The thermochemistry of aromatic radicals, biradicals, and triradicals
Abstract
A number of σ localized aromatic radicals, biradicals, and triradicals were studied in the gas phase. The heats of formation of α- and β-naphthyl radicals, chloro-substituted benzynes, and the three benzene triradicals were determined using thermochemical cycles. Their homolytic C-H bond dissociation energies were derived in order to describe their electronic structure. The heat of formation of the (σ,σ,σ) triradical 1,3,5- tris(dehydro)benzene was derived from five independent thermochemical measurements leading from 3,5-dichlorobenzoic acid to the triradical. The third C-H bond dissociation energy (BDE) of benzene, i.e. the C-H BDE of m-benzyne at the five position, was determined from the triradical heat of formation. This BDE is similar to the first and ca. 20 kcal/mol greater than the second, suggesting that the triradical is best described as a lone phenyl radical opposite an m-benzyne moiety. This structure is consistent with a Jahn-Teller distorted C2v structure. The three BDEs of benzene were compared with those of cyclopropane, stressing the similarities between 1,3,5-tris(dehydro)benzene and cyclopropenyl radical. The heats of formation 1,2,3- and 1,2,4-tris(dehydro)benzene were derived from dichlorobenzenes via four independent thermochemical measurements in an approach similar to that carried out for the 1,3,5-triradical. The third BDE of benzene to form the 1,2,4-triradical from o-benzene is comparable to that to form the 1,3,5-triradical from m-benzene. This indicates that this triradical is also a lone phenyl radical, which, in this case, is separated from an o-benzene moiety on the other side of the aromatic ring. The third BDE of benzene to form the 1,2,3-triradical, however, is somewhat lower. This is attributed to a σ “allylic” interaction that stabilizes the singly occupied orbital with respect to the phenyl radical-type singly occupied orbitals in the other two triradicals.
Degree
Ph.D.
Advisors
Wenthold, Purdue University.
Subject Area
Organic chemistry|Chemistry
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