Extent Of Delocalization Research Articles

Probing magnetic interactions in columnar phases of a paramagnetic gold dithiolene complex

A novel radical gold dithiolene complex exhibits a hexagonal columnar mesophase, as confirmed by optical microscopy, DSC analysis and X-ray diffraction. The extent of delocalization of the spin density in such a complex was analyzed by EPR. Temperature dependent magnetization measurements reveal that the global magnetic moment is remarkably affected at the liquid-crystalline phase transition with a marked hysteresis signature, rare behavior among the few described paramagnetic discotic phases. In addition, these molecules were found to strongly aggregate in solution into one-dimensional fibers with a mean diameter of 60 nm extending over micrometres, leading to the formation of gel-like structures. These fibers are stable and can be isolated on surfaces. The gelation of the system can also be detected by temperature-dependent magnetic measurements.

Parallelism between charge redistribution and delocalization. Applications to organic reactions

The study is devoted to interrelations between two perturbative expansions for molecules and molecular systems, namely between terms of power series for one-electron density matrices on the one hand, and those for respective representation matrices of non-canonical (localized) molecular orbitals (MOs) on the other hand, as well as to the relevant implications. As the most outstanding example of the latter, simple proportionalities are established between alterations in populations of basis orbitals due to chemical interaction and delocalization coefficients of respective localized MOs (LMOs). The proof of these proportionalities is valid for members of power series to within the fifth order inclusive that were shown previously to be sufficient for investigations of the most important organic reactions. As a result, classical interpretations of early stages of these reactions in terms of shifts of respective localized pairs of electrons (cf. the so-called ‘curly arrow chemistry’) acquire a quantum-chemical support. Moreover, the results of the present study allow comparisons of relative extents of delocalization of LMOs for alternative routes of the same process. On this basis, predominant (allowed) routes of organic reactions are shown to be characterized by enhanced delocalization of respective principal pairs of electrons as compared to alternative (forbidden) routes. Raktažodžiai: localized molecular orbitals, Brillouin theorem, organic reactions, delocalization, curly arrow chemistry

Electronic Absorption Spectra of Some Triazolopyrimidine Derivatives

The electronic absorption spectra of triazolo pyrimidine and some of its derivatives were measured in polar as well as nonpolar solvents. Assignment of the observed transitions is facilitated via molecular orbital calculations. Charge density distributions, dipole moments, and the extent of delocalization of the MOS were used to interpret the observed solvent effects. The observed transitions are assigned as charge transfer (CT), localized, and delocalized according to the contribution of the various configurations in the CI‐states. The correspondence between the calculated and experimental transition energies is satisfactory.

Signatures of coherent vibrational energy transfer in IR and Raman line shapes for liquid water

We calculate theoretical IR and Raman line shapes for the OH stretch region of liquid water, using mixed quantum/classical and electronic-structure/molecular-dynamics methods. Our approach improves upon the time-averaging approximation used earlier for the same problem, and our results are in excellent agreement with experiment. Previous analysis of theoretical results for this problem considered the extent of delocalization (over local OH stretch excitations) of the instantaneous vibrational eigenstates. In this work we present a complementary analysis in the time-domain, by decomposing the appropriate response functions into diagonal and off-diagonal contributions (in the local mode basis). Our analysis indicates that all vibrational spectra show signatures of coherent vibrational energy transfer. This is manifest in different (IR, isotropic and depolarized Raman) experiments to different extents, because of the competition between coherent energy transfer and rotational disorder.

