Remote substituent effects on homolytic bond dissociation energies

In the study we tried to answer two questions. First, does X-Z homolytic bond dissociation energy (BDE) of Y-C6H4-X-Z obey the Hammett relationship? Second, if it does what factors determine the magnitude and sign of the slope (rho+) of Hammett regression against substituent sigma(p)(+) constants? We collected a large number of X-Z BDEs for over one-thousand Y-C6H4-X-Z systems using the RMP2/6-311++G**//UB3LYP/6-31G* method. We found that remote substituent effects on X-Z BDEs are determined by both the ground effect (i.e. stabilization/destabilization of X-Z by the substituents) and the radical effect (i.e. stabilization/destabilization of X. by the substituents). The ground or radical effect is determined by the electron demand of X-Z or X. in the same way as the deprotonation enthalpy of HOOC-C6H4-X-Z or HOOC-C6H4-X. is affected by X-Z or X. . As a result, rho+ (BDE) for X-Z bond homolysis can be quantitatively predicted by using the change in deprotonation enthalpy from HOOC-C6H4-X-Z to HOOC-C6H4-X. .     

Triplet-state energies and substituent effects of excited aroyl compounds in the gas phase

Triplet-state energy values obtained from the gas phase are still scarce. In this study, the triplet-state energies of 58 aroyl compounds, introduced as gas chromatographic peaks at atmospheric pressure and typically 473 K, have been determined from the 0-0 bands of their n --> pi* type phosphorescence spectra in excited nitrogen. Correlations of those gas-phase triplet-state energies with Hammett constants could be observed for substituted acetophenones, benzaldehydes and benzophenones.     

Remote substituent effects on bond dissociation energies of para-substituted aromatic silanes

UB3LYP/6-31G(d) and ROMP2/6-311++G(d,2p) methods were used to calculate the Si-X bond dissociation energies (BDEs) of a number of para-substituted aromatic silanes (4-Y-C(6)H(4)-SiH(2)X, where X = H, F, Cl, or Li). It was found that the substituent effect on the Si-H BDE of 4-Y-C(6)H(4)-SiH(3) was small, as the slope (rho(+)()) of the BDE- regression was only 0.09 kJ/mol. In comparison, the substituent effect on the Si-F BDE of 4-Y-C(6)H(4)-SiH(2)F was much stronger, whose rho(+ )()value was -2.34 kJ/mol. The substituent effect on the Si-Cl BDE of 4-Y-C(6)H(4)-SiH(2)Cl was also found to be strong with a rho(+)() value of -1.70 kJ/mol. However, the substituent effect on the Si-Li BDE of 4-Y-C(6)H(4)-SiH(2)Li was found to have a large and positive slope (+9.12 kJ/mol) against. The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituent to stabilize or destabilize the starting material (4-Y-C(6)H(4)-SiH(2)X) as well as the product (4-Y-C(6)H(4)-SiH(2)* radical) of the homolysis. Therefore, the direction and magnitude of the effects of Y-substituents on the Z-X BDEs in compounds such as 4-YC(6)H(4)Z-X should have some important dependence on the polarity of the Z-X bond undergoing homolysis. This conclusion was in agreement with that from earlier studies (for example, J. Am. Chem. Soc. 1991, 113, 9363). However, it indicated that the proposal from a recent work (J. Am. Chem. Soc. 2001, 123, 5518) was unfortunately not justified.     

