bluuebunny/crossref_metadata_2025 huggingface dataset:
Intrazeolite photochemistry. VI. Diffuse reflectance laser flash photolysis and product studies of diphenylmethyl radicals on solid supports
Authors: Kelly Graeme , Willsher C. J. , Wilkinson F. , Netto-Ferreira J. C. , Olea A. , Weir D. , Johnston L. J. , Scaiano J. C.
Published: May 2006

Diphenylmethyl radicals have been generated by 266 nm laser excitation of 1,1,3,3-tetraphenylacetone adsorbed on silica gel and included in NaX and Silicalite zeolites and have been studied using diffuse reflectance laser flash photolysis techniques. The spectrum for the radical shows λmax at ~335 nm in all three supports and is similar to that in solution. The radicals decay over time scales that vary from hundreds of nanoseconds to minutes and there are indications that some radicals may be decaying on shorter time scales than we can monitor. The efficiency of oxygen quenching increases in going from Silicalite to NaX to silica gel, consistent with the greater accessibility of oxygen to silica gel pores as compared to the narrow channels in Silicalite. Laser dose and ketone loading effects were also examined for the various supports. Potential applications of a kinetic treatment of the data based on dispersive reaction kinetics are also discussed as a means of dealing with the problem of decay kinetics that occur over a wide range of time scales. Keywords: zeolites, kinetics, diphenylmethyl radicals, laser photolysis.
Electron transfer reactions. Reaction of nitrogen heterocycles with potassium
Authors: Muneer Mohammed , Kamat Prashant V. , George Manapurathu V.
Published: May 2006

The results of our studies on potassium-induced transformations of some selected nitrogen heterocycles are presented. The substrates under investigation include 2,3-diphenylindole (1a), 2,3-diphenyl-1-methylindole (1b), 1,2,3-triphenylindole (1c), 2,3,4,5-tetraphenylpyrrole (5a), 1,2,3,5-tetraphenylpyrrole (5b), 1-benzyl-2,3,5-triphenylpyrrole (5c), 2,4,5-triphenyloxazole (15a), 4,5-diphenyl-2-methyloxazole (15b), 2,4-diphenyl-5-methyloxazole (15c), and 2,4,5-triphenylimidazole (19). Treatment of 1a with potassium in THF gave 9H-dibenzo[a,c]carbazole (3a), whereas 1c gave a mixture of 9-phenyl-9H-dibenzo-[a,c]carbazole (3c) and 2,3-diphenylindole (1a). Under identical conditions, 1b gave only the cleavage product 1a. In contrast, when the reactions of 1a,c were carried out with potassium in THF saturated with oxygen, and with potassium superoxide in benzene containing 18-crown-6, a mixture of 2-benzamidobenzophenone (4a), the carbazoles 3a,c, and 1a was formed. Although no product was isolated on treatment of 5a with potassium in THF, the reaction of 5a with potassium in THF saturated with oxygen gave a mixture of tetraphenylpyrazine (7a), the benzoylaminostilbene 8a, the lactam 12a, benzamide (11a), and benzoic acid (14). Similar results were obtained in the reaction of 5a with potassium superoxide. The reaction of N-substituted pyrroles such as 5b,c with potassium gave the NH pyrrole 9b in each case, whereas the reaction of 5b,c with potassium in THF, saturated with oxygen, gave a mixture of 9b, the butanone 10b, the 1,4-dione 13b, the lactam 12b, the amides 11a–c, and benzoic acid (14). Attempted reactions of 5b,c with potassium superoxide did not give any isolable product; most of the starting material could be recovered unchanged in each case. A mixture of N-(1,2-diphenylethyl)benzamide (18a) and benzoic acid (14) was formed in the reaction of the oxazole 15a with potassium, whereas 15b,c, under analogous conditions, gave the N-vinylamides 17b,c and benzoic acid (14). In contrast, treatment of the imidazole 19 with potassium in THF did not give any product; however, when the reaction of 19 was carried out with potassium in THF saturated with oxygen, and with potassium superoxide, dibenzamide (21) was isolated, in each case.Radical ions have been invoked as intermediates in the transformation of the different substrates to the observed products. Cyclic voltammetric studies have been carried out to measure the reduction potentials of these radical anion intermediates. These radical anions have also been generated by pulse radiolysis in methanol, and their absorption spectra recorded. Keywords: nitrogen heterocycles, radical ions, potassium-induced transformations, pulse radiolysis, cyclic voltammetry.
Ionization potentials of multiply charged ions from generalized exchange local-spin-density functional theory
Authors: Guo Yufei , Whitehead M. A.
Published: May 2006

The self-interaction-corrected generalized exchange local-spin-density functional theory (LSD-GX-SIC) with Gopinathan, Whitehead, and Bogdanovic (GWB) Fermi-hole exchange parameters is used to calculate the ionization potentials of the multiply charged ions for the elements C, Al, Cl, Ar, Ca, Fe, and Br. The effect of the correlation correction on the ionization potentials is considered. The calculated ionization potentials are compared with Hartree–Fock, recent Xα, and experiment. The correlation correction is important in the calculations. The results in the LSD-GX-SIC theory with GWB Fermi-hole exchange parameters and the Vosko, Wilk, and Nusair correlation correction are in excellent agreement with experiment. The LSD-GX-SIC theory can predict unknown ionization potentials for the multiply charged ions with acceptable computational time, since there is no need to optimize the exchange parameters. Keywords: ionization potentials, multiply charged ions, density functional theory.
Left- and right-handed chiral nematic mesophase of (trityl)(alkyl)cellulose derivatives
Authors: Harkness Brian R. , Gray Derek G.
Published: May 2006

