bluuebunny/crossref_metadata_2025 huggingface dataset:
Remarks on the internal motion in diphenyl ether. Fluorophenyl ethers
Authors: Schaefer Ted , Penner Glenn H. , Takeuchi Craig , Tseki Potlaki
Published: May 2006

The 13C nuclear magnetic resonance chemical shifts and the 13C,19F spin–spin coupling constants are reported for 4,4′-difluorophenyl ether and 4-fluorophenyl phenyl ether in CS2 and in acetone-d6 solutions. An estimate of 6J90, the extremum in the σ–π coupling constant between the 19F nucleus on one ring and the ipso13C nucleus on the other, is obtained from measurements on 2,6-dibromo-4-fluorophenyl phenyl ether. The ensuing estimates of [Formula: see text], the expectation values of sin2 θ as obtained from 6J(13C,19F), are compared with those obtained from STO-3G MO computations for diphenyl ether and its 4-fluoro derivatives. These computations give conformational energies at 30° intervals of the angles of twist about the two C—O bonds. In rough agreement with C-INDO computations, interconversion of the helical forms is calculated to occur most easily by the so-called one-ring flip mechanism; the barrier to interconversion is less than 1 kJ/mol in the ether and its 4-fluoro derivatives. It appears that the conformational behaviour of these derivatives is unaltered by passage from CS2 to acetone solutions at 300 K. Furthermore, [Formula: see text] values from 6J(13C,I9F) in solution are very similar to those obtained from the computations on the free molecules. If this agreement is not accidental, then it may arise from a high degree of flexibility of the molecules in which, by a disrotatory or one-ring flip mechanism requiring a very low energy of activation, one helical or C2 conformation can be converted to another. The other conformations have considerably higher energies and the solvents do not appear to lower these energies enough to favor their populations significantly at 300 K.
Influence of alkylation of the amino group on the reactions of adenine with the methylmercury(II) cation
Authors: Grenier Lucie , Charland Jean-Pierre , Beauchamp André L.
Published: May 2006

Reaction of the CH3Hg+ cation with 6-dimethylamino- and 6-diethylaminopurine (HL) yielded compounds of the types [(CH3Hg)L], [(CH3Hg)2L]X (X = NO3, ClO4), and [(CH3Hg)2(HL)](NO3)2. Metal binding to N(9) in [(CH3Hg)L] was deduced from the nmr spectra. The coordination sites in the other types of compounds were determined from X-ray diffraction studies on two complexes of 6-dimethylaminopurine. Crystals of [(CH3Hg)2L]ClO4 are monoclinic, space group P21/c, a = 9.319(3) Å, b = 9.581(8) Å, c = 19.624(8) Å, β = 118.23(3)°, and Z = 4. The structure was refined on 1521 nonzero MoKα reflections to R = 0.040. [(CH3Hg)2(HL)](NO3)2•H2O belongs to the same space group, a = 11.012(3) Å, b = 23.669(7) Å, c = 6.748(4) Å, β = 95.25(4)°, Z = 4, 1781 MoKα reflections, R = 0.053. In both complexes, the CH3Hg+ groups are bound to the N(9) and N(3) sites of the purine ring. In the [(CH3Hg)2L]+ cation, the lone pair of N(7) is free, whereas it is occupied by a proton in [(CH3Hg)2(HL)]2+. The 1H and 13C nmr spectra show that the solid state structures are retained in DMSO solutions. The 2J(199Hg—1H) coupling constants suggest that protonation on N(7) produces for N(9) and N(3) average decreases of basicity corresponding to ~3 pK units.
Extraction of Zn(II), Cd(II), and Hg(II) by dodecyloligo(oxyethylene) carboxylic acids
Authors: Strzelbicki Jerzy , Charewicz Witold , Beger Jorg , Hinz Lutz
Published: May 2006

