Reports: UR651272-UR6: Understanding Diradical Behavior in Asphaltene Model Compounds

Carol Parish, PhD, University of Richmond

Singlet Oxygen (2+4) Cycloaddition to Heteroaromatic Compounds

The (2+4) cycloaddition reaction of singlet molecular oxygen (1Δg) with a series of aromatic heterocyclic compounds, to form the corresponding endoperoxides, was investigated at both the MP2/6-311++G(d, p) and B3LYP/6-311++G(d, p) levels of theory. Thiophene was used as a prototype for the molecular constituents of asphaltenes and the addition of singlet oxygen to various substituted thiophenes was investigated. Alkyl (methyl, ethyl, and propyl) and halogen (-F, -Cl, -Br- and -I) substituents were examined as well the effects of benzannulation and the nature of the ring heteroatom (thiophene, pyrrole, furan and benzene). All of the molecules studied undergo concerted (2+4) cycloaddition of singlet O2 to exothermically produce the corresponding endoperoxide product, except for benzothiophene and dibenzothiophene which endothermically form highly strained, multicyclic products. All of the reactions are preceded by the formation of weakly bound complexes containing favorable intermolecular interactions between the oxygen atoms of 1O2 and the carbon atoms on the heterocyclic rings. These complexes exhibit charge-transfer character owing to the strong electrophilic nature of 1O2. Trends in activation energies and product stabilities can be understood in terms of the electronic structures and geometries of the reacting heteroaromatic rings. This work is in preparation for submission to the Journal of Physical Chemistry A.

Nine undergraduate students worked on these and other projects full-time this past summer.  We continue to focus on multireference characterization of diradical species and quantum evaluation of reaction mechanisms important to combustion.