Theoretical studies of the benzene oxide—oxepin valence tautomerism

We have calculated the geometry and energy of the valence tautomers benzene oxide and oxepin using the semiempirical AM1 model and the 6–31G and 6–31G^* basis sets utilizing full geometry optimization. In the oxide the folding angle, the angle between the epoxide ring and the adjacent plane containing four carbon atoms, is about 106°. The carbon skeleton is almost planar, the folding angle, the angle between the two four-carbon atom planes being about 175°. In contrast, oxepin is found to have a marked boat-shaped structure with the corresponding and angles about 137° and 159°, respectively. The AM1, 6–31G, and 6–31G^* calculations give –11.4, –10.8, and –2.9 kcal mol^–1 for the energy change that accompanies the valence tautomerism, oxide-oxepin, compared to an experimental value of about +0.3 kcal mol^–1. Single point calculations of the energies at the 6–31 G^* geometry using Møller-Plesset perturbation theory to second order (MP2/6–31 G^*) and third order (MP3/6–31G^*) give E _T =+3.3 and +0.8 kcal mol^–1. The values for the energy change in the transfer of epoxide oxygen from ethylene oxide to benzene using AM1, 6–31G, and 6–31G^* are in good agreement, viz., +31.1, +34.5, and +33.6 kcal mol^–1, respectively. A large positive energy change is to be expected in view of the loss of benzene aromaticity.     

An Ab Initio Molecular Orbital Theory and Density Functional Theory (DFT) Study of Conformers and Rotamers of 4-Substituted (Methyl, Hydroxymethyl, Methanoyl, Ethanoyl, Cyano, Fluoro, Chloro, Bromo, Acetoxy) Tetrahydro-2H-thiopyran-1,1-dioxides

The structures and energies of axial and equatorial conformers and rotamers of 4-substituted tetrahydro-2H-thiopyran-1,1-dioxides (tetrahydrothiopyran-1,1-dioxides, thiacyclohexane-1,1-dioxides, thiane-1,1-dioxides, and 1,1-dioxothianes; CH₃, CH₂OH, CHO, COCH₃, CN, F, Cl, Br, and OCOCH₃) were calculated using the hybrid density functionals B3LYP, B3P86, and B3PW91, as well as MP2 and the 6-31G(d), 6-31G(2d), 6-31G(3d), 6-31G(d,p), and 6-31+G(d) basis sets. MP2/6-31+G(d)/ /HF/6-31+G(d) [–G° = 1.73 kcal/mol], B3P86/6-31G(d) [–G° = 1.75 kcal/mol], and B3PW91/6-31G(d) [–G° = 1.85 kcal/mol] gave conformational free energy (G°) values at 180 K for 4-methyltetrahydro-2H-thiopyran-1,1-dioxide which were similar to the reported experimental values for methylcyclohexane (–G° = 1.80 kcal/mol), 4-methyltetrahydro-2H-thiopyran (–G° = 1.80 kcal/mol), and other 4-methyl-substituted heterocycles. All levels of theory showed that the conformational preferences of the 4-methanoyl (4-formyl), 4-ethanoyl (4-acetyl), and 4-cyano substituents were small. The HF calculations gave conformational free energy (G°) values for 4-chlorotetrahydro-2H-thiopyran-1,1dioxide which were closer to the experimental value than the MP2 and density functional methods. The best agreement with available experimental data for 4-bromotetrahydro-2H-thiopyran-1,1-dioxide was obtained from the HF/6-31G(2d), HF/6-31G(3d), and B3LYP/6-31G(2d) calculations, and, for 4-acetoxytetrahydro-2H-thiopyran-1,1-dioxide, from the HF/6–31G(3d) calculations. The conformational free energies (G°) and relative energies (E) of the conformers and rotamers have been compared with the correspondingly substituted cyclohexanes and tetrahydro-2H-thiopyrans and are discussed in terms of dipole–dipole (electrostatic) interactions and repulsive nonbonded interactions (steric) in the most stable axial and equatorial conformers. The axial S=O bond lengths are shorter than the equatorial S=O bond lengths and the C2–C3 bond lengths in the substituents with carbon-bonded to the ring are shorter than the C3–C4 and C4–C-5 bond lengths. In contrast, the C2–C3 bond lengths in the 4-halogen and 4-acetoxy substituents are longer than the C3–C4 and C4–C-5 bond lengths.     

