Analysis of kinetic models for the tunnel electron transfer reactions. Reaction kinetics for various radial and angular dependences of the tunneling probability

A new version of the “stepwise” approximation used in kinetic models for the long range electron transfer reactions is suggested. Using this approach the effects on the kinetics of these reactions of such factors are examined as a more complicated dependence (compared to the commonly used exponential one) of the tunneling probability, w, on the distance between the reagents as well as the angular dependence of w. For many practically important situations taking account of the above factors is shown to have no significant effect on the form of the reaction kinetics for both the pairwise and random spatial distributions of the reagents. However, the relations between the parameters a_ef and ν_cf in the expression for the tunneling distance R* = ₂¹a_ef In ν_cft, which can be determined from the experimental kinetics, and similar theoretical parameters ν_o and a that are involved in the expressions for w are found to be different for various types of the radial and the angular dependences of w. It is shown that when the orbitals between which the electron transfer takes place are located at sites removed from the geometric centers of the reactants the values of ν_ef can exceed by many orders of magnitude the characteristic frequency of the electron movement in molecules ν_e ≈ 10¹³ s^?1.     

Kinetics of tunneling electron transfer in the presence of scatter in the parameters of the tunneling rate constant

In the case when kinetics of tunneling electron transfer are controlled predominantly by scatter in separations of donors and acceptors, the presence of even large scatter in the parameters v and a of the corresponding rate constant is shown not to affect markedly the shape of the kinetics, the latter being well described by a single pair of effective parameters.     

The role of long range tunneling in electron transfer processes in condensed media

Experimental evidence is given for the importance of long range tunneling in electron transfer reactions in condensed media: the unusually weak effect of electrostatic repulsion on the rate of some electron transfer and spin exchange processes; electron transfer between distant (up to ≈ 30 A) species in solids at a rate considerably exceeding that of thermal diffusion; the unusual concentration dependence of radiation yields in the presence of scavengers, etc. The concept of long range tunneling is shown to permit quantitative explanation and correlation of experimental data on electron transfer in quite different fields.The factors determining the efficiency of tunneling, as well as some peculiar features of tunneling kinetics are considered.The role of long range tunneling in various chemical processes involving electron transfer in condensed media are discussed (ion reactions in solutions, photochemistry, radiation chemistry, reactions with polymers, some biochemical reactions).     

Simultaneous electron diffraction and mass-spectrometric study of the structure of the MoF5 molecule

Electron diffraction data for the MoF₅ molecule are analyzed in terms of an r_a structure. Three models of the geometrical structure, which have D_3h, C_4v, and C_2v symmetry, are considered. It is confirmed that a distorted bipyramid of C_2v symmetry is the best model that is in agreement with experimental electron diffraction data. The model has three different types of nuclear Mo-F distances: r_α(Mo-F_1eq) = 1.720(5) Å, r_α(Mo-F_2eq) = 1.826(7) Å, r_α(Mo-F_ax) = 1.825(7) Å. The bond angle between the pseudoaxial bonds is 168.1(0.6)?, and the angle between the Mo-F_2eq pseudoequatorial bonds is 122.6(0.8)?. The r_a(Mo-F_leq) and r_a(Mo-F_2eq) distances differ significantly. Possible reasons for this are discussed.     

Gas-phase electron diffraction study of the molecular structure of tungsten oxytetrafluoride,WOF4

The molecular structure of tungsten oxytetrafluoride has been studied in the gas phase by electron diffraction. A square pyramidal model with molecular symmetryC_4v, as indicated by vibrational spectroscopy, gives a good fit to the experimental data. Least squares refinement on the molecular intensity curves gives the following results for the principal geometrical parameters (uncertainties in parentheses are 2σ):r_a(W=O) = 1.666 (0.007)Å,r_a(W-F)= 1.847 (0.002)Å, ∠OWF = 104.8 (0.6)°, ∠FWF = 86.2(0.3)°.     

