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Journal ArticleDOI

Radical cations from dipyridinium derivatives: A combined EPR and DFT study

TL;DR: The monoelectronic reduction of 1,1'-dimethyl-2,2'-dicyano-4,4'-bipyridinium (DCMV++) bis-methylsulphate gave the signal of the corresponding radical cation, whose interpretation has been carried out with the aid of density functional theory (DFT) calculations run at different levels.
About: This article is published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.The article was published on 2006-06-01. It has received 8 citations till now. The article focuses on the topics: Dication & Radical ion.
Citations
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Journal ArticleDOI
TL;DR: In this paper, 1,1,1′-Bis (1-methyl pyridinium-2-yl)-4,4′-dipyrinium dichloride di-iodide (TPy) and 1, 1′-dimethyl-4, 4′-Dipyrithinium di-ideal (DPy) were synthesized and used as corrosion inhibitors for ferritic type 430 stainless steel in 0.5 M H 2 SO 4 solution.

21 citations

Journal ArticleDOI
TL;DR: In this paper, the authors examined the attenuation coefficient β for a series of molecules which contain a central conjugated moiety (phenyl, viologen or α-terthiophene) connected on either side to alkane chains of varying length, with each end terminated by thiol or thiomethyl anchor groups.
Abstract: If the factors controlling the decay in single-molecule electrical conductance G with molecular length L could be understood and controlled, then this would be a significant step forward in the design of high-conductance molecular wires. For a wide variety of molecules conducting by phase coherent tunnelling, conductance G decays with length following the relationship G = Ae−βL. It is widely accepted that the attenuation coefficient β is determined by the position of the Fermi energy of the electrodes relative to the energy of frontier orbitals of the molecular bridge, whereas the terminal anchor groups which bind to the molecule to the electrodes contribute to the pre-exponential factor A. We examine this premise for several series of molecules which contain a central conjugated moiety (phenyl, viologen or α-terthiophene) connected on either side to alkane chains of varying length, with each end terminated by thiol or thiomethyl anchor groups. In contrast with this expectation, we demonstrate both experimentally and theoretically that additional electronic states located on thiol anchor groups can significantly decrease the value of β, by giving rise to resonances close to EF through coupling to the bridge moiety. This interplay between the gateway states and their coupling to a central conjugated moiety in the molecular bridges creates a new design strategy for realising higher-transmission molecular wires by taking advantage of the electrode–molecule interface properties.

19 citations

Journal ArticleDOI
TL;DR: Differences in the resonant Raman and UV/vis absorption spectra of two diradicaloïd compounds are attributed to the presence in the experimental spectrum of surface-enhanced effects due to adsorption on the electrodes and signatures of the extension of the effective conjugation length as well as of the increase in Diradical character.
Abstract: The resonant Raman and UV/vis absorption spectra of two diradicaloid compounds, methyl viologen and phenylene-extended viologen in their neutral state, have been simulated using multiconfigurational wavefunction methods. For methyl viologen, a good agreement with experiment is evidenced for the UV/vis absorption vibronic structure, provided dynamic correlation is accounted for to get the vibrational frequencies and normal modes. To some extent, the agreement with experiment is also good for the RR spectrum and the differences have been attributed to the presence in the experimental spectrum of surface-enhanced effects due to adsorption on the electrodes. As a result of inserting a phenylene group between the pyridinium units, the simulations have demonstrated that (i) in the UV/vis absorption spectrum, the relative intensity of the second band with respect to the 0–0 band increases, (ii) additional strong bands are observed in the RR spectrum, and (iii) the RR excitation profiles of the phenylene-extended viologen present less structure than in the case of methyl viologen where the relative mode intensities can strongly depend on the incident light wavelength. These differences are signatures of the extension of the effective conjugation length as well as of the increase in diradical character.

17 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used the LC-BLYP range-separated hybrid exchange-correlation functional with different range separating parameters (μ) to simulate the UV/vis absorption spectra and resonance Raman spectra of methyl viologens and p-phenylene-extended Viologens.
Abstract: The UV/vis absorption spectra and resonance Raman spectra of methyl viologens and p-phenylene-extended viologens have been simulated by using the LC-BLYP range-separated hybrid exchange-correlation functional with different range-separating parameters (μ). In most cases, the shape of the UV/vis absorption bands and the relative Raman intensities, which find their common origins in the geometrical relaxation upon electronic excitations, depend strongly on the μ value: the larger the μ values, the more long-range Hartree–Fock (HF) exchange, the larger the geometrical relaxations, and the more complex the UV/vis absorption bands, whereas the resonance Raman signatures depend more on the presence or not of a dominating normal coordinate of relaxation. For the first excited state of the methyl viologen/p-phenylene-extended viologen radical cations, the μ values matching best the experimental UV/vis spectra are 0.03/0.13 bohr–1, whereas for the second excited state, they amount to 0.28–0.33/0.18 bohr–1. The fir...