A series of novel specifically substituted 6-O-trityl-2,3-O-alkyl cellulose derivatives have been prepared by the reaction of triphenylmethyl (trityl) cellulose in DMSO with powdered sodium hydroxide and an alkyl iodide (iodohexane, iodopentane, and iodobutane). These derivatives form lyotropic chiral nematic mesophases that exhibit both left- and right-handed twists. The dilute solution circular dichroism due to the phenyl chromophores along the chain does not correlate with the twist sense of the tertiary structures. However, a striking difference in the optical activity of the phenyl chromophores was observed in mesophases with the opposite twist sense. This may reflect some molecular factor responsible for the change in the macroscopic twist sense, or it may be due to the chirality of the helicoidal tertiary structure of the mesophase. Keywords: cellulose, trityl, mesophase, cholesteric, chiroptical.
Complexes of carbohydrates with metal cations. XVI•Di-D-fructose and di-L-sorbose dianhydrides
Authors: Angyal Stephen John , Craig Donald Chadwick , Defaye Jacques , Gadelle Andrée
Published: May 2006

Di-D-fructose 2′, 1:2,1′-dianhydrides form complexes with metal cations if the two anomeric carbon atoms have the same configuration. Such anomers have the central 1,4-dioxane ring in a flexible form; complex formation involves O-1, O-1′, O-3, and O-3′, as confirmed by the X-ray crystal structure of the strontium complex of the β,β-dipyranose anomer. The α,β-anomers, and dianhydrides containing only furanose rings, do not form such complexes. Keywords: difructose dianhydrides, complex formation with cations, crystal structure of complex, chromatographic separation on ion-exchange resins.
Two-dimensional POMMIE carbon–proton chemical shift correlated 13C NMR spectrum editing
Authors: Coxon Bruce
Published: May 2006

Two pulse sequences are described for acquisition of two-dimensional, carbon–proton chemical shift correlated 13C NMR spectra by the "phase oscillations to maximize editing technique". One of these sequences provides two-dimensional, carbon–proton chemical shift correlated spectra in which the 1H–1H coupling constants are present in the 1H chemical shift dimension, whereas the other sequence includes a bilinear rotation decoupling unit that removes the vicinal 1H–1H couplings in this dimension. Extensions of these techniques to generation of two-dimensional, carbon–proton chemical shift correlated CH, CH2, and CH313C NMR subspectra from linear combinations of three two-dimensional data sets are described. Decreased residual signals in the edited 2D subspectra have been achieved by Pascal programs that include six floating point coefficients, and a method for their calibration is discussed. Results are reported for troleandomycin (1). Keywords: 13C nuclear magnetic resonance, carbon–proton chemical shift correlation, DEPT, Pascal programs, POMMIE, two-dimensional NMR spectrum editing, troleandomycin.
Synthesis of optically active cyclohexanone analogs of the plant hormone abscisic acid
Authors: Lamb Nancy , Abrams Suzanne R.
Published: May 2006

The abscisic acid analogs (−)-(4R,5R)-, (+)-(4S,5S)-, and (−)-(4S,5R)-4(1E,3Z)-4-(4-carboxy-3-methyl-1,3-butadienyl)-4-hydroxy-3,3,5-trimethylcyclohexanones (dihydroabscisic acids), and (−)-(4S,5R)-, (+)-(4R,5S)-, and (−)-(4R,5R)-4(Z)-4-hydroxy-4-(5-hydroxy-3-methylpent-3-en-1-ynyl)-3,3,5-trimethylcyclohexanones were synthesized from a common precursor, (−)-(6R)-2,2,6-trimethylcyclohexan-1,4-dione, which was readily prepared by the fermentation of oxoisophorone with bakers' yeast. Keywords: abscisic acid, dihydroabscisic acid, acetylenic analogs, optically active analogs, plant growth regulators.
NMR and conformational study of branched oligosaccharides containing 2,3-disubstituted residues of α-L-rhamnose
Authors: Lipkind Grigory M. , Nifant'ev Nikolay E. , Shashkov Alexander S. , Kochetkov Nikolay K.
Published: May 2006

1H and 13C NMR spectra have been recorded and the nuclear Overhauser enhancements (in D2O), after pre-irradiation of the anomeric protons, have been measured for tri- and tetrasaccharides containing 2,3-disubstituted residues of α-L-rhamnose and the corresponding disaccharides. The proximity of the conformational states of the disaccharide units in these branched oligosaccharides and in the corresponding disaccharides has been shown, on the basis of both the nOe experimental data and theoretical conformational analysis. This conclusion is also confirmed by data for glycosylation effects in the 13C NMR spectra of the oligosaccharides. Keywords: branched oligosaccharides, 1H and 13C NMR spectra, conformations.
Synthesis and multinuclear magnetic resonance study of some tin(IV) complexes of pyridine-2-thiolate and related ligands, and the X-ray structural analysis of Sn(C5H4NS)4•C5H5NS
Authors: Damude Leslie C. , Dean Philip A. W. , Manivannan Veeragathy , Srivastava Radhey S. , Vittal Jagadese J.
Published: May 2006

