Paramagnetic features of the molecular complexes of poly (phenylacetylene)

The complexes of poly(phenylacetylene) (PPA) with electron acceptors are examined. The concentration of paramagnetic centers is estimated, together with the line-shape parameters and the size of the delocalization region. The temperature dependence of the ESR intensity is used to show that the paramagnetism of complexes of PPA with weak acceptors is due solely to the formation of local defect centers. PPA forms strong charge-transfer complexes with iodine, tetracyanoethylene, and tetracyanoquinodimethane; these contain ion radicals. The activation energy for charge transfer in these complexes is 0.12–0.14 eV. An ESR signal has also been observed from the complex of PPA with the electron donor naphthacene.     

Structure-reactivity relationships in reactions of alkane radical cations: Study of the proton transfer from alkane radical cations to alkane molecules in γ-irradiated disordered CCl3F/alkane systems by ESR spectroscopy at cryogenic temperatures

A summary is presented of ESR results obtained in γ-irradiated disordered CCl₃F/alkane systems at cryogenic temperatures, with respect to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The nature of the alkyl radicals formed by proton transfer is indicative for the site of proton donation and is derived unambiguously from ESR results by comparison with powder spectra of authentic isomeric alkyl radicals, obtained by γ-irradiation of various chloro and bromoalkanes in perdeuterated cis-decalin. The experiments can be divided into two main classes. (i) Experiments on n-alkane radical cations in the extended all-trans conformation, i.e. ESR results on the system CCl₃F/heptane. The ESR spectrum of γ-irradiated CCl₃F/heptane consists of a triplet due to heptane radical cations in the extended all-trans conformation. In this conformation, the unpaired electron is delocalized over the carbon-carbon σ-bonds as well as the two chain-end carbon-hydrogen bonds that are in the plane of the C---C skeleton. Superimposed on the ESR triplet is a low-intensity spectrum due to heptyl radicals, which increases drastically with increasing heptane concentration. The formation of these heptyl radicals can be attributed unambiguously to proton transfer from heptane radical cations to heptane molecules, taking place in small heptane clusters to which positive-hole transfer still occurs efficiently. At the onset of proton transfer with increasing heptane concentration only primary heptyl radicals are present, clearly showing that the proton transfer takes place selectively from a chain-end position, in accordance with the electronic structure of the reacting radical cations. At higher heptane concentration secondary heptyl radicals also appear as a result of intermolecular radical-site transfer, i.e. the nature of the heptyl radicals becomes governed by their thermodynamic stability. (ii) Experiments on n-alkane radical cations in the gauche-at-C₂ conformation, i.e. ESR results on the system CCl₃F/octane. The ESR spectrum of γ-irradiated CCl₃F/octane indicates that octane radical cations are largely in the gauche-at-C₂ conformation in this matrix, with large unpaired-electron (and positive-hole) density on one planar chain-end C---H bond and one planar penultimate C---H bond at the other side of the radical cation. Careful investigation of ESR spectra with increasing octane concentration clearly reveals that in this case secondary octyl radicals are present from the very onset of proton transfer, in accordance with the electronic structure of the reacting radical cations. The results clearly point to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules and to a strict dependence of the site of proton donation on the electronic structure and conformation of the reacting radical cations.     

Spectrophotometric and spectroscopic studies of complexation of 8-hydroxyquinoline with π acceptor metadinitrobenzene in different polar solvents

The complexation of electron donor–acceptor complexes of 8-hydroxyquinoline (8HQ) and metadinitrobenzene (MNB) have been studied spectrophotometrically and thermodynamically in different polar solvent at room temperature. A new absorption band due to charge transfer (CT) transition is observed in the visible region. A new theoretical model has been developed which take into account the interaction between electronic subsystem of 8HQ and MNB. The results indicate the extent of charge transfer complexes (CTCs) formation to be more in less polar solvents. Stoichiometry of the complex was found to be 1:1 by straight line method and ¹H NMR between donor and acceptor at the maximum absorption bands. Ionization potential (I_D) and resonance energy (R_N) were determined from the CT transition energy in different solvents. The formation constants of the complexes were determined in different polar solvents from which ΔG° formation of the complexes was estimated and also extinction coefficient of the charge transfer complex (CTC) was calculated. Oscillator strength, transition dipole strengths and maximum wavelength of the CTC (λ_CT) in various solvents and IR spectra of the CTC have also been discussed. It has been observed that all parameters described above changed with change in polarity and concentration of donor.     