Ion-pair extraction of tetracyclines

Summary Ion-Pair Extraction of Tetracyclines Tetracyclines form ion-pair complexes with dyes that can be extracted into chloroform and re-extracted into 0.1N hydrochloric acid and finally quantified by spectrophotometry. Of the dyes studied methyl red, methyl orange, bromophenol blue and bromothymol blue form complexes with cationic form of tetracycline (pH 8–9) and crystal violet with the anionic form (pH 9–10). 98% of tetracycline can be extracted from the body fluids and solution containing 10g/ml of tetracycline can be conveniently analyzed.     

Photochemistry of benzimidazolocyanine dyes in solvents of different polarity

The effect of ion pair formation on the kinetics of the decay of the photoisomers and triplet states of cationic benzimidazolocyanine dyes is studied by flash photolysis. An increase in the rate constant of the reversecis-trans isomerization of the photoisomers is observed when ion pairs are formed (in nonpolar solvents). In the case of benzimidazolocyanine dyes with the I^– anion, ion-pair formation causes an increase in the rate constant of decay of the triplet state. Acceleration of S₁ S₀ internal conversion is discovered for the dyes with I^–1 Translated fromIvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 507–512, March, 1993.     

Kinetics of ferrocenoylacetonato substitution from (1,5-cyclooctadiene)(ferrocenoylacetonato)rhodium(I) with 2,2′-dipyridyl and derivatives of 1,10-phenanthroline

Treatment of MeOH solutions of [Rh(cod)(fca)] (cod = 1,5-cyclooctadiene, fca = ferrocenoylacetonato) with seven derivatives of 1,10-phenanthroline (N,N), as well as with the (N,N) ligand 2,2-dipyridyl, gave [Rh(cod)(N,N)]⁺. The kinetics of these reactions follow the rate law: Rate = k[Rh(cod)(fca)[N,N] The temperature dependence of all the studied substitutions resulted in activation entropies, S , more negative than –100 J K^–1 mol^–1 which is indicative of associative mechanisms. The pK _a's of the incoming phenanthroline derivatives were between 3.03 and 6.31 but did not influence the reaction rate to any significant extent. This implies that the rate determining step during the substitution involves Rh—O bond breaking and not Rh—N bond formation. Substitution of fca with 2,2-dipyridyl was slightly faster (k = 118 dm³ mol^–1 s^–1) than with the 1,10-phenanthroline derivatives (k _average = 14.2 dm³ mol^–1 s^–1) and may be attributed to the free rotation capability of the two pyridyl rings about the 1-1 carbon–carbon axis in 2,2-dipyridyl. 1,10-Phenanthroline cannot rotate about the corresponding carbon axis.     

Nonempirical calculations of NQR spectrum parameters of azetidine

The components of the¹⁴N electric field gradient (EFG) tensor, the corresponding nuclear quadrupole coupling constant (NQCC) , and the asymmetry parameter of azetidine were calculated using the restricted Hartree-Foek-Roothaan method, The geometry of azetidine was optimized with the 4–31G basis set, and the values of the ring puckering angle () and the angle between the N-H bond and the CNC plane () were refined with the 6–31G^* basis set. The effect of choice of geometry on calculated NQR parameters was studied. To clarify the origin of EFG at the nitrogen atom nucleus, the contributions from individual bond orbitals and lone electron pairs to the EFG tensor componentseq _ii were calculated in the framework of the LMO approach. It was demonstrated that the 4-31G + 6–31G^*//6–31G^* level calculations give NQCC and values of azetidine that are in good agreement with the results of MW spectroscopy.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No, 12, pp. 2886–2889, December, 1996.     

Pyrylocyanines. 31. Benzopyrylocarbocyanines with bridged groupings in the chromophore

4,4-Tetramethylene- and 4,4-pentamethylenebisflavylium salts and their sulfur- and selenium-containing analogs were synthesized. Trimethinecyanine dyes with ethylene and trimethylene bridged groupings in the chromophore were obtained from these salts. The factors that affect the color of these dyes were analyzed thoroughly.For Communication 30 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1319–1323, October, 1993.     

