Alain Fritsch

Synthesis and complexation properties of two new curcuminoid molecules bearing a diphenylmethane linkage

Abstract Bis-curcuminoids 3 and 4 , bearing a diphenylmethane bridge for both compounds and a crown ether chain adapted to complex Li + for compound 4 , were synthesized and characterized by mass and NMR spectroscopies. By their preorganized geometry, they represent a new class of curcuminoids able to complex transition metal cations. Their absorption spectra in DMF by comparison with those of curcumin 1 and dimethylcurcumin 2 have shown that the phenolate forms in the two halves are in interaction in compound 3 and are independent in compound 4 . Complexation studies have revealed a poor selectivity of curcuminoids for transition metals. Nevertheless complexation of Cu (II), studied by UV–visible absorption spectroscopy, has shown subtle differences between curcumin 1 and bis-curcuminoid 3 . These observations were supported by quantum mechanic calculations to establish the most probable structures of non- and complexed compounds.

A Valence-Bond/Hartree-Fock method to determine the extended Hubbard parameters in organic conductors

The relative magnitude of the parameters describing the interaction between conduction electrons in organic conductors is not yet firmly established. A mixed Valence-Bond/Hartree-Fock method applied to all valence electrons calculations on finite size clusters was thus derived in order to size up the magnitude of the coulombic terms. An effective Hamiltonian built upon a single site orbital per molecule reproduces the whole set of VB energy levels. Coulombic parameters describing the interactions in clusters of organic molecules derived of TTF (tetrathiafulvalene) may be thus extracted. Intrachain and interchain terms for a series of quasi-1D and quasi-2D representative salts were evaluated. The magnitude of the coulombic terms (U ≅ 4 eV; V(nearest-neighbour) ≅ 2–3 eV) differs qualitatively from previous estimations and evidences the long range part of the potential.

Transfer integrals, anion-cation interaction and molecular vibrations in the organic salts (RTCF)2X: Structural and theoretical investigation

Abstract The transfer integrals of a large series of organic salts, including symmetrical and unsymmetrical molecules and anions of different geometries are presented. We outline the variation of these integrals with the physical constraints and show that even small geometrical changes may induce large variations of the electronic interactions. The general features of the delocalized model are presented on the grounds of modifications of the Fermi surface. These results are compared with the localized approach based on interacting, isolated dimers. An attempt to correlate these models is made, showing that the TMTTF salts may behave differently from their selenium analogs. A careful examination of crystallographic structures shows that the molecular vibrations (especially those of the C=C and CS or CSe bonds) which interact with the molecular orbitals originating in the electronic band might be linked on adjacent chains by the anion atoms. Model calculations, based on a SCF semi-empirical method, of these interactions are presented, and the possible implications of this mechanism on the competition between electronic states are discussed.

Parametrization of an effective correlated hamiltonian for hole transfer in large DNA strands

The hybrid valence-bond/Hartree–Fock model (VB/HF) [F. Castet, A. Fritsch and L. Ducasse, J. Phys. I (France), 1996, 6, 583; F. Castet, L. Ducasse and A. Fritsch, Chem. Phys., 1998, 232, 37] is used to derive an effective model for conduction holes along DNA nucleoside stacks. The extraction procedure which gives the transfer integrals from the off-diagonal elements of the VB/HF matrix was presented in the first paper of this series [G. Brunaud, F. Castet, A. Fritsch, M. Kreissler and L. J. Ducasse, Phys. Chem. B, 2001, 105, 12 665.]. The remaining electrostatic parameters, introduced here, provide a complete set of transferable intrasite and intersite parameters for the effective hamiltonian. The sensitivity of the effective parameters to the chemical nature of the interacting nucleosides as well as to geometrical fluctuations of the DNA strands is analyzed. It is shown that conformational changes can induce sizeable variations on the effective parameters, in particular on penetration integral values which play a leading role in hole localization properties [G. Brunaud, F. Castet, A. Fritsch and L. Ducasse, Phys. Chem. Chem. Phys., 2002, 4, 6072]. However, these variations do not affect the nature of the most efficient trapping sites, so that the experimental sampling of the charge on guanine sites are quite satisfactorily simulated using the effective parameters calculated from canonical geometries. This point is illustrated in the last part of the paper, in which electron distributions in the ground state of charged DNA sequences are calculated and compared to experimental data.

