Intramolecular Coulomb Interactions Research Articles

Role of Dynamical Electron Correlation in the Differences in Bonding between CaAlH3 and MgAlH3.

The most important factor behind the intriguing differences between the geometries of the M′AlH3 (M′ = Mg, Ca) molecules is shown to be dynamical electron correlation and not intramolecular Coulombic interactions, as previously thought. Spin-coupled generalized valence bond (SCGVB) calculations reveal the different bonding situations in the two molecules at their optimal geometries but do not explain why these geometries differ so much; the solution to this conundrum comes instead from detailed analysis of coupled-cluster (CCSD(T)) energies at model and optimal geometries.

Many-Body State Description of Single-Molecule Electroluminescence Driven by a Scanning Tunneling Microscope.

Electron transport and optical properties of a single molecule in contact with conductive materials have attracted considerable attention because of their scientific importance and potential applications. With the recent progress in experimental techniques, especially by virtue of scanning tunneling microscope (STM)-induced light emission, where the tunneling current of the STM is used as an atomic-scale source for induction of light emission from a single molecule, it has become possible to investigate single-molecule properties at subnanometer spatial resolution. Despite extensive experimental studies, the microscopic mechanism of electronic excitation of a single molecule in STM-induced light emission has yet to be clarified. Here we present a formulation of single-molecule electroluminescence driven by electron transfer between a molecule and metal electrodes based on a many-body state representation of the molecule. The effects of intramolecular Coulomb interaction on conductance and luminescence spectra are investigated using the nonequilibrium Hubbard Green's function technique combined with first-principles calculations. We compare simulation results with experimental data and find that the intramolecular Coulomb interaction is crucial for reproducing recent experiments for a single phthalocyanine molecule. The developed theory provides a unified description of the electron transport and optical properties of a single molecule in contact with metal electrodes driven out of equilibrium, and thereby, it contributes to a microscopic understanding of optoelectronic conversion in single molecules on solid surfaces and in nanometer-scale junctions.

Extended Bis(anthraoxa)quinodimethanes with Nine and Ten Consecutively Fused Six-Membered Rings: Neutral Diradicaloids and Charged Diradical Dianions/Dications.

We report the challenging synthesis of two very long bis(anthraoxa)quinodimethanes with nine (ABA) and ten (ANA) consecutively fused six-membered rings. The former is stable with negligible diradical character, while the latter with a moderate diradical character ( y0 = 25.0%) is reactive and an unexpected trifluoroacetic substituted product (ANA-TFA) was isolated. X-ray crystallographic analysis revealed a planar backbone with a typical quinoidal character for both. Their dications can be regarded as the isoelectronic structures of the respective nonacene and decacene. The dication ABA2+ and dianion ABA2- are open-shell singlet diradicaloids, while the longer dication ANA-TFA2+ and dianion ANA2- have closed-shell ground state, which can be explained by the different intramolecular Coulomb interactions. Both dianions have a bent backbone and can be considered as an isoelectronic structure of the tetraanion of nonacene and decacene, respectively.

Intermolecular coupling and superconductivity in PbMo6S8 and other Chevrel phase compounds

To understand superconductivity in Chevrel phase compounds and guide the search for interesting properties in materials created with Chevrel phase molecules as building blocks, we use ab-initio methods to study the properties of single Mo$_6$X$_8$ molecules with $X=S$, $Se$, $Te$ as well as the bulk solid PbMo$_6$S$_8$. In bulk PbMo$_6$S$_8$, the different energy scales from strong to weak are: the band kinetic energy, the intra-molecular Coulomb interaction, the on-molecule Jahn-Teller energy and the Hund's exchange coupling. The metallic state is stable with respect to Mott and polaronic insulating states. The bulk compound is characterized by a strong electron-phonon interaction with the largest coupling involving phonon modes with energies in the range from 11 meV to 17 meV and with a strong inter-molecule (Peierls) character. A two-band Eliashberg equation analysis shows that the superconductivity is strong-coupling, with different gaps on the two Fermi surface sheets. A Bergman-Rainer analysis of the functioanl derivative of the transition temperature with respect to the electron-phonon coupling reveals that the Peierls modes provide the most important contribution to the superconductivity. This work illustrates the importance of inter-molecular coupling for collective phenomena in molecular solids.

