Synthesis, properties and reactivity of the phenylcarbene anion in the gas phase†

Abstract

The gas-phase synthesis and characterization of phenylcarbene anion, PhCH^˙– (1^˙–), are reported. This archetypal arylcarbene radical anion is produced in isomerically-pure form by dissociative electron ionization of phenyldiazirine in a flowing afterglow–triple quadrupole instrument. The structure and purity of PhCH^˙– are established by authentication of its oximate derivative, PhCHNO^–, by collision induced dissociation (CID). Ab initio calculations indicate a ²A″ ground state for 1^˙–, with a π-radical/σ-anion electronic configuration, which is 11–12 kcal mol^–1 lower in energy than the π-anion/σ-radical ²A′ state. Reactions of PhCH^˙– with a series of small molecules are described that reveal its strong base and nucleophilic character. PhCH^˙– abstracts a sulfur-atom from CS₂ and OCS, N-atom from N₂O, and H-atom from (CH₃)₂NH, propene and (CH₃)₂S. Nucleophilic displacement of Cl^– from CH₃Cl by 1^˙– occurs at 41% of the collision rate; more than forty times faster than for the benzyl anion. PhCH^˙– exchanges both the carbene hydrogen and a hydrogen in the phenyl ring upon reaction with ND₃ or D₂O, which is consistent with theoretical predictions that indicate similar energies for PhCH^˙– and α,n-didehydrotoluene anion isomers. Acid–base bracketing and Cooks’ kinetic method were used to determine the proton affinity and electron binding energy of PhCH^˙– to be 392.0 ± 2.1 kcal mol^–1 and 27.8 ± 0.7 kcal mol^–1, respectively. These quantities were used to derive the heat of formation of phenylcarbene. The value obtained, 103.8 ± 2.2 kcal mol^–1, is in excellent agreement with that recently measured by energy-resolved collision-induced dissociation (102.8 ± 3.5 kcal mol^–1: J. C. Poutsma, J. J. Nash, J. A. Paulino and R. R. Squires, J. Am. Chem. Soc., 1997, 119, 4686).