The synthesis of a series of n-dodecyloligo(oxyethylene) carboxylic acids of general formula C12H25(OCH2CH2)nOCH2COOH (n = 0 to 5) and application of these compounds for the separation of Zn(II), Cd(II), and Hg(II) in extraction are reported. Negligible extractability of the metal ions investigated was found using dodecyloxyacetic acid (n = 0) which indicated that complexation resulted from metal–cation interaction with oxygen atoms of the polyoxyethylene chain incorporated into the molecule of complexon. In extractions from aqueous solutions containing ZnCl2, CdCl2, HgCl2, and NaOH (pH regulator), loading of the organic phases increased for extractants with more oxyethylene units. In particular, Hg(II) extractability and molecular structure of the extractant are related. The selectivity of extraction increased drastically if NaCl was added to the initial aqueous phase, as the extractabilities of Cd(II) and, particularly, Hg(II) declined while the extractability of Zn(II) remained high. Selectivity of extraction is produced by the complex chemical equilibria resulted from complexation of the metal cation by an alkyloligoether carboxylic acid in the organic phase and by Cl− and OH− anions in the aqueous phase. Thus, aqueous solution parameters greatly influence the efficiency and the selectivity of the extraction process and suitable experimental conditions are very important.
Ambident reactivity in the reaction of phenoxide ion with 2-N-(2′,4′-dinitrophenyl)- and 2-N-(4′-nitrophenyl)-4,6-dinitrobenzotriazole 1-oxides, new superelectrophiles
Authors: Buncel Erwin , Dust Julian M.
Published: May 2006

Reaction of the novel superelectrophiles 2-N-(2′,4′-dinitrophenyl)- and 2-N-(4′-nitrophenyl)-4,6-dinitrobenzotriazole 1-oxides, 3, and 4, possessing two electrophilic centres, with the ambident nucleophile potassium phenoxide in (CD3)2SO was followed by 400 MHz 1H nuclear magnetic resonance spectroscopy. A dichotomy in the reaction pathways has been observed. With MeO−, attack at C-7 leads to reversible adduct formation, while attack at C-1′ results in irreversible N-2: C-1′ bond scission via the metastable C-1′ adduct. In contrast, the reaction of 3 and 4 with PhO− proceeds by a two-pronged attack: formation of C-7 carbon-bonded phenoxide adducts via the ortho and para carbon sites, and oxygen-based cleavage products by attack at the C-1′ position, accompanied by N-2:C-1′ bond scission, in accord with the ambident reactivity of PhO−. Significantly, in this case reaction of both C-7 and C-1′ is effectively irreversible. Moreover, the reaction of phenoxide with either 3 or 4 shows striking differences compared to the reaction of PhO− with 2-N-(picryl)-4,6-dinitrobenzotriazole 1-oxide, 1. Reaction of PhO− with 1 resulted only in O-attack at C-1′ and N-2:C-1′ bond scission; there was no evidence for C-7 adduct formation via O- or C-attack. This marked difference in behaviour can be attributed to the decreased susceptibility to C-1′ attack exhibited by 3 and 4 as compared to 1 and arises from the successive removal of electron-withdrawing nitro groups from the 2-N′-nitroaryl moiety in the series 1 → 3 → 4. The reactions are discussed on the basis of selectivity considerations and an activation energy/reaction coordinate profile comparing the pathways for both C-attack at C-7 and O-attack at C-l′ as electrophilicity (delocalizability) is progressively modulated in the reaction series.
The solution conformation of the synthetic tubulin fragment Ac-tubulin-α(430–441)-amide based on two-dimensional ROESY experiments
Authors: Otter Albin , Kotovych George
Published: May 2006

Ac-tubulin-α(430–441)-amide, a synthetic fragment of tubulin, has been studied by 300-MHz 1H nuclear magnetic resonance spectroscopy in 80% CD3OH/20% D2O solution at neutral pH. The results, based on two-dimensional phase-sensitive techniques such as COSY, RELAY, and ROESY, reveal that the dodecapeptide exists in a non-random conformation. The conformation can be described as an α helix for the N-terminal five residues, followed by an extended section. The extended part exhibits a significant change in the nuclear Overhauser enhancement (nOe) pattern between units number 9 and 10. Together with long-range nOe data this can be attributed to a folding back that allows the close proximity of the C-terminal half of the extended chain and amino acids 3 to 5 of the α-helical turn. The existence of an α helix is in good agreement with previously published circular dichroism data.
Spatial intensity distributions of vacuum ultraviolet photon beams measured by thermoluminescent image films
Authors: Kikukawa Nobuyuki , Makita Tsutomu
Published: May 2006