Pyrylocyanines

Unsymmetrical selenaflavylocyanines containing flavylium, thiaflavylium, 1,3,3-trimethylindoleninium, or 1-ethylbenzothiazolium rings were synthesized. The deviations for the dyes obtained, which were calculated from the absorption maxima and by the band moment method, were compared with the same values obtained for similar dyes with flavylium or thiaflavylium rings in place of the selenaflavylium ring. The results indicate that the basicity of the heterorings in the polymethine dyes increases in the order selenaflavylium < thiaflavylium < flavylium. This is confirmed by a comparison of the deviations in hemicyanines and styryls.See [1] for communication II.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1177–1181, September, 1971.     

Galactans and galactan-containing polysaccharides of higher plants

Research on the chemical structure and physiological activity of galactan and galactan-containing polysaccharides of higher plants is reviewed. The principal chain in galactan-containing polysaccharides consists of 1–3, 1–4, 1–6, and - and -bonded D-galactopyranoses. The side chains contain separate or bonded chains of galactose, arabinose, glucose, rhamnose, and uronic acids. The relationship of chemical structure and physiological activity of the polysaccharides of higher plants is discussed.S. Yu. Yunusov Institute of the Chemistry of Plant Substances. Academy of Sciences of the Republic of Uzbekistan, Tashkent. fax (99871) 120 64 75. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 185–197, May–June, 2000.     

An ab initio study of the structures and enthalpies of the hydrogen-bonded complexes of the acids H2O,H2S,HCN, and HCl with the anions OH−, SH−, CN−, and Cl−

Ab initio calculations at second-order Møller-Plesset perturbation theory with the 6-31 + G(d,p) basis set have been performed to determine the equilibrium structures and energies of a series of negative-ion hydrogen-bonded complexes with H₂O, H₂S, HCN, and HCl as proton donors and OH^–, SH^–, CN^–, and Cl^– as proton acceptors. The computed stabilization enthalpies of these complexes are in agreement to within the experimental error of 1 kcal mol^–1 with the gas-phase hydrogen bond enthalpies, except for HOHOH^–, in which case the difference is 1.8 kcal mol^–1. The structures of these complexes exhibit linear hydrogen bonds and directed lone pairs of electrons except for complexes with H₂O as the proton donor, in which cases the hydrogen bonds deviate slightly from linearity. All of the complexes have equilibrium structures in which the hydrogen-bonded proton is nonsymmetrically bound, although the symmetric structures of HOHOH^– and ClHCl^– are only slightly less bound than the equilibrium structures. MP2/6-31 + G(d,p) hydrogen bond energies calculated at optimized MP2/B-31 + G(d,p) and at optimized HF/6-31G(d) geometries are similar. Using HF/6-31G(d) frequencies to evaluate zero-point and thermal vibrational energies does not introduce significant error into the computed hydrogen bond enthalpies of these complexes provided that the hydrogen-bonded proton is definitely nonsymmetrically bound at both Hartree-Fock and MP2.     

LC-MS/MS method for the confirmatory determination of aromatic amines and its application in textile analysis

A confirmation method for the determination of 18 aromatic amines originating from azo dyes after reductive cleavage was developed. The method is based on the use of high-performance liquid chromatography/tandem mass spectrometry with atmospheric-pressure chemical ionization. For the identification of the analytes one precursor ion and two daughter ions (multi-reaction monitoring, MRM) were selected and the LC-MS/MS parameters optimized to obtain high sensitivity and selectivity. The linear ranges varied from 0.1–1 to 30–50 g mL^–1 with correlation coefficients of 0.99 or better. The applicability of the method to determine o-tolidine (3,3-dimethylbenzidine) and 3,3-dimethoxybenzidine in textiles following reductive cleavage of acid red 114, trypan blue, and Chicago sky blue 6B was demonstrated.     

