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For the prototypical dyad (TCNE2)-•, previous in vacuo calculations indicate that sizable distortion of the equilibrium gas-phase structure may be required to reduce the donor/acceptor electronic coupling element (HDA) to the solution-phase experimental estimates. We employ the polarizable continuum model (PCM) to simulate the solvation environment for several polar solvents, finding noticeable structure change associated with the promotion of charge localization due to solvation. We have extended the counterpoise (CP) correction procedure so as to include fragment relaxation energies within the PCM model, and it would be of interest to incorporate this approach into schemes for optimizing coordinates on CP-corrected energy surfaces. The calculations include face-to-face encounter geometries as well as several lateral and twist distortions of the face-to-face structures. In proceeding from vacuum to solution, the calculated stabilization energy is reduced from −18 to −3 kcal/mol, and the calculated energy surface becomes flatter, with a somewhat larger minimum-energy separation of the monomer units (rDA). The corresponding minimum-energy structures are, respectively, delocalized and charge-localized. Using TD-DFT, spin-projected MP2 (PUMP2), and state-averaged two-configuration SCF (SA-TCSCF) calculations to evaluate HDAfor symmetric encounter complex geometries (models for transition-state structures) indicates that HDAhas comparable magnitude in the gas phase and in solution for a given dimer structure. SA-TCSCF calculations comparing HDAbased on symmetric charge-delocalized structures and their asymmetric (minimum-energy) charge-localized counterparts (at a given rDA) yield very similar values. Even with account taken of the energetically accessible configurations probed by the PCM calculations, the HDAvalues remain significantly higher than the experimental estimates inferred from solution spectra and assumption of rDAbased on crystal data. Clearly, additional calculations based on molecular-level solvent models would be of value in helping to characterize the intermolecular structures accessible to the encounter complex in polar solution. [ABSTRACT FROM AUTHOR]

This work provides a detailed account of the application of our multichromophoric Förster resonance energy transfer (MC-FRET) theory (Phys. Rev. Lett.2004, 92, 218301) for the calculation of the energy transfer rate from the B800 unit to the B850 unit in the light harvesting complex 2 (LH2) of purple bacteria. The model Hamiltonian consists of the B800 unit represented by a single bacteriochlorophyll (BChl), the B850 unit represented by its entire set of BChls, the electronic coupling between the two units, and the bath terms representing all environmental degrees of freedom. The model parameters are determined, independent of the rate calculation, from the literature data and by a fitting to an ensemble line shape. Comparing our theoretical rate and a low-temperature experimental rate, we estimate the magnitude of the BChl−Qytransition dipole to be in the range of 6.5−7.5 D, assuming that the optical dielectric constant of the medium is in the range of 1.5−2. We examine how the bias of the average excitation energyof the B800-BChl relative to that of the B850-BChl affects the energy transfer time by calculating the transfer rates based on both our MC-FRET theory and the original FRET theory, varying the value of the bias. Within our model, we find that the value of bias 260 cm-1, which we determine from the fitting to an ensemble line shape, is very close to the value at which the ratio between MC-FRET and FRET rates is a maximum. This provides evidence that the bacterial system utilizes the quantum mechanical coherence among the multiple chromophores within the B850 in a constructive way so as to achieve efficient energy transfer from B800 to B850. [ABSTRACT FROM AUTHOR]

Closed-form rate expressions are derived on the basis of the stationary-phase approximation for the Fermi golden rule expression of the quantum electron-transfer (ET) rate. First, on the basis of approximate solutions of the stationary-phase points near G 0, −, and , where Gis the reaction free energy and is the reorganization energy, three closed-form rate expressions are derived, which are respectively valid near each value of G. Numerical tests for a model Ohmic spectral density with an exponential cutoff demonstrate good performance of the derived expressions in the respective regions of their validity. In particular, the expression near G −, which differs from the semiclassical approximation only by a prefactor quadratic in G, works substantially better than the latter. Then, a unified formula is suggested, which interpolates the three approximate expressions and serves as a good approximation in all three regions. We have also demonstrated that the interpolation formula can serve as a good quantitative means for understanding the temperature dependence of the quantum ET rate. [ABSTRACT FROM AUTHOR]

The main structural and electronic factors playing a role in intramolecular dissociative electron transfer of a simple donor−peptide−acceptor (D−peptide−A) model have been investigated by an integrated computational protocol based on the density functional theory, its time-dependent extension, and the polarizable continuum model. Our results allow us to elucidate the electronic states involved in the process and how they are perturbed by the orientation of the donor and the acceptor with respect to the peptide chain and by the presence of the solvent. We also report a semiquantitative estimation of the rate constant governing electron transfer obtained by a direct quantum mechanical evaluation of all the terms entering the kinetic expressions based on the Marcus theory and its extensions. [ABSTRACT FROM AUTHOR]

Diabatic states for donor (D) and acceptor (A) interactions in electron transfer (ET) processes are formulated and evaluated, along with coupling elements (HDA) and effective D/A separation distances (rDA), for reduced electronic spaces of variable size, using the generalized Mulliken Hush model (GMH), applicable to an arbitrary state space and nuclear configuration, and encompassing Robin−Day class III and as well as class II situations. Once the electronic state space is selected (a set of n≥ 2 adiabatic states approximated by an orbital space based on an effective 1-electron (1-e) Hamiltonian), the charge-localized GMH diabatic states are obtained as the eigenstates of the dipole moment operator, with rotations to yield locally adiabatic states for sites with multiple states. The 1-e states and energies are expressed in terms of Kohn−Sham orbitals and orbital energies. Addressing questions as to whether the estimate of HDA“improves” as one increases n, and in what sense the GMH approach “converges” with n, we carry out calculations for three mixed-valence binuclear Ru complexes, from which we conclude that the 2-state (2-st) model gives the most appropriate estimate of the effectivecoupling, similar (to within a rms deviation of ≤15%) to coupling elements obtained by superexchange correction of HDAvalues based on larger spaces (n= 3−6), and thus yielding a quasi-invariant value for HDAover the range explored in the calculations (n= 2−6). An analysis of the coupling and associated D and A states shows that the 2-st coupling involves crucial mixing with intervening bridge states (D and A “tails”), while increasingly larger state spaces for the same system yield increasingly more localized D and A states (and weaker coupling), with HDAtending to approach the limit of “bare” or “through space” coupling. These results help to reconcile seemingly contradictory assertions in the recent literature regarding the proper role of multistate frameworks in the formulation of coupling for both intra- and intermolecular ET systems.The present results are compared in detail with other reported results. [ABSTRACT FROM AUTHOR]