59th Annual Report on Research 2014

Singlet Oxygen (2+4) Cycloaddition to Heteroaromatic Compounds. The (2+4) cycloaddition reaction of singlet molecular oxygen (¹△_g) 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.

Quantum mechanical characterizations of tetraradical-producing cyclizations, and an extended multireference electronic structure study of 2,5,7,10-tetradehydronaphthalene. The focus of this tetraradical study is on the fate of tetraethynylethene, specifically on whether it tends to cyclize via a 1,5 or 1,6 path. Preliminary scans of the potential energy surface were conducted using the computationally efficient B3LYP/6-31G** method followed by further refinement using highly correlated single- and multi-reference wavefunction techniques available in the Q-Chem software package. Singlet, triplet and quintet spin states were explored. Based on our evidence, the 1,5 and 1,6 cyclizations are likely to be competitive. Current data suggests that the cyclization has multiple steps whereby the enediynes cyclize in succession, rather than simultaneously. Additionally, high level, multi-reference calculations were performed using the COLUMBUS software package on the 2,5,7,10-tetradehydronaphthalene to understand the electronic structure of this tetraradical product. Effects of through bond coupling have been observed, evidenced by the ground state low-lying singlet. This work is in preparation for submission to the Journal of Chemical Physics.

A Mechanism for Annealing Bicyclic Polyyne Rings into Fullerenes. Theoretical quantum chemistry is the framework that provides an atomic and molecular level description of chemical structure and reactivity. Through the utilization of quantum mechanical methods, this investigation evaluated a mechanism, proposed by Hunter, for the formation of Buckminster Fullerene molecules by annealing together bicyclic polyyne precursors. Buckminster Fullerene (Buckyballs) molecules are spherical ball shaped structures made up of interlocking twenty hexagons and twelve pentagons containing all carbon atoms. Calculations were performed using the Q-Chem software package. Preliminary scans of the potential energy surface were done using a computationally efficient density functional theory (DFT) approach with a standard basis set (UB3LYP/6-31G**). Energies and structures of various reactants, transition states and diradical intermediates were then refined using single- and multi-reference wavefuntion methods. Results indicate that the proposed mechanism of annealing bicyclic polyyne is kinetically favorable under typical fullerene formation conditions of temperatures exceeding 2000 K. This work is in preparation for submission to the Journal of Organic Chemistry.

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.