Electron diffraction and quantum chemical study of the molecular structure of 4-methylbenzene sulfochloride

A combined electron diffraction and mass spectrometric study was carried out to investigate the molecular structure of 4-methylbenzene sulfochloride at 330(2) K. An analysis of the electron diffraction data was performed in terms of the r_α structure. Several models of geometrical structure having different orientations of the sulfochloride group relative to the plane of the benzene ring are treated. The following values of structural parameters were obtained: r_α(C-H)_meth= 1.104(41)Å, r_a(C-H)/_phen = 1.103(27)Å, r_a(C-C)_phen = 1.403(7) Å, r_a(C-C)_meth = 1.512(25) Å, r_a(C-S) =1.758(6) Å, r_a(S = O) = 1.419(3) Å,r _a(S-Cl) = 2.049(5) Å, ∠CCH_meth = 106.9(47)?, ∠CSO = 110.5(6)?, ∠CSCl = 101.3(6)°, ∠OSO = 120.5(9)°. The angle between the plane of the benzene ring and the plane of the S-Cl bond was found to be 83°. Ab initio and semiempirical quantum chemical calculations were accomplished to estimate the geometrical and energy parameters and compare them with electron diffraction data.     

Predissociation of tif by tunneling

The UV spectra of the B ³Π₁-X ¹Σ⁺ transition in TIF were recorded applying frequency-doubled laser radiation and fluorescence or direct absorption detection. The observed J' -dependent predissociation in the upper levels v' = 2 and v' = 3 is explained quantitatively by invoking tunneling through a potential hump. The line positions and widths are fitted to a potential curve of the B ³Π₁ state using the semiclassical approximation for bound and quasibound levels.     

Effect of dioxane on the binding of competitive inhibitor proflavin and catalytic activity of bovine pancreatic α-chymotrypsin

The binding of competitive inhibitor proflavin by α-chymotrypsin in water-dioxane mixtures over the entire range of thermodynamic activities of water a _w was studied. The data on the degree of binding of proflavin were compared to the results on the catalytic activity of the enzyme preliminary incubated in water-dioxane mixtures. An analysis of the behavior of the concentration dependences of these characteristics demonstrated that, at low a _w values, the behavior of the interprotein contacts in the enzyme formed during its drying largely governs its functional properties, while at high a _w values, they are determined by the interaction of the enzyme with the organic solvent. Interplay of these two factors is responsible for the observed complex shape of the isotherm of binding of proflavin, with the maximum degree of binding being attained at moderate a _w values.     

On the structure and conformational composition of 1,1-bis(methylthio)ethylene

1,1-bis(methylthio)ethylene has been studied in the gaseous phase by electron diffraction and in the solid and liquid phases by Raman spectroscopy. While there is apparently only one conformer in the solid, the fluid phases consist of probably three forms, two of these have a non-planar skeleton. Average values for the bond lengths are: r_a(C_eth—S) = 1.767 Å, r_a(C_met—S) = 1.815 Å, r_a(CC) = 1.348 Å.     

Does the electron spin affect the rates of electron tunneling in electrochemical systems?

An effect of spin degeneracy of electron energy levels in the metal electrode on the observable characteristics of electrochemical systems in the absence of magnetic field is discussed. Single-electrode outer-sphere electron transfer reactions are considered as well as redox-mediated electron tunneling in electrochemical contacts. Particular attention is paid to the difference between the spin-less model and the limit of infinitely large Coulomb repulsion of the electrons occupying the same valence orbital in the redox group. Adiabatic and non-adiabatic regimes of the transitions are studied and the expressions for the tunnel current are obtained.     

The molecular structure of difluorophosphine selenide,determined using a combination of gas electron diffraction and liquid-crystal NMR data

The molecular structure of difluorophosphine selenide has been determined by a combined analysis of gas-phase electron diffraction data and dipolar couplings obtained for a solution in a nematic phase. Geometrical parameters (r_a) are: _r(PSe) 202.6(4), r(P-F) 155.7(3), r(P-H) 142.2(7) pm, ∠SePF 116.8(3), ∠FPF 98.1(7), ∠SePH 118.6(7)°.     

Electron diffraction study and CNDO/2 calculations on the complex of aluminium trichloride with ammonia,Cl3Al.NH3

The molecular geometry of the complex of aluminium trichloride with ammonia, Cl₃Al.NH₃, has been studied by electron diffraction. The most important internuclear distances in terms of r_a parameters are as follows: r(Al-Cl) = 2.100±0.005 Å, r(Al-N) = 1.996±0.019 Å, r(Cl·Cl) = 3.569±0.011 Å and r(Cl·N) = 3.165±0.012 Å. The Cl-Al-Cl bond angle in terms of an approximate r_a structure is 116.9°. The assumptions of a staggered model in the structure analysis was justified by CNDO/2 calculations. The experimental data indicate strong linkage between the donor and acceptor parts. The flat pyramidal average configuration of the AlCl₃ part of the complex suggests planar equilibrium structure for free AlCl₃. Variations in the bond configurations of the donor and acceptor parts, as compared with those of the respective free molecules, are discussed.     