Reactivity of co-micellar systems based on dimeric functionalized tetraalkylammonium surfactant in phosphoryl and sulfonyl group transfer processes

The reactivity of co-micellar systems based on dimeric functionalized tetraalkylammonium surfactant in phosphoryl and sulfonyl group transfer processes is lower than the reactivity of systems based on analogous imidazolium surfactant. The observed difference in the kinetics of acyl group transfer promoted by dimeric surfactants differing by the nature of the cationic center is likely to be general, and it results from both different nucleophilicities of their oximate fragments and different modes of solubilization of hydrophobic substrates. The most probable reasons for the latter are differences in intramolecular Coulomb interactions and packing of surfactant molecules in co-micelles.

Effects of Counterions with Multiple Charges on the Linear and Nonlinear Optical Properties of Polymethine Salts

Counterions with multiple charges have been used to organize polymethines into multichromophore salt complexes. The intramolecular Coulombic interactions between multiply charged counterions and polymethines can play a significant role in modifying intermolecular interactions (i.e., aggregation). Here, we report a detailed study of the linear and nonlinear optical properties of such complexes with dications and hexacations over a large range of molecular concentrations. Our results have demonstrated that, despite strong intramolecular interactions, the preorganization of chromophores into the multichromophore salt constructs with multiple charges can provide a steric repulsive effect that can mitigate intermolecular interactions in the solid state. This results in a more efficient translation between microscopic and macroscopic optical properties for highly polarizable polymethines, which is essential for all optical signal switching.

Calcium Ions to Remotely Control the Reversible Switching of Secondary and Quaternary Structures in Bioconjugates

A biomimetic strategy to modulate inter- and intramolecular Coulomb interactions of peptides allows for the reversible regulation of functions of peptide–polymer conjugates. Calcium ions are exploited as triggers to screen peptide charges, thereby switching the peptide secondary structure. This approach results in a reversible control mechanism for bioconjugate self-assembly. Disassembly is feasible by regulating the Ca2+ levels using competitive Ca2+ binders.

ChemInform Abstract: Use of Intramolecular Coulombic Interactions to Achieve “Impossible” Reactions. Photochemical Cleavage of 4-Nitrophenyl Ethers.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Mesoscale hydrodynamic simulation of short polyelectrolytes in electric fields

The dynamical, conformational, and transport properties of short flexible polyelectrolytes are studied in salt-free solution under the influence of an external electric field taking hydrodynamic interactions into account. A coarse-grained polymer model is applied and the multiparticle collision dynamics approach is adopted to account for the solvent. We consider various Coulomb interaction strengths and polymer lengths. The scaling behavior of the diffusion coefficient with respect to polymer length strongly depends on the charge interaction strength due to changes in the polyelectrolyte conformations by counterion condensation. The diffusion coefficients at weak Coulomb interaction strengths can well be described by the expression of rodlike objects, whereas at stronger electrostatic interactions, we observe a Zimm-like behavior. The polyelectrolyte mobilities obey the Nernst-Einstein relation at weak charge interactions. For large interaction strengths and long polymers, the mobilities are independent of molecular weight because of the screening of the intramolecular Coulomb interactions by counterions. Our studies demonstrated that the dynamics of polyelectrolytes in dilute solution and in an external field is governed by an interplay of hydrodynamic interactions and counterion condensation.

Correlation mechanism of negative differential resistance for tunneling through a molecule

A model for electron transport through a single molecule with two electronic states, one of which is strongly hybridized with contacts and the other is localized, has been investigated. A solution for this model in the limit of strong intramolecular Coulomb interaction is found. A mechanism of negative differential resistance caused by strong intramolecular correlations is revealed.

The influence of intramolecular charge redistribution at the photoexcitation stage on the rate of subsequent electron transfer

The two-stage model of photoinduced electron transfer includes an explicit description of photoexcitation and nonradiative charge transfer. The model takes into account intramolecular reorganization and Coulomb interaction between the electron being transferred and medium polarization fluctuations. We studied the influence of intramolecular electron density redistribution on the rate of photoinduced electron transfer in donor-acceptor complexes. Excitation pulse frequency and donor dipole moment variations in the excited state changed the nonthermal and thermal electron transfer rates by 2–3 orders of magnitude.

