Quantum chemical investigations of charge transfer interactions in relation to the electronic theory of cancer. IV. The interaction of formamide and the enol tautomers of several glyoxals

The results of ab initio “supermolecule” calculations of the charge transfer between formamide and the enol forms of methylglyoxal, ethylglyoxal, dimethylglyoxal, and propenylglyoxal are compared for several different conformations of the constituent molecules. The enols were found to be poorer electron acceptors than their respective keto isomers.     

Modelling charge transfer reactions with the frozen density embedding formalism

The frozen density embedding (FDE) subsystem formulation of density-functional theory is a useful tool for studying charge transfer reactions. In this work charge-localized, diabatic states are generated directly with FDE and used to calculate electronic couplings of hole transfer reactions in two π-stacked nucleobase dimers of B-DNA: 5'-GG-3' and 5'-GT-3'. The calculations rely on two assumptions: the two-state model, and a small differential overlap between donor and acceptor subsystem densities. The resulting electronic couplings agree well with benchmark values for those exchange-correlation functionals that contain a high percentage of exact exchange. Instead, when semilocal GGA functionals are used the electronic couplings are grossly overestimated.     

The interaction of chemical bonds. III. Perturbed strictly localized geminals in LMO basis

The formalism of strictly localized geminals (SLGs ) is summarized. It is shown that the SLG wave function serves as an appropriate multiconfigurational reference state that can easily be improved by perturbational, CI - or coupled cluster-type procedures. The possibility of expanding the geminals in the basis set of localized Hartree-Fock molecular orbitals (LMOs ) is discussed. Sample calculations on H₄, CH₄, H₂O, and He…?He systems are reported. © 1994 John Wiley & Sons, Inc.     

The charge capacitance of the chemical bond: Application to bonds containing metals

The charge capacitance of metal containing complexes are studied. For molecules with multiple bonding between the metal atoms it is found that the charge capacitance is correlated to the maximum bond order, natural bond order, and to some extent the effective bond order. Furthermore the charge capacitance of some methylidene metal dihydride complexes are studied. These molecules have agostic interactions of varying strength, and it is concluded that this strength is very well reflected in the charge capacitances of the systems. In accordance with the definition of agostic interactions it is therefor concluded that the charge capacitance holds information about the strength of covalent interactions. The effect therefore on the agostic interactions upon substitution of one of the hydrogen atoms with fluorine in the methylidene metal complexes is studied, and found to reduce the agostic interactions. It is also demonstrated that there is an agostic interaction in an ArCrCrAr complex. The distance dependence of the charge capacitance is also discussed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Modulation of fragmental charge transfer via hydrogen bonds. Direct measurement of electronic contributions

Hydrogen bonds play an important role in an overwhelming variety of fields from biology to surface and supramolecular chemistry. The term "hydrogen bond" refers to a wide range of interactions with various covalent and polar contributions. In particular, hydrogen bonds have an important role in the folding and packing of peptides and nucleic acids. Recent studies also point to the importance of hydrogen bonding in the context of second-shell interactions, in metal binding and selectivity in metalloproteins, and in controlling the dynamics of membrane proteins. In this study, we demonstrate and quantify the modulation of fragmental charge transfer from hydrogen-bonded ligands to a metal center, by employing our recently introduced molecular potentiometer. The molecular details that affect this type of fragmental charge transfer are presented and a path for transferring chemical information is demonstrated. We found that H-bond interactions in the extended positions of axial ligands provide an effective means of modulating the amount of fragmental charge transfer to a metal center, thereby dramatically influencing the electronic properties of the ligand, the binding affinity, and the binding of additional ligands. The magnitude of fragmental charge-transfer modulation induced by a single ligand-solvent H-bond interaction is comparable to those induced by covalent substitution, although H-bond enthalpy is only on the order of several kilojoules per mole. Importantly, we find a significant change in the ligand electronic properties, even for weak C-H...O=C H-bond formation, where the bond enthalpy is substantially lower than for conventional H-bond interactions. The excess fragmental charge transferred to the metal center, deduced from the spectroscopic measurements, correlates well with the computationally determined values. Our findings underscore the importance of second-shell interactions in the active sites of enzymes, beyond the structural and electrostatic importance that is widely recognized today.     