The thermoluminescent phosphor, calcium sulfate activated by manganese, was found to be a suitable material for imaging films to record the spatial distributions and reflections of vacuum ultraviolet (vuv) photon beams used for high-resolution photoelectron spectrometers. He I 584 Å photons radiated in a microwave discharge tube were collimated through capillary tubes, and the phosphor films were placed at the position of the photoionization chamber. When the irradiated phosphor films were heated up to 200 °C in a darkroom, faint green glow images were observable on the phosphor films and these images were recorded by photographic films. When the films were placed 118 mm from the exit of the collimating capillary tube, the photon beam images were almost circular, about 4.5 mm in diameter. The photon distribution in the collimated beam was interpreted with the aid of computer simulation. The experimental results revealed that the reflections of vuv photons at planes of various kinds were almost specular if the surface was clean and smooth and the glancing angles were small.
Design of novel mesomorphic compounds methyl-substituted N,N′-dialkanoyl-1,3-benzenediamines
Authors: Kawada Hiromitsu , Matsunaga Yoshio , Takamura Takumi , Terada Masahiro
Published: May 2006

Calorimetric, X-ray diffraction, and broad-line proton nuclear magnetic resonance studies on the thermal behaviour of N,N′-dialkanoyl-1,3-benzenediamines carrying one, two, three, and four methyl groups disclosed that the enthalpy change at the transition to an isotropic liquid can be considerably diminished by this substitution and that various mesomorphic states can be obtained by this approach. Thus, the 2-methyl compounds with decanoyl to octadecanoyl groups exhibit hexagonal disordered columnar phases. Furthermore, the enthalpy changes in the 2,4,6-trimethyl compounds with octanoyl to octadecanoyl groups are so much reduced that their mesophases are nematic in type. On the other hand, the viscous birefringent fluids formed by the 4-methyl, 2,4-dimethyl, 4,6-dimethyl, and 2,4,5,6-tetramethyl compounds give X-ray diffraction patterns indicating highly ordered structures.
The complementary use of factor analysis and spectral deconvolution in a study of the CD3COOD–D2O system
Authors: Ng Jesse B. , Petelenz B. , Shurvell H. F.
Published: May 2006

The carbonyl stretching region of fully deuterated acetic acid CD3COOD in D2O has been studied by means of Raman spectroscopy. The number of linearly independent spectral components was determined to be four by factor analysis. Fourier self-deconvolution of the spectra revealed another major component band which was assigned to an overtone and/or combination vibration of the acid monomer. The overall band envelope was fitted with five Gauss–Lorentz functions using a non-linear least squares procedure. The abnormally high intensity of the band assigned to the overtone/combination is discussed in terms of Fermi resonance.
Electron spin resonance and electron spin echo spectroscopic studies of Cu2+ sites in synthetic cancrinite zeolites
Authors: Raghunathan P. , Kevan L.
Published: May 2006

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) studies of Cu2+ sites introduced by ion exchange are reported in two types of synthetic cancrinite zeolites, one of them a more hydrated sample, Na6 Al6Si6O24•8H2O (denoted C1), and the other a less hydrated sample with entrained calcium carbonate, Na6Al6Si6O24•CaCO3•8H2O (denoted C2). ESR spectra reveal that C1 activated at 100 °C contains Cu2+ sites of tetrahedral symmetry with [Formula: see text] and [Formula: see text] while at a higher activation temperature of 400 °C Cu2+ sites with a different symmetry are produced corresponding to [Formula: see text] and [Formula: see text] C2 activated at 100 °C contains almost exclusively Cu2+ sites of trigonal symmetry with [Formula: see text] and [Formula: see text] Two- and three-pulse ESEM experiments at 4 K give 2D and 27Al modulation patterns that corroborate these ESR assignments. Possible locations of the Cu2+ sites in the C1 and C2 structures are proposed.
Magnetic field effects upon photochemistry of bichromophoric chain molecules containing nitroaromatic and arylamino moieties: Elucidation of reaction mechanism and control of reaction yields
Authors: Nakagaki Ryoichi , Mutai Kiyoshi , Hiramatsu Mitsuo , Tukada Hideyuki , Nakakura Saburo
Published: May 2006

The photochemistry of bichromophoric species containing nitroaromatic and arylamino moieties has been studied in the presence and absence of an external magnetic field. Photolysis products are an unsubstituted arylamine, a nitrosoaromatic species (Product 1), and a nitroaromatic compound (Product 2). On application of the magnetic field, the formation yield for Product 1 decreases and that for Product 2 correspondingly increases, whereas the consumption of the starting material and the release of the unsubstituted arylamine are independent of the magnetic field. Analysis of the magnetic field effects shows that the hyperfine coupling mechanism predominates and that the photoreaction takes place in the triplet manifold of the nitroaromatic moiety. Application of the magnetic field may cause an appreciable change in the relative yield of cage and escape products, i.e., the branching ratio of competitive processes.
Molybdenum–tin and molybdenum–copper complexes derived from the molybdenum tricarbonyl anions, LMo(CO)3− (where L = HBpz3 or HBpz″3; pz = pyrazolyl, N2C3H3; and pz″ = 3,5-dimethylpyrazolyl, N2C5H7)
Authors: Liu Yan-Yun , Mar Arthur , Rettig Steven J. , Storr Alan , Trotter James
Published: May 2006