Calculations of intermode coupling constants and simulations of amide I, II, and III vibrational spectra of dipeptides

Amide I, II, and III vibrations of polypeptides are important marker modes whose vibrational spectra can provide critical information on structure and dynamics of proteins in solution. The extent of delocalization and vibrational properties of amide normal mode can be described by the amide local mode frequencies and intermode coupling constants between a pair of amide local modes. To determine these fundamental quantities, the previous Hessian matrix reconstruction method has been generalized here and applied to the density functional theory results for various dipeptide conformers. The calculation results are then used to simulate IR absorption, vibrational circular dichroism, and 2D IR spectra of dipeptides. The relationships between dipeptide backbone conformations and these vibrational spectra are discussed. It is believed that the present computational method and results will be of use to quantitatively simulate vibrational spectra of complicated polypeptides beyond simple dipeptides

Dynamical simulations of charged soliton transport in conjugated polymers with the inclusion of electron-electron interactions

We present numerical studies of the transport dynamics of a charged soliton in conjugated polymers under the influence of an external time-dependent electric field. All relevant electron-phonon and electron-electron interactions are nearly fully taken into account by simulating the monomer displacements with classical molecular dynamics and evolving the wave function for the pi electrons by virtue of the adaptive time-dependent density matrix renormalization group simultaneously and nonadiabatically. It is found that after a smooth turn on of the external electric field the charged soliton is accelerated at first up to a stationary constant velocity as one entity consisting of both the charge and the lattice deformation. An Ohmic region (6 mV/A</=E(0)</=12 mV/A) where the stationary velocity increases linearly with the electric field strength is observed. The relationship between electron-electron interactions and charged soliton transport is also investigated in detail. We find that the dependence of the stationary velocity of a charged soliton on the on-site Coulomb interactions U and the nearest-neighbor interactions V is due to the extent of delocalization of the charged soliton defect.

Stabilities and reactivities of the cyano-bridged binuclear complexes of trans -isonicotinamidetetraammineruthenium and hexacyanoferrate

The binuclear complexes, trans-(isn)(NH3)4RuNCFe (n = 2−, 1−, 0), were prepared both in aqueous solutions and as isolated salts. Their properties, particularly that of the mixed-valence species (M), were characterized by UV-Vis, infrared, electrochemical and kinetic studies. The results suggested that Ru(III)–Fe(II) was both thermodynamically and kinetically stable oxidation states. The extent of delocalization of M was estimated by analysis of the intervalence band on the basis of Hush's theory. The stabilization of M with respect to its oxidation state isomer (M′) was investigated and it was found that the electrostatic effect associated with the net charges of metal moieties constituted the dominant factor governing the stability of M.

Poly(thiophenylanilino) and poly(furanylanilino) polymers

Novel hybrid polymers with thiophenylanilino and furanylanilino backbones and substituted phenyl side groups are reported. The new monomers bis-(4-heterocyclic-2-yl-phenyl)-aryl-amine (heterocyclic = thiophen with aryl = 4-benzoyl (2a), 4-nitro-phenyl (2b) and furan with aryl = 4-benzoyl-phenyl (3a), 4-nitro-phenyl (3b)) were prepared by monosubstituting triphenylamine under electrophilic aromatic conditions affording 4-nitrotriphenylamine and 4-benzoyltriphenylamine. Di(bromination) of the latter compounds followed by Stille cross-coupling reactions with 2-tributylstannylthiophene or 2-tributylstannylfuran produces the new monomers 2a–b and 3a–b in high yield. The monomers are electrochemically polymerized at relatively low potentials (<0.8 V versus Ag+/AgCl) in acetonitrile electrolytes resulting in electroactive films. All the new polymers can be repeatedly oxidized and reduced with little loss of electrochemical activity. Vibrational spectroscopy reveals that the monomer units are connected predominately via coupling of the thiophenyl or furanyl rings yielding the novel polymers. Single-crystal molecular structure determinations of 4-nitrotriphenylamine and monomer 3b indicate the importance of the electron-withdrawing groups on the pendent phenyl groups in determining the extent of delocalization of the extended multi-ring systems. Molecular orbital calculations suggest that the HOMO of 2b is delocalized about both anilino and thiophenyl portions of the molecule.

