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44764-B6
Computational Studies of the Atmospheric Chemistry of Alkene Ozonolysis Intermediates

Keith T. Kuwata, Macalester College

My laboratory has focused on two projects with PRF support.  Both have involved the use of quantum chemistry and statistical rate theory to predict quantities that can be tested against experiment.

            (1) Hydroxyl Radical Production from Thermalized Carbonyl Oxides. It is well established that a chemically activated syn carbonyl oxide (1) isomerizes to a vinyl hydroperoxide (3, Reaction 1), which decomposes quantitatively to afford ·OH. 

However, the atmospheric fate of thermalized syn carbonyl oxides is not clear, because Reaction 1 will compete with the bimolecular reaction with water (Reaction 2):

We are using highly accurate composite electronic structure methods and variational transition state theory to predict rate constants for Reactions 1 and 2b.  Table 1 summarizes MCG3 energetics based on QCISD/MG3 (or QCISD/6-31G(d)) optimized geometries.  Density functional theory always predicts reaction barriers (not shown) lower than the MCG3//QCISD barrier, although B3LYP/6-31+G(d,p)  typically is within 1 kcal/mol of the higher level barriers.

TABLE 1: Relative Energies (in kcal/mol) for the Species in Reactions 1 and 2

            Preliminary kinetics results, based on B3LYP/6-31+G(d,p) energies, gradients, and Hessians, are reported for Reaction 1 in Table 2 and for Reaction 2b in Table 3.  Rate constants were calculated using conventional transition state theory (TST) and canonical variational transition state theory (CVT).    All vibrational modes with frequencies less than 200 cm^-1 were treated as hindered internal rotors. Transmission coefficients were estimated using Truhlar's multidimensional optimized tunneling method (mOMT) and by fitting the minimum energy path with the asymmetric Eckart potential (Eckart).

TABLE 2.  Rate Constants and Transmission Coefficients for Reaction 1

TABLE 3.  Rate Constants and Transmission Coefficients for Reaction 2b

The variational effect on Reaction 1 is negligible, but tunneling accelerates the reaction enormously at low temperatures, up to 5-6 orders of magnitude at 200 K.  This is consistent with our previous study of the 1,4-hydrogen shift in the 2-hydroxyethoxy radical (J. Phys. Chem. A 2007, 111, 5032).  In contrast, Reaction 2b is only slight accelerated by tunneling, but variational optimization of the transition state lowers the 200 K rate constant by ~33%.  Moreover, treating all vibrational modes as harmonic oscillators leads to a significant underestimate of the rate constant (results not shown); at 2000 K, the TST rate constant under a purely harmonic oscillator model is ~85% lower than the rate constant reported in Table 3.

(2) Reactions of Thermalized Carbonyl Oxides and Sulfur Dioxide.  Landmark experimental work in the 1970's and 1980's provided evidence that a reliable measure of thermalized carbonyl oxide formed in an ozonolysis experiment was the amount of sulfuric acid aerosol formed (Scheme 1):

However, the only recent quantum chemical study of this system indicates that SO₂ is capable of catalyzing the isomerization of carbonyl oxide 8 via a cyclic adduct 10 (Reaction 3):

We have undertaken a comprehensive study of the reaction mechanism using the B3LYP/6-31+G(d,p) model chemistry.  Our calculations have identified not only the five-membered ring intermediate 10, but also a four-membered ring (11) and an open diradical (12) (Scheme 4):

RRKM/Master equation simulations, based on the mechanism in Scheme 2 and B3LYP/6-31+G(d,p) quantum chemical data, predict the branching ratios shown next to each arrow.  Around 60% of the SO₂ reacting with CH₂OO should be oxidized to SO₃, while the other 40% drives the isomerization of CH₂OO to the carboxylic acid.  None of the intermediates is predicted to be collisionally stabilized under atmospheric conditions.

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