Tin metal reacts with 2,2′-dithiobis (pyridine) in refluxing toluene to give the pyridine-2-thiolate complex Sn(C5H4NS)4, 1a. Multinuclear (77Se, 119Sn) magnetic resonance shows that ligand redistribution occurs in 1a:Sn(EPh)4 (E = S or Se) mixtures to give Sn(EPh)4−x(C5H4NS)x. From the pattern of the 119Sn chemical shifts of these series, compared with that of Sn(EPh)4−x(SR)x (R = simple alkyl or aryl group), the complexes are shown to be five-coordinate Sn(EPh)3(C5H4NS-N,S) when x = 1 and six-coordinate Sn(EPh)4−x(C4H4NS-S)x−2(C5H4NS-N,S)2 when x = 2–4. Reaction of Sn(SPh)2 and (C5H4NS)2 yields pure Sn(SPh)2(C5H4NS)2. The 13C NMR spectrum of Sn(C5H4NS-S)2 (C5H4NS-N,S)2 is an exchange average down to 178 K, probably as a result of fluxionality. The series Sn(EPh)4−x,(L′)x (E = Se, L′ =5-nitropyridine- or quinoline-2-thiolate, x = 0–4; E = S or Se, L′ = pyridine-2-thiolate-N-oxide, x = 0–2) can be prepared insitu by redox between Sn(EPh)4 and the disulfide (L′)2. Treatment of 1a with X2 (X = Br or E- gives SnX4−x(C5H4NS-N,S)x(x = 2 or 3), which have been characterized by NMR, and provides a new synthesis of the known dihalo complexes.In a recrystallization of 1a from CH2Cl2:hexane Sn(C5H4NS)4•C5H5NS, 2, was obtained, probably as a result of hydrolysis. The structure of 2 has been solved by single crystal X-ray diffraction techniques. The compound crystallizes in the orthorhombic space group Pbca (No. 61) with a unit cell of dimensions a = 17.132(2) Å, b = 15.337(2) Å, c = 21.268(3) Å, and Z = 8. Full-matrix least-squares on F led to agreement factors R = 0.0439 and Rw = 0.0426 for 3145 independent reflections with Fo > 2.5σ (Fo) and 206 variables. The molecule Sn(C5H4NS-S)2(C5H4NS-N,S)2 has highly distorted octahedral coordination about the tin atom, with the monodentate ligands in cis positions and the bidentate ligands attached with mutually cis nitrogen atoms. The Sn—S distances are 2.463(2)–2.542(2) Å and the Sn—N distances are 2.332(2) and 2.324(2) Å. The N—Sn—S angles in the chelate rings are 64.2(2) and 65.1(1)°. The molecule of pyridine-2-thione in the lattice is hydrogen-bonded to the N atom of one of the monodentate ligands in the tin complex, with a [Formula: see text] hydrogen bond distance of 1.92(7) Å. Keywords: pyridine-2-thiolate, selenium-77 NMR, tin-119 NMR, tin(IV) complexes, X-ray analysis.
The crystal structure and thermal expansion of the CCl4 adduct of Dianin's compound
Authors: Abriel Walter , Bois André Du , Zakrzewski Marek , White Mary Anne
Published: May 2006

The crystal structure of the title compound has been determined by single crystal X-ray diffraction data collected at 293 K, and refined to a final Rw of 0.057. The crystals are rhombohedral, space group [Formula: see text], with a = 27.134(8) Å, c = 10.933(2) Å, and Z = 18. The mole ratio of Dianin's compound (4-p-hydroxyphenyl-2,2,4-trimethylchroman) to CCl4 is 6:1. The guest molecules are disordered. X-ray powder diffraction was carried out in the temperature range from 10 to 300 K. From this, the thermal expansion coefficients for the a- and c-axes and the volume have been determined. Keywords: thermal expansion, crystal structure, clathrate.
X-ray structural and nuclear magnetic resonance study of open chain and macrocyclic thioether complexes
Authors: Lucas C. Robert , Liu Shuang , Newlands Michael J. , Gabe Eric J.
Published: May 2006

Preparations of the thiophenophane and open chain thioether complexes MX2•BBTE(M = Pd; X = Cl, I)(M = Pt; X = Cl) (BBTE = 1,2-bis(benzylthio)ethane) and MX2•L (M = Pd; X = Cl, Br, I, SCN) (M = Pt; X = Cl) (L = 2,5,8-trithia[9](2,5)thiophenophane) are described. The molecular structure of PdBr2•L which contains a weak thiophene-sulfur-to-palladium interaction has been determined: space group P21/n, a = 8.3569(3), b = 16.3254(15), c = 11.1462(3) Å, β = 92.833(4)°, Z = 4, Rf = 0.060, Rw = 0.058. The electronic, low frequency ir, and 13C nmr spectra are described. Variable temperature 1H nmr spectra are discussed and it is concluded that the open chain complexes undergo rapid conformational exchange at room temperature but are configurationally rigid unless the temperature is increased. In contrast, the macrocyclic complexes undergo very limited conformational and no configurational exchange up to their decomposition temperatures. They are also nonfluxional in the same temperature range. Chemical exchange of acidic ligand hydrogens and a metal–ligand dissociative equilibrium were also detected when X = SCN. Keywords: thiophene, thioether, macrocyclic metal complexes.
1H and 19F NMR conformational studies of the monofluorostyrenes in solution. Comparison with theory and vapor phase behavior
Authors: Schaefer Ted , Sebastian Rudy
Published: May 2006

The parameters for the high resolution 1H and 19F NMR spectra of 2-, 3-, and 4-fluorostyrene are reported for solutions in CS2 and acetone-d6 at 300 K. The populations of the planar cis and trans conformers of 2- and 3-fluorostyrene are deduced from the long-range coupling constants involving the meta and α protons. These populations are insensitive to solvent and appear to be in reasonable agreement with previous 6-31G MO computations for the free molecule; they are also compared with populations deduced from recent rotational and vibronic spectra. The long-range coupling constants for the protons of 4-fluorostyrene imply an internal barrier to rotation about the exocyclic carbon–carbon bond very similar to that in styrene, in agreement with the 6-31G results. The signs of the coupling constants involving 19F and the protons in the side chain are reported and discussed in terms of coupling mechanisms for the three molecules. An earlier surmise, of a positive a electron contribution to the coupling constant over six bonds in an all-trans arrangement, is confirmed for the meta and trans-β protons in 3-fluorostyrene. Keywords: monofluorostyrenes, 1H and 19F NMR, conformations, long-range coupling mechanisms, MO calculations of internal barriers.
Oxidative coupling of amines and carbon monoxide catalyzed by palladium complexes. Mono- and double carbonylation reactions promoted by iodine compounds
Authors: Pri-Bar Ilan , Alper Howard
Published: May 2006