Supramolecular fullerene/porphyrin charge transfer interaction studied by absorption spectrophotometric method

A detailed UV–Vis spectrometric and thermodynamic studies were done to look insight into the nature of molecular interactions of the electron donor–acceptor complexes of C₆₀ and C₇₀ with 5,10,15,20-tetrakis(octadecyloxyphenyl)-21H,23H-porphyrin (1) in chloroform and toluene. Charge transfer (CT) absorption bands were located in the visible region and vertical ionization potential of 1 was determined utilizing CT transition energy. Low values of oscillator and transition dipole strengths suggested that the complexes were almost of neutral character in ground states. The high binding constant value for the C₇₀–1 complex indicated high selectivity of 1 molecule towards C₇₀. Experimental as well as theoretically determined of enthalpies of formation value substantiated the trend in K values for fullerene–1 complexes.     

Small-angle X-ray scattering from a dual-component organogel to exhibit a charge transfer interaction

The structure of a dual-component organogel consisting of methyl 4,6-O-(p-aminobenzylidene)-alpha-D-glucopyranoside and methyl 4,6-O-(p-nitrobenzylidene)-alpha-D-glucopyranoside in diphenyl ether was investigated with small-angle X-ray scattering (SAXS). The individual components gelatinized the solvent to yield a colorless gel and the gel fiber consisted of the crystal, providing the crystalline peaks at the same diffraction angles as those of the solid samples. When the components were mixed in equimolar ratio and dissolved in diphenyl ether, a yellow gel was formed and the crystalline peaks disappeared. For all compositions, the SAXS profiles were well fitted by a cylinder model. The cross-sectional radius of gyration, r(c), was determined from the cross-sectional Guinier plot (qI vs q(2), where I and q are the scattering intensity and the magnitude of the scattering vector). The value of r(c) reached a minimum of 3.0 nm at the equimolar composition. By correcting the data for the thermal scattering background, we obtained the entire SAXS profile for the equimolar dual-component gel. From this profile, the radial electron density distribution was determined and the radius of the cylinder was estimated to be 2.6 nm. The electron density distribution thus obtained revealed that four gelator molecules are packed in the sectional direction. This model was consistent with the size of the gelator molecules.     

HEK-TCNQ的光诱导电荷转移与HEK-DDQ的基态电荷转移

合成了电荷转移复合物HEK-TCNQ和HEK-DDQ(HEK=9-hydroxyethylcarbazole)。拉曼光谱和吸收光谱测定表明: 光电导为10^-^1^1s.cm^-^1的HEK-TCNQ在514.5nm激光照射下可发生电荷转移, 生成HEK^+TCNQ^-, 其光电导显著增大。X射线结构分析和红外光谱表明: HEK与DDQ之间基态电荷转移量为0.1~0.2。     

Chromophoric systems based upon through-bond interaction IV : Intramolecular charge-transfer interaction via three sigma bonds

Ketones containing a β-substituent with readily ionizable lone pair or π-electrons are studied by UV spectroscopy. When the geometry favours through-bond interaction between the carbonyl group and the γ-substituent a sigma-coupled transition is observed in the near UV region. Replacement of the CO group by more electronegative systems such as the gem-dicyanovinylidene group is shown to enhance the intensity of the sigma-coupled transition and to shift it bathochromically. These phenomena are interpreted to result from a charge-transfer character of the sigma transition. Furthermore it is shown that intensity borrowing from locally excited states plays an important role in determining the intensity of this sigmal-coupled transition.     

