The relative energies of radical cation states on nucleobases in DNA are considerably affected by the local distribution of water molecules and counterions. In fact, fluctuations of ΔG are large enough to render electron‐hole transfer from G+ to A energetically feasible, thus allowing a changeover from the generally accepted G‐hopping mechanism to A‐hopping (see picture).

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The solvent reorganization energy λs can have a significant effect on the activation energy for charge transfer in DNA and its dependence on donor (D)−acceptor (A) distance R DA. To estimate λs and the resulting effective contribution β s to the falloff parameter β for the overall transfer rate constant, the Poisson equation was solved numerically for several systems representing DNA duplexes, 5‘-GGGT n GGG-3‘, in a realistically structured heterogeneous dielectric, as determined by molecular dynamics (MD) simulations. The charge transfer was modeled primarily for holes localized on single guanine bases. Effects of thermal fluctuations on λs were taken into account via structures for a given duplex sampled from MD trajectories. Calculated values of λs were found to be rather insensitive to thermal fluctuations of the DNA fragments but depended in crucial fashion on details of the dielectric model (shape and dielectric constants of various zones) that was used to describe the polarization response of the DNA and its environment to the charge transfer. λs was calculated to increase rapidly at small R DA values (<15 Å), and accordingly the falloff parameter β s (defined as a local function of R DA) decreases appreciably with increasing R DA (from 1.0 Å-1 with only one intermediate base pair between D and A to 0.15 Å-1 for systems with five intervening pairs). Calculated λs values were accurately fitted (standard deviation of ∼0.5 kcal/mol) to a linear function of 1/R DA, including all cases except contact (R DA = 3.4 Å), where some overlap of D and A sites may occur. A linear fit to an exponential (of form exp(−βs R DA)) gave comparable accuracy for the entire R DA range. λs based on D and A holes delocalized over two adjacent guanine bases was uniformly ∼12 kcal/mol smaller than the corresponding results for holes localized on single guanines, almost independent of R DA. The internal reorganization energy for hole transfer between GC pairs was calculated at 16.6 kcal/mol at the B3LYP/6-31G* level.

In the present work we present a comprehensive study of the magneto-structural correlations of a series of ferromagnetic triply heterobridged Cu(II) dinuclear compounds containing [Cu(2)(mu-O(2)CR)(mu-OH)(mu-X)(L)(2)](2+) ions (where X = OH(2), Cl(-), OMe(-) and L = bpy, phen, dpyam) which have the particularity of being all ferromagnetic. The present theoretical study, based on hybrid density functional theory (DFT) calculations, leads to strong conclusions about the role of the pentacoordination geometry of the Cu(II) ions (square base pyramidal (SP) or trigonal bipyramidal (TBP) coordination) and the nature of the third bridging ligand in determining the final value of the magnetic coupling constants in this series of compounds. These investigations point toward the existence of a maximum value for the ferromagnetic interaction and may offer some useful information to synthetic chemists aiming at obtaining new compounds with enhanced ferromagnetism.