Iodine is an effective promoter for the carbonylation of primary and secondary amines to ureas using palladium acetate as the catalyst and a base (e.g. K2CO3) in acetonitrile (3 h at 95 °C and 2.7 atm). Oxamides are formed in excellent yields when secondary amines are carbonylated in the presence of iodide ion and oxygen, while primary amines give ureas as the principal product at 95 °C, and oxamide at room temperature. Keywords: oxamides, ureas, double carbonylation, amines.
Evidence for stereoregulations in the polymerisation of phenylsilane by bis (indenyl)dimethylzirconium complexes
Authors: Gauvin F. , Harrod J. F.
Published: May 2006

Bis(indenyl)dimethylzirconocene, 1, and [1,2-bis(indenyl)ethane]dimethylzirconocene, 2, both catalyze the dehydropolymerization of phenylsilane to a mixture of low molecular weight cyclic oligosilanes (PhSiH)n (n = 6–9). The limited number of sharp resonances in the 29Si and 1H NMR spectra of the polymer produced with 2 as catalyst indicates that the catalyst exerts a significant stereoregulating influence on the polymerization. Keywords: polysilanes, catalytic dehydrocoupling, stereoregulation, bis(indenyl)zirconium complexes.
The enthalpy and entropy of cavity formation in liquids and Corresponding States Principle
Authors: Dionísio Madalena S. , Ramos Joaquim J. Moura , Gonçalves Raquel M.
Published: May 2006

A new method to calculate the enthalpy and entropy of cavity formation in liquids is proposed. The reference cavity formation process is identified with the vaporization of liquid in the absence of order and Corresponding States Principle is used to eliminate the order contribution to vaporization enthalpy. The proposed method agrees very well with Claverie's modification of the Pierotti's method but strongly disagrees with the Sinanoglu's method, particularly in the context of entropy of cavity formation.The new method is checked by applying it to the description of solution process in alkane binary mixtures at infinite dilution and is used to study the solvent effect on the solvolysis of t-butyl chloride and bromide. Keywords: cavity formation, enthalpy and entropy of cavity formation, Corresponding States Principle, alkanes, intermolecular interactions, acentric factor, solvent effect.
Oxidation of substituted anilines by horseradish peroxidase compound II
Authors: Huang Jin , Dunford H. Brian
Published: May 2006

The kinetics of oxidation of eight monosubstituted anilines catalyzed by horseradish peroxidase compound II has been studied at pH 7.00 and 7.60. With p-toluidine the rates of oxidation by compound II have been measured at 21 pH values between 3.60 and 10.25. The rate–pH profile indicates that an acidic form of compound II and the electrically neutral, unprotonated form of p-toluidine are reactive. The correlation of rate constants for the substituted anilines with the substituent constant σ in the Hammett equation suggests that the aromatic amine donates an electron to compound II in the rate-controlling step and loses a proton simultaneously. The value of ρ, the susceptibility factor in the Hammett equation, is −6.0 ± 0.7. The reactivity of anilines with HRP-II observed in this study is lower than that of anilines with HRP-I observed previously, although the value of ρ is the same within experimental error (D. Job and H. B. Dunford. Eur. J. Biochem. 66, 607 (1976)). The difference in reactivity is explained by the relative complexities of the reactions of compounds I and II. Keywords: horseradish peroxidase, peroxidase compound II, aniline oxidation, Hammett correlation, enzymatic oxidation.
Skeletal scrambling of sulphur diimide radical anions
Authors: Bestari Ketut , Oakley Richard T. , Cordes A. Wallace
Published: May 2006

In the presence of a catalytic quantity of alkali metal mixtures of sulphur diimides RNSNR and R′NSNR′(R, R′ = Ph, 4-C6H4Me, 4-C6H4OMe, SiMe3, SPh) undergo a rapid scrambling of the R and R′ groups. When R and R′ are significantly different (e.g., R = Ph, R′ = SiMe3 or SPh) the equilibrium is shifted to favour the unsymmetrical sulphur dimmide (R ≠ R′); the procedure thus represents an effective method for preparing such derivatives. A mechanism involving the centrosymmetric association and rearrangement of two sulphur diimide radical anions is suggested for the observed ligand scrambling. The synthesis and X-ray structure determination of the mixed diimide (4-NO2C6H4)SNSN(4-C6H4OMe) is reported. The crystals are monoclinic, space group P21/n; the molecules stack as plates along the a axis in a head-to-head fashion, producing an interplanar spacing between consecutive S2N2 units of 3.421 Å. Key words: sulphur diimide radical anions, skeletal rearrangements, (4-C6H4NO2)SNSN(4-C6H4OMe), X-ray structure.
The influence of dielectric permittivity of the nonaqueous phase on the photooxidation of water at the interface of two immiscible liquids in the presence of a hydrated oligomer of chlorophyll. The role of a proton acceptor
Authors: Kandelaki Mikhail D. , Volkov Alexander G.
Published: May 2006

We have investigated polarographically the dependence of the dioxygen yield on the concentration of 2,4-dinitrophenol in photooxidation of water in an octane/water system containing hydrated oligomer of chlorophyll, a water-soluble electron acceptor ferricyanide and a buffer. A linear correlation between the rate of dioxygen evolution and the photopotential at the interface between the two liquid phases is observed. Investigation of the dependences of photopotential and rate of dioxygen evolution on the dielectric permittivity of the nonaqueous phase has shown that with increasing dielectric permittivity, one can observe a decrease in both the reaction rate and the potential difference. The lifetime of chlorophyll in the course of water photooxidation reaction has been measured. Key words: chlorophyll, oxygen, water photooxidation, oil/water interface, artificial photosynthesis.
Synthesis, structure, and magnetic properties of diphenylphosphinates of cobalt(II) and manganese(II). The crystal and molecular structures of the γ forms of poly-bis(μ-diphenylphosphinato)cobalt(II) and manganese(II)
Authors: Du Jing-Long , Rettig Steven J. , Thompson Robert C. , Trotter James
Published: May 2006

