Synthesis, properties and reactivity of the phenylcarbene anion in the gasphase

The gas-phase synthesis and characterization of phenylcarbene anion, PhCH.- (1(.-)), are reported. This archetypal arylcarbene radical anion is produc ed in isomerically-pure form by dissociative electron ionization of phenyld iazirine in a flowing afterglow-triple quadrupole instrument. The structure and purity of PhCH.- are established by authentication of its oximate deri vative, PhCHNO-, by collision induced dissociation (CID). Ab initio calcula tions indicate a (2)A " ground stale for 1(.-), with a pi-radical/sigma-ani on electronic configuration, which is 11-12 kcal mol(-1) lower in energy th an the pi-anion/sigma-radical (2)A' state. Reactions of PhCH.- with a serie s of small molecules are described that reveal its strong base and nucleoph ilic character. PhCH.- abstracts a sulfur-atom from CS2 and OCS, N-atom fro m N2O, and H-atom from (CH3)(2)NH, propene and (CH3)(2)S. Nucleophilic disp lacement of Cl- from CH3Cl by 1(.-) occurs at 41% of the collision rate; mo re 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 alpha,n-didehydrotoluene anion isomers. Aci d-base bracketing and Cooks' kinetic method were used to determine the prot on 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 u sed to derive the heat of formation of phenylcarbene. The value obtained, 1 03.8 +/- 2.2 kcal mol(-1), is in excellent agreement with that recently mea sured 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. Sec, 1997, 119, 4686).