Electron transfer in the photochemical reactions of phenothiazine with halomethanes

Photochemical transformations of phenothiazine (PTA) in solutions of halomethanes CH_nX_4–n (X = Cl, Br; n = 0, 1, 2) and in n-hexane—CH_nX_4–n mixtures under the irradiation with = 337 and 365 nm were studied. The rate constants of quenching of PTA fluorescence with halomethanes (k _q) are 4·10⁵—1.3·10¹⁰ L mol^–1 s^–1. The process occurs due to electron transfer with the C—X bond cleavage in the radical anion fragment of the primary radical ion pair. This results in the formation of the stable radical cation salt (PTA^·+X^–). The plot of k _q vs. free energy of electron transfer corresponds to the Rehm—Weller empirical equation for a one-electron process and is satisfactorily described in terms of the theory of nonradiative electron transitions in the approximation of one quantum vibration.     

A theoretical investigation of the electronic and geometrical structure of silicon fluorides SiF n and their anions SiF n −,n=1−6

The electronic and geometrical structure of the ground and low-lying excited states of the SiF _n and SiF_n ^– series (n = 1-6) are calculated using the density functional method. Energies of fragmentation through different decay channels were evaluated for both series and found to be in good accord with the experimental data and results of nonempirical calculations. The adiabatic electron affinity (EA) of the neutral series is estimated for the first time. The SiF₄ ^– anion is shown to be stable toward dissociation though its neutral precursor possesses adiabatic EA close to zero. The SiF₅ ^– and SiF₆ ^– anions are stable toward dissociation in the gas phase; however, the neutral radical SiF₅ is near the stability threshold and SiF₆ is unstable as regards dissociation to SiF₄+F₂. An interesting peculiarity of the silicon fluoride anions is their similar energy of F-detachment, i.e. the affinities of all the neutral SiF_n, (n = 0-5) for the fluoride anion are similar.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 44–53, January, 1993.     

Reactions in clusters of acetone and fluorinated acetones triggered by low energy electrons

Electron attachment to clusters of acetone (A), trifluoroacetone (TFA) and hexafluoroacetone (HFA) is studied in a crossed beam experiment with mass spectrometric detection of the anionic products. We find that the electron attachment properties in A change dramatically on going from isolated molecules to clusters. While single acetone is a very weak electron scavenger (via a dissociative electron attachment (DEA) resonance near 8.5 eV), clusters of A capture electrons at very low energy (close to 0 eV). The final ionic products consist of an ensemble of molecules (M) subjected to the loss of two neutral H₂ molecules ((M_n−2H₂)⁻, n ≥ 2). Their formation at low energies can only be explained by invoking new cyclic structures and polymers. In clusters of TFA, anionic complexes containing non-decomposed molecules (M_n⁻) including the monomer (M⁻) and ionic products formed by the loss of one and two HF molecules are observed. Loss of HF units is also interpreted by the formation of new cyclic structures in the anionic system. HFA is a comparatively stronger electron scavenger forming a non-decomposed anion via a narrow resonant feature near 0 eV in the gas phase. In HFA clusters, the non-decomposed parent anion is additionally observed at higher electron energies in the range 3–9 eV. The M⁻ signal carries signatures of self-scavenging processes, i.e., inelastic scattering by one molecule and capture of the completely slowed down electron by a second molecule within the same cluster. The scavenging spectrum is hence an image of the electronically excited states of the neutral molecule.     

Reaction mechanism and chemical polarization of phosphorus nuclei when diethyl phenylazophosphonate is reacted with sodium ethylate

Conclusions The action of ethoxide anion on diethyl phenylazophosphonate leads to nucleophilic substitution of the phenyldiazenyl anion, the cleavage of nitrogen from it, and subsequent electron transfer from the formed phenyl anion to the starting phosphonate molecule. Singlet-triplet evolution in the singlet radical pair that is formed here and its recombination give diethyl diphenylhydrazophosphates, which exhibit the³¹P CPN effect.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2796–2799, December, 1976.     

