. This research will be the first to determine kO2 at temperatures less than 298 K for methoxy radical. It will also be the first temperature-dependent determination of branching ratio for production of normal and deuterated formaldehyde in the CH2DO* + O2 reaction, and only the second study of this branching ratio. The calculations will provide benchmarks for reliable calculations of kO2(T) for a diverse range of alkoxy radicals of tropospheric interest.
By enhancing other researchers' abilities to calculate reliable values of kO2(T), this research will lead to a much better understanding, not just of alkoxy radical chemistry, but also of the overall mechanisms of degradation of many classes of volatile organic compounds (VOCs). This will help improve representations of VOC degradation processes in models of air pollution and global tropospheric chemistry, contributing to more effective ozone abatement plans and better modeling of climate-chemistry feedbacks. The results will also constrain the mechanism of deuterium enrichment of molecular hydrogen in the atmosphere, and improve our understanding of the atmospheric budget of molecular hydrogen. This will help to understand the potential impacts of a hydrogen economy on microbial communities, stratospheric and local ozone, and the abundance of greenhouse gases. Two graduate students and several undergraduates working on this project will grow intellectually and professionally, and gain advanced technical knowledge of diverse experimental and computational methods for kinetics.
In NSF award 0937626, Dr. Theodore S. Dibble (www.esf.edu/faculty/dibble/) and collaborators Geoffrey S. Tyndall and John J. Orlando (http://www2.acd.ucar.edu/) worked to understand the reaction: CH3O + O2 → H2C=O + HOO (R1a) and analogous reactions where deuterium (D) was substituted for normal hydrogen (H) either partially or completely. This reaction is, itself, a key step in the production of ozone from the oxidation of methane (CH4) in polluted air. Reaction R1 is also model for reactions central to the oxidation of larger gas-phase organic compounds (a process which also leads to production of ozone in polluted air). The reaction CH2DO + O2 can make two sets of products: CH2DO + O2 → HDC=O + HOO (R1b) CH2DO + O2 → H2C=O + DOO (R1c) We determined that R1b was about7-8 times faster than R1c at room temperature. We further determined that the temperature dependence (from -23 ºC to about +60 ºC) of the ratio of these two reaction rates was well fit by a common equation known as the Arrhenius equation. The fact that this ratio followed Arrhenius implies that quantum mechanical tunneling is not accelerating the rate of R1b over that of R1c. This is sort of odd, since substituting D for H in reactions like R1 that shift a hydrogen atom often causes a large decrease in tunneling near room temperature. In order to determine how fast R1a and R1d occur: CD3O + O2 → D2CO + DOO (R1d) we determined the ratio of the rates of R1a to R2: CH3O + NO2 → CH3ONO2 (R2) In another experiment, we directly determined the rate coefficient for R2 (and the analogous reaction of CD3O). These two sets of data allow us to determine the rate coefficient for R1a and R1d. As with R1b and R1c, the manner in which the ratio of the rate coefficients R1a to R1d varies with temperature implies that quantum mechanical tunneling is not occurring to a great degree. At the same time we were using computational chemistry to understand reactions R1a-d. The most obvious factor controlling the reaction rate coefficient is the activation barrier, which is the energy difference between the reactants and the saddle point. The saddle point (see Figure 1) is the high point along the mountain pass connecting the valley of reactants with the valley of products. We know from experiment that this mountain pass is fairly low (about 2.5 kcal/mole). Various theoretical approximations give values ranging from 6 to 20 kcal/mole. The difference between 2.5 kcal/mole and 6 kcal/mole corresponds to a factor of over 350 in the rate coefficient. Despite this poor performance, we use one of the more promising methods to compute the extent of tunneling in reactions R1a-d. Note that the error in the computed activation barrier cancels out when calculating the rate constant in the absence of tunneling. The calculations suggest that tunneling increases the rate coefficient for R1 by a factor of 2 at room temperature, but that deuterium substitution lowers the rate coefficient by less than expected: only about 20%. So the experimental results on the effects of deuterium substitution need to be re-evaluated: it is not that tunneling is so small, but that substituting D for H does not reduce tunneling by as much as is typical. Theory and experiment agree very well for the ratio of rate coefficients reactions R1b to R1c. This means that we can rely on the calculations to predict this ratio at temperatures down to -80 ºC that are relevant to the atmosphere. Reactions R1b and R1c compete to strongly influence the ratio of HDC=O and H2C=O in the atmosphere. These molecules are major sources of HD and H2 in the atmosphere. The computed ratio of rate coefficients reactions R1b to R1c will be useful to modelers trying to use HD : H2 ratios to understand how biological versus non-biological processes control production and destruction of molecular hydrogen. One composite method used by others previously obtained a fairly reliable activation barrier. This method should be useful in computing the rate constant for the reaction of larger analogs of CH3O with O2, where reaction with O2 competes with the analog molecules rearranging or breaking apart. These larger analogs are very important in ozone formation in polluted air, and we know much less about the rates of their reactions than we do about the rates of reactions of CH3O. So we hope this research will lead to a better understanding of the atmospheric fate of the analog compounds, their ultimate products, and their contribution to ozone and particulate matter.