RuGavSnw Nowotny Chimney Ladder Phases and the 14-Electron Rule

A series of ruthenium gallium stannides have been prepared with stoichiometry RuGa_vSn_w, where 8+3v+4w=14 and 0<v<0.70. These samples have been analyzed by X-ray powder diffraction and transmission electron microscopy (TEM). The data show that these compounds are Nowotny chimney ladder phases with both commensurate and incommensurate structures. We show that there are special characteristics in chimney ladder powder diffraction patterns that allow one to determine the ratio of main group atom-sites to transition metal atom-sites to high accuracy. Our results confirm earlier work which suggest that both the stoichiometry and the structure of chimney ladder phases are dominated by electronic factors. The structures reported in this paper adhere to the 14-electron rule, i.e., there are 14 valence electrons per transition metal ion. The interplay of main group and transition metal structures leads to a pseudo c-axis, the presence of which is confirmed by the TEM data. We discuss the relation between these phases and the Fibonacci sequence.     

Quantum-chemical calculations for silyl- and alkyl-pseudohalides X3YNCZ (X = H or CH3; Y = C or Si; Z = O,S, or Se) and electron diffraction study of the equilibrium molecular structure of (CH3)3SiNCSe

The equilibrium structures and force fields of the twelve simplest silyl- and alkyl-pseudo halides are calculated by means of B3LYP and MP2(full) quantum-chemical methods with the use of the aug-cc-pVTZ basis. Some regularities in their structure are established. Using these data, the equilibrium structure of the (CH₃)₃SiNCSe molecule with symmetry C _3v is described experimentally for the first time via gas electron diffraction. The following values of the main r _e parameters are determined (uncertainty 3σ is in parentheses): C=Se, 1.709(14) Å; N=C, 1.190(10) Å; N-Si, 1.767(15) Å; Si-C, 1.847(13) Å; N-Si-C, 106.4°; C-Si-C, 112.4°.     

An Organic Solvent-Free Method for the Extraction of Ellagic Acid Compounds from Raspberry Wine Pomace with Assistance of Sodium Bicarbonate

Industrial processing of raspberry juice and wine generates considerable byproducts of raspberry pomace. Ellagic acids/ellagitannins, being characterized by their antioxidant and antiproliferation properties, constitute the majority of polyphenolics in the pomace and are valuable for recovery. In the present study, we developed a novel procedure with sodium bicarbonate assisted extraction (SBAE) to recover ellagic acid from raspberry wine pomace. Key parameters in the procedure, i.e., sodium bicarbonate concentration, temperature, time and solid/liquid (S/L) ratio, were investigated by single factor analysis and optimized subsequently by Response Surface Methodology (RSM). Optimal parameters for the SBAE method here were found to be 1.2% (w/v) NaHCO₃, 1:93 (w/v) S/L ratio, 22 min and 100 °C. Under these conditions, the ellagic acid yield was 6.30 ± 0.92 mg/g pomace with an antioxidant activity of 79.0 ± 0.96 μmol Trolox eq/g pomace (DPPH assay), which are 2.37 and 1.32 times the values obtained by extraction with methanol–acetone–water solvent, respectively. The considerable improvement in ellagic acid extraction efficiency could be highly attributed to the reactions of lipid saponification and ellagitannin hydrolysis resulted from sodium bicarbonates. The present study has established an organic solvent-free method for the extraction of ellagic acid from raspberry wine pomace, which is feasible and practical in nutraceutical applications.     