An effective hamiltonian for hole transfer along B-DNA double strands

A new electronic effective hamiltonian based on a quantum cell formalism is used to calculate the charge distributions when a single hole is delocalised along B-DNA double strands. This scheme allows one to deal with very large sequences while including electrostatic interactions. The ionisation potentials and electronic distributions are compared to ab initio and experimental results. The hole localisation properties are very sensitive to the modulation of the site energies by electrostatic interactions, which are sequence dependent.

A Valence-Bond/Hartree–Fock method to determine the Hubbard transfer integrals in organic conductors

Abstract In previous reports [F. Castet, A. Fritsch, L. Ducasse, J. Phys. (Paris) I, 6 (1996) 583; L. Ducasse, A. Fritsch, F. Castet, Synth. Metals 85 (1997) 1627], we presented a new mixed Valence-Bond/Hartree–Fock (VB/HF) method to extract the Extended Hubbard Coulombic parameters in organic conductors. In our approach, calculations (restricted to all valence electrons) are performed on finite size clusters using crystal data and molecular orbitals (MOs) localised on fragments. The optimized MOs depend on the charge of the fragment. The present paper describes the calculation scheme for matrix elements between distinct Valence-Bond configurations involved in charge transfer processes, i.e. the transfer integrals. Calculations were performed for different representative salts with either quasi one-dimensional or two-dimensional character, using different semi-empirical hamiltonians.

Spectroscopic and theoretical investigations of phenolic acids in white wines

Model solutions of white wines containing phenolic acids have been investigated by means of UV-vis, laser induced fluorescence and Raman spectroscopic techniques. In order to interpret the spectra, density functional theory calculations of phenolic acids have been performed. This work demonstrates that only hydroxynamic acids are in resonance with a laser excitation line with 325nm wavelength and are therefore at the origin of the strong enhancement of the Raman light scattering. Real white wines also display such resonance Raman scattering so that their content in hydroxycinnamic acids may be quite precisely determined. The analysis of the Raman spectrum of a real dry white wine reveals qualitatively the preponderance in its composition of p-coumaric and caftaric acids.

Anion–cation induction coupling in organic superconductors

Within the framework of the valence bond/Hartree–Fock (VB/HF) formalism, [Castet et al., J. Phys. I (France) 6, 583 (1996); L. Ducasse et al., Synth. Metals 85, 1627 (1997); F. Castet et al., Chem. Phys. 232, 37 (1998); Synth. Metals 103, 1799 (1999)] anion-cation induction interactions are evaluated in organic superconductors derived from the Bechgaard salts. The calculation scheme is based on an extension of the VB/HF model, so as to incorporate the effect of the distinct anion polarization states in the calculation of the VB Hamiltonian matrix elements. The induction mechanism involves a charge displacement on the counteranions in the electrostatic field of adjacent positively charged organic molecules. Anion-cation interactions are calculated for the β-BEDTTTF2I3 structure [A. J. Schultz et al., J. Am. Chem Soc. 108, 7853 (1986)], which involves highly polarizable anions.

NON-ORTHOGONAL ORBITALS FOR LOCALIZED ELECTRONS. II. VARIATIONAL OPTIMIZATION BASED ON AN EFFECTIVE SPIN HAMILTONIAN

Abstract The derivation of an effective spin hamiltonian is achieved for a single configuration Spin-Coupled Valence Bond wavefunction built upon weakly overlapping non-orthogonal orbitals. The effective Spin-Coupled Magnetic Hamiltonian (SCMH) serves for the variational optimization of both orbital and spin parts of the wavefunction for larger numbers of electrons than genuine quantum-chemical methods. Optimized magnetic coupling terms are thus obtained. The method is applied to localized electrons within the one-dimensional half-filled Hubbard model yielding magnetic coupling constants that are in good agreement with reference Spin-Coupled calculations on finite size clusters and perturbation theory in the localized regime.

Non-orthogonal orbitals for localized electrons III. Application to periodic one dimensional lattices

Abstract A new hybrid method associating an effective spin hamiltonian with an approximate spin function is applied to an infinite s =1/2 magnetic chain. The effective spin hamiltonian is derived from the single configuration spin-coupled valence bond wave function built upon weakly overlapping non-orthogonal orbitals. The approximate spin function gives a zeroth order description of the short range spin correlation while insuring a proper translational symmetry. The variationally optimized magnetic coupling integrals are extracted using the long range Pariser–Parr–Pople model and the influence of the exchange and Coulomb interactions cut off is shown.