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Lauricella, J. C. Bouteiller and P. Tordo, J. Chem. Soc., Perkin Trans. 2, 1999, 2777. 14 C. Rizzi, V. Roubaud, R. Lauricella, J. C. Bouteiller and B. Tuccio, J. Chem. Soc., Perkin Trans. 2, 2001, 1912. 15 C. Rizzi, R. Lauricella, J. C. Bouteiller, V. Roubaud and B. Tuccio, Magn. Reson. Chem., 2002, 40, 273. 16 A. M. Wasserman, Usp. Khim., 1994, 63, 391 (Russ. Chem. Rev ., 1994, 63, 373). Received: 17th January 2006; Com. 06/2650 Optically detected ESR spectrum of decafluorobiphenyl radical anions Maria M. Vyushkova,* a Victor P. Vysotsky, b Irina V. Beregovaya, b Lyudmila N. Shchegoleva, b Victor A. Bagryansky a and Yuri N. Molin a a Institute of Chemical Kinetics and Combustion, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation. Fax: + 7 383 330 7350; e-mail: barlukova@kinetics.nsc.ru b N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation. Fax: + 7 383 330 9752; e-mail: sln@nioch.nsc.ru DOI: 10.1070/MC2006v016n03ABEH002326 The radical anion of decafluorobiphenyl was detected by the OD ESR technique for the first time; the spectrum was interpreted on the basis of quantum-chemical calculations as a result of fast exchange among equivalent asymmetrical structures. Fluoroarene radical ions attract much attention as key inter- mediates responsible for the chemical transformations of fluoro- arenes. 1 For instance, such synthetically important polyfluoro- arene reactions as S RN 1 nucleophilic substitution and reductive dehalogenation involve the stage of radical anion formation. 2,3 The route of these reactions is determined by the structure of radical anion intermediates. Thus, the regularities of fluoride ion elimination from polyfluoroarene radical anions determine the regioselectivity of polyfluoroarene dehalogenation. 4 Interest in these radical ions is due to their electronic and spatial struc- tures. The large values of hyperfine coupling (hfc) constants of fluoroarene radical anions testify to the disturbance of their planar structure due to vibronic coupling between the anion ground and low-lying excited states (pseudo-Jahn–Teller effect). 5 The out-of-plane distortions are caused by a strong perturbation of the aromatic molecule -system by fluorine atoms, which generates the low-lying excited -states of radical anions. One of the main obstacles to the ESR study of fluoroarene radical ions is the instability of these particles. For instance, the fragmentation rate constant of the radical anions of pentafluoro- phenol, pentafluoroaniline and polyfluorinated benzoates in water exceeds 10 6 s –1 . 6,7 At the same time, the optically detected ESR (OD ESR) spectroscopy, being highly sensitive for radical ion pairs, provides the detection of unstable radical ions with life- times as short as 10 ns. 8–10 The extremely high sensitivity of this method is due to optical detection through the fluorescence of excited molecules, which are produced by the recombination of radical ion pairs in irradiated solution. Here, the OD ESR method was used to detect the ESR spectrum of the decafluorobiphenyl radical anion in a liquid squalane (2,6,10,15,19,23-hexamethyltetracosane) solution at room temperature. The solution contained deuterated p-terphenyl used as a positive charge acceptor and an emitting probe. The measurements were performed using an OD ESR spectrometer 11 based on an ER-200D Bruker ESR spectrometer equipped with a source of ionizing radiation (an X-ray tube) and a photomulti- plier to measure fluorescence intensity. Modulation amplitude and microwave power of 0.36 mT and 2.5 W, respectively, were used to increase the intensity of very expanded ESR spectrum of the decafluorobiphenyl radical anion. Under these conditions, the individual line widths were determined mainly by the micro- wave power broadening. Experimental results were compared with the hfc values pre- dicted by quantum-chemical calculations. The calculations were performed with the GAMESS program. 12 The geometry of the (C 6 F 5 ) 2 · – radical anion was optimised at the PBE/6-31+G* level of calculations. The hfc constants were estimated at the PBE geometries by the B3LYP method (UB3LYP/EPRIII//PBE/ 6-31+G*). According to the calculations, the structure of the (C 6 F 5 ) 2 · – radical anion exhibits pseudo-Jahn–Teller distortions of two types: out-of-plane deviation of fluorine atoms (which is peculiar to radical anions of fluorobenzenes) and the nonequivalence of aromatic rings. The former distortion results from the interaction between the ground and the excited state of the radical anion, while the latter one arises from the interaction between the ground and excited states. The disturbance of fragment equivalence in radical ions of symmetric molecules was discussed previously. 5,13 The energy minimum corresponds to the totally asymmetric C 1 structure (Figure 1), where a dihedral angle between the planes of aromatic rings is ~45°, the out-of-plane deviations of 0.8 0.8 0.1 4.3 4.0 13.5 7.1 0.4 2.1 –0.04 F F F F F F F F F F Figure 1 Structure corresponding to local PES minima of the (C 6 F 5 ) 2 · – radical anion and respective calculated 19 F hfc constants (mT).