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Highest occupied molecular orbital (HOMO) of a Ru2+ cation in aqueous solution. This 4d6 transition metal aqua cation is low spin and forms a coordination shell consisting of six H2O molecules which in solution retains to a good approximation octahedral symmetry (as can be seen from the near t2g symmetry of the HOMO in the picture). Oxidation of the complex proceeds via an outer-sphere reorganization mechanism, preserving the octahedral coordination, and the Ru2+/Ru3+ couple is considered as a text book example of a system to which the Marcus theory of electron transfer applies. This system has been therefore our favourite model for the development of the Marcus theory based method for the computation of redox and reorganization free energies([JPCB05a, TCA05]). We have also studied the electronic absorption spectrum of aqueous Ru(II) using TDDFT methods[JPCB05b]

Lowest unoccupied molecular orbital (LUMO) of a Ag1+ cation in aqueous solution (ion is in the center). Since the 4d10 configuration of Ag(I) is closed shell, the LUMO is the 5s orbital which in solution is hybridized with a delocalized virtual state of the solvent (water) as shown in the picture (compare picture on the main page). The Ag1+ aquaion is also of interest because oxidation to Ag2+ increases the coordination number (on average) by one H2O molecule. This introduces non-linearities in the solvent response placing the Ag1+/Ag2+ outside the Marcus regime. Both these features are in contrast to Ru(II) which is the reason that Ag(I) has become a second model system to which we have returned repeatedly in calculations of redox [JACS04, JPCB04a, JCP06] and optical properties[JCP04].

Two model quinones, benzoquinone (BQ) and duro quinone (DQ), forming stable radical anions. The reaction free energy (redox potential) of the BQ + DH•- → BQ•- + DQ redox reaction was computed by simulating the DQ•- → DQ + e- and BQ•- → BQ + e- half reactions and subtracting the oxidation free energies. Two different non-aqueous solvents, methanol and acetonitrile were used and the results for the reaction and reorganization free energy compared[Angew. Chem.]

Ab initio molecular dynamics model system used for the simulation of the BQ + e- → BQ•- half reaction in methanol solution. The green contours indicate the spin density of the unpaired electron in the BQ•- radical anion.

Two organosulfur compounds, tetrathiafulvalene (TTF) and tianthrene (TH), forming stable radical cations. We computed the reaction free energy change (redox potential) and reorganization free energy of the TTF + TH•+ → TTF•+ + TH redox reaction in acetonitrile solution[JPCB2005c]

Electronic polarization of a TTF•+ radical cation created by vertical ionization of a TTF molecule in acetonitrile solution. Picture shows the difference of the charge density in cationic and neutral state for fixed atomic positions. Note the polarization of the solvent molecules (green indicates that charge has been removed).

Spin polarization of the TTF•+ radical cation of the picture above (so same atomic configuration). The unpaired electron is localized exclusively on the solute.

Model system of aqueous uracil used for the ab initio MD computation of the infra-red absorption spectrum of this nucleic base. The spectrum was determined by Fourier transformation of the time fluctuations of the polarization as computed from maximally localized Wannier functions[JPCB03]. For a similar calculation of the IR spectrum of NMA see [JCTC05].

OH radical in solution. This highly reactive species was simulated using a special self-interaction corrected BLYP functional. The corrections are applied to the unpaired electron only. Without these corrections (using the regular BLYP functional) the radical even attacks the solvent forming a spurious hemi-bond with the O atom of a neighbouring water. With these corrections the OH was found to be hydrogen bonded to the solvent accepting two to three weak H-bonds and donating one strong H-bond[PCCP05]

Pentahydroxyphosphorane (P(OH)5) molecule in aqueous in solution (periodic boundary conditions applied). Using this rather small model system we were able to compute the relative equilibrium constants (pKa's) for axial and equatorial acid dissociation[JACS2002]. The pKa's were obtained from the reversible work needed to transfer a proton from a phosphorane OH group to the solvent. The challenge is now to find a method for computation of acid dissociation constants which is consistent with our half-reaction scheme for computing redox potentials, i.e. we will have to find a way to completely eliminate the proton from the system or insert a proton. This will enable us to compute reaction free energies for proton coupled redox reactions, which are the rule for redox reactions of organic molecules.

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