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43407-GB4
Reaction of Hydroxyl Radical with Polycyclic Aromatic Hydrocarbons
The research of this group relates primarily to the chemistry of hydroxyl radicals with aromatic hydrocarbons, particularly with respect to the reactivity and selectivity of reactions.
Much of the studies of reactivity of hydroxyl with aromatic hydrocarbons have involved gas-phase (so-called “smog chamber”) or aqueous phase (pulse radiolysis) chemistry, and each of these methods have limitations with respect to volatility or solubility. Recently, a laser flash photolysis method has been developed for small aromatic hydrocarbons, but is inapplicable for larger PAH due to the broad UV-Visible absorbance ranges of such compounds.
Our work falls roughly into two categories:
1) Product analysis of reaction mixtures.
We have developed an experimental methodology which allows us to study the selectivity of addition of hydroxyl to aromatic rings utilizing standard GC techniques. In our experiment, isomeric intermediate radicals generated from hydroxyl radical addition to, say, toluene, are scavenged by persistent aminoxyl radicals to generate hydroxylamines and the isomeric cresols, which are amenable to GC analysis. We are currently utilizing this method to investigate mono- and bicyclic compounds. In addition, we can utilize competitive reaction experiments to determine relative reactivities of hydrocarbons.
In preliminary studies, we have found that hydroxyl radical attack on toluene is some 3 times faster than attack on benzene in hydrocarbon solvents at 298K (compared with a factor of 4 in acetonitrile and 5 in water). This dependence is consistent with published work on solvent polarity dependence of hydroxyl radical addition reactions. In terms of selectivity we find product ratios for toluene addition (cresols) are 4:1:1 (o-:m-:p-) Such observations tie in well with the generally held view that hydroxyl radical is an electrophilic species.
In the case of naphthalene, we find that there is a 1.5:1 ratio of 1-naphthol:2-naphthol at 298K. Attack on naphthalene is a factor of 10 faster than attack on benzene. Our aim is to expand this methodology to more sophisticated PAHs, and to develop HPLC-based methods for identifying the products of hydrogen abstraction.
2) Reporter Molecules for Hydroxyl Radical
The acquisition of a laser flash photolysis instrument at Ball State University allows us to investigate the kinetics of hydroxyl radical reactions with arenes on the nanosecond timescale. The laser flash photolysis projects are as follows.
Firstly, we wish to adopt existing methods to expand the scope of our understanding of the kinetics of hydroxyl radical reactions with arenes, and particularly to mono- and bicyclic heterocycles, which are also of environmental significance.
Secondly, current LFP/pulse radiolysis methods use polar solvents (acetonitrile and water respectively). We are currently investigating the possible modification of the existing methods to non-polar solvents.
Finally, the current LFP method is not entirely satisfactory. The hydroxyl radical is generated by the photolysis of N-hydroxypyri-2-thione, but cannot be detected directly. Instead, it reacts with a substrate (in this case stilbene) to generate an intermediate with a detectable absorbance – a “reporter” species. Unfortunately, in the current method, there is spectral interference between the reporter and the pyrithiyl radical generated by photolysis. We are seeking to discover novel reporter species that are less susceptible to spectral interference.
In addition to LFP studies, we have also undertaken a theoretical study aimed at understanding the chemistry of hydroxyl radical with thiocyanate anion. Thiocyanate is a common reporter molecule for hydroxyl radical reactions, due to the formation of a dithiocyanatyl radical anion that absorbs at 475 nm in water. Despite the common usage of this reaction, the mechanism is not clearly characterized. Our preliminary DFT studies show that the reaction between hydroxyl radical and thiocyanate to give thiocyanatyl and hydroxide appears to proceed via an addition intermediate, rather than direct electron transfer, and there is spectroscopic evidence to suggest this. Hydroxyl radical attacks at carbon, rather than sulfur as indicated in the literature. Further studies with higher accuracy methods are underway.