Molecular terms, magnetic moments, and optical transitions of molecular ions C60m±

Starting from a multipole expansion of intramolecular Coulomb interactions, we present configuration interaction calculations of the molecular energy terms of the hole configurations (hu+)m, m=2–5, of C60m+ cations, of the electron configurations t1un, n=2–4, of the C60n− anions, and of the exciton configurations (hu+t1u−), (hu+t1g−) of the neutral C60 molecule. The ground state of C602− is either T1g3 or Ag1, depending on the energy separation between t1g and t1u levels. There are three close (∼0.03 eV) low lying spin triplets T1g3, Gg3, T2g3 for C602+, and three spin quartets T1u4, Gu4, T2u4 for C603+, which can be subjected to the Jahn–Teller effect. The number of low lying nearly degenerate states is largest for m=3 holes. We have calculated the magnetic moments of the hole and electron configurations and found that they are independent of molecular orientation with respect to an external magnetic field. The coupling of spin and orbital momenta differs from the atomic case. We analyze the electronic dipolar transitions (t1u)2→t1ut1g and (t1u)3→(t1u)2t1g for C602− and C603−. Three optical absorption lines (3T1g→3Hu, 3T1u, 3Au) are found for the ground level of C602− and only one line (4Au→4T1g) for the ground state of C603−. We compare our results with the experimental data for C60n− in solutions and with earlier theoretical studies.

Protein molecular dynamics with electrostatic force entirely determined by a single Poisson-Boltzmann calculation.

The electrostatic force including the intramolecular Coulombic interactions and the electrostatic contribution of solvation effect were entirely calculated by using the finite difference Poisson-Boltzmann method (FDPB), which was incorporated into the GROMOS96 force field to complete a new finite difference stochastic dynamics procedure (FDSD). Simulations were performed on an insulin dimer. Different relative dielectric constants were successively assigned to the protein interior; a value of 17 was selected as optimal for our system. The simulation data were analyzed and compared with those obtained from 500-ps molecular dynamics (MD) simulation with explicit water and a 500-ps conventional stochastic dynamics (SD) simulation without the mean solvent force. The results indicate that the FDSD method with GROMOS96 force field is suitable to study the dynamics and structure of proteins in solution if used with the optimal protein dielectric constant.

Heteroorganic betaines. 8.* Synthesis and structures of silicon- and germanium-containing organoarsenic betaines R13As+-CR2R3-EMe2-S- (E = Si,Ge)

The reactions of ylides R13As=CHR2 with hexamethyl-2,4,6-trisila- and hexamethyl-2,4,6-trigermatrithiacyclohexanes afforded betaines R13As+—CHR2—SiMe2—S– (2) (R1 = Et; R2 = Ph (a), Me3Si (b); R1 = R2 = Ph (c)) and Et3As+—CH(SiMe3)—GeMe2—S– (3), respectively. Betaines 2a,b and 3 were characterized by multinuclear NMR spectroscopy. According to the X-ray diffraction data, in the crystals the As+—C—E—S– main chain (E = Si or Ge) of molecules 2a,b and 3 adopts a twisted cis conformation due to strong intramolecular Coulomb interactions between the anionic and cationic centers. The equilibrium geometries of isolated molecules 2a and 3, which were calculated within the framework of the density functional theory (the PBE functional, the TZ2P basis set), are in qualitative agreement with the X-ray data. In solutions, betaines 2a (in the absence of Li salts) and 2c (in the presence of LiBr) selectively decomposed according to the Corey—Chaykovsky reaction, which was accompanied by elimination of R3As and, probably, the intermediate formation of silathiirane. The subsequent transformation of the latter afforded 2,2,4,4-tetramethyl-5-phenyl-2,4-disila-1,3-dithiolane.

A model for the electronic structures of β and κ-(BEDT-TTF) 2X

A simple microscopic model is obtained for β and κ-(BEDT-TTF) 2 X starting from the PPP model with hyperconjugation of BEDT-TTF. UHF calculations for clusters with intramolecular electron Coulomb interaction show that only HOMOs are important in the crystals and for most value of the screened electron Coulomb interaction in the HOMO, U, the systems can be represented by dimers consisting of Hubbard type units. The dimers arrange in triangular lattices. The gap in a dimer is small but the electron correlation measured by U t is very strong.

