Flowing afterglow studies of hydrocarbon triradicals and nitrene radicals

Tamara Elizabeth Munsch, Purdue University

Abstract

Mixed σ and π aromatic triradicals and nitrene radicals were studied in the gas phase. The hemolytic C-H bond dissociation energies (BDE) at the 5-position of m-xylylene and the heats of formation of 2-, 3-, and 4-dehydrophenylnitrenes were determined by using negative ion thermochemical cycles. Bond dissociation energies derived from thermochemical measurements were used to assess the electronic structures of the triradicals. The heat of formation of the 5-dehydro-1,3-quinodimethane (5-dehydro- m-xylylene, DMX) was determined by measuring the BDE at the 5-position of m-xylylene, which is obtained from the thermochemical cycle that combines the gas-phase acidity at the C5 position in m-xylylene and the electron affinity of DMX. Combination of the measured EA and gas-phase acidity gives a C--H bond dissociation enthalpy of 112 ± 4 kcal/mol at the 5-postion for m-xylylene, indistinguishable from the first C--H BDE for benzene, 112.9 ± 0.5 kcal/mol. By using the measured BDE along with the previously reported heat of formation of m-xylylene, 80.2 ± 2.4 kcal/mol, the heat of formation of DMX is found to be 141 ± 5 kcal/mol. The measured BDE indicates an interaction of 1 ± 4 kcal/mol between the unpaired electrons in the σ and π systems of the triradical, consistent with what has been found previously in other organic systems such as the dehydrotoluenes and the dehydrophenylnitrenes. Additionally, the electronic structure of the corresponding triradical anion, DMX− , was investigated. Reactivity and thermochemical properties of this ion are in good agreement with a phenyl-like anion with two benzylic radical sites, which inconsistent with theoretical predictions. Absolute heats of formation for 2-, 3-, and 4-dehydrophenylnitrenes have been determined from three independent thermochemical measurements. From the collision-induced dissociation threshold energies for dissociation of odohhenylnitrene radical anions, the heats of formation for the dehydrophenylnitrenes were found to be 170 ± 4 kcal/mol. Based on the measured heats of formation, the triradical stabilization energy (TSE) was calculated to be -5 ± 5 kcal/mol, indicating a destabilizing interaction between the triradical's third electron and the phenylnitrene moiety. These results are consistent with an electronic structure combining a phenylnitrene and phenyl radical moiety.

Degree

Ph.D.

Advisors

Wenthold, Purdue University.

Subject Area

Organic chemistry

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