ESR study on copper(II) macrocyclic polythia complexes

ESR measurements have been made on complexes formed by copper(II) with [12]anS₄, [13]anS₄, iso-[14]anS₄, and [18]anS₆; they show intramolecular reduction for the central ion by electron transfer from the donor sulfur atoms, and free cation-radicals are formed in solution. The covalency parameter ² indicates that the copper-sulfur bonds are mainly covalent. The electronic structures are analogous to those of the active centers in copper-bearing blue proteins.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 6, pp. 737–742, November–December, 1989.     

Opening of the chelate ring in paramagnetic 3,6-di-tert-butylpyrocatechate trifluorosilicate by the action of electron-donor molecules

Conclusions The changes in the ESR spectra of 3,6-di-tert-butylpyrocatechate trifluorosilicate in the presence of electron-donor Ph₃P, MeCN, and PhCN molecules indicate coordination of these molecules with the central silicon atom and subsequent opening of the chelate pyrocatechate ring with formation of phenoxyl complexes. Migration of the heteroorganic group occurs between the oxygen atoms of the pyrocatechate ligand at elevated temperatures.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1426–1429, June, 1986.     

Quantum-chemical simulation of the active center of vitamin B12

The electronic structure of porphin and corrin complexes of cobalt differing with respect to the oxidation state of the central ion has been investigated by the MO-LCAO-SCF-CNDO method with the Kai-Nishimoto parameters for transition metals. On the basis of an analysis of the distribution of the electron density and the structure of the energy spectrum, it has been shown that the oxidation-reduction processes of the complexes are accompanied by restructuring of the energy spectrum, and the differences between the electronic structures of porphin and corrin complexes have been discussed. It has been established that cobalt(I) porphin has stronger nucleophilic properties than does cobalt(I) corrin. The electronic structure of hexacoordinate complexes in which an imidazole molecule and a molecule of L (L = H₂O, CH₃ ⁺, CN^–) are axially coordinated has been calculated. The mechanisms of the dissociation of cobalt alkyl complexes and the differences between the processes of the heterolytic dissociation of porphin and corrin complexes have been discussed. It has been shown that the elimination of a CH₃ ⁺ cation, which plays an important role in biomethylation reactions, is more favorable in corrin complexes.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 4, pp. 400–409. July–August, 1986.     

ESR,spectroscopic, and quantum-chemical studies on the electronic structures of complexes formed by Cu(I) with radicals

The optical and ESR spectra have been examined for complexes of Cu(I) with various radicals, which contain various numbers of Cl ions in the central-atom coordination sphere. The spin-Kamiltonian parameters have been determined for all these radical complexes, and the observed ESR spectra have been compared with those calculated with allowance for second-order effects. The observed values for the isotropic and anisotropic components of the HFI constant from the central ion have been used to estimate the contributions from the 4s and 3d² _z orbitals of the copper ion to the unpaired-electron MO. Quantum-chemical calculations have been performed by the INDO method on the electronic structures and geometries of complexes formed by CH₂OH with Cu(I) for various Cl contents in the coordination sphere. The radical is coordinated by the orbital on the carbon atom, and the stabilities of the radical complexes decrease as the number of Cl ions in the coordination sphere increases. A geometry close to planar is proposed for the CuCl₄ ^–3 fragment in a complex containing four Cl ions.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 32–38, January–February, 1986.     

The reactivity of the electron formed in the radiolysis of aerated alkaline aqueous solutions containing tetracycline hydrochloride,at 77 K

The present study deals with the radiolysis of tetracycline hydrochloride dissolved in aerated alkaline aqueous solutions containing 0.1, 0.5 and 1M NaOH, at 77 K, as followed by ESR. The rate constants for the reactions between the electron and physical and chemical traps which are present in these solutions are calculated. These values are k_ph= =9.6·10¹⁵ l·mol^–1·s^–1 and k_ch=1.3·10¹⁰ l·mol^–1·s^–1. The reactivity of electrons that are formed in the radiolysis of water decreases in the following proportions: physical traps: chemical traps: molecules of water: 4.8·10¹⁴: 6.5·10⁸: 1.0. The electrons react preferentially with the solute instead of the solvent.From a thesis submitted by S.M.L.Guedes to the University of São Paulo in partial fulfillment of the requirements for a Doctor of Science Degree in Nuclear Technology.     