Perturbations to the Geometric and Electronic Structure of the CuA Site: Factors that Influence Delocalization and Their Contributions to Electron Transfer

Using a combination of electronic spectroscopies and DFT calculations, the effect of pH perturbation on the geometric and electronic structure of the CuA site has been defined. Descriptions are developed for high pH (pH = 7) and low pH (pH = 4) forms of CuA azurin and its H120A mutant which address the discrepancies concerning the extent of delocalization indicated by multifrequency EPR and ENDOR data (J. Am. Chem. Soc. 2005, 127, 7274; Biophys. J. 2002, 82, 2758). Our resonance Raman and MCD spectra demonstrate that the low pH and H120A mutant forms are essentially identical and are the perturbed forms of the completely delocalized high pH CuA site. However, in going from high pH to low pH, a seven-line hyperfine coupling pattern associated with complete delocalization of the electron (S = 1/2) over two Cu coppers (I(Cu) = 3/2) changes into a four-line pattern reflecting apparent localization. DFT calculations show that the unpaired electron is delocalized in the low pH form and reveal that its four-line hyperfine pattern results from the large EPR spectral effects of approximately 1% 4s orbital contribution of one Cu to the ground-state spin wave function upon protonative loss of its His ligand. The contribution of the Cu-Cu interaction to electron delocalization in this low symmetry protein site is evaluated, and the possible functional significance of the pH-dependent transition in regulating proton-coupled electron transfer in cytochrome c oxidase is discussed.

Mixed valent sites in biological electron transfer

Many of the active sites involved in electron transfer (ET) in biology have more than one metal and are mixed valent in at least one redox state. These include Cu(A), and the polynuclear Fe-S clusters which vary in their extent of delocalization. In this tutorial review the relative contributions to delocalization are evaluated using S K-edge X-ray absorption, magnetic circular dichroism and other spectroscopic methods. The role of intra-site delocalization in ET is considered.

Synthesis, Luminescence Properties, and Explosives Sensing with 1,1-Tetraphenylsilole- and 1,1-Silafluorene-vinylene Polymers

The syntheses, spectroscopic characterizations, and fluorescence quenching efficiencies of polymers and copolymers containing tetraphenylsilole- or silafluorene-vinylene repeat units are reported. These materials were prepared by catalytic hydrosilylation reactions between appropriate monomeric metallole alkynes and hydrides. Trimeric model compounds methyl(tetraphenyl)silole-vinylene trimer (1), methyl(tetraphenyl)silole-silafluorene-vinylene cotrimer (2), and methylsilafluorene-vinylene trimer (3) were synthesized to provide detailed structural and spectroscopic characteristics of the polymer backbone and to assess the extent of delocalization in the luminescent excited state. Poly(tetraphenylsilole-vinylene) (4), poly(tetraphenylsilole-silafluorene-vinylene) (5), and poly(silafluorene-vinylene) (6) maintain a regio-regular trans-vinylene Si−C backbone with possible ground state σ*−π and excited state σ*−π* conjugation through the vinylene bridge between metallole units. Fluorescence spectra of the polymers show an ∼13 nm bathochromic shift in λflu from their respective model compounds. Molecular weights (Mn) for these polymers and copolymers are in the range of 4000–4500. Detection of nitroaromatic explosives by solution-phase fluorescence quenching of polymers 4−6 was observed with Stern−Volmer constants in the range of 400−20 000 for TNT, DNT, and picric acid (PA). A surface detection method for the analysis of solid particulates of TNT, DNT, PA, RDX, HMX, Tetryl, TNG, and PETN is also described for silafluorene-containing polymers. Polymer 6 exhibited detection for all the preceding types of explosive residues with a 200 pg cm−2 detection limit for Tetryl. Polymers 4 and 5 exhibited only luminescence quenching with nitroaromatic explosives, revealing that the excited-state energy of the sensor plays a key role in the fluorescence detection of explosives.

The Geometric and Electronic Structure of a One‐Electron‐Oxidized Nickel(II) Bis(salicylidene)diamine Complex

Class III delocalization? The isolation and analysis of the ligand-radical title complex has allowed description of the changes in bonding from the reduced form as well as the extent of delocalization. The intense low-energy absorption of the oxidized complex can be described as a ligand-radical transition of a highly delocalized class III mixed-valence compound. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2007/z701194_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Cyclobutadiene: The Antiaromatic Paradigm?