Quantum-chemical study of the electronic structure of adamantane and the 1-adamantyl cation,anion, and radical

The spatial and electronic structures of the adamantane molecule (AdH), the 1-adamantyl radical (1-Ad^.), the 1-adamantyl cation (1-AD⁺), and the 1-adamantyl anion (1-Ad^–) have been calculated by the SCF-MO-LCAO method in the all-valence-electron MINDO approximation. The calculated heats of formation attest to the fact that the homolytic cleavage of the C-H bond is energetically most advantageous in the gaseous phase or in nonpolar inert solvents. The formation of a 1-Ad^–, H⁺ pair is associated with the largest expenditure of energy due to the large ionization potential of the hydrogen atom.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 357–360, May–June, 1989.     

Length of Soliton Wave in the Excited State in the Cations of Symmetrical Pyridocyanines and Their Carbo and Hetero Analogs

Quantum-chemical calculations in the AM1 approximation were undertaken for the optimized geometry of the cations of polymethine dyes [R⁺—(CH=CH) _n —CH=R], where R represents phenyl substituents or pyridinium, pyrylium, and thiopyrylium groups and n = 1-6, in the ground state and for the electron density distribution in the Frank – Condon excited state. It was found that excitation of the molecule by a light quantum leads to a decrease in the length of the charge wave (soliton). It was shown that the length of the soliton depends on the electron-donating character of the terminal groups R and on the length of the conjugated chain, while shortening of the soliton in the thiopyrylocyanines leads to disruption of its symmetry.     

Theoretical study of nonrigid rotation of the tetrahedral cation and anion in NH4BH4

Segments of the potential energy surface of NH₄BH₄ corresponding to nonrigid rotations of the NH₄ ⁺ cation and BH₄ ^– anion relative to each other were calculated using the SCF/3–21G approximation with complete geometrical optimization and MP3/6–31G^* approximation for special points. The barriers for these motions are only several kcal/mole and the molecule is very nonrigid relative to several types of intramolecular rotations (rotation about the B-N bond, rotations with change in the dentation of the cation and anion, and rotations of the cation and anion facilitating the formation of some H₃N-N..H-BH₃ bonds and the cleavage of other such bonds). The strong geometric deformation of the NH₄ ⁺ and BH₄ ^– ions, the polarization of the electron density in these ions, and the H-H interaction between the hydrogen atoms belonging to different ions were discussed. The structural nonrigidity of NH₄BH₄ is closely related to its stability relative to decomposition.Institute of New Chemical Products, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 5, pp. 27–34, September–October, 1989.     

ESR study of free radicals formed upon UV irradiation of nitropentafluoroacetone in various solvents

The formation of CF₃C(O)CF₂N(O^.)O₂CF₂C(O)CF₃ free radicals upon the UV irradiation of nitropentafluoroacetone (1) in toluene and mesitylene is established by ESR. The most likely cause of their formation is the one-electron oxidation of the solvents by photoexcited1 followed by decay of the radical anion formed from1 with the expulsion of an NO₂ anion and attachment of the radical to a molecule of original1. The irradiation of 1 in triethylsilane results in the elimination of a fluoride ion and fixation of a CF₃COCFN(O^–)O' radical. UV irradiation of ketone1 in pentane results in the abstraction of a hydrogen atom from the solvent and the formation of a CF₃COCF₂N(OH)O ' radical.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 512–514, March, 1993.     

Structural peculiarities and the possibility of the existence of small water cluster anions (H2O) n − withn≤4

Structures of (H₂O) _n ⁻ anions withn≤4 were optimized at the UHF/4-31++G^** level and their stability was estimated at the MP2/4-31++G^** level. The trimer anion has a chain-like structure while the tetramer anion can exist either in a chain-like or a cyclic configuration. In the dimer anion and in the chain-like anions, the excess electron density is localized on the terminal water molecule, an acceptor of the H-bond proton. In the cyclic anion, it is uniformly distributed over the free hydrogen atoms. All considered anions have energy values higher than those of the corresponding neutral oligomers. The detachment of an electron from the anions should proceed with the liberation of energy. However, trimer and larger anions are stable against dissociation into individual water molecules and a free electron. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 41–46, January, 1997.     