Electronic and ESR spectroscopic study of thiocyanate-containing coordination compounds of titanium(III) in alcohol solutions

In an investigation of electronic and ESR spectra, it has been established that titanium(III) with the thiocyanate ion (in ethanol) forms distorted tetrahedral and octahedral complexes. The tetrahedral compounds, which are hydroxo complexes of titanium(III) that contain one hydroxyl group and three thiocyanate ions in the coordination sphere, give signals in the ESR spectra with g_o=1.960, A_o=15.0·10^–4 cm^–1 g=1.958, A ^Ti =26.4·10^–4cm^–1, g= 1.691, A^Ti=9.0·10^–4 cm^–1. In the ESR spectra of liquid solutions, additional splitting on the nitrogen atoms is manifested(A _o ^N =2.2·10^–4 cm^–1), which indicates bonding of thiocyanate ions through the nitrogen atom. The pseudo-octahedral complexes of titanium(III), containing from two to six thiocyanate ions in the coordination sphere, are characterized by d-d absorption bands in the 520–590 nm region; and at 77°K, they give anisotropic ESR signals. On the basis of the temperature dependences of the equilibrium constants for the reaction of conversion of the tetrahedral complexes to the octahedral complexes, and also the lifetimes of the tetrahedral complexes, values have been estimated for the heat of reaction (H=26.8 kJ/mole) and the entropy change (S=–1.66 kJ/mole·°K) of the equilibrium process, and also the activation energy for the reaction of titanium(III) thiocyanate formation (E=37 kJ/mole).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 560–567, September–October, 1985.     

Application of perturbation theory to the description of the lowest excited states of porphyrin molecules

Consideration has been given to the effect of one-electron perturbation on the energy of the lowest excited singlet and triplet states of high symmetry (D_4h) of porphyrins, corresponding to the transfer of an electron from the pair of quasi-degenerate highest occupied MO a_1u and a_2u to the pair of degenerate unoccupied MO e_g. It is assumed that the MO considered satisfy the SCF equation. With the help of orthogonal transformations of the highest occupied and lowest unoccupied MO it is shown that the secular fourth-order problems for the S- and T-states have exact solutions. By comparing with experimental data and with the results of quantum-chemical calculations for the S₁, S₂ and T₁-levels the accuracy of the results obtained has been analyzed for two series of compounds (metal complexes of the azaporphyrins, and compounds with different numbers and positions of the central hydrogen atoms).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 257–267, May–June, 1990.     

Adsorption of Cu(II) and Co(II) complexes on a silica gel surface chemically modified with 2-mercaptoimidazole

The isotherms of adsorption of MeX₂ (Me = Cu²⁺, Co²⁺; X = Cl^–, Br^–, ClO ₄ ^– ) by silica gel chemically modified with 2-mercaptoimidazole (SiMI) were studied in acetone and ethanol solutions, at 25 °C. Covalently attached 2-mercaptoimidazole molecule to silica gel surface adsorbs MeX₂ from solvent by forming a surface complex. The metal is bonded to the surface through the nitrogen atom of attached 2-mercaptoimidazole. At low loading, the electronic and ESR spectral parameters indicated that the Cu²⁺ complexes are in a distorted-tetragonal symmetry field. The d-d electronic transition spectra showed that for Cu(ClO₄)₂ complex, the peak of absorption did not change for any degree of metal loading and for Cl^– and Br^– complexes, the peak maxima shifted to higher energy with lower metal loading. The CoX₂(X = Cl^–, Br^–, ClO ₄ ^– ) analogues possess a distorted-tetrahedral field.     

Theoretical analysis of electron-transfer crossover reactions between complex ions. Communication 4. Reactions involving ethylenediamine complexes of cobalt and ruthenium

Conclusions Calculations have been made within the bounds of the quantum-mechanical theory of chemical reactions of the rate constants and activation enthalpies of a number of electron transfer reactions involving the ethylenediamine complexes of cobalt and ruthenium. An estimate has been obtained of the inner-sphere reorganization energy for the Co(en)₃ ^3+/2+ complexes. The transmission coefficients for electron transfer in redox systems incorporating ethylenediamine complexes of cobalt and ruthenium have been compared, which enables a conclusion to be drawn on the degree of transferred-electron delocalization in the complexes being compared.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 353–358, February, 1984.     

Stereospecificity of quenching of excited aromatic molecules by electron acceptors

On the example of fluorescence quenching of perylene by organic electron acceptors we have shown that the radius of the quenching sphere depends on the electron affinity of the oxidizing agent. The deactivation of excited molecules of m-dinitrobenzene was achieved by tunnel electron transfer at a distance, which was significantly greater than the sum of the radii of the reagent molecules. The fluorescence quenching with phthalic anhydride, a weaker electron acceptor, occurs on contact between molecules. The kinetics of this process are described by the Smoluchowski theory of diffusion-controlled reactions. The rate of fluorescence quenching with carbon tetrachloride and nitromethane in polar solvents also increases with the increase of the coefficients of mutual diffusion, but the formal reaction radii of these processes are smaller than the sum of the radii of the interacting particles, which is due to the stereospecifity of the reaction. It was shown that the degree of stereospecificity increases with the decrease of the reaction radius.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 5, pp. 585–590, September–October, 1986.     