The synthesis of the β and γ forms of M(Ph2PO2)2 where M = Co and Mn are described and the compounds are characterized by infrared spectroscopy, differential scanning calorimetry, X-ray powder diffraction, and low-temperature (2–80 K) magnetic susceptibility studies. Single crystal X-ray diffraction studies are reported on the γ forms. Crystals of the γ forms of poly-bis(μ-diphenylphosphinato)cobalt(II) and poly-bis(μ-diphenylphosphinato)manganese(II) are isomorphous, crystallizing with 4 formula units per unit-cell in the monoclinic space group P21/c. Lattice constants are a = 8.080(2), 8.161(1), b = 23.550(6), 23.751(1), c = 11.726(3), 11.6946(6) Ǻ, and β = 92.88(2), 93.026(8)° for the Co and Mn derivatives respectively. The structures were solved by heavy atom methods and were refined by full-matrix least-squares procedures to R = 0.039 and 0.045 for 4041 and 2878 reflections with I ≥ 3σ(I), respectively. Both crystal structures consist of double phosphinate-bridged chain polymers containing tetrahedrally coordinated metal atoms: Co—O = 1.950(2)–1.963(2) Ǻ, O—Co—O = 104.81(8)–117.77(9)°, Mn—O = 2.016(3)–2.033(3) Ǻ, O—Mn—O = 103.2(1)–114.7(1)°. All four compounds exhibit antiferromagnetic coupling and magnetic susceptibilities have been analyzed according to two Heisenberg models for linear chains of metal ions with S = 3/2 for cobalt and S = 5/2 for manganese. The Weng model (with values for the Wagner and Friedberg model in parentheses) gives –J = 0.25 (0.26) cm−1 and 0.55 (0.60) cm−1 for the β and γ forms, respectively, of Co(Ph2PO2)2, and 0.34 (0.36) cm−1 and 0.17 (0.17) cm−1 for the β and γ forms, respectively, of Mn(Ph2PO2)2. Key words: crystal structure, diphenylphosphinates of cobalt(II) and manganese(II), magnetic properties.
Determination of the absolute stereochemistry of the fungal metabolite (R)-(−)-2-(4′-hydroxyphenyl)-2-hydroxyethanoic acid (pisolithin B)
Authors: Tsantrizos Youla S. , Ogilvie Kelvin K.
Published: May 2006

The antifungal antibiotic pisolithin B (p-hydroxymandelic acid, 2-(4′-hydroxyphenyl)-2-hydroxyethanoic acid, 1a) was shown to have the absolute (R) configuration. The stereochemistry was established via comparison of its optical rotation to that of its synthetic (R) and (S) enantiomers. The synthetic samples were prepared by the stereospecific reduction of the prochiral α-keto acid, p-hydroxybenzoylformic acid (2-(4′-hydroxyphenyl)-2-oxoethanoic acid, 2a), with (R) or (S)-2,2′-dihydroxy-1,1′-binaphthyl lithium aluminum hydride (BINAL-H). The absolute configuration and enantiomeric purity of both products were determined using the 1H NMR of their isobutyl esters in the presence of the chiral solvating agent (R)-(−)-2,2,2-trifluoro-1-(9-anthryl)ethanol. Key words: pisolithin B, p-hydroxymandelic acid, antifungal, absolute configuration.
Synthetic conversion of bryostatin 2 to bryostatin 1 and related bryopyrans
Authors: Pettit George R. , Sengupta Dipanjan , Herald Cherry L. , Sharkey Nancy A. , Blumberg Peter M.
Published: May 2006

Bryostatin 2 (1a) has been converted to bryostatin 1 (1e) and bryostatin 12 (1i) by a selective protection and deprotection involving the C-26 hydroxyl group. The new bryostatins 1g,1k, and 1m were also prepared starting from bryostatin 2. The C-7 substituents of natural bryostatins 4 and 5 were revised from isovalerate → pivalate employing comparative 1H and 13C NMR studies of the semi-synthetic bryostatins 1k and 1m and the natural products. Key words: bryostatin 2 → 1, selective conversion, bryostatins 4 and 5, pivalates.
Negative hyperconjugation in hexachloro-3-cyclopentenylaminosulphenyl halides: Preparation and X-ray structure of C5Cl6NSBr and synthesis of C5Cl6NS+AsF6−
Authors: Apblett Allen , Chivers Tristram , Fait James F. , Vollmerhaus Rainer
Published: May 2006

The reaction of hexachlorocyclopentadiene with (NSCl)3 at 65 °C in the absence of a solvent produced hexachloro-3-cyclopentenylidenaminosulfenyl chloride, C5Cl6NSCl, in good yield. The derivatives C5Cl6NSX(X = Br, I, C5Cl6N, NPr2i, Ph) were prepared by treatment of C5Cl6NSCl with Me3SiBr, Me3SiI, Ph3Sb, iPr2NH, and Ph2Hg, respectively. The crystal structure of C5Cl6NSBr was determined by X-ray crystallography. Crystals of C5Cl6NSBr are triclinic, space group [Formula: see text] with a = 9.626(2), b = 9.810(2), c = 13.736(2) Å, α = 79.68(1), β = 88.62(1), γ = 74.73(1)°, Z = 4, final R and Rw values were0.075 and 0.063, respectively, for 4850 unique observed reflections. The > C=NSBr moiety adopts a syn conformation and there are two different molecules of C5Cl6NSBr in the asymmetric units. The bond lengths are d(C=N) = 1.278(8) and 1.253(8), d(N—S) = 1.560(6) and 1.569(6), and d(S—Br) = 2.229(2) and2.235(2) Å. The bond angles at nitrogen are 146.7(4) and 148.6(4) and those at sulfur are 116.8(2) and 116.5(2)°. The unusual geometry of the < C=NSBr moiety is explained in terms of negative hyperconjugation (nN → σSBr*). A variable temperature 13C NMR of C5Cl6NSCl provides evidence for a fiuxional process (ΔG*298 = 68.1 ± 1.2 kJ mol−1) in solution. The dark purple compound C5Cl6NS+AsF6− was synthesized from the reaction of C5Cl6NSCl and AgAsF6 in SO2. Key words: hexachloro-3-cyclopentenylidenamino sulfides, crystal structure, hexachloro-3-cyclopentenylidenaminosulfenyl bromide.
The conformational distribution of 2-cyanobenzaldehyde and 3-cyanobenzaldehyde in solution and in the vapor
Authors: Schaefer Ted , Cox Kerry J. , Sebastian Rudy
Published: May 2006