The electrostatic contribution to the B to Z transition of DNA.

In this paper, the finite difference nonlinear Poisson-Boltzmann (NLPB) equation is used to calculate the electrostatic contribution to the B to Z transition of DNA using detailed molecular structures of each DNA form. The electrostatic transition free energy is described as a balance between the change in intramolecular Coulombic interactions and charge-dependent interactions between the DNA and the solvent. As in many prior studies, we find that the larger electrostatic repulsions among the more closely spaced Z-DNA phosphates destabilize this form compared to B-DNA in the absence of solvent. However, as a result of the more compact three-dimensional geometry of Z-DNA, both water and salt are found to strongly stabilize this conformation to the extent that the total electrostatic free energy favors the B to Z transition in aqueous solution. Water acts not only by screening the inter-phosphate repulsions but also by solvating both charged and polar groups on Z-DNA more favorably than B-DNA. In addition, Z-DNA is stabilized by a substantially higher concentration of nearby counterions than B-DNA. The relative stabilization of Z-DNA by salt increases with increasing bulk salt concentration, leading to the high-salt B to Z transition. We find that the salt dependence of the B to Z transition free energy calculated with the NLPB equation agrees reasonably well with experimental results. Since electrostatic interactions are found to favor the Z-form, nonelectrostatic forces must be responsible for the relative stability of B-DNA in solution. An analysis of these forces suggests that the conformational entropy may play an important role.

Use of intramolecular coulombic interactions to achieve impossible reactions. Photochemical cleavage of 4-nitrophenyl ethers

Use of intramolecular coulombic interactions to achieve impossible reactions. Photochemical cleavage of 4-nitrophenyl ethers

Zwitterionic polymers—III. Salt effects on the solubility of poly(vinyl sulphobetaines) and poly(vinyl betaines) in aqueous solution

Critical salt concentrations were measured for dissolution of poly[1-vinyl-3-(2-sulphoethyl)imidazolium betaine] and poly[1-vinyl-3-carboxy-ethylimidazolium betaine] in aqueous solutions. It was found that the critical salt concentration varies considerably with the nature of the anion and follows a Hofmeister series. The differences within a series of cations were rather small and only by changing a metal cation to a tetraalkylammonium cation was there a considerable change in critical salt concentration. These results are interpreted in terms of differences in hydration of the individual charges of the zwitterionic polymer. By comparison of the critical salt concentrations within a series of poly(vinyl sulphobetaines) with different numbers of methylene units between the charges, evidence was obtained for the operation of intramolecular Coulombic interactions as the dominating factor in the solubility behaviour of the zwitterionic polymers in aqueous solutions.

A small U approach to the 4k F reflections in organic linear chain compounds

It is shown that a lattice distortion of wavevector 4 k F with an essentially one-dimensional correlation occurs in a one-dimensional metal when the intramolecular Coulomb interaction U is small compared with the band-width 4 t; ( U 4t ⋍ 1 4 ). This one-dimensional property suppresses the real phase-transition temperature well below the mean-field transition, where fluctuative 4 k F distortions build up. This accounts for the 4 k F reflections observed in TTF-TCNQ and several other charge transfer compounds. The strong locking of the electrons to the lattice when the band is 1 4 (or 1 2 ) full, and weak locking when the band is 1 3 (or 2 3 ) full, are also accounted for.

Charge transfer in electrostatically coupled chains: Effects of the nearest neighbor Coulomb correlations

The effects of the nearest neighbor Coulomb correlations on the character of the charge transfer between donor and acceptor molecular chains are investigated. We use a modified Hubbard model for the individual donor and acceptor chains. In this model the intramolecular Coulomb interactions are taken to be infinitely large, and the transfer integrals are considered to be infinitely small. All the intermolecular interactions except those between the nearest neighbors along the chains are treated within the mean field approximation. The free energy of this model can be calculated for an arbitrary strength of the nearest neighbor interaction and results are given for the temperature dependences of the charge transfer and the zero field paramagnetic spin susceptibility.