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Design and preparation of neutral substituted fluorene- and carbazole-based platinum(ii)–acetylide complexes

Various combinations of mono- or diethynyl-substituted fluorene or carbazole building blocks were connected via a σ-bonded ethynyl linkage to ortho-metallated Pt(II) fragments, giving rise to phosphorescent mono- and dinuclear complexes for which the solubility and extent of delocalization could be tuned by the chemistry on the acidic methylene position of the fluorene.

Cyclobutadiene: The Antiaromatic Paradigm?

Cyclobutadiene: The Antiaromatic Paradigm?

Time-Domain Calculations of the Polarized Raman Spectra, the Transient Infrared Absorption Anisotropy, and the Extent of Delocalization of the OH Stretching Mode of Liquid Water

The polarized Raman spectrum and the time dependence of the transient infrared (TRIR) absorption anisotropy are calculated for the OH stretching mode of liquid water (neat liquid H2O) by using time-domain formulations, which include the effects of both the diagonal frequency modulations (of individual oscillators) induced by the interactions between the dipole derivatives and the intermolecular electric field, and the off-diagonal (intermolecular) vibrational coupling described by the transition dipole coupling (TDC) mechanism. The IR spectrum of neat liquid H2O and the TRIR anisotropy of a liquid mixture of H2O/HDO/D2O are also calculated. It is shown that the calculated features of these optical signals, including the temperature dependence of the polarized Raman and IR spectra, are in reasonable agreement with the experimental results, indicating that the frequency separation between the isotropic and anisotropic components of the polarized Raman spectrum and the rapid decay (approximately 0.1 ps) of the TRIR anisotropy of the OH stretching mode of neat liquid H2O are mainly controlled by the resonant intermolecular vibrational coupling described by the TDC mechanism. Comparing with the time evolution of vibrational excitations, it is suggested that the TRIR anisotropy decays in the time needed for the initially localized vibrational excitations to delocalize over a few oscillators. It is also shown that the enhancement of the dipole derivatives by the interactions with surrounding molecules is an important factor in generating the spectral profiles of the OH stretching Raman band. The time-domain behavior of the molecular motions that affect the spectroscopic features is discussed.

Effects of Intermolecular Vibrational Coupling and Liquid Dynamics on the Polarized Raman and Two-Dimensional Infrared Spectral Profiles of Liquid N,N-Dimethylformamide Analyzed with a Time-Domain Computational Method

A time-domain method for calculating polarized Raman and two-dimensional infrared (2D-IR) spectra that includes the effects of both the diagonal frequency modulations (of individual molecules in the system) and the off-diagonal (intermolecular) vibrational coupling is presented and applied to the case of the amide I band of liquid N,N-dimethylformamide. It is shown that the effect of the resonant off-diagonal vibrational coupling and the resulting delocalization of vibrational modes is clearly seen as the noncoincidence effect in the polarized Raman spectrum and some spectral features (especially as asymmetric intensity patterns) in the 2D-IR spectra. The type of 2D-IR spectra (concerning the polarization condition) most appropriate for observing this effect is discussed. On the basis of the agreement between the observed and calculated band profiles of the polarized Raman spectrum, the time dependence of the transient IR absorption anisotropy is also calculated. The method of evaluating the extent of delocalization of vibrational modes that is relevant to the features of these optical signals in the time and frequency domains is discussed. The nature of the molecular motions (concerning the liquid dynamics) that are effective on the diagonal frequency modulations is also examined.