The 1H nuclear magnetic resonance spectra of 2-cyanobenzaldehyde (2CNB) and 3-cyanobenzaldehyde (3CNB) in CS2/C6D12 and acetone-d6 solutions at 300 K yield precise stereospecific long-range proton–proton coupling constants. These are used to establish the conformational population of the o-cis and o-trans conformers of these relatively polar molecules. For example, the fractional o-cis population of 2CNB changes from 0.12(4) in CS2/C6D12 to 0.46(6) in acetone-d6, whereas that of 3CNB is 0.48(2) in both solvents. Extrapolation to the vapor phase, using a dielectric model, implies a negligible concentration of the o-cis conformer of 2CNB and a roughly 50% abundance of each conformer of 3CNB. Computations at various levels of molecular orbital theory provide estimates of the rotational barrier of the aldehyde moiety and confirm the planar structure of each conformer. The geometries of three conformers are given as obtained from the 6-31G MO basis and may be useful to molecular spectroscopists. Theoretical and experimental dipole moments are interpolated to yield estimates of their magnitudes for the four planar conformers. Somewhat less precise 1H nmr spectral parameters (than for the above solutions) are also obtained for dilute solutions in benzene-d6 at 300 K. The conformational distributions based on these parameters are compared with their only other measurement, based on dipolar moments in benzene at 298 K. Good agreement between the results of the two methods is found for 3CNB but not for 2CNB. It is suggested that specific interactions occur between benzene solvent and solute molecules, particularly for 3CNB, for which these interactions stabilize the conformer having a low dipole moment. Remarkable changes in the intraring proton–proton coupling constants occur in going from CS2/C6D12 to acetone-d6 solution. Key words: 2- and 3-cyanobenzaldehyde (2CNB and 3CNB): 1H NMR, conformations, long-range spin–spin coupling constants, MO computations.
β-Substitution de la méso-tétraphénylporphyrine de zinc par voie électrochimique
Authors: Giraudeau Alain , Kahef Lana El
Published: May 2006

The electrochemical oxidation of the zinc tetraphenylporphyrin complex in a mixed acetonitrile–dichloroethane solution in the presence of nucleophiles (Py, 3-Pic, [Formula: see text], SCN−) leads to the formation of the corresponding monosubstituted metalloporphyrin. For each of these nucleophiles the substitution occurs at a pyrrole carbon atom (β-substitution). The electrochemical conditions of these substitutions are discussed and an overall reaction is proposed. Key words: porphyrins, electrochemical reactions, nucleophilic substitution.
Reactions of Muonium with simple biochemical solutes in water and micelles
Authors: Barnabas Mary V. , Walker David C.
Published: May 2006

Rate constants are reported for the reaction of muonium atoms in water with some 36 organic compounds, many of interest in biology. These kM values range from < l05 to 7 × 1010 M−1 s−1, according to the type of reaction involved, with the sugar–phosphate backbones of nucleic acids being at the low end and their bases at the high end. They are compared with corresponding published H-atom data (kH), where possible, and show kinetic-isotope-effects ranging over five orders of magnitude. Since all kH data were obtained at pH = 1, while kM values refer to pH ~ 7 of the natural state, the pH-dependence of kM was examined in representative cases. The changes found result from protonation of the solute rather than a changed reactivity of Mu on being converted to MuH+. On localizing the solutes in the hydrophobic phase of dilute micelles, the reactivity of Mu was again measured (kM(mic)). The resulting "enhancement" factor was considered in terms of: the reaction mechanism, its dependence on microenvironment (solvation), and the concentrating effect of mutual confinement to small sections of a biphasal system. Key words: kinetic isotope effects, muonium, biochemicals, micelles.
Voltammetric determination of Al(III) with adsorptive preconcentration of the pyrocatechol violet complex
Authors: Vukomanovic Dragic V. , Page John A. , Vanloon Gary W.
Published: May 2006

A method is described for the determination of aluminium in aqueous samples by linear scan voltammetry after adsorptive accumulation of the pyrocatechol violet (PCV) complex. A pH 6.5 triethanolamine/perchloric acid electrolyte is used for the determination with adsorption onto a mercury drop electrode at −0.50 V (vs a Ag/AgCl/satd KCl reference). Scanning cathodically after adsorption gave a current peak at −0.70 V for reduction of adsorbed PCV and a peak at −0.90 V for reduction of the adsorbed Al-PCV complex. The peaks were well resolved. Using 1.0 μM PCV and a 60 s adsorption period gave linear calibration curves for Al in the range 1 to 25 ng mL−1 Al. The detection limit was 0.1 ng mL−1 Al. For analysis, calibration by the method of standard additions is required. Most other metal species do not interfere, but vanadium (V(IV) and V(V)) forms a complex with PCV that is adsorbed and reduced at the same potential as Al–PCV. The interference may be eliminated by complexation of the V with citrate. Al and V present together may be determined by analysis in the absence of citrate (total: Al + V) and in the presence of citrate (Al); V is determined as the difference in the two values.The analysis of a range of samples with Al concentrations from 6 ng mL−1 to 3 μg mL−1 gave results which were in good agreement with those obtained by other methods. Key words: aluminium, pyrocatechol violet, adsorption, voltammetry, environmental.
Hydrogen-bonded complexes of perfluoro-t-butanol with acetone and nitromethane in low temperature solutions and matrices
Authors: Schriver Louise , Schriver André , Peil Stefan , Schrems Otto
Published: May 2006