Influence of the linking chain length on the recombination kinetics of covalently bonded triplet radical pairs and magnetic field effects

Nanosecond laser flash photolysis technique is used to study the formation and decay kinetics of covalently linked triplet radical pairs (RP) formed after photoinduced electron transfer in the series of 21 zinc porphyrin—chain—viologen (Pph—Sp_n—Vi²⁺) dyads, where the number of atoms (n) in the chain increases from 2 to 138. In poorly viscous polar solvents (acetone, CHCl₃—CH₃OH (1 : 1) mixture), the dependence of the rate constant of RP formation on n can be described by the equation k _e = k _e ⁰ n ^–a at k _e ⁰ = 2.95·10⁸ s^–1 anda = 0.8. In the zero magnetic field, the RP recombination rate constant (k _r(B = 0)) is significantly lower than k _e and ranges from 0.7·10⁶ to 8·10⁶ s^–1. The dependence of k _r(B = 0) on n is extreme. The dependence k _r(B = 0) reaches a maximum at n = 20. In the strong magnetic field (B = 0.21 T), the significant retardation of triplet RP recombination is observed. The chain length has an insignificant effect on k _r(B = 0.21 T), which ranges from 0.3·10⁶ to 0.9·10⁶ s^–1. The regularities found are discussed in terms of the interplay of molecular and spin dynamics.     

Perfluoroalkylphosphines and arsines obtained by Pd-catalyzed cross-coupling reaction with organoheteroatom stannanes

Pd-catalyzed cross-coupling reaction of organoheteroatom stannanes containing elements of the groups 15 (P, As) and 16 (Se) with perfluoroalkyl iodides (R_fI) was studied. Herein, a one-pot two-step reaction to form P–R_f, As–R_f and Se–R_f bonds is reported. The stannanes n-Bu₃SnZPh_n (Z = P, As, Se; n = 1–2) were generated in situ by the reaction of the Ph_nZ⁻ anion with n-Bu₃SnCl. The cross-coupling reactions of these stannanes with R_fI afforded C-heteroatom products, new perfluoroalkylarsines and perfluoroalkylselenides in good yields (47–90%) and perfluoroalkylphosphines in moderate yields (15–48%).     

Enzymatic oxidation of lignin and compounds modeling it. III. Formation of singlet oxygen in the peroxidase oxidation of lignin

The main emitters of radiation in the aerobic oxidation of lignin are the carbonyl groups in an excited state and singlet oxygen. It has been shown that the main source of O₂(¹) may be the radical anion O^–·₂. Singlet oxygen and the radical anion are by-products of the radical oxidation of lignin.Siberian Scientific-Research Institute of Cellulose and Board, Bratsk. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 510–514, July–August, 1984.     

Nature of the Chemical Bond and the Mutual Influence of Ligands in Co(III) Dioximinates

The results of MWG calculations of the electronic structures of real Co(III) complexes [Co(HD)₂L¹L²] ⁿ were used to analyze the electron density distribution and to determine the charges on atoms and configurations, where nis the charge of the complex and HD is the acid residue of dimethylglyoxime (H₂D); L¹= NH₃at L²= NH₃, Cl^–, Br^–, or I^–and L¹= L²= Cl^–; and L¹= H₂O or NO^– ₂at L²= NO^– ₂, with self-consistency over all atoms of the system and over d, s, and pconfigurations of cobalt. The mutual influence of the ligands (trans- and cis-) was shown to be determined by the atomic charges and bond orders on the axial coordinate and in the equatorial plane of the complex. The following order of the trans-effect was proposed: I^–> Br^–> Cl^–> NO^– ₂> NH₃> H₂O. The effects of the electronic factors on distorsion and conformational processes in the complexes were discussed.     

