The influence of alkyl substituents on the chromatographic indicator of self-association of<Emphasis Type="Italic">N</Emphasis>-containing heterocyclic compounds

The chromatographic indicator of the ability of substances to form associates in a pure liquid δT _b.p. proposed in our previous study was used to estimate the capacity for self-association of alkyl-substituted imidazoles, pyrazoles, pyrroles, oxazoles, isoxazoles, pyridazines, pyrimidines, pyrazines, and pyridines. Alkyl substituents introduced in heterocyclic compounds decrease the δT _b.p. values, which is consistent with the data on the heats of self-association of heterocyclic compounds in pure liquids. The reasons for the difference between the boiling point of a compound at standard pressure and its “gas-chromatographic boiling point” are discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp 319–322, February, 2000.     

Temperature dependence of the free energy of sorption ofn-alkanes and energy contribution of their methylene groups on columns with fullerene C60

The partial molar free energy, enthalpy, and entropy of sorption of C₁₁−C₂₃ n-alkanes were calculated on the basis of the GC data obtained on the glass capillary column coated with fullerene C₆₀ (Ful-60) as stationary phase. The thermodynamic parameters ofn-alkane sorption on a column with Ful-60 and a fused silica capillary column with polydimethylsiloxane OV-1 were determined and compared. The enthalpy-entropy compensation effect for the sorption ofn-alkanes on Ful-60 and OV-1 was found. A linear dependence of the partial molar free energy ofn-alkane sorption on the temperature of analysis and carbon chain length was found. The free energy contributions of the methylene groups were calculated, and their temperature dependences were studied. The differences in the temperature dependences of the energy contributions of methylene groups ofn-alkanes on Ful-60 and OV-1 were revealed. The entropy contribution is 68–82% of the enthalpy contribution which indicates a substantial role of the number of contacts with Ful-60 in retention ofn-alkanes. The ability of Ful-60 for dispersive interactions is similar to those of nonpolar liquid phases and substantially differs from that for carbon adsorbents. Fullerene columns were shown to be convenient for analysis of highly boiling organic substances in aqueous and organic solutions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1490–1495, August, 1999.     

Kinetics of heat release during the reaction ofn-decane with nitrogen dioxide in the liquid phase

The rates of heat release in the nitrogen dioxide—n-decane system at a molar ratio of nitrogen oxides ton-decane (β) from 2.4·10⁻³ to 3.1 and gaseous volumes per mole ofn-decane (V(g)) equal to 0.05–4.5 were studied in the 55.2–92.8 °C temperature range. The initial rate of the process is determined by the interaction of NO₂ withn-decane. The equilibrium constants of dissociation of N₂O₄ inn-decane and Henry's constants of NO₂ and N₂O₄ in ann-decane solution were determined by complex analysis of the thermodynamic equilibrium in the NO₂—n-decane system and dependences of the initial rates onV(g) and β. The experimentally observed self-acceleration of the process in the region of high β and lowT values was suggested to be due to the reaction of N₂O₄ with intermediate oxidation products. The rate constants of the reaction of NO₂ withn-decane were compared with analogous values determined in its mixtures with HNO₃ solutions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1789–1794, October, 1997.     

Estimation of hydrogen bond energy in n-alkanols from the gas chromatography data

The retention factors of C1—C9 n-alkanols on a capillary column with the SE-30 nonpolar phase were determined for temperatures between 320 and 400 K. From the temperature dependence of the free energy change, variations in the enthalpy of sorption of n-alkanols on the SE-30 polymethylsiloxane phase were calculated. The difference between the enthalpies of condensation and sorption was used to estimate the contribution of hydrogen bonding to self-association of pure liquid alcohols. The calculated energies of hydrogen bonds in liquid n-alkanols are in good agreement with the published data.     

Sodium and silver phosphate glasses doped with chlorides of Fe,Mn and Zn

A number of samples of sodium and silver phosphate glasses doped with various compositions of some transition metals viz. iron, manganese and zinc chlorides alongwith undoped samples of sodium and silver phosphate glasses were synthesized and characterized by X-ray diffraction, IR spectral, electrical conductivity and differential scanning calorimetry (DSC). The glass transition temperature (T _g) and crystallization temperature (T _c) values obtained from DSC curves were found to increase with increasing concentration of the dopant Fe/Mn/Zn chlorides in both sodium and silver phosphate glasses and the following sequence is observed: T _g(–FeCl₃)>T _g(–MnCl₂)>T _g(–ZnCl₂) T _c(–FeCl₃)>T _c(–MnCl₂)>T _c(–ZnCl₂) The increase in T _g and T _c values indicate enhanced chemical durability of the doped glasses. The electrical conductivity values and the results of FTIR spectral studies have been correlated with the structural changes in the glass matrix by the addition of different transition metal cations as dopants.     

