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 (ΔH°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.
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14.
All thermochemical data unless otherwise noted comes from BartmessJ.E., in Secondary NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Ed by MallardW.G.LinstromP.J., National Institute of Standards and Technology, Gaithersburg, MD, USA (http://webbook.nist.gov).
15.
Uncertainties in the proton and electron affinity measurements were assigned using the following relationship: Δ = [{(y1 – y2)1/2}2 + ({(Δy12 + Δy22)1/2}/2)2]1/2 where y1 ± Δy1 and y2 ± Δy Δy2 are the values for the two limiting reference compounds. This approach accounts for the difference in the bracketing values and incorporates the uncertainties in the reference compounds.
16.
MOLPRO version 2002.1, a package of ab initio programs designed by WernerH.J.KnowlesP.J., was used to compute the CCSD(T) energy of 1 – H•. AmosR.D.BernhardssonA.BerningA.CelaniP.CooperD.L.DeeganM.J.O.DobbynA.J.EckertF.HampelC.HetzerG.KnowlesP.J.KoronaT.LindhR.LloydA.W.McNicholasS.J.ManbyF.R.MeyerW.MuraM.E.NicklassA.PalmieriP.PitzerR.RauhutG.SchützM.SchumannU.StollH.StoneA.J.TarroniR.ThorsteinssonT.WernerH.J.,
17.
WentholdP.G.SquiresR.R.LinebergerW.C., “Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p-Benzyne Negative Ions. Electron Affinities and Singlet–Triplet Splittings for o-, m-, and p-Benzyne”, J. Am. Chem. Soc.120, 5279 (1998).
18.
The heat of formation of 1 was obtained using the experimental value for 1,2,4,5-tetrafluorobenzene (–647 ± 3 kJ mol−1) (see Reference 14) and the ZPE-corrected B3LYP/6-31+G(d) energy difference at 298 K between the 1,2,4,5- and 1,2,3,4-isomers (0.4 kJ mol−1, the former compound is more stable). This leads to an estimate of ΔHf° (1) = −646 kJ mol−1, which is in accord with an estimate of −638 ± 1 kJ mol−1 given in Reference 19.
19.
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(a) DavicoG.E.BierbaumV.M.DepuyC.H.EllisonG.B.SquiresR.R., “The C–H Bond Energy of Benzene”, J. Am. Chem. Soc.117, 2590 (1995); (b) ErvinK.M.DeTuriV.F., “Anchoring the Gas-Phase Acidity Scale”, J. Phys. Chem. A106, 9947 (2002).
21.
WentholdP.G.HuJ.SquiresR.R., “o-, m- and p-Benzyne Negative Ions in the Gas Phase: Synthesis, Authentication and Thermochemistry”, J. Am. Chem. Soc.118, 11865 (1996).
22.
B3LYP predicts a singlet ground state for the m-isomer and a triplet ground state for the p-species. In contrast, both compounds are predicted to be ground state singlets at the CCSD(T) level.
23.
For p-benzyne we used ΔH°f = 577 ± 6 kJ mol−1, which is the average of two experimental determinations (577 ± 12 and 577 ± 4 kJ mol−1). (a) Reference 19; (b) RothW.R.HopfH.HornC., “1,3,5-Cyclohexatrien-1,4-diyl und 2,4-Cyclohexadien-1,4-diyl”, Chem. Ber.127, 1765 (1994).