Ions with a strong symmetric H-bond in solutions of sodium acetate in acetic acid

The multiple attenuated total reflection IR spectra of solutions of sodium acetate in acetic acid have been recorded in the range from 9000 to 4000 cm^–1. The CH₃COO^– anion and an acid molecule form be complex (CH₃COO...H...OOCCH₃) with a strong symmetric H-bond.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp, 1854–1857, July, 1996.     

Activation Enthalpy and Entropy for the Decay of 9-Cyanophenanthrene Exciplexes

Activation parameters were studied for the decay of 9-cyanophenanthrene exciplexes with some weak electron donors (the Gibbs energy of electron transfer G ^* _et ranging from –0.02 to –0.09 eV), which displayed fairly high emission in both nonpolar and polar aprotic solvents. It was shown that the activation enthalpy of decay for the exciplexes is low, while the activation entropy reaches –(100–150) J mol^–1 K^–1, which is consistent with the two possible decay mechanisms: by dissociation into free radical ions or by intersystem crossing into the triplet state.     

Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

On the coupled oxidation-reduction mechanism of molecular nitrogen fixation

A new mechanism for the catalytic reduction of N₂ was proposed. According to the mechanism, reduction is preceded by the oxidation step with the formation of N₂O. The mechanism allows the participation of weaker reducing agents than those in purely reductive processes. Probable individual steps are considered, in particular, the oxygen atom transfer from the superoxide radical anion O₂ ^–· in a cyclic complex containing the N₂ molecule in the coordination sphere of a metal. The proposed mechanism can explain N₂ reduction involving recently discovered nitrogenase in which O₂ ^–· acts as an electron donor and N₂ reduction in purely chemical systems including the air nitrogen and relatively weak reducing agents.     

Interaction of iron carbonyls with Lewis bases

The interaction of iron carbonyls Fe _n (CO) _m (wheren = 1,m = 5;n = 2,m = 9;n = 3,m = 12) with anionic Lewis bases (H^–, F^–, Cl^–, Br^– , I^–, CN^–, SCN^–, N₃ ^–, MeSO₃ ^–, MeCO₂ ^–, CF₃CO₂ ^–, S₂ ^–, CO₃ ^2–, and SO₄ ^2–) passes through two-stage redox-disproportionation. The first stage is the formation of an iron carbonyl-base complex, [Fe _n (CO) _m–1C(O)L]^–, and the second is a single-electron reduction of this complex by another molecule of the initial iron carbonyl, giving rise to Fe(l) and Fe(–l) derivatives.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 248–249, January, 1996.     

Vibrational spectroscopic and density functional theory studies on ion solvation and ion association of lithium tetrafluoroborate in N,N-dimethylcarbamoyl chloride-based solvents

Solvation and association interactions in solutions of LiBF₄/DMCC (DMCC for N,N-dimethylcarbamoyl chloride) and LiBF₄/DMCC–DME (DME for 1,2-dimethoxyethane) have been studied as a function of concentration of lithium tetrafluoroborate by infrared and Raman spectroscopy. Strong interactions between Li⁺ and solvent molecules or BF₄⁻ anions are observed. The apparent solvation numbers of Li⁺ in LiBF₄/DMCC solutions were deduced. Band-fitting to the B–F stretching band of BF₄⁻ anion permits detailed assess of the ion pairing. Based on the calculations of density function theory, optimal structures of Li⁺(DMCC)_n (n = 1–3) were suggested. It is found that the lithium ion was preferentially solvated by DME in DMCC–DME binary solvents. This finding is supported by quantum chemistry calculations.     

Effect of the Charge of Complex Anions on the Rate of Their Delivery to the Cathode in a High-Voltage Electrolysis

The depression of mass transfer of metal-containing anions MeL _n ^1–n , which discharge at the cathode simultaneously with hydrogen from solutions containing a weak acid as the supporting electrolyte, is studied by a numerical method. The anion's charge has an ambiguous effect on the depression. Depending on the anion concentration, at a constant hydrogen evolution rate, the depression may both increase and decrease with increasing charge of the anion. The degree of dissociation of the supporting acid also affects the depression. These effects are explained within a modern theory of correlation interactions between diffusion–migration fluxes of parallel processes.