Linear free energy relationships for polyhalogenated alkane transformationby electron-transfer mediators in model aqueous systems

Linear free energy relationships (LFERs) based on Marcus theory were genera ted for transformation of C-1- and C-2-polyhalogenated alkanes (PHAs or R-X , where X = H, F, Cl, Br) in model aqueous systems containing bulk reductan ts and the electron-transfer mediators iron porphyrin or mercaptojuglone (5 -hydroxy-2-mercapto-1,4-naphthoquinone). The model systems are representati ve of common natural environments where iron species and natural organic ma tter serve as electron shuttles from bulk reductants to pollutants such as PHAs. Seven ab initio computational theories were tested for their ability to generate rapid, accurate, and precise estimates of the R-X bond dissocia tion energy, the largest energetic term in the Marcus equation. The descrip tors for the LFERs were computed using B3LYP/6-311++g(d, p) theory/basis se t. The LFERs that had the highest correlation coefficients for the two mode l systems were log(k(FcP)) = -0.0777(+/-0.0105)D(R-X)' -0.00804(+/-0.00961) Delta G(o)' + 21.7(+/-2.82) (adj r(2) = 0.946; n = 16) and log(k(Jug)) = -0 .103(+/-0.0308)-D(R-X)' - 0.00958(+/-0.00513)LUMO + 22.7(+/-9.72) (adj r(2) = 0.955; n = 12). D(R-X)' is the bond dissociation energy of the R-X bond that dissociates the transition state, Delta G degrees' is the standard fre e energy of one-electron reduction, LUMO is the energy of the lowest unoccu pied molecular orbital of the PHA, and the numbers in parentheses are 95% c onfidence limits of the regression coefficient estimates. All coefficients were significant at 90% confidence. These results support earlier hypothese s based on PHA kinetic results, reaction intermediates, and products in the model systems that the initial, rate-limiting step in the reaction in both model systems is a dissociative one-electron transfer. The study supports previous studies that showed, for electron-transfer reactions involving hom olytic bond dissociation, the overall reorganization energy term in the Mar cus equation is composed primarily of the bond dissociation energy. Correla tion of rate constants of polyhalogenated aliphatic compounds measured in r elated aqueous systems with D(R-X)' and Delta G degrees' suggests one-elect ron transfer may, at least partially, limit disappearance rates in those sy stems.