Theoretical studies of electron transport in thiophene dimer: effects of substituent group and heteroatom

The electron-transport properties of various substituted molecules based on the thiol-ended thiophene dimer (2Th1DT) are investigated through density functional theory (DFT) combined with nonequilibrium Green's function (NEGF) method. The current-voltage (I-V) curves of all the Au/2Th1DT/Au systems in this work display similar steplike features, while their equilibrium conductances show a large difference and some of these I-V curves are asymmetric distinctly. The results reveal the dependence of conductance on the energy level of the substituted 2Th1DT molecules. Rectification ratios are computed to examine the asymmetric properties of the I-V curves. The rectifying behavior in the 2Th1DT molecule containing the amino group close to the molecular end is more prominent than that in the other molecules. The rectifying behavior is analyzed through transmission spectra and molecular projected self-consistent Hamiltonian (MPSH) states. Slight negative differential resistance (NDR) can be observed in some of the systems. The electron-transport properties of 2Th1DT molecules containing different heteroatoms are also investigated. The results indicate that the current in heteroatom-containing molecules is larger than that in their pristine analogues, and lighter heteroatoms are more favorable than heavier heteroatoms for electron transport of the thiophene dimer.     

Hopping charge transport in organic materials

General properties of the transport of charge carriers (electrons and holes) in disordered organic materials are discussed. It was demonstrated that the dominant part of the total energetic disorder in organic material is usually provided by the electrostatic disorder, generated by randomly located and oriented dipoles and quadrupoles. For this reason this disorder is strongly spatially correlated. Spatial correlation directly governs the field dependence of the carrier drift mobility. Shape of the current transients, which is of primary importance for a correct determination of the carrier mobility, is considered. A notable feature of the electro-static disorder is its modification in the vicinity of the electrode, and this modification takes place without modification of the structure of the material. It is shown how this phenomenon affects characteristics of the charge injection. We consider also effect of inter-charge interaction on charge transport.     

Similar substituent effects in the oxidations of primary aliphatic alcohols with dichromates and halochromates of heterocyclic bases

The reaction constants for the oxidations of primary aliphatic alcohols with 12 dichromates and halochromates of heterocyclic bases do not differ significantly indicating operation of a common mechanism. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 37: 5–9, 2005     

Bond lengths and reorganization energies in fullerenes and their ions

A very simple tight binding method with bond-length-dependent couplings, similar to the Su-Schrieffer-Heeger model, is described and applied to fullerenes in different charge states. The bond lengths, calculated from the π bond orders, are in good agreement with results obtained by ab initio methods which include correlation, where comparisons can be made. The Jahn-Teller distortions are found to be small for the examples C₂₀, C₂₈-T_d, and C₆₀-I_h. Often many possible distortions are found, for example two different D_5d distortions for the C₂₀ spin triplet state. Bond reorganization energies (λ_b) for reduction and oxidation, which are measures for trapping efficiency and vibronic coupling, are obtained using a parabolic approximation of the energy surface. Received: 3 December 1996 / Accepted: 3 March 1997     

Intermolecular charge transfer between heterocyclic oligomers. Effects of heteroatom and molecular packing on hopping transport in organic semiconductors

For electron or hole transfer between neighboring conducting polymer strands or oligomers, the intrinsic charge-transfer rate is dictated by the charge-resonance integral and by the reorganization energy due to geometric relaxation. To explain conduction anisotropy and other solid-state effects, a multivariate, systematic analysis of bandwidth as a function of intermolecular orientations is undertaken for a series of oligoheterocycles, using first-principles methods. While cofacial oligomers show the greatest bandwidths at a given intermolecular C-C contact distance, for a fixed center-to-center intermolecular distance, tilted pi-stacking increases pi-overlap (particularly for LUMO orbitals) and decreases electrostatic repulsion, yielding optimum tilt angles for packing of approximately 40-60 degrees at small intermolecular separations. The calculations also reveal that bandwidths and intrinsic mobilities of holes and electrons in conjugated oligoheterocycles can be quite comparable.     

Impact of replacement of the central benzene ring in anthracene by a heterocyclic ring on electronic excitations and reorganization energies in anthratetrathiophene molecules

The impact of changing the central benzene ring on the electronic excitations and reorganization energies (λ) of the anthratetrathiophene (ATT) molecules is studied by density functional theory (DFT) and time‐dependent DFT (TD‐DFT) quantum chemical calculations. The effect of changing the position of the sulfur atom at the periphery of anthracene on the optical and charge transfer properties is also studied. The calculated results suggest that the HOMO, LUMO, HOMO–LUMO energy gap, ionization potential (IP), electron affinity (EA), hole extraction potential (HEP), electron extraction potential (EEP), and reorganization energies (λ) are affected by replacing the central ring with different heterocyclic rings and the position of the sulfur atom. In addition, all molecules show good hole‐ and electron‐transport properties. This work may be helpful for future design and preparation of high‐performance charge‐transport materials.     

