CHEMPHYSCHEM 2003,4,61±66 ¹ 2003 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 1439-4235/03/04/01 $ 20.00+.50/0 61 On the Dissociation of Aromatic Radical Anions in Solution Damien Laage, [a] Ire ¡ne Burghardt, [b] Thomas Sommerfeld, [c] and James T. Hynes* [a, d] A new theoretical formulation is given for the reaction rate and path for the important reaction class of aromatic radical anion dissociation in solution [Ar  X]  . !Ar .  X  , and is illustrated for the case of the cyanochlorobenzene radical anion [CN FCl]  . in dimethylformamide. Among the theory's novel features is the inclusion of the conical intersection aspect of this ground electronic state problem, which is key in allowing the reaction to occur and which has a significant impact on the reaction barrier height. Reasonable agreement with the experimental rate is found. KEYWORDS: aromatic radical ions ¥ charge transfer ¥ conical intersections The cleavage of an aromatic radical anion into the correspond- ing aromatic radical plus an anionic nucleophile, Equation (1), is of importance in a range of chemical contexts, most prominently in connection with the S RN 1 radical chain mechanism of nucleophilic substitution, and thus to fundamental questions in nucleophile±electrophile chemistry. [1, 2] [Ar X]  . !Ar .  X  (1) Further, radical anion dissociation is implicated [3] in the well- known Grignard reaction, of central importance in synthetic chemistry, and is possibly involved for halo-uracil compounds connected to DNA defects induced by ionizing radiation. [4] The dissociation of aromatic radical anions [Equation (1], has been the object of extensive experimental [2, 5] and theoretical [6, 7] investigation. Here, we present a new theoretical description [8] of the dissociation Equation(1) in solution, and illustrate it with a calculation of the reaction path and rate constant for the reaction of the cyanochlorobenzene radical anion ([CN FCl]  . ), here- after CCB . , in solution. Among the novel features of this description is the inclusion of a conical intersection (CI), which is critical for the reaction. In the adiabatic picture, a CI between two electronic state surfaces is associated with the topology of a double cone and gives rise to non-adiabatic coupling effects in this region. [9±11] In a complementary diabatic picture for the CCB . molecule of interest here, the two electronic states in question are electronically uncoupled in a particular geometry due to symmetry reasons, but become so coupled by virtue of displace- ment in a symmetry-breaking coordinate (see for example ref.[11] for a more general characterization in the diabatic picture). In the present case, this coordinate is the C Cl out of plane bending, or wagging, angle [6, 8, 12] q, which allows coupling of the p*orbital, where the electron chiefly resides, and the s* orbital involved in the bond breaking (see Figure 1). While CIs are of great current photochemical interest in connection with transitions from excited electronic states to the Figure 1. Schematic representation of the p* and s* orbitals of CCB, and the wagging motion responsible for the electronic coupling (q is the wagging angle). The p* orbital is represented perpendicular to the ring plane; the s* orbital is along the C Cl axis. ground state, [9±11] and recently their impact on ground-state reactions (in the absence of solvent) has received attention, [13] the present effort appears to be the first explicit treatment for the impact of a CI on a ground-electronic-state reaction in solution; as will be seen, this impact is large both for the reaction path, in which the CI point is avoided, and for the magnitude of the reaction activation free energy. The only previous related discussion of which we are aware is the qualitative one by Rettig [14] of excited-state twisting charge-transfer (TICT) reactions in solution; a contribution to which we will return. The inclusion [a] J. T. Hynes, D. Laage De ¬partement de Chimie CNRS UMR 8640 PASTEUR Ecole Normale Supe ¬rieure, 24 rue Lhomond 75231 Paris Cedex 05 (France) Fax: ( 33)1-44-32-33-25 E-mail: hynes@chimie.ens.fr [b] I. Burghardt De ¬partement de Chimie CNRS UMR 8642, Ecole Normale Supe ¬rieure 24 rue Lhomond, 75231 Paris Cedex 05 (France) [c] T. Sommerfeld Theoretische Chemie, Physikalisch-Chemisches Institut Universit‰t Heidelberg, Im Neuenheimer Feld 229 69120 Heidelberg (Germany) [d] J. T. Hynes Department of Chemistry and Biochemistry University of Colorado, Boulder, CO 80309-0215 USA