Fluorescence quenching by charge transfer interaction. Exciplexes of fluorene and hetero-analogues with electron donors and acceptors

The fluorescence quenching of fluorene, dibenzofuran and dibenzothiophen by aliphatic amines (electron donors) and aromatic nitriles (electron acceptors) has been studied in different solvents. Emission spectra, solvent effects and the observed relationship between the quenching rate constants and ionization potentials of the donors or reduction potentials of the acceptors support an electron-transfer mechanism with exciplex formation. Carbazole does not enter in this scheme and is probably quenched via a different mechanism.     

The chemical structure matrix and a new formalism for the QSPR problem

The notion of the chemical structure matrix (CSM) is introduced. The columns of the CSM represent the numbers of occurrences of different subgraphs in molecular graphs and are treated as vectors in a linear space. Any topological indices and physicochemical properties can also be treated as vectors in the same linear space. The QSPR problem is formally reduced to the search for linear correlations between vectors. A simple procedure for solving the problem is proposed. A novel method for establishing QSAR is outlined. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1447–1453, September, 2006.     

Dielectric and elastic properties of a nematic phase induced by charge transfer interaction in a binary system containing disc-shaped molecules

The dielectric constants and the elastic coefficients for splay (K₁) and bend (K₃) of the charge transfer induced nematic (N_c) phase of tridecyl pentakis(phenylethynyl)phenyl ether (1) doped with different amounts of 2,4,7-trinitrofluorenone (2) were determined by studying the electric field induced bend deformation using the capacitance method. A negative dielectric anisotropy was observed. For the bend elastic constant K₃ values up to 22 × 10^-12 N are found which are one order of magnitude higher than the respective values of discotic nematic (N_D) phases. Values of 0·6-0·8 are obtained for the ratio K₁/K₃; these show a minimum for the equimolar complex.     

Dielectric and elastic properties of a nematic phase induced by charge transfer interaction in a binary system containing disc-shaped molecules

Abstract

The dielectric constants and the elastic coefficients for splay (K ₁) and bend (K ₃) of the charge transfer induced nematic (N_c) phase of tridecyl pentakis(phenylethynyl)phenyl ether (1) doped with different amounts of 2,4,7-trinitrofluorenone (2) were determined by studying the electric field induced bend deformation using the capacitance method. A negative dielectric anisotropy was observed. For the bend elastic constant K ₃ values up to 22 × 10^?12 N are found which are one order of magnitude higher than the respective values of discotic nematic (N_D) phases. Values of 0·6–0·8 are obtained for the ratio K ₁/K ₃; these show a minimum for the equimolar complex.     

Molecular ordering of photoreactive nonmesogenic 1,3,5-triazine compounds into columnar mesophases by charge transfer interaction

1,3,5-Triazine compounds having diacetylenic groups and their charge transfer complexes with 2,4,7-trinitrofluoren-9-one (TNF) were prepared. The morphological changes were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) measurement. Although the 1,3,5-triazine compounds did not exhibit a mesophase, the charge transfer (CT) complexes with various mole ratios of TNF assembled into liquid crystalline phases. The CT complexes were UV-irradiated in the liquid crystalline (LC) state to yield oligomers.     

Synthesis of a novel perylene diimide derivative and its charge transfer interaction with C60

A novel organic electron acceptor, N,N′-dipyrimidinyl-3,4,9,10-perylene-tetracarboxylic diimide (DMP), was designed and synthesized. The molecular structure was characterized by FTIR spectrum and elemental analysis. By cyclic voltammetry measurements, DMP was found to possess a lower LUMO energy level than N,N′-diphenyl-3,4,9,10-perylene-tetracarboxylic diimide due to the stronger electron-withdrawing pyrimidinyl group than the phenyl group. Fluorescence quenching is observed in a dual-layer film consisting of a DMP layer and a C₆₀ layer and was attributed to the charge transfer at the interface due to the energy level offset between DMP and C₆₀.