Ligand reorganization and activation energies in nonadiabatic electron transfer reactions

The activation energy and ligand reorganization energy for nonadiabatic electron transfer reactions in chemical and biological systems are investigated in this paper. The free energy surfaces and the activation energy are derived exactly in the general case in which the ligand vibration frequencies are not equal. The activation energy is derived by free energy minimization at the transition state. Our formulation leads to the Marcus-Hush [J. Chem. Phys. 24, 979 (1956); 98, 7170 (1994); 28, 962 (1958)] results in the equal-frequency limit and also generalizes the Marcus-Sumi [J. Chem. Phys. 84, 4894 (1986)] model in the context of studying the solvent dynamic effect on electron transfer reactions. It is found that when the ligand vibration frequencies are different, the activation energy derived from the Marcus-Hush formula deviates by 5%-10% from the exact value. If the reduced reorganization energy approximation is introduced in the Marcus-Hush formula, the result is almost exact.     

Alkoxysilanes and disiloxanes comprising mesoionic sydnon-4-yl heterocyclic substituent

Russian Chemical Bulletin -     

Anomalous reorganization free energies in optical electron transfer in solution

Autoionization bands are observed in the photoelectron emission spectroscopy of aqueous solutions of cyanometalate complexes (Mn, Fe, W, Mo), anions (NO₃, CIO⁻₄ and cations (Ag⁺ TI⁺. Reorganization free energies for autoionization bands are anomalously low in absolute value (by≈1 eV) in comparison with direct transitions to the continuum. Interpretation is based on potential energy profiles and model calculations for the reorganization free energy.     

Structural motifs in phenylbismuth heterocyclic carboxylates--secondary interactions leading to oligomers

The reaction of triphenylbismuth [BiPh(3)] with several heterocyclic carboxylic acids was explored. Seven crystalline compounds, [PhBi(2-O(2)C-3-(OH)C(5)H(3)N)(2)(2-O(2)C-3-(OH)C(5)H(3)NH)] (5), [(Bi(2-O(2)C-3-(OH)C(5)H(3)N)(4))(C(5)H(5)NH)(C(5)H(5)N)] (7), [PhBi(2-O(2)C-C(4)H(3)N(2))(2)(2-O(2)C-C(4)H(4)N(2))·H(2)O] (8), [PhBi(2-O(2)C-C(9)H(6)N)(2)·H(2)O] (9), [Ph(2)Bi(O(2)C-C(4)H(3)O)] (10), [Ph(2)Bi(O(2)C-C(4)H(3)S)] (11) and [PhBi(O(2)C-C(4)H(3)S)(2)](2) (12), were prepared by simple reactions using BiPh(3) and the corresponding acids, 3-hydroxypicolinic acid, pyrazine-2-carboxylic acid, quinoline-2-carboxylic (quinaldic) acid, furan-2-carboxylic acid and thiophene-2-carboxylic acid. Compound 5 primarily exhibits a coordination number of six with pentagonal pyramidal geometry at bismuth, but an additional weak Bi···O interaction in the direction of the lone pair of electrons is present. This feature leads to a weakly bound dimer. The use of pyridine as the solvent in a similar reaction, however, led to 7, in which all of the Bi-Ph bonds are cleaved. In this compound, bismuth exhibits a coordination number of eight and distorted dodecahedral geometry. In compound 8, the geometry around bismuth is primarily a pentagonal pyramid, however, clear-cut but weak secondary Bi···N interactions leading to a dimeric formulation are discernible in the structure. The quinaldate compound 9 exhibits a lower formal coordination number of five for bismuth, with square pyramidal geometry, but again two secondary Bi···O interactions for each bismuth in the direction of the lone pair lead to a dimer. A similar secondary Bi···O interaction involving furan oxygen is present in the furoate compound 10, which is a polymeric chain (one dimensional coordination polymer). Although the thiophene carboxylate 11 is also a polymeric chain, no Bi···S interactions are present. Unlike the previously reported tetrameric biscarboxylate [PhBi(2-O(2)C-C(5)H(3)N)(2)](4), the thiophene carboxylate [PhBi(O(2)C-C(4)H(3)S)(2)](2) (12) is a dimer considering only primary interactions. However, these dimers are arranged in such a way that there are secondary Bi···S interactions in the structure in the expected direction of the lone pair of electrons on bismuth. Thus, these studies suggest that the stereochemical activity (or inactivity) of the bismuth lone pair of electrons need to be judged more cautiously. TGA studies are consistent with the presence of Bi-Ph groups in all of the compounds, except 7, as indicated by their formulae.     