The pyrazolylborate molybdenum tricarbonyl anions, (HBpz3)Mo(CO)3− and (HBpz″3)Mo(CO)3−, have been utilized to prepare a number of heteronuclear bimetallic species containing Mo—Sn and Mo—Cu bonds. Crystals of [hydrotris(1-pyrazolyl)borato-N,N,N](triphenylstannyl)tricarbonylmolybdenum are monoclinic, a = 17.570(2), b = 14.077(2), c = 12.180(1) Å, β = 96.985(8)°, Z = 4, space group P21/n, those of [hydrotris(1-3,5-dimethylpyrazolyl)borato-N,N,N](chlorodimethylstannyl)tricarbonylmolybdenum are monoclinic, a = 11.044(3), b = 12.061(2), c = 20.218(4) Å, β = 100.43(1)°, Z = 4, space group P21/c, and those of [hydrotris(1-pyrazolyl)borato-N,N,N]tricarbonylmolybdenum(N,N,N′,N′-tetramethylethylenediamine)copper dichloromethane solvate are orthorhombic, a = 19.027(2), b = 16.995(2), c = 16.136(2) Å, Z = 8, space group Pbca. The structures were solved by conventional heavy atom methods and refined by full-matrix least-squares procedures to R = 0.044, 0.042, and 0.059 for 2084, 2502, and 1891 reflections, respectively. The two tin structures have a 3:3:1 capped octahedral structure with Mo—Sn bonds of 2.864(1) and 2.823(1) Å. The copper complex has a 3:4 piano stool arrangement of ligands about Mo with a Mo—Cu bond of 2.639(2) Å, two semibridging carbonyl ligands, and one terminal carbonyl ligand.
Tetracyclic triterpenes. X. Solvent effect in reactions of tetrasubstituted triterpenoidal olefins with ozone. An allylic oxidation
Authors: Paryzek Zozislaw , Martynow Jacek
Published: May 2006

The highly hindered 8,9 double bond in lanostane derivatives was found susceptible to oxidation with ozone. The reaction depends on the polarity of the solvent. It is proposed that the structure of the initial complex formed between the olefin and ozone is influenced by the reaction medium. Reaction of 3β-acetoxy-5α-lanost-8-ene with ozone gives 8α, 9α-epoxide in methylene chloride, while 3β-acetoxy-5α-lanost-8-en-7-one, an allylic oxidation product, is the main compound formed in ethyl acetate.
Prototropic reactions for 1-, 2-, and 4-fluorenamides in the ground and excited singlet states
Authors: Manoharan Ramasamy , Dogra Sneh K.
Published: May 2006

Absorption and fluorescence spectra of 1-, 2- and 4-fluorenamides (1-FAm, 2-FAm, 4-FAm) were studied in different solvents and at various acid concentrations. This study indicates that the long wavelength absorption band is long-axis polarised in 2-FAm whereas in 1-FAm and 4-FAm, it is composed of long- and short-axis polarised transitions. The —CONH2 group in 1-FAm and 4-FAm is coplanar with the fluorene moiety, whereas the same group in 2-FAm is non-coplanar with the fluorene ring in non-polar solvents, both in S0 and S1 states, but attains coplanarity in the polar and hydrogen bonding solvents in the S1 state. Hammett's acidity scale is not valid for the protonation reaction of amides and a medium effect is observed on the absorption spectra of the monocations in strong sulphuric acid. The prototropic equilibrium for the protonation of ring carbon atom is not established in the S1 state, due to the slower rate of protonation reaction. Proton-induced fluorescence quenching of the monocations is followed before the monocations are being protonated.
Polynuclear silver complexes of 2,5-dimethyl-2,5-diisocyanohexane (TMB). Influence of the anions
Authors: Dartiguenave Michèle , Dartiguenave Yves , Mari Alain , Guitard André , Olivier Marc J. , Beauchamp André L.
Published: May 2006