12 citations

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TL;DR: It is demonstrated both experimentally and theoretically that additional electronic states located on thiol anchor groups can significantly decrease the value of β, by giving rise to resonances close to EF through coupling to the bridge moiety.
Abstract: If the factors controlling the decay in single-molecule electrical conductance G with molecular length L could be understood and controlled, then this would be a significant step forward in the design of high-conductance molecular wires. For a wide variety of molecules conducting by phase coherent tunneling, conductance G decays with length following the relationship G = Aexp-\b{\eta}L. It is widely accepted that the attenuation coefficient \b{\eta} is determined by the position of the Fermi energy of the electrodes relative to the energy of frontier orbitals of the molecular bridge, whereas the terminal anchor groups which bind to the molecule to the electrodes contribute to the pre-exponential factor A. We examine this premise for several series of molecules which contain a central conjugated moiety (phenyl, viologen or {\alpha}-terthiophene) connected on either side to alkane chains of varying length, with each end terminated by thiol or thiomethyl anchor groups. In contrast with this expectation, we demonstrate both experimentally and theoretically that additional electronic states located on thiol anchor groups can significantly decrease the value of \b{eta}, by giving rise to resonances close to EF through coupling to the bridge moiety. This interplay between the gateway states and their coupling to a central conjugated moiety in the molecular bridges creates a new design strategy for realising higher-transmission molecular wires by taking advantage of the electrode-molecule interface properties.

8 citations

References
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Journal ArticleDOI
TL;DR: In this article, a semi-empirical exchange correlation functional with local spin density, gradient, and exact exchange terms was proposed. But this functional performed significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
Abstract: Despite the remarkable thermochemical accuracy of Kohn–Sham density‐functional theories with gradient corrections for exchange‐correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact‐exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange‐correlation functional containing local‐spin‐density, gradient, and exact‐exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first‐ and second‐row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.

87,732 citations

Journal ArticleDOI
TL;DR: Numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, show that density-functional formulas for the correlation energy and correlation potential give correlation energies within a few percent.
Abstract: A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.

84,646 citations

Book
01 Jan 1989
TL;DR: In this paper, a review of current studies in density functional theory and density matrix functional theory is presented, with special attention to the possible applications within chemistry, including the concept of an atom in a molecule, calculation of electronegativities from the Xα method, pressure, Gibbs-Duhem equation, Maxwell relations and stability conditions.
Abstract: Current studies in density functional theory and density matrix functional theory are reviewed, with special attention to the possible applications within chemistry. Topics discussed include the concept of electronegativity, the concept of an atom in a molecule, calculation of electronegativities from the Xα method, the concept of pressure, Gibbs-Duhem equation, Maxwell relations, stability conditions, and local density functional theory.

14,008 citations

Journal ArticleDOI
TL;DR: In this paper, an analysis of the performances of a parameter free density functional model (PBE0) obtained combining the so-called PBE generalized gradient functional with a predefined amount of exact exchange is presented.
Abstract: We present an analysis of the performances of a parameter free density functional model (PBE0) obtained combining the so called PBE generalized gradient functional with a predefined amount of exact exchange. The results obtained for structural, thermodynamic, kinetic and spectroscopic (magnetic, infrared and electronic) properties are satisfactory and not far from those delivered by the most reliable functionals including heavy parameterization. The way in which the functional is derived and the lack of empirical parameters fitted to specific properties make the PBE0 model a widely applicable method for both quantum chemistry and condensed matter physics.

13,411 citations

Journal ArticleDOI
TL;DR: In this paper, a new implementation of the conductor-like screening solvation model (COSMO) in the GAUSSIAN94 package is presented, which allows Hartree−Fock (HF), density functional (DF) and post-HF energy, and HF and DF gradient calculations: the cavities are modeled on the molecular shape, using recently optimized parameters, and both electrostatic and nonelectrostatic contributions to energies and gradients are considered.
Abstract: A new implementation of the conductor-like screening solvation model (COSMO) in the GAUSSIAN94 package is presented. It allows Hartree−Fock (HF), density functional (DF) and post-HF energy, and HF and DF gradient calculations: the cavities are modeled on the molecular shape, using recently optimized parameters, and both electrostatic and nonelectrostatic contributions to energies and gradients are considered. The calculated solvation energies for 19 neutral molecules in water are found in very good agreement with experimental data; the solvent-induced geometry relaxation is studied for some closed and open shell molecules, at HF and DF levels. The computational times are very satisfying: the self-consistent energy evaluation needs a time 15−30% longer than the corresponding procedure in vacuo, whereas the calculation of energy gradients is only 25% longer than in vacuo for medium size molecules.

7,616 citations