Bogdan Frecus

Density Functional Restricted-Unrestricted/Molecular Mechanics Theory for Hyperfine Coupling Constants of Molecules in Solution

A density functional restricted-unrestricted approach, capable of evaluating hyperfine coupling constants with the inclusion of spin polarization effects in a spin-restricted Kohn-Sham method, has been extended to incorporate environmental effects. This is accomplished by means of a hybrid quantum mechanics/molecular mechanics formalism which allows for a granular representation of the polarization and electrostatic interactions with the classically described medium. By this technique, it is possible to trace the physical origin of hyperfine coupling constants in terms of spin polarization and spin density contributions and disentangle the dependence of these contributions on molecular geometry and solvent environment, something that increases the prospects for optimal design of spin labels for particular applications. A demonstration is given for the nitrogen isotropic hyperfine coupling constant in di-tert-butyl nitroxide solvated in water. The results indicate that the direct spin density contribution is about 5 times smaller than the spin polarization contribution to the nitrogen isotropic hyperfine coupling constant and that the latter contribution is solely responsible for the solvent shift of the constant. The developed approach is found capable of achieving satisfactory accuracy in prediction of the hyperfine coupling constants of solvated di-tert-butyl nitroxide and other similar nitroxides without the inclusion of solvent molecules in the quantum region provided polarizable force fields are used for the description of these molecules.

Encapsulation Influence on EPR Parameters of Spin-Labels: 2,2,6,6-Tetramethyl-4-methoxypiperidine-1-oxyl in Cucurbit[8]uril.

Encapsulation of a nitroxide spin label into a host cavity can prolong the lifetime of the spin label in biological tissues and other environments. Although such paramagnetic supramolecular complexes have been extensively studied experimentally, there is yet little understanding of the role of the encapsulation on the magnetic properties of the spin labels and their performance at the atomistic level. In this work, we approach this problem by modeling encapsulation induced changes of the magnetic properties of spin labels for a prototypical paramagnetic guest-host complex, 2,2,6,6-tetramethyl-4-methoxypiperidine-1-oxyl, enclosed in the hydrophobic cavity of cucurbit[8]uril, using state-of-the-art hybrid quantum mechanics/molecular mechanics methodology. The results allow a decomposition of the encapsulation shift of the electronic g-tensor and the nitrogen isotropic hyperfine coupling constant of nitroxide radical into a set of distinct contributions associated with the host cavity confinement and with changes of the local solvent environment of the spin label upon encapsulation. It is found that the hydrophobic cavity of cucurbit[8]uril only weakly influences the electronic g-tensor of the 2,2,6,6-tetramethyl-4-methoxypiperidine-1-oxyl but induces a significant encapsulation shift of the nitrogen hyperfine coupling constant. The latter is caused by the change of topology of the hydrogen bonding network and the nature of the hydrogen bonds around the spin label induced by the hydrophobic cavity of the inclusion host. This indirect effect is found to be more important than the direct influence of the cavity exerted on the radical. The ramification of this finding for the use of approximate methods for computing electron paramagnetic resonance spectra of spin labels and for designing optimal spin labels based on guest-host templates is discussed.

On exchange coupling and bonding in the Gd2@C80 and Gd2@C79N endohedral dimetallo-fullerenes

A series of computational experiments performed with various methods belonging to wave-function and density functional theories approaches the issue of bonding regime and exchange coupling in the title compounds. Gd2@C80 is computed with a very weak exchange coupling, the sign depending on the method, while Gd2@C79N has resulted with a strong coupling and ferromagnetic ground state, irrespective of the computational approach. The multi-configuration calculation and broken symmetry estimation are yielding closely coincident coupling constants, of about J ∼ 400 cm−1. No experimental estimation exists, but the ferromagnetic ground state of Gd2@C79N is confirmed from paramagnetic resonance data. The different behaviour is due to particularities of electron accommodation in the orbital scheme. The exchange effects localised on atom lead to preference for parallel alignment of the electrons placed in the 4f and 5d lanthanide shells, determining also a ferromagnetic inter-centre coupling. The structural insight is completed with a ligand field analysis of the density functional theory results in the context of frozen density embedding. The energy decomposition analysis of bonding effects is also discussed. Finally, with the help of home-made codes (named Xatom+Xsphere), a model for the atom encapsulated in a cage is designed, the exemplified numeric experiments showing relevance for the considered endohedral metallo-fullerene issues.

