The hydroxyl radical is formed primarily via the reaction between atomic oxygen and water. Atomic oxygen in an excited state (1D2) is formed by the decomposition of ozone from UV-B radiation:
(1) O3 + photon ( l < 320 nm) --> O2* (1Dg) + O.* (1D2)The hydroxyl radical serves to initiate the oxidation of atmospheric hydrocarbons via either hydrogen abstraction or addition; hydroxyl additions are limited to unsaturated compounds and have much faster rates than reactions involving hydrogen abstractions via hydroxyl radicals.
(2) O.* (1D2) + H2O --> 2 OH.
(3) OH. + R-H --> H2O + R. Hydrogen AbstractionFor both types of hydroxyl reactions, a neutral hydrocarbon molecule is turned into a free radical, a much more reactive species. The follow-on series of oxidation reactions eventually form carbon dioxide and occur fairly rapidly once the initial free radical has been formed. Understanding the role of the hydroxyl radicals in initiating the oxidation of atmospheric hydrocarbons is a critically important part of understanding ozone formation in urban regions during summer months.
(4) OH. + C=C --> H2O + .C-C-OH Hydroxyl Addition to Multiple Bond
In general, the lower the energy of the newly formed radical (compared to its neutral counterpart), the faster the reaction that will occur. This assumes thermodynamic control, a situation that is normally the case for atmospheric free radical reactions that involve tropospheric ozone formation. One effective tool is to use computational methods to calculate the total energy difference between reactants and products for a given hydroxyl radical / neutral organic reaction. Experimental data on this rates are limited, particularly at low concentrations due to inherent limitations of smog chambers.
The energetics of non-isodesmic reactions are not well modeled by semi-empirical computational methods, and ab initio algorithms, preferably with methods that account for correlation effects, are often used. Hartree-Fock (HF) techniques treat electron-electron interactions as a single electron interacting with a smeared average of all other electrons. Because of this, Hartree-Fock calculations do not take electron-electron correlation into account and HF energies tend to be high since they overestimate electron-electron repulsions. Since these electron coupling effects do not tend to cancel for nonisodesmic reactions, methods (such as Moller-Plesset 2 (MP2) perturbation) which account for electron-electron correlation are necessary to determine reaction energetics.
A comparison of product and reactant energies can be made by searching for the specific geometry that is the lowest energy structure for each reactant and product species. Geometry optimization requires varying bond angles and lengths for a number of trial geometries, calculating the energy for each, and then continuing the process with new geometries (that are in the direction of lower energy) until a global energy minimum has been found. This geometry optimization process can require the energy calculation of 10-100 individual structures for each molecule. Since these types of calculations have to be repeated time and again, efficient geometry optimization methods are necessary.. Unfortunately, MP2 energy calculations can quickly become computationally expensive.
This problem can be addressed by using an uncorrelated method (such as HF) to determine optimum geometries and then using a more time-consuming and reliable electron correlation method (such as MP2) to predict the energy of each optimized structure. This two step process allows the efficient calculation of each energy in a manner that incorporates the advantages of correlated methods. Previous work has shown that uncorrelated methods perform just as effectively as correlated techniques in determining optimum geometries.
Table 1. Most prevalent ambient hydrocarbons found in Charlotte urban air during summer ozone season for the years 1995-1999.
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Isopentane 2,2,4-Trimethylpentane m/p-Xylene Ethylene Ethane Propane Benzene n-Butane 2-Methylpentane |
Isopentane m/p-Xylene Propane Ethane Ethylene n-Butane 2,2,4-Trimethylpentane Benzene n-Pentane |
Isopentane Acetylene Propane m/p-Xylene Isoprene Ethylene Ethane 2,2,4-Trimethylpentane n-Pentane |
Toluene Propane Ethane Acetylene m/p-Xylene n-Butane Ethylene n-Pentane 1,2,4-Trimethylbenzene |
Isopentane Propane Ethane m/p-Xylene Ethylene Acetylene n-Pentane n-Butane 2,2,4-Trimethylpentane |
Top Ten Hydrocarbons Ranked by Reactivity-Weighted Concentrations |
m/p-Xylene Toluene Isoprene 1,2,4-Trimethylbenzene Propylene Isopentane o-Xylene 3-Methyl-1-Butene 2-Methyl-1-Pentene |
Toluene Ethylene Isoprene 1,2,4-Trimethylbenzene Propylene o-Xylene Isopentane 1,3,5-Trimethylbenzene 3-Methyl-1-Butene |
m/p-Xylene Ethylene Toluene Propylene 1,2,4-Trimethylbenzene Isopentane o-Xylene 1,3,5-Trimethylbenzene trans-2-Pentene |
m/p-Xylene 1,2,4-Trimethylbenzene Isoprene Toluene Propylene o-Xylene Isopentane 1,3,5-Trimethylbenzene trans-2-Pentene |
m/p-Xylene Toluene Isoprene Propylene o-Xylene Isopentane trans-2-Pentene cis-2-Pentene Ethylbenzene |