Estimation of Standard Reduction Potentials of Halogen Atoms and Alkyl Halides Abdirisak A. Isse, ‡ Ching Yeh Lin, † Michelle L. Coote, † and Armando Gennaro* ,‡ Department of Chemical Sciences, UniVersity of PadoVa, Via Marzolo, 1 - 35131 PadoVa, Italy, and ARC Centre of Excellence for Free-Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National UniVersity, Canberra ACT 0200, Australia ReceiVed: October 7, 2010; ReVised Manuscript ReceiVed: December 6, 2010 Standard reduction potentials, SRPs, of the halogen atoms have been calculated in water on the basis of an appropriate thermochemical cycle. Using the best up-to-date thermodynamic data available in the literature, we have calculated E X • /X - o i values of 3.66, 2.59, 2.04, and 1.37 V vs SHE for F • , Cl • , Br • , and I • , respectively. Additionally, we have computed the SRPs of Cl • , Br • , and I • in acetonitrile (CH 3 CN) and dimethylformamide (DMF) by correcting the values obtained in water for the free energies of transfer of X • and X - from water to the nonaqueous solvent S and the intersolvent potential between water and S. From the values of E X • /X - o i in CH 3 CN and DMF, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other important processes such as pollution abatement have been calculated in these two solvents. This has been done with the aid of a thermochemical cycle involving the gas-phase homolytic dissociation of the C-X bond, solvation of RX, R • , and X • , and reduction of X • to X - in solution. Introduction Injection of a single electron into an organic halide often leads to the rupture of a carbon-halogen bond. 1 This process follows either a concerted mechanism in which electron transfer and bond breaking occur in a single step, or a stepwise mechanism involving the intermediate formation of a radical anion. 1,2 Both homogeneous 3 and heterogeneous 4,5 reduction of most alkyl halides follows the concerted mechanism, which can be written as where D •- stands for a homogeneous donor, which might be an aromatic compound or a metal complex. Reductive cleavage of alkyl halides plays an important role in several important processes such as reductive dehalogenation of recalcitrant pollutants, 6-8 atom transfer radical polymerization, 9 and various other processes of synthetic importance. 10 Reduction of alkyl halides has also attracted much interest as a model for the investigation of the mechanism and dynamics of dissociative electron-transfer processes. 11 Information on the kinetics and thermodynamics of the reductive cleavage of carbon-halogen bonds is important to the full comprehension of the mechanism of the above- mentioned processes; these data are also useful in the optimiza- tion of synthetic and catalytic processes. The standard reduction potentials, SRPs, of alkyl halides undergoing concerted dis- sociative electron transfer (eq 1) cannot be experimentally measured. In fact, an irreversible reduction peak (or wave) occurring at potentials that are considerably more negative than the standard potential, E o i RX/R • +X - , is observed because of the sluggishness of the electron transfer. However, E o i RX/R • +X - may be calculated if one incorporates reaction 1 into a thermochemi- cal cycle involving reactions with known thermodynamic data. 4a,12 The most simple thermochemical cycle, which is also the most widely used one, involves homolytic dissociation of the C-X bond followed by 1e - reduction of X • to X - . Therefore, estimation of E o i RX/R • +X - requires knowledge of the RX bond dissociation energy, BDE, and the standard potential of the X • / X - couple. Although BDE values are available in the literature for a large number of alkyl halides, 13 only a few contrasting values have been reported for E o i X • /X - . 4c,14 The aim of this paper is to calculate the standard reduction potentials of the halogen atoms in water as well as in two popular organic solvents, namely, acetonitrile and dimethylfor- mamide. From an appropriate thermochemical cycle, E o i X • /X - in water will be first calculated on the basis of the most up-to- date available thermodynamic data. The ensuing values will then be used to calculate E o i X • /X - in nonaqueous solvents. The second objective of the paper is to critically examine the commonly used method of calculation of SRPs of alkyl halides in aprotic solvents and to apply it for the calculation of E o i RX/R • +X - of a series of important alkyl halides. Computational Methods Free energies of solvation for the studied species in water, acetonitrile, and dimethylformamide were calculated from the conductor-like polarizable continuum model, CPCM, the uni- versal solvation model based on solute electron density, SMD, and conductor like screening model for realistic solvents, COSMO-RS. 15 CPCM 16 and SMD 17 calculations were performed in Gaussian 03 and 09, respectively, 18 whereas COSMO-RS calculations were performed in ADF. 19 These calculations were carried out with theoretical proce- dures for which they were parametrized, namely, HF/6-31+G(d) with UAHF radii for CPCM, HF/6-31G(d) for SMD and BP/ TZVP for COSMO-RS, all geometries being optimized at the same level. For species containing third-row atoms, Br and Kr, we adopted Rassolov’s 6-31G 20 rather than the default * Corresponding author. E-mail: armando.gennaro@unipd.it. ‡ University of Padova. † Australian National University. R - X + e - f R • + X - (1) R-X + D •- f R • + X - + D (2) J. Phys. Chem. B 2011, 115, 678–684 678 10.1021/jp109613t  2011 American Chemical Society Published on Web 12/27/2010