In this paper we analyze using Density Functional Theory (DFT) the electronic structure of the Creutz−Taube ion, [(NH3)5Ru-pyrazine-Ru(NH3)5]5+, a classical example of a mixed-valence complex. A complete structural and bonding analysis, description of the low-lying excited states, and a vibronic coupling calculation along both the symmetric and the antisymmetric Ru-pyrazine-Ru stretching mode are presented. The results are in very good agreement with the reported experimental data and we are able to assign unambiguously this compound as a Class III system, following the Robin and Day classification. In this work we demonstrate that DFT does offer a unique tool for handling the localization−delocalization of electrons in mixed-valence systems and therefore we expect that this approach can play an important role in characterizing electron-transfer systems. Localized Ru−N(pyrazine) stretching in the range 254−275 cm-1 has been estimated from our calculations, which should be used in future model Hamiltonians which aim to include the Ru−pyz bond vibration to describe the spectral features of the Creutz−Taube ion.

A three-site, three-harmonic oscillator vibronic model Hamiltonian that includes correlation effects is considered in order to study the optical absorption spectra of the Creutz-Taube ion at various values for the total charge. The model is made by a two band Hubbard Hamiltonian, plus a vibronic interaction that is linear in the nuclear coordinates. W e show that the main features of absorption spectra observed experimentally in the near-IR-visible are well reproduced by this model, as far as band position and shape are concerned. An approximate approach to the vibronic problem reveals that, in the present case, the vibronic interaction is strong and good results can be obtained only by solving the full vibronic problem.

Eumelanins, the chief photoprotective pigments in man and mammals, owe their black color to an unusual broadband absorption spectrum whose origin is still a conundrum. Excitonic effects from the interplay of geometric order and disorder in 5,6-dihydroxyindole (DHI)-based oligomeric/polymeric structures play a central role, however the contributions of structural (scaffold-controlled) and redox (π-electron-controlled) disorder have remained uncharted. Herein, we report an integrated experimental-theoretical entry to eumelanin chromophore dynamics based on poly(vinyl alcohol)-controlled polymerization of a large set of 5,6-dihydroxyindoles and related dimers. The results a) uncover the impact of the structural scaffold on eumelanin optical properties, disproving the widespread assumption of a universal monotonic chromophore; b) delineate eumelanin chromophore buildup as a three-step dynamic process involving the rapid generation of oxidized oligomers, termed melanochromes (phase I), followed by a slow oxidant-independent band broadening (phase II) leading eventually to scattering (phase III); c) point to a slow reorganization-stabilization of melanochromes via intermolecular redox interactions as the main determinant of visible broadband absorption.

Articles you may be interested inA hybrid quantum mechanical force field molecular dynamics simulation of liquid methanol: Vibrational frequency shifts as a probe of the quantum mechanical/molecular mechanical coupling Quantum mechanics on multiply connected manifolds with applications to anyons in one and two dimensions AIP Conf.We present simulations of wave-packet dynamics for a model of a conical intersection in two dimensions. The potential energy surfaces and couplings are functions of a total symmetrical coordinate and of a symmetry breaking one. The wave packet crosses the coupling region once, moving essentially in the direction of the symmetrical coordinate. The dynamics are determined by two methods, one quantum mechanical and the other semiclassical, based on trajectories and surface hopping. The semiclassical approximation is quite adequate for low coupling strengths in the diabatic representation, less so for larger couplings. Approximate analytic solutions for the two-dimensional problem and for one-dimensional analogs are provided, in order to generalize the numerical results and to analyze the reasons of the discrepancies between semiclassical and quantum mechanical results.

An integrated computational approach built on quantum mechanical (QM) methods, purposely tailored inter-and intra-molecular force fields and continuum solvent models combined with time-independent and timedependent schemes to account for nuclear motion effects is applied to the spectroscopic investigation of pyrimidine in the gas phase as well as in aqueous and CCl 4 solutions. Accurate post-Hartree-Fock methodologies are employed to compute molecular structure, harmonic vibrational frequencies, energies and oscillator strengths for electronic transitions in order to validate the accuracy of approaches rooted into density functional theory with emphasis also on hybrid QM/QM 0 models. Within the time-independent approaches, IR spectra are computed including anharmonicities through perturbative corrections while UV-vis line-shapes are simulated accounting for the vibrational structure; in both cases, the environmental effects are described by continuum models. The effects of conformational flexibility, including solvent dynamics, are described through time-dependent models based on purposely DFT-tailored force fields applied to molecular dynamics simulations and on QM computations of spectroscopic properties. Such procedures are exploited to Dedicated to Professor Vincenzo Barone and published as part of the special collection of articles celebrating his 60th birthday.Electronic supplementary material The online version of this article (simulate IR and UV-vis spectra of pyrimidine in the gas phase and in solutions, leading in all cases to good agreement with experimental observations and allowing to dissect different effects underlying spectral phenomena.