Two series of complexes have been prepared by reaction of 2,5-dimethyl-2,5-diisocyanohexane (TMB) with AgX [Formula: see text]. Complexes of the first series, Ag(TMB)X, have been obtained with [Formula: see text]. The structures have been determined by X-ray diffraction for [Formula: see text] and [Formula: see text]. The crystals contain infinite chains of silver atoms alternating with bridging TMB ligands. When [Formula: see text], the Ag atom is best described as (2 + 2)-coordinated, with two of the coordination sites filled by [Formula: see text] acting as a bidentate ligand. When [Formula: see text], the Ag—TMB chain is of a different type and Ag can be considered two-coordinated and weakly interacting with a monodentate [Formula: see text] ion. The second series includes Ag2(TMB)3X2 complexes whose structure consists of pairs of chains cross-linked by extra TMB ligands leading to a ladder-like pattern. A weak unidentate interaction involving [Formula: see text] or [Formula: see text] is also apparent. The infrared data are in agreement with the X-ray results, since the highest v(C≡N) value is obtained for Ag(TMB)BF4, where Ag—CN distance is the shortest. The conformation of TMB is always trans about the central CH2—CH2 bond. The methyl groups render one of the staggered orientations of the —C(CH3)2(NC) groups unfavorable, leaving two conformations for coordinated TMB, both of which are observed in the Ag complexes.
Ion–dipole complexes in the unimolecular reactions of isolated organic ions. Fragmentation of protonated and cationated formate and acetate esters
Authors: McMahon Adam W. , Harrison Alex. G.
Published: May 2006

The unimolecular fragmentation reactions occurring on the metastable ion time scale have been determined for protonated alkyl formates and acetates and for the adducts of CD3+ and C2H5+ with these esters. The protonated esters, R1CO2RH+, fragment to form R+ + R1CO2H or, by internal hydrogen transfer, R1CO2H2+ + (R – H) with the relative yields depending on the identity of R1 and R. Using deuterium labelling it has been shown that the added proton interchanges with hydrogens of the R group prior to fragmentation. The methyl-d3-cationated esters, R1CO2R•CD3+, mostly fragment to give R1CO2CD3, H+ by elimination of (R – H), although R+ formation is observed in a few cases. The ethyl-cationated esters fragment primarily to give R1CO2C2H5•H+ by elimination of (R–H); specifically no elimination of C2H4 is observed. All of the results can be rationalized in terms of a step-wise mechanism of fragmentation involving initial formation of an ion–dipole complex of the alkyl ion R+ with a carbonyl compound. This complex may either fragment to form R+ or may proceed by way of an ion–dipole complex of a protonated carbonyl and an olefin to the protonated species plus (R – H). The results indicate that primary alkyl ions are not formed but rather that rearrangement to the more stable secondary or tertiary structure occurs. There is some evidence that s-butyl cations are not formed, but that these too rearrange to the t-butyl structure.
Enhanced nucleophile selectivity in the photoaddition to styrene. Comparison with the thermal addition
Authors: Anderson Steven W. , Yates Keith
Published: May 2006

The reactivity–selectivity properties of the sec-phenethyl carbenium ion and its analogues have been examined in binary-aqueous solutions of alcohols (ROH; R = Me, Et, CF3CH2, n-Bu, t-Bu, n-Pr, i-Pr), acetic acid, and in solutions containing external nucleophiles such as thiophenol, ethanethiol, acetate, and benzenesulfinate. Selectivity (S = k(Nuc)/k(H2O)) is observed to be a function of the nucleophile, the ring substituent, thermal or photochemical conditions, irradiation time, and medium acidity. In the case of styrene, the photochemically generated (usually 254 nm, 1 h) species exhibits greater selectivity (up to two orders of magnitude) than its thermal counterpart, generated (usually 80 °C, 2−4 d) in identical media, particularly in aqueous-rich solvent compositions. A rationale for these differences, supported by fluorescence quenching experiments, and the other effects upon selectivity will be discussed.
Density measurements in the succinonitrile–water system
Authors: Ng Lee Hong , Sadoway Donald Robert
Published: May 2006

The variation of density with temperature and composition in the succinonitrile–water system was measured by the direct Archimedean method. At fixed composition, density was found to vary linearly with temperature. At constant temperature, density was found to vary linearly with composition. Along the phase boundary there is a density reversal: at temperatures above 45 °C the water-rich phase is denser; below 45 °C the succinonitrile-rich phase is denser.
Étude structurale de l'amino-5 dibenzyl-4,4 one-3 2H-thiadiazine-1,2,6 dioxyde-1,1
Authors: Brouant Pierre , Barbe Jacques , Goya Pilar , Ochoa Carmen