Infrared spectra are reported for binary complexes between perfluoro-tert-butanol as proton donor and nitromethane and acetone as bases. The complexes have been investigated in low temperature solutions and in cryogenic matrices. The spectra have been evaluated in terms of frequency shifts (ΔVOH), half widths (FWHH) and anharmonicities (κ) caused by the hydrogen bonding of these complexes. The influence of the environment (solvents and solid matrices) as well as temperature on the spectra of the complexes has also been studied and is discussed in detail. Key words: hydrogen bonding, temperature effects, solutions, matrix.
Molecular orientation in thin Langmuir–Blodgett films of dipalmitoylphosphatidylcholine as studied by FTIR transmission and reflection–absorption spectroscopy
Authors: Okamura Emiko , Umemura Junzo , Takenaka Tohru
Published: May 2006

Langmuir–Blodgett (LB) films of DPPC were prepared by the horizontal lifting technique at various surface pressures encompassing the plateau region of its surface pressure (π) – area (A) isotherm. Molecular orientation in the LB films was quantitatively evaluated by a method recently proposed by Umemura et al. (J. Phys. Chem. 94, 62 (1990)) using Fourier transform infrared (FTIR) transmission and reflection–absorption (RA) spectra. From the results obtained, it is apparent that (i) in the LB films deposited at higher surface pressures above the plateau, the hydrocarbon chains are of all-trans conformation and well oriented at the tilting angle of ca. 20° from the surface normal; (ii) at lower surface pressures below the plateau, the chain segments are almost randomly oriented (like in a liquid-crystalline state) with an increased amount of the gauche conformers; and (iii) in the films deposited within the plateau, the hydrocarbon chains are in an intermediate state between the above two states. It is therefore concluded that the plateau corresponds to a process of the reorientation of the hydrocarbon chains from an almost randomly-oriented state to a highly-oriented one with the increase in the number of the all-trans conformers. Key words: dipalmitoylphosphatidylcholine, Langmuir–Blodgett film, molecular orientation, infrared reflection–absorption spectra, surface pressure–area isotherm.
Computers and infrared spectroscopy: evolution and revolution
Authors: Moffatt D. J. , Kauppinen J. K. , Mantsch H. H.
Published: May 2006

A brief history of the relationship between computer and infrared spectroscopist is given with emphasis on the use of the Fourier transform. Subsequently, an algorithm is developed that may be used to devise an objective Fourier self-deconvolution procedure which depends only on the input data and is not subject to the biases that are often introduced in such subjective techniques. Key words: deconvolution, Fourier transform, maximum entropy method.
A study of the phase transition in semiconducting 1:2 tetracyanoquinodimethane charge transfer complexes with dipropylamine derivatives: X-ray structure and polarized reflection spectra
Authors: Truong Kim D. , Pépin Claude , Bandrauk André D. , Drouin Marc , Michel André , Banville Marcel
Published: May 2006

The donors dipropylamine (DPA), methyldipropylamine (MDPA), and ethyldipropylamine (EDPA) from 1:2 charge transfer complexes with the acceptor tetracyanoquinodimethane (TCNQ). All crystals in the above series are semiconductors and undergo a first-order phase transition at the temperature around 210 K as obtained from conductivity measurements. The X-ray structure of DPA–(TCNQ)2 was obtained as a function of temperature, and shows a distinct order–disorder transition in the hydrogen-bond network between donors and acceptors. Polarized reflection spectra on single crystals of these compounds were measured as a function of temperature in the frequency range from 400 to 4000 cm−1. A detailed study of the C≡N modes of TCNQ indicates this mode is strongly influenced by the transition due to it being hydrogen bonded to the donors. The reflection coefficients of the IR active Ag modes of TCNQ also undergo a sharp discontinuity at the transition temperature. These results are interpreted as due to a cooperative order–disorder and electronic spin – Peierls transition. Key words: organic semiconductors, X-ray structure, polarized reflection spectra, phase transition, cooperative order–disorder transition, spin–Peierls transition.
Physical characteristics and polarographic reduction mechanism of some oxazolones
Authors: Ismail M. I.
Published: May 2006

The redox potential (ΔE1/2), electron affinity (EA), ionization potential (IP), coulomb repulsion integral (J12), and electronic transition energy (ECT) of several oxazolone derivatives in DMF were computed. A linear correlation was shown to exist between ΔE1/2 and ECT• The polarographic reduction was investigated in ethanolic-Theil buffer solutions. At pH ≤ 7.2, two waves were obtained representing the uptake of 2- and 4-electron steps respectively. In alkaline media, a third wave appeared seemingly a result of the hydrolysis of oxazolones. The rate of hydrolysis was determined and the electrode mechanism was elucidated and confirmed via spectrophotometric and coulometric analysis. Key words: oxazolones, polarography, electrochemistry, electrode reaction, spectrophotometry, redox potential.
Complexing behavior of rutin and quercetin
Authors: Escandar Graciela M. , Sala Luis F.
Published: May 2006

The equilibrium reactions between rutin, 3-[6-O-(6-deoxy-α-L-mannopyranosyl)-(β-D-glucopyranosyl)oxy]-2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-4H-1-benzopyran-4-one, and quercetin (rutin aglycone) with iron(III), copper(II), cobalt(II), and nickel(II) have been studied by potentiometric and spectrophotometric methods in aqueous solution. All measurements were carried out at t = 20 °C and μ = 0.10 M, and the corresponding stability constants were calculated by applying computational methods. The interactions between the proposed cations with both biological ligands and other related ones are compared, with simultaneous postulation of the probable structures. Key words: chelate stability constants of rutin, chelate stability constants of quercetin, transition metal ion interaction with flavonoids, stability constant determinations, potentiometric and spectrophotometric studies.
Ion–molecule reactions of transition-metal ions with nitriles in the gas phase: competitive formation of both "end-on" and "side-on" coordination
Authors: Chen Lin-Zhi , Miller Jack M.
Published: May 2006