Perturbation theory of nonbonding orbitals in polymeric dyes

In terms of the approximation of an extremely long polymethine chain a relation was obtained between the electron-donating power ₀ and the effective length L of the terminal groups in cyanine dyes, on the one hand, and the form of their asymptotic nonbonding orbital, on the other. A simple general procedure for determining the coefficients of such orbitals is indicated. Expressions were obtained through these coefficients for the derivatives of ₀ and L with respect to the elements of the topological Hamiltonian of the terminal group, and this makes it possible to assess the effect of structural perturbations of the terminal groups on ₀ and L. The possibility of refinement of the absorption region of the dyes with a short polymethine chain is considered.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 3, pp. 257–262, May–June, 1988.     

Experimental study of sensitized photooxidation reaction of compounds with anthracene structure in poly(methyl methacrylate)

Sensitized photooxidation of a series of compounds with an anthracene structure in a block poly(methyl methacrylate) (PMMA) containing a tetrapyrrole pigment (TPP) or anthraquinone dyes as spectral sensitizers, was studied. The values of changes in the molar refraction at equal to 441 and 633 nm were determined experimentally. They satisfactorily agreed with the theoretical values calculated from dispersional relationships. The initial quantum yields of the photooxidation (⁰ox) of the anthracene compounds were measured both in solution and in PMMA blocks. The lack of correlation between the ⁰ox values in the solid polymers and in the liquid solution indicates the dominating role of the polymeric matrix in determining the effectiveness of photooxidation process taking place. In the selection of the optimal polymerization conditions for the synthesis of the polymeric blocks, the values of ⁰ox of certain anthracene compounds can be up to 1. In the series of the anthraquinone dyes used, 1,4-dihydroxy and 2-aminoanthraquinones, whose ⁰ox reaches the values of 0.2–0.25 have the highest sensitizing ability.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 235–240, March–April, 1986.     

Ab initio calculations of potential energy surface for reaction of nucleophilic addition of H− ion to methylacetylene

Within the framework of the Hartree-Fock-Roothaan Method, using double- basis sets 3-21++G and (6-31-H-G//3-21++G), the minimum energy paths (MEPs) have been calculated for reactions of nucleophilic addition of the hydride ion H^– to the methylacetylene molecule: CH₃-CCH+H^–[CH₃-CH=CH]^– (1) CH₃-CCH+H^–[CH₃-C=CH₂]^– (2). It has been established that the activation energy for reaction (2) is 7.02 kJ/mole lower than for reaction (1). An analysis has been made of the character of electron density distribution along the MEP of each reaction. It has been shown that distortion of geometry of the reactants plays an important role in intensifying the interaction of the frontier orbitals. The reasons for nonfulfillment of Markownikoff's rule for these reactions have been determined. The results from the calculations are compared with calculations reported in the literature for the related reaction of nucleophilic addition of the hydride ion H^– to the acetylene molecule: HCCH+H^–[CH₂=CH]^–.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 21, No. 3, pp. 303–309, May–June, 1985.     

Mechanism of the chemical conversions of amino acids with various chain lengths on a dehydrated silica surface

The joint use of methods of IR spectroscopy and mass spectrometry with bombardment by fast atoms to identify products of chemisorption of --amino acids on a dehydrated silica surface is described. It was established that amino acids undergo multistage chemical conversions in this case, with the formation of various products of intra- and intermolecular condensation — lactams, linear and cyclic peptides. The composition of the condensation products depends on the chain length of the amino acids.Translated from Teoreticheskaya i Éksperimental/naya Khimiya, Vol. 26, No. 5, pp. 628–633, September–October, 1990.     

Acid-base properties and stability of pyrylium polymethine dyes in chlorohydrocarbon solutions

The reactions of pyrylocyanine dyes with aqueous alcoholic alkali and with acetic acid have been studied. It has been shown that decoloration of such dyes in chlorohydrocarbon solutions only occurred under the action of alkali and was linked with the formation of an intermediate acyclic form of the dye which was the addition product of hydroxyl ion at the position of the pyrylium ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 312–314, March, 1986.     