DFT study of electronic structure and optical properties of some Ru- and Rh-based complexes for dye-sensitized solar cells

The paper reports Time Dependent Density Functional Theory (TD DFT) calculations providing the structure, electronic properties and spectra of [Ru(II)(bpy)3− n (dcbpy) n ]2+ and [Rh(III)(bpy)3− n (dcbpy) n ]3+ complexes, where bpy = 2,2′-bipyridyl, dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl, and n = 0, 1, 2, 3, studied as possible pigments for dye-sensitized solar cells. The role of the metallic ion and of the COOH groups on the optical properties of these complexes are compared and contrasted and their relevance as dyes for hybrid organic–inorganic photovoltaic cells is discussed. It was found that the optical spectra are strongly influenced by the metallic ion, with visible absorption bands for the Ru(II) complexes and only ultraviolet bands for the Rh(III) complexes. Upon excitation, the extra positive charge of the Rh3+ centre tends to draw electrons towards the metal ion, facilitating some charge transfer from the ligand to the metal, whereas in the case of the Ru2+ ion the electron transfer is clearly from the metal to the ligand. The carboxyl groups play an important role in strengthening the absorption bands in solution in the visible region. Of the complexes studied, the most suited as pigments for dye-sensitized solar cells are the [Ru(II)(bpy)3− n (dcbpy) n ]2+ complexes with n = 1 and 2. This is based on the following arguments: (i) their intense absorption band in the visible region, (ii) the presence of the anchoring groups allowing the bonding to the TiO2 substrate and the charge transfer, and (iii) the good energy level alignment with the conduction band edge of the semiconducting substrate and the redox level of the electrolyte.

Noble gas endohedral fullerenes, Ng@C60 (Ng=Ar, Kr): a particular benchmark for assessing the account of non-covalent interactions by density functional theory calculations

Abstract This work is dedicated to a special test, checking the capability of density functional theory computations in the account of long-range effects. The particular case of endohedral noble gas atoms in C60 fullerene puts several methodological challenges, such as the numeric problem in the balance of covalent versus non-covalent interactions. We designed a procedure based on the shifts of vibrational frequencies in C60 versus Ng@C60 couples. The energy scale of the investigated vibrations is comparable to that of the extracted van der Waals interaction parameters, achieving then a well-tempered description. A phenomenological model, based on transparent analytical formulas of the totally symmetric modes, is outlined and used to assess the computational results on series of functionals belonging to different classes (genuine forms, long-range corrected and with empiric dispersion ingredients). While the vast majority of the tested functionals undergo failures, good results are obtained for some long-range-corrected functionals (LC-BLYP and LC-wPBE), which follow the Tsuneda and Hirao’s scheme, provided that richer basis sets (with diffuse components) are used. Successes are obtained also for the Grimme B97D functional, when coupled with the D2 and D3 dispersion scheme, the results being rather independent from the basis set, as expected from the empirical nature of this type of amendment.

Theoretical Calculations of Structure and Exchange Coupling of a Room‐Temperature Molecular Magnet

We report here quantum chemical calculations advancing structural hypotheses and corresponding methodological concerns devoted to the elucidation of magneto‐structural features of the V[TCNE]2 molecular magnet (TCNE = tetracyanoethylene). V[TCNE]xy(CH2Cl2) is the first room‐temperature molecular magnet discovered and the only one with an active organic component. Despite previous detailed magnetic, spectroscopic and conductivity studies, the mechanism for the strong exchange coupling in the family M[TCNE]xy(solvent) has remained an open question, the difficulty being related to the absence of structural data for these amorphous compounds. Starting from a structural model that we propose, we report the results of Density Functional Theory (DFT) calculations providing the optimized geometry of a periodic lattice. The DFT methods retrieve the long range magnetic ordering of the system, but overestimate the absolute values of exchange constants. We analyze the DFT results assessing the intrinsic limitations i...