Visualization and Characterization of the Infrared Active Amide I Vibrations of Proteins

To facilitate the analysis of frequency-structure correlations in the amide I vibrational spectroscopy of proteins, we investigate visualization methods and spatial correlation functions that describe delocalized vibrations of proteins and protein secondary structures. To study those vibrational modes revealed in infrared spectroscopy, we characterize frequency-dependent bright states obtained from doorway mode analysis. Our visualization methods pictorially color code amplitude and phase of each oscillator within the structure to reveal spatially varying patterns characteristic of excitations within sheets and helices. Spatial correlation functions in the amplitude and phase of amide I oscillators quantitatively address the extent of delocalization and the alpha helical and beta sheet character of these modes. Specifically, we investigate the vibrations of idealized antiparallel beta sheets and alpha helices and perform case studies on three proteins: concanavalin A, myoglobin, and ubiquitin.

Diffusion Monte Carlo approaches for investigating the structure and vibrational spectra of fluxional systems

Recent advances in diffusion Monte Carlo (DMC) are reviewed within the context of the vibrational motions of systems that undergo large amplitude motions. Specifically, the authors describe the DMC approach for obtaining the ground state wave function and zero-point energy (ZPE) of the system of interest, as well as extensions to the method for evaluating probability amplitudes, rotational constants, vibrationally excited states and methods for obtaining vibrational spectra. The discussion is framed in terms of the properties of several systems of current experimental and theoretical interest, specifically complexes of neon atoms with OH or SH, , , and . The results of the DMC simulations provide the information necessary to characterize the extent of delocalization of the probability amplitudes, even in the ground vibrational states. Methods for evaluating expectation values and vibrationally excited states are explored, and, when possible, the results are compared with those from other approaches. Finally, the methods for evaluating intensities are described and existing and future challenges for the approach are reviewed. Contents PAGE 1. Introduction 78 2. Systems 79 2.1. Nen · XH complexes 79 2.2. and 80 2.3. 83 3. Ground state wave functions and energies 84 3.1. Theory 84 3.2. Results 86 4. Obtaining properties from DMC 89 4.1. Averaging by Pair Counting (AVPC) 89 4.2. Adiabatic DMC 89 4.3. Descendent weighting 91 4.4. Example 1 – Bond lengths in 91 4.5. Example 2 – Bond length distributions for and 91 4.6. Example 3 – Rotational constants 93 5. Excited states 96 5.1. Example 4 – The stretch fundamental in Ar3 99 5.2. Example 5 – The fundamentals in Ne2XH 100 5.3. Example 6 – Fundamental vibrations in and 102 6. Using DMC to interpret spectra 103 7. Summary and future prospects 104 Acknowledgements 105 References 105

Time-Domain Theoretical Analysis of the Noncoincidence Effect, Diagonal Frequency Shift, and the Extent of Delocalization of the CO Stretching Mode of Acetone/Dimethyl Sulfoxide Binary Liquid Mixtures

A time-domain method for simulating vibrational band profiles that simultaneously takes into account both the diagonal and off-diagonal effects is developed and applied to the C=O stretching bands of neat liquid acetone and the acetone/dimethyl sulfoxide (DMSO) binary liquid mixtures. By using this method, it is possible to examine the influence of liquid dynamics on the noncoincidence effect (NCE), which arises from the off-diagonal vibrational interactions, as well as the frequency shifts and band broadening, which are related to both the diagonal and off-diagonal effects. It is shown that the simulations for the C=O stretching bands of acetone in acetone/DMSO binary liquid mixtures on the basis of this method can reproduce the experimentally observed concave curvature of the concentration dependence of the NCE and the unusually large frequency shift of the anisotropic Raman band. The widths of the infrared, isotropic Raman, and anisotropic Raman bands calculated for neat liquid acetone are also in good agreement with those observed. Based on these calculations, the extent of delocalization of the C=O stretching vibrational motions is examined by referring to two quantitative measures of this property, one calculated in the frequency domain and the other in the time domain. It is shown that the extent of delocalization gets larger as the mole fraction of acetone increases, the C=O stretching vibrations being delocalized over a few tens of molecules in neat liquid acetone. It is also shown that the extent of delocalization is related to the quantity called NCE detectability, which is the ratio between the magnitude of NCE and the bandwidth. It is therefore suggested that the extent of delocalization of vibrational motions may be estimated from observable features of Raman band profiles.