Simultaneous determination of cobalt,nickel and iron by multi-wavelength spectrophotometry

A method for the simultaneous spectrophotometric determination of cobalt, nickel and iron based on the formation of their complexes with 1,5-bis(di-2-pyridylmethylene) thiocarbonohydrazide is proposed. The absorption curves of these complexes overlap severely in the scanning range 390–510 nm. The analyte concentrations are calculated by a least squares fit of the pure spectra to the mixture spectra, which therefore makes the simultaneous determination of these metallic ions possible without tedious pretreatment. The detection limits afforded by the proposed method range from 0.05 g/ml for Fe and Ni to 0.1 g/ml for Co. Root-mean-squared errors of prediction of 0.085 g/ml for Co, 0.048 g/ml for Ni and 0.1172 g/ml for Fe were obtained using the wavelength range 400–510 nm and 0.147 g/ml for Co, 0.107 g/ml for Ni and 0.127 g/ml for Fe using the wavelength range 420–434 nm. The effect of interferences is studied and the proposed method is applied to analysis for the above elements in synthetic samples and real samples, such as biological materials and alloys.     

Spectrophotometric procedures for the determination of roxithromycin in pharmaceutical formulations

Four simple and sensitive visible spectrophotometric procedures for the assay of roxithromycin (RXT) have been developed. Procedures A and B are based on the formation of ion-pairs of roxithromycin with the dyes supracen violet 3B and tropaeolin 000, which are extracted into chloroform and have absorption maxima at 590 nm (SV 3B) and 490 nm (TP 000). Procedures C and D are based on condensation between the hydrolysis product of RXT in the presence of 3.6M hydrochloric acid in acetic acid medium and vanillin orp-dimethylaminobenzaldehyde (PDAB) to form coloured products with _max at 500 nm. Regression analysis of the Beer's plot showed good correlation in the concentration ranges 5–60 (A), 5–40 (B) and 5–50 (C & D) g/ml. No interference was observed from excipients and the validity of the procedures was tested by analysing pharmaceutical formulations. Recoveries were 99.0–102.0%. The concentration measurements are reproducible within a relative standard deviation of 1.0%.     

Triterpene glycosides of Thalictrum squarrosum. III. Structures of squarrogenins 1 and 2

Two genins — squarrogenin 1 and squarrogenin 2 — have been isolated from nodding meadow rue by the hydrolysis of squarrosides A1 and A2. The compounds are epimeric at C-21 and have the following structures: 1 — (21R, 22S, 23R)-21-methoxy-21,23-epoxycycloart-24-ene-3,22-30-triol, C₃₁H₅₀O₅, mp 169–171°C (hexane-acetone), [] ₅₄₆ ²⁰ –11.06° (c 4.52; pyridine); and 2 — (21S, 22S, 23R)-21-methoxy-21,23-epoxycycloart-24-ene-3,22,30-triol, C₃₁H₅₀O₅, mp 190–193°C (hexane-acetone), [] ₅₄₆ ²⁰ +106.6° (c 0.3; pyridine). The results of¹H and¹³C NMR spectroscopy and of mass spectrometry for the new compounds are given.Irkutsk Institute of Organic Chemistry, Siberian Division of the USSR Academy of Sciences. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 510–516, July–August, 1989.     

A quantum-mechanical study of the chain-length dependent stability of the extended and 310-helix conformations in dehydroalanine oligopeptides

Summary A quantum-chemical study of the chain-length dependent stability of the extended, 2₇-ribbon and 3₁₀-helix conformations in dehydroalanine (Ala) oligopeptides has been performed by using both semiempirical AM1 and ab initio 4–31G methodologies. The validity of both methods in the study of the conformational properties of Ala oligopeptides was tested first on the dipeptide. The results of this test showed that 4–31G and AM1 calculations are in good agreement with 6–31G^* calculations and experimental data. In order to monitor the conformational conversions, Ala oligopeptides comprising two to six residues were constructed. Molecular geometries were fully optimized using AM1, and the final conformations were verified to be minima by analysis of the corresponding second-derivative matrices. Conformational studies revealed that the 3₁₀-helix is stabilized with respect to the 2₇-ribbon when the number of residues is three or four, at the AM1 and ab initio 4–31G level respectively, while the extended form is the most stable in all the calculations performed. On the other hand, if a linear behaviour is assumed for longer chains, our calculations show a trend that would predict a conversion from extended form to 3₁₀-helix in oligopeptides with around six (ab initio 4–31G) or eight (AM1) Ala residues. In order to explain these conformational changes, the cooperative effects for the different conformers were investigated. Large cooperative energy effects were found for the 3₁₀-helix conformation.