Hopping transport and energy transfer in substitutionally disordered media

We present an alternative derivation of configuration-averaged equations governing transport and trapping in a three-component disordered system. It is assumed that migration of the initial excitation is described by the usual master equation. Using the Zwanzig projection operator technique, we were able, after averaging, to separate the contribution of the transport and transfer processes. We found finally that the averaged master equation including sinks is equivalent to a hierarchy of coupled equations.     

The use of calculated reorganization energies in experimental electrochemical kinetics

Calculated reorganization energies in solution using the polarizable continuum model implemented in Gaussian 03 are compared to experimental values of the heterogeneous rate constants for the reduction of a wide variety of neutral molecules in dimethylformamide. The calculated reorganization energies are fully consistent with the experimental data; the computational procedure may in fact be quite useful for estimating the reorganization energies for outer-sphere electrochemical reactions. From the comparison between the calculated reorganization energy values and the experimental values for the heterogeneous rate constants, the adiabaticity of the redox couples' is discussed.     

Solvent-dependent vibrational frequencies and reorganization energies of two merocyanine chromophores

Absorption and resonance Raman spectra have been measured over a wide range of solvents for two merocyanine dyes containing the indoline ("Fischer" base) electron donor group with different accepting groups. One appears to be near the cyanine limit (equal contributions of the neutral and zwitterionic resonance forms to both ground- and excited-state structures) based on electrooptic absorption data showing a very small dipole moment change upon electronic excitation. The resonance Raman spectra of both molecules show significant frequency shifts and intensity redistributions that evolve monotonically with increasing solvent polarity and are consistent with increasing zwitterionic character of the ground-state structure. The vibrational reorganization energies of both molecules, obtained by simulating the absorption band shapes, are smaller in polar solvents than in nonpolar or weakly polar ones, consistent with a more cyanine-like structure at higher solvent polarities. However, the vibrational reorganization energies of both molecules exceed 700 cm(-1) in all solvents, larger than in many true cyanine dyes, and the optical absorption maxima do not correlate well with either solvent polarity or vibrational reorganization energy. This indicates some limitations to the structural conclusions that can be reached from the two-state model for pi-conjugated donor-acceptor systems.     

Star-shaped pyrrole-fused tetrathiafulvalene oligomers: synthesis and redox, self-assembling, and conductive properties

A series of star-shaped pyrrole-fused tetrathiafulvalene (TTF) oligomers 1-3 was synthesized via an S(N)Ar reaction of fluorinated benzenes with the pyrrolyl sodium salts. Electrochemical and chemical oxidations of 1-3 revealed that a radical cation moiety on each TTF unit was successfully accumulated in all oligomers. Self-assembled structures of neutral and oxidized species were characterized by SEM and XRD, and their conductive properties of the iodine-doped 1-3 as well as an intermolecular mixed-valence ion radical salt were investigated.