Production of Microbial Cellulose by a Bacterium Isolated from Fruit

This study presents the production of bacterial cellulose (BC) by a bacterium isolated from a rotten fruit and its process optimization. Here, isolation and screening of potent cellulose producers were carried out from different natural sources, viz., soil, rotten fruits, and vegetables and vinegar. A total of 200 bacterial isolates were obtained, which were screened for cellulose production using Hestrin?CSchramm medium. A novel and potent cellulose-producing bacterium was newly isolated from a rotten fruit and identified as Gluconacetobacter sp. F6 through 16S ribosomal DNA sequencing and morphological, cultural, and biochemical characteristics. After optimization of culture conditions, including pH, temperature, agitation, carbon/nitrogen sources, and inducers, the BC production was greatly increased from 0.52 to 4.5?g/l (8.65-fold increase). The optimal culture medium contained 1% (w/v) glucose, 1.5% (w/v) yeast extract, 0.5% (w/v) peptone, 0.27% (w/v) disodium hydrogen phosphate, 0.115% (w/v) citric acid, and 0.4% (w/v) ethanol. BC produced was analyzed for the presence of cellulose fibrils by epiflourescent microscopy using Calcofluor white stain and scanning electron microscopy and confirmed by NMR. There are very scanty reports about the optimization of BC production by bacteria isolated from rotten fruits.     

A gas-phase electron diffraction study op the molecular structure of 1,1,1,2-tetrafluoroethane

The molecular structure of 1,1,1,2-tetrafluoroethane is studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument. Effective least-squares refinement of the geometry is achieved with values for vibrational amplitudes transferred from normal coordinate calculations on related molecules. The following values for the main independent geometrical parameters are obtained (r_a values with e.s.d. in parentheses): C-C = 1.501(4) Å, C-H = 1.077 (15) Å, C-F(CH₂F) = 1.389(6) Å, C-F(CF₃) = 1.334 (2) Å, ∠CCH= 106.1(12)°, ∠CCF(CH₂F)= 112.3(4) Å, ∠CCF(CF₃)= 110.4(2). Other angles are ∠FCF = 108.6 (2)° and ∠FCH = 111.4(15)°, with ∠HCH constrained at 109.4°. The r_a bond lengths of all the fluoroethanes are compared.     

An electron diffraction determination of the molecular structures of phosphoryl bromide and thiophosphoryl bromide

Gas-phase electron diffraction structures have been determined for phosphoryl bromide (OPBr₃ thiophosphoryl bromide (SPBr₃Normal coordinate analyses were carried out for the two molecules using a valence force field, and the resulting amplitude terms used for transformations between r_a and r_ga. An unconstrained refinement of the OPBr₃ intensities gives the parameters r_g(PO) = 1.455(7) Å and r_g(PBr) = 2.175(3) Å. The weighted average, geometrically-consistent valence angles derived from the four internuclear distances, r_α, are θ_α(OPBr) = 114.4(2)° and θ_α(BrPBr) = 104.1(2)°. For SPBr₃ a constrained fit to a self-consistent rα structure gives the parameters r_g(PS) = 1.895(4) Å, r_g(PBr) = 2.193(3) Å, θ_α(SPBr) = 116.2(2)°, and θ_α(BrPBr) = 101.9(2)°. Electron diffraction and spectroscopic vibrational amplitudes are reported for both molecules. The electron diffraction structures are compared with those predicted by simple models previously developed to describe main group V trihalides and trihalogen oxides and sulfides. Treatment of valence angles in four-coordinate molecules is found to be the least satisfactory feature of these models.     

Physical-Mechanical Behavior and Water-Barrier Properties of Biopolymers-Clay Nanocomposites

The preparation and characterization of biodegradable films based on starch-PVA-nanoclay by solvent casting are reported in this study. The films were prepared with a relation of 3:2 of starch:PVA and nanoclay (0.5, 1.0, and 1.5% w/v), and glycerol as plasticizer. The nanoclays before being incorporated in the filmogenic solution of starch-PVA were dispersed in two ways: by magnetic stirring and by sonication. The SEM results suggest that the sonication of nanoclay is necessary to reach a good dispersion along the polymeric matrix. FTIR results of films with 1.0 and 1.5% w/v of sonicated nanoclay suggest a strong interaction of hydrogen bond with the polymeric matrix of starch-PVA. However, the properties of WVP, tensile strength, percentage of elongation at break, and Young’s modulus improved to the film with sonicated nanoclay at 0.5% w/v, while in films with 1.0 and 1.5% w/w these properties were even worse than in film without nanoclay. Nanoclay concentrations higher than 1.0 w/v saturate the polymer matrix, affecting the physicochemical properties. Accordingly, the successful incorporation of nanoclays at 0.5% w/v into the matrix starch-PVA suggests that this film is a good candidate for use as biodegradable packaging.