Study of the general properties of the adiabatic potential energy of molecules using the scaling transformation

General relations derived from the invariance of the electronic energy of molecules relative to the scaling of the electronic coordinates using the multiplier (R/R_o), which depends on the internuclear distance, and relative to the translation of the nuclear core in the Hamiltonian were studied. Universal inequalities correct for the energy and its derivative over the internuclear distance R were obtained; the question of the allowed forms of the approximations for the adiabatic potentials of molecules was investigated; the problem of the determination of the optimal form of the electronic Hamiltonian for the calculation of the second derivative of the energy of molecules over R using the perturbation theory was formulated and solved. The use of the optimal Hamiltonian form makes it possible to decrease, by 6–9 times, the absolute value of the second-order relaxation contribution for light molecules (H₂ ⁺ and H₂). With increase in the nuclear charge and the number of electrons, this gain increases rapidly, and the optimal value approaches 1.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 3, pp. 257–267, May–June, 1987.     

Energies of Association of Carbenium and Silylium Cations with Oxygen-Containing Molecules

The energies of association of Me_nH_3-n X⁺ cations (n = 0-3) with molecules of oxygen-containing bases (water, methanol, dimethyl ether) were calculated by the B3LYP/6-31G(d) method. These energies decrease with an increase in the number n of methyl groups at the X atom; this trend is more pronounced for X = C, which is due to a lower degree of charge transfer in the cation from the occupied orbitals of the methyl group to the vacant orbital of X in the case of X = Si. An increase in the association energy with an increase in the number of methyl substituents at the oxygen atom is due to an increase in the energy level of lone electron pairs of oxygen upon methyl substitution. As a result, the energy gap between the electronic levels of the unoccupied orbital of the cation and orbitals of the oxygen lone electron pairs becomes narrower, which makes the interaction between the unoccupied and occupied orbitals more efficient.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005, pp. 754–756.Original Russian Text Copyright © 2005 by Ignat’ev, Kochina.     

Quenching of the fluorescence of riboflavin by the histidinates and alaninates of nickel,copper, and zinc

The formation of complexes of histidinates and alaninates of Ni(II), Cu(II), and Zn(II) with riboflavin (RF) in the ground state and the quenching of the fluorescence of RF by these compounds has been investigated. It has been found that the quenching of the fluorescence of RF by the Cu²⁺ and Ni²⁺ complexes is caused mainly by the nonemissive energy transfer from the donor (RF) to the metal ions. In the case of the Cu²⁺, Ni²⁺, and Zn²⁺ histidinates the formation of nonfluorescing unstable complexes (K_stab — 3–10) of the metal histidinates with RF in the ground state also contribute to the quenching. Free histidine and zinc histidinate quench the fluorescence of RF by the transfer of an electron to the excited molecule of the flavin with the formation of nonfluorescing reduced RF.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 4, pp. 488–493, July–August, 1985.     

Vibrational energy storage in high pressure mixtures of diatomic molecules

CO/N₂, CO/Ar/O₂, and CO/N₂/O₂ gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T_V=1900–2300 K for N₂, T_V=2600–3800 K for CO, and T_V=2200–2800 K for O₂. The translational–rotational temperature of the gases does not exceed T=700 K. Line-of-sight averaged CO vibrational level populations up to v=40 are inferred from infrared emission spectra. Vibrational level populations of CO (v=0–8), N₂ (v=0–4), and O₂ (v=0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement.     

Electronic structure,spectra, and classification of excited electronic states of the molecules MoCl4 and MoBr4

The absorption spectra of gaseous molybdenum tetrachloride and tetrabromide have been measured in the visible and UV regions. The SCF-X-SW method has been used to calculate the electronic structure, the spectra, and the system of first excited electronic states of these molecules. The results are compared with published data for the molecules CrCl₄ and CrBr₄.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 5, pp. 606–610, September–October, 1989.     

Investigation of the kinetics and equilibrium of the transfer of protons from dithiophosphoric acids to amines

The transfer of protons from dithiophosphoric acids R₂PSSH to substituted pyridines and proton exchange between the SH groups of the same acids and NH of protonated pyridines in chloroform solution were studied. The results obtained are compared with the analogous data for monothiocarboxylic acids RCOSH. The mechanism of SH-NH exchange was established; its slow step, just as for RCOSH, is transfer of a proton in the ion pairs. In the transition from RCOSH to R₂PSSH, an increase in the thermal effect of the deprotonation of the acids by amines and the activation energy E of SH-NH proton exchange is observed, which is due to the greater stability of the R₂PSS^– anions in comparison with RCOS^– on account of the greater delocalization of charge on the two sulfur atoms. With increasing excess of acid (above two-fold), the values of E are increased, since the excess acid molecules more effectively solvate the ion pairs than the original molecular complexes.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 676–681, November–December, 1985.