Gas chromatographic indicator of the ability of five- and six-membered heterocyclic nitrogen-containing compounds for self-association in pure liquids

The retention indices of five- and six-membered nitrogen-containing heterocyclic compounds (pyrrole, pyrazole, imidazole, 1,2,4-triazole, pyridine, pyridazine, pyrimidine, pyrazine, 1,3,5-triazine, isoxazole, and oxazole) were determined on a capillary column with the OV-101 nonpolar stationary phase. The difference between the experimentally determined boiling point of the sorbate and that calculated from GC data (δT _b.p.) was proposed as a gas chromatographic indicator sensitive to the ability of substances to form self-associates in pure liquids. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 730–733, April, 1999.     

A study of alkyl radicals in the matrix of polycrystallinen-alkane γ-irradiated at 77 K

The composition of alkyl radicals (AR) formed by γ-radiolysis (T=77 K) of polycrystallinen-alkanes with different lengths of the carbon chain (C(5), C(7), C(10), C(11), and C(18)) and their polymeric analog (polyethylene) was estimated from the ESR spectra. The ESR spectra of the irradiatedn-alkanes are superpositions of the signals from the H₃CC^.HCH₂− and −CH₂C^.HCH₂− radicals, whose HFS constants with α and β protons as well as the equilibrium conformation are independent of the chain length of then-alkane molecule. A dependence of the concentration of the radicals on the chain length ofn-alkane was found. The absence of the −CH₂C^.H₂ radicals that may arise upon H atom elimination from the Me fragments of then-alkane molecules is most likely related to the transfer of excitation energy from the Me group to the neighboring methylene fragment and the transformation of the −CH₂C^.H₂ radicals into H₃CC^.HCH₂− radicals. With account for this, the concentrations of the AR formed were suggested to be proportional to the number of H atoms at the corresponding C atom. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1034–1037, June, 2000.     

Evaluation of the polarity and boiling points of nitrogen-containing heterocyclic compounds by gas chromatography

Experimental gas-chromatographic data for 55 aromatic and aliphatic N-containing heterocyclic compounds and C₅–C₁₃ n-alkanes are analyzed. The polar constitutents of the boiling points (T _pol) were calculated as the difference between the boiling points measured directly and the boiling points calculated from GC data obtained on a nonpolar column. Depending on the number, nature, and position of heteroatoms and alkyl groups in the rings and the structures of the molecules, theT _pol values vary from −11° to +82°. The possibility of usingT _pol values to estimate the physicochemical properties of compounds is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp., 323–326, February, 2000.     

Effect of the nature of zeolite and modifying additives on the activity of zeolite-containing catalysts inn-butane isomerization

Isomerization ofn-butane on various types of zeolites (ZVM, ZVK, mordenite, and Y) modified with transition metals and cationic and anionic additives was investigated. Under the conditions studied, H-forms of zeolites are inactive. Pt-containing systems based on the H-form of ZVM (HZVM) are the most efficient catalysts forn-butane isomerization, and the yield of isobutane reaches 20–26 wt.% at a selectivity of 40–45%. Modification of this catalyst with Ga and Fe compounds or with an aqueous solution of HCl increases the selectivity with respect to isobutane up to 70–90%. Introduction of Zn²⁺ cations or F⁻ and SO₄ ²⁻ anions into the Pt-containing HZVM system decreases the selectivity and yield of isobutane due to the formation of very strong acidic centers on which disproportionation and hydrocracking ofn-butane mainly occur. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1281–1285, July, 1999.     

An equation for the calculation of retention indices in temperature-programmed gas chromatography with allowance for the nonlinear variation of the retention parameters ofn-alkanes

In view of the nonlinear variation of the temperature increments ofn-alkanes found previously, the accuracy of the calculations of the retention indices (I _pr) of substances in temperature-programmed capillary gas chromatography carried out in terms of six known equations was verified. A new four-parameter equation was proposed, and a general method for the calculation of its coefficients, suitable for all stationary phases, based on the adjusted retention times ofn-alkanes was suggested. The coefficients of the equation for 12 temperature variation programs were determined. Using the homologous series of methyl esters of fatty acids as an example, it was shown that the proposed equation ensures the minimum error of determination ofI _pr under various conditions. The equation also makes it possible to carry out interpolation and extrapolation calculations. The coefficients of the equation are found using the least-squares method based on data for any 4–5 referencen-alkanes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 323–327, February, 1997.     

