Flowing afterglow studies of biradicals, clusters and open -shell anions

Silvi Ann Chacko, Purdue University

Abstract

Electron-promoted Cope cyclization of 2,5-phenyl-1,5-hexadiene radical anions was studied in the gas phase. The heat of formation of m-quinomethane biradical was derived from negative ion thermochemical cycle and the electronic properties of o-and p-biradicals were examined. The in situ generation hydrogen cyanide clusters was studied in the flowing afterglow mass spectrometer. Hydrogen cyanide (HCN) for use in ion preparation can be generated in the gas phase by the neutral-neutral reaction of trimethylsilyl cyanide (Me 3SiCN) and water in a flowing afterglow mass spectrometer. We demonstrate that the approach can be used to generate a wide range of HCN solvated ions such as F-(HCN), Cl-(HCN), CN-(HCN), PhNO2·- (HCN), Me3SiO- (HCN) and PhSiF4- (HCN), many of which are otherwise difficult to generate. The bond dissociation energy of CN -(HCN), generated by using this approach, has been measured by using energy-resolved collision-induced-dissociation (CID) to be 0.87 ± 0.07 eV. The electron-promoted Cope cyclization of 2,5-phenyl-1,5-hexadiene radical anions was studied in a flowing afterglow triple quadrupole mass spectrometer. The electronic properties of the hexadienes have been systematically modified by using aromatic substituents at the 2- and 5- position of hexedienes. Structures of the molecular radical anions were probed to determine whether they undergo cyclization to a cyclohexane-1,4-diyl anion structure by examining chemical reactivity with neutral reagents including carbon dioxide, carbon disulfide, and nitric oxide. Based on the reactivity results, a thermochemical model has been developed, which predicts the reaction thermochemistry by using thermochemical properties of model systems. The enthalpy of formation of m-quinomethane has been determined by energy-resolved collision-induced dissociation measurements involving chloro-substituted phenoxide ions. The chloride affinity of m-quinomethane is measured to be 177 ± 8 kJ/mol, whereas the chloride affinities of o- and p-quinomethane are 57 ± 5 and 57 ± 5 kJ/mol, respectively. The difference in the chloride affinities reflects the difference in the stabilities of the Kekule and non-Kekule structures. Combination of the dissociation energy of the meta-isomer with the gas-phase acidity of 3-(chloromethyl)phenol, measured by using the kinetic method to be 1432 ± 6 kJ/mol, results in an enthalpy of formation of 141 ± 11 kJ/mol for the m-quinomethane diradical.

Degree

Ph.D.

Advisors

Wenthold, Purdue University.

Subject Area

Analytical chemistry|Organic chemistry

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