ab initio Modeling of Hydroxyl Radical Reactions with Prevalent Hydrocarbons

Objectives

Background

Summertime formation of ground-level ozone is one of the major environmental problems facing emerging metropolitan centers such as the Charlotte-Gastonia-Rock Hill Metropolitan Statistical Area (MSA).  For each of the past two summer, the American Lung Association has ranked this region as the 8th worst in the nation for tropospheric ozone formation.  High ozone levels interfere with respiratory function and directly affect the health of many citizens in this area. On high ozone days, there are more emergency admissions for patients having respiratory problems.  Respiratory illnesses are currently the 3rd leading cause of death in the US behind heart disease and cancer.  The incidence of asthma has more than doubled over the past several decades.  In addition to human health concerns, ozone adversely affects plant growth and agriculture productivity.

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)
(2)   O.* (1D2)   +   H2O   -->   2 OH.
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.
    (3)   OH.  + R-H  -->  H2O   +   R.                           Hydrogen Abstraction
    (4)   OH.  + C=C  -->  H2O   +   .C-C-OH              Hydroxyl Addition to Multiple Bond
For 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.

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.

Computational Aspects

Since the reactions of hydroxyl radicals with neutral organic molecules to form organic radicals involve the breaking and forming of different types of bonds; these are not isodesmic ("equal bonds") type of reactions.  Fortunately, the hydroxyl reactions do involve the same total number of bonds and the same number of lone electron pairs between reactants and products; thus, they are somewhat "gentler" and more amenable to modeling than many other nonisodesmic reactions such as  bond dissociations.

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.

Problem

Table 1 (developed by Winthrop student Steve Hudson as part of a project for the NC Division of Air Quality) lists the most prevalent hydrocarbons in this region during summer months in recent years, both in terms of relative concentration and ozone reactivity.  The goal of this lab project is to use computational techniques to learn more about the energetics and geometries of the structures involved from the reactions of the hydroxyl radicals with these substances.

Table 1.  Most prevalent ambient hydrocarbons found in Charlotte urban air during summer ozone season for the years 1995-1999.
Year
1995
1996
1997
1998
1999
Top Ten 
Hydrocarbons
Ranked by
Concentrations
Toluene
Isopentane
2,2,4-Tmp*
m/p-Xylene
Ethylene
Ethane
Propane
Benzene
n-Butane
2-Methylpentane
Toluene
Isopentane
m/p-Xylene
Propane
Ethane
Ethylene
n-Butane
2,2,4-Tmp*
Benzene
n-Pentane
Toluene
Isopentane
Acetylene
Propane
m/p-Xylene
Isoprene
Ethylene
Ethane
2,2,4-Tmp*
n-Pentane
Isopentane
Toluene
Propane
Ethane
Acetylene
m/p-Xylene
n-Butane
Ethylene
n-Pentane
1,2,4-Tmb*
Toluene
Isopentane
Propane
Ethane
m/p-Xylene
Ethylene
Acetylene
n-Pentane
n-Butane
2,2,4-Tmp*
Top Ten
Hydrocarbons
Ranked by 
Reactivity-Weighted
Concentrations
Ethylene
m/p-Xylene
Toluene
Isoprene
1,2,4-Tmb*
Propylene
Isopentane
o-Xylene
3-Methyl-1-Butene
2-Methyl-1-Pentene
m/p-Xylene
Toluene
Ethylene
Isoprene
1,2,4-Tmb*
Propylene
o-Xylene
Isopentane
1,3,5-Tmb*
3-Methyl-1-Butene
Isoprene
m/p-Xylene
Ethylene
Toluene
Propylene
1,2,4-Tmb*
Isopentane
o-Xylene
1,3,5-Tmb*
trans-2-Pentene
Ethylene
m/p-Xylene
1,2,4-Tmb*
Isoprene
Toluene
Propylene
o-Xylene
Isopentane
1,3,5-Tmb*
trans-2-Pentene
Ethylene
m/p-Xylene
Toluene
Isoprene
Propylene
o-Xylene
Isopentane
trans-2-Pentene
cis-2-Pentene
Ethylbenzene
*2,2,4-Tmp is 2,2,4-Trimethyle Pentane; 1,2,4-Tmb is 1,2,4-Trimethylbenzene; 1,3,5-Tmb is 1,3,5-Trimethylbenzene
 

Requirements

Proposed Schedule