Isomerization ofn-pentane andn-hexane on modified zeolites

The activity and selectivity of catalysts based on TsVM (an analog of ZSM-5), Beta, and La-H-Beta zoelites modified by Pt, Pt−Fe, and Pt−Ga were studied in the isomerization of C₅ and C₆ linear alkanes. The Pt/HTsVM, Pt/H-Beta, and Pt/La-H-Beta catalysts are efficient inn-pentane isomerization, whereas the Pt/H-Beta and Pt/La-H-Beta are most active inn-hexane isomerization. Nearly equilibirum isoparaffin yield at a selectivity of at least 95–96% is reached on these catalysts unlike other zeolite systems. The overall yield of 2,2-and 2,3-dimethylbutanes is 22 wt.%. The hexane isomers are not formed over the Pt/HTsVM catalyst due to the molecular-sieve properties of this type of zcolites. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya No. 11, pp. 1866–1869, November, 2000.     

Microstructure and surface composition of corn starch cryogels with sorbed organic flavoring agents

The surface composition and surface microtopography of corn starch cryogels obtained from a 3% starch sol containing organic flavoring agents were studied by X-ray photoelectron spectroscopy and scanning electron microscopy. The cryogels withn-octanol,n-octyl methyl ketone, andn-octyl acetate have a well-developed microtopography and their surface layer (5–20 nm) is enriched in sorbed compounds.n-Octyl acetate, unliken-butyl acetate, influences the composition and microstructure of the cryogel surface. The sorbedn-octyl acetate occupies 20–25% of the surface, which has a well-developed topography and a porous structure. Drying and evacuation do not change the concentration of the organic sorbates having a C₈ alkyl group in the surface layer. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 312–314, February 2000.     

N-(n-Alkyl)-N′-diphenylphosphorylureas as a new type of extractants for preconcentration of actinides and lanthanides

N-(n-Alkyl)-N′-diphenylphosphorylureas Ph₂P(O)NHC(O)NHC _n H_2n+1 (n = 6–10) were synthesized by the reaction of diphenylphosphoryl isocyanate with primary n-alkylamines C _n H_2n+1NH₂. The products (especially N-diphenylphosphoryl-N′-n-octylurea) are efficient extractants capable of extracting actinides and lanthanides from nitric acid solutions with high distribution coefficients. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 374–379, February, 2008.     

Thermodynamic studies of (R)-BINOL-menthyl dicarbonates

The (R)-BINOL-menthyl dicarbonates, one of the most important compounds in catalytic asymmetric synthesis, was synthesized by a convenient method. The molar heat capacities C _p,m of the compound were measured over the temperature range from 80 to 378 K with a small sample automated adiabatic calorimeter. Thermodynamic functions [H _T–H _298.15] and [S _T–S _298.15] were derived in the above temperature range with a temperature interval of 5 K. The thermal stability of the substance was investigated by differential scanning calorimeter (DSC) and a thermogravimetric (TG) technique.     

Photoreduction ofortho-benzoquinones in the presence ofpara-substitutedN,N-dimethylanilines

Photoreduction ofo-benzoquinones irradiated at the wavelengths λ_max ≈ 400 and 600 nm corresponding to the S(π → π*) and S(n → π*) electron transitions in the >C=0 groups, respectively, in the presence ofN,N-dimethylaniline and its derivatives was studied. The apparent rate constants of the photoreduction (k _H) ofo-quinones are determined by the free energy of electron transfer from the amine molecule to a photoexcitedo-quinone molecule (ΔG _e.t). The ΔG _e.t. values are calculated as the sums of the energies of the 0→0 transitions of the lowest triplet excited state ofo-quinones, the reduction energies ofo-quinones, and the oxidation energies of amines (the last two terms are numerically equal to the corresponding redox potentials). The maximum rate of photoreduction was found for ΔG _e.t≈0. The reaction mechanism is proposed, in which the reversible formation of a triplet exiplex is the rate-determining stage and hydrogen transfer proceeds in parallel with electron transfer within the exiplex. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1515–1521, September, 2000.     

Temperature dependence of excess enthalpies for systems containing normal hexadecane

Excess enthalpies, and heat capacities derived therefrom, have been obtained between 25 and 65 or 75°C at a constant concentration for cyclohexane and octamethylcyclotetrasiloxane mixed with normal hexadecane and with a highly branched C₁₆ isomer, 2,2,4,4,6,8,8-heptamethylnonane, and also forcis-andtrans-decalin mixed withn-C₁₆. Theh ^E values withn-C₁₆ are positive and much larger than with the branched-C₁₆. They decrease rapidly withT so thatc _p ^E is large and negative. These results imply the presence of orientational order in then-C₁₆, which is destroyed on mixing with the other component and which decreases withT. Theh ^E fortrans-decalin+n-C₁₆ is much smaller than forcis-decalin+n-C₁₆, and becomes negative with increase ofT. This change of sign, which is unexplained by current theory, is interpreted as due to an interference of the flat, plateliketrans-decalin molecule with the molecular motion of then-C₁₆ chain.     

