Tetrafluorobenzyne Thermochemistry: Experiment and Theory

Abstract

Gas-phase thermodynamic properties of 1,2,3,4-tetrafluorobenzyne (1 – H2) were determined by Fourier transform mass spectrometry and ab initio and density functional theory methods. 1,2,3,4-Tetrafluorobenzyne radical anion was generated by abstraction of a proton and a hydrogen atom upon reaction of 1,2,3,4-tetrafluorobenzene (1) with O−•. The resulting structure was confirmed by converting it to a species which could be independently prepared. Bracketing results provided the proton affinity of 1,2,3,4-tetrafluorobenzyne radical anion and the electron affinities of 1,2,3,4-tetrafluorobenzyne and 1,2,3,4-tetrafluorophenyl radical. These measured values were combined in a thermodynamic cycle to provide the heat of hydrogenation of 1 – H2 (Δ H°hyd = 367 ± 18 kJ mol−1) and the first and second C–H bond dissociation energies of 1 (481 ± 11 and 321 ± 13 kJ mol−1). The same approach failed for the meta and para isomers, but their energetics were examined using B3LYP and CCSD(T) computations.