First field-free region B/E linked scan studies of metastable and particularly collision-induced dissociations show that two competitive coordination processes ("end-on" and "side-on") exist in the complexation of metal ions (Ni+, Co+, Fe+, and Mn+) with alkanenitriles in the FAB source of an E/B configuration mass spectrometer. Analysis of the B/E linked scan CA spectra gives approximate percentages of the end-on coordination mode in the reactive complexes. An unusual β-methyl migration, involved in the elimination of CH4 from end-on complexes, may be in competition with β-H migration/elimination of H2 and β-C—C bond cleavage/elimination of alkene. For the side-on complexes, the metal ions can insert into various C—C bonds followed by β-H transfer/competitive elimination of alkene and alkane or by a direct C—M+ bond cleavage to give an alkyl radical. Key words: FAB mass spectroscopy, metal-ion/nitrile reactions, collisional activation, coordination modes, ion–molecule reactions.
The crystal structure of W(CO)5piperidine: Implications for photoreactivity of W(CO)5L complexes
Authors: Moralejo Carol , Langford Cooper H. , Bird Peter H.
Published: May 2006

The X-ray crystal structure of W(CO)5piperidine is reported and assignments are given for the LF and CT band of W(CO)5piperidine and W(CO)5pyridine. The [Formula: see text](axial), and [Formula: see text] (equatorial) bond lengths for W(CO)5pip are 2.330(5), 1,963(6), and 2.04(7), respectively. The longer [Formula: see text] bond length in W(CO)5pip compared to W(CO)5pyr corresponds to greater photoreactivity observed under LF triplet excitation for W(CO)5pip but cannot account for the reverse trend observed under LF singlet irradiation. The structure also fails to support a model based on differences in π back donation from W. It is concluded that differences between singlet and triplet photoreaction originate in the relative rates of radiationless decay. Key words: W(CO)5piperidine, photoreactivity, crystal structure.
Catalytic activity of vanadyl phosphate supported on TiO2 (anatase) and SiO2 (silica)
Authors: Martinez-Lara M. , Moreno-Real L. , Pozas-Tormo R. , Jimenez-Lopez A. , Bruque S. , Ruiz P. , Poncelet G.
Published: May 2006

Oxidation of n-butane was investigated on vanadyl phosphate (VP) prepared in the presence of titania (anatase) and silica as supports. In the case of silica, α-VOPO4 was synthesized with no sign of interaction between the silica surface and vanadium phosphate. On the contrary, titania was coated with amorphous VP (P/V = 1), preventing the crystallization of VOPO4 at least up to the VP content of 21%. n-Butane was completely converted into CO2 for the VP/TiO2 catalytic system. Upon impregnation with metal sulfates, maleic anhydride (MA) was produced with selectivities depending on the nature of the added metallic species, the best effect being observed with Fe2+ (V/Fe = 3). Selectivities to MA were influenced by the P/V ratio, with a maximum at P/V = 1.2. Keywords: vanadyl phosphate, maleic anhydride, butane oxidation, anatase, rutile.
Antigenic and structural relationships within group 19 Streptococcuspneumoniae: chemical characterization of the specific capsular polysaccharides of types 19B and 19C
Authors: Beynon Linda M. , Richards James C. , Perry Malcolm B. , Kniskern Peter J.
Published: May 2006

The specific capsular polysaccharides of Streptococcuspneumoniae types 19B and 19C (American types 58 and 59) were investigated by a combination of 1H, 13C, and 31P nuclear magnetic resonance spectroscopy and analytical methods based on mass spectrometry. The two polysaccharides were found to be high molecular weight polymers composed of L-rhamnose, 2-amino-2-deoxy-D-mannose, D-ribose, D-glucose, and phosphate. Homo- and heteronuclear chemical shift correlation techniques and nuclear Overhauser enhancement experiments led to the unambiguous assignment of the 1H and 13C resonances from the glycose residues and established their sequence within repeating oligosaccharide units. The oligosaccharide units are polymerized through phosphate diester linkages.[Formula: see text]Both polysaccharides share a common hexasaccharide structural unit and they differ only in the degree of substitution at the branched 2-acetamido-2-deoxy-β-D-mannopyranosyl residue: the 19C polysaccharide is O-6 linked by β-D-glucopyranosyl end groups to form a heptasaccharide repeating unit, while the 19B polysaccharide is unsubstituted at that position. The serologic cross-reactivity between S. pneumoniae serotypes 19B and 19C can now be related to the structural similarity of the antigenic capsular polysaccharides. Keywords: Streptococcuspneumoniae, capsular polysaccharide, immuno chemistry.
Molecular recognition IX. The synthesis of the H-type 2 human blood group determinant and congeners modified at the 6-position of the N-acetylglucosamine unit
Authors: Petrakova Eva , Spohr Ulrike , Lemieux Raymond U.
Published: May 2006

The synthesis of the methyl glycosides of the H-type 2 human blood group determinant (α-L-Fuc-(1c → 2b)-β-D-Gal-(1b → 4a)-β-D-GlcNAc-OMe) and its 6a-deoxy, 6a-O-methyl, 6a-chloro-6a-deoxy, 6a-deoxy-6a-fluoro, 6a-amino-6a-deoxy, 6a-acetamido-6a-deoxy, and 6a-deoxy-6a-pivalamido congeners is reported. The compounds were prepared to test the hypothesis that the binding of the H-type 2 trisaccharide by the lectin I of Ulexeuropaeus requires OH-6a to become intramolecularly hydrogen bonded to the neighboring O-5a ring atom. The results obtained in the binding studies are reported in an accompanying publication (Spohr, Paszkiewicz-Hnatiw, Morishima, and Lemieux) where it is demonstrated that the formation of the intramolecular hydrogen bond is not required for effective binding. In view of the results reported in another accompanying paper (Nikrad, Beierbeck, and Lemieux), OH-6a most likely remains in contact with the aqueous phase near the periphery of the combining site. Keywords: N-acetylglucosamine derivatives, H-type 2 blood group determinant and congeners, oligosaccharide synthesis.