α- and β-cyclodextrin complexes with n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates. A molecular mechanics study of 1:1 and 1:2 associations

Molecular mechanics (MM) methods were employed to evaluate stabilization upon formation of inclusion compounds between two different guest molecules and α- and β-cyclodextrins (CDs) for two different stoichiometries 1:1 and 1:2. The two guest molecules studied were n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates with variety of chain lengths. The computed stability for the inclusion compounds between α-CDs and n-alkyl carboxylic acids reproduced experimental data reported in the literature. The transition between 1:1/1:2 complexes occurred at an alkyl chain length of n_C=9. It was previously demonstrated by diffusion coefficients measures that a stable 1:2 stoichiometry inclusion compound could be formed between n-alkyl p-hydroxy benzoates and α-CD for the chain length n_C>4. The computed results reproduced the experimental ones. The combination between OPLS and GB/SA resulted in better agreements with experiments than those obtained with MM2 and MM3.     

Electrochemical characteristics of La0.6Sr0.4CoO3-δ, Pr0.6Sr0.4CoO3-δ and Gd0.6Sr0.4CoO3-δ on Ce0.85Sm0.15O1.925 electrolyte

Cyclic voltammetry, chronoamperometry and electro-chemical impedance have been used for the analysis of the following medium temperature half-cells: Ce_0.85Sm_0.15O_1.925| La_0.6Sr_0.4CoO_3-δ, Ce_0.85Sm_0.15O_1.925| Pr_0.6Sr_0.4CoO_3-δ and Ce_0.85Sm_0.15O_1.925| Gd_0.6Sr_0.4CoO_3-δ. The influence of the atomic mass of the A–site cation in the perovskite cathode on the oxygen reduction kinetics has been discussed. The total polarisation resistance, obtained from the Z′′, Z′-plots, increases with the rise of atomic mass of the cation in the A-site position. Two different time constants have been obtained for the oxygen electroreduction process, and the replacement of La³⁺ by Gd³⁺ in the cathode material decreases somewhat the surface catalytic activity, but the noticeably higher low-frequency series resistance, i.e. mainly diffusion-like mass transfer resistance, values have been obtained. However, the mainly diffusion-limited process at T≤773 K for Gd_0.6Sr_0.4CoO_3-δ and the kinetically mixed process (diffusion + charge transfer) for Pr_0.6Sr_0.4CoO_3-δ and La_0.6Sr_0.4CoO_3-δ have been established. At higher temperature (T≥993 K) and more negative potentials, the O₂ reduction process is limited mainly by the heterogeneous charge transfer step. Presented at the fourth Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005.     

Charge transfer in complexes of C60 and C70 in solutions and solid state

Electronic absorption spectra of complexes of C₆₀ and C₇₀ fullerenes with donors, tetrathiafulvalene and pyranylidene derivatives, were studied in solutions and in the solid state. Charge transfer bands were found in the 680–1300 nm range. The charge transfer energies (hv _ct) for the C₆₀ and C₇₀ complexes in solutions are close and almost independent of the solvent polarity. For the C₆₀ complexes in the solid state, the dependence ofhv _ct on the ionization potential (IP) of donors was found to behv _ct=0.82IP–3.93 eV. In the C₆₀ complexes in the solid state, thehv _ct values are 0.15–0.20 eV lower than those in the solution. The linear dependences ofhv _ct onIP of donors for the C₆₀ complexes lie 0.6–0.7 eV higher than those in the complexes with tetracyanoethylene (TCNE). This is associated with lower values of the electron affinity of C₆₀ and the energy of the electrostatic interaction in the fullerene complexes as compared to those of the TCNE complexes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 478–483, March, 1999.     

The Merrifield–Simmons index in (<Emphasis Type="Italic">n</Emphasis>, <Emphasis Type="Italic">n</Emphasis> + 1)-graphs

A (n, n + 1)-graph G is a connected simple graph with n vertices and n + 1 edges. In this paper, we determine the upper bound for the Merrifield–Simmons index in (n, n + 1)–graphs in terms of the order n, and characterize the (n, n + 1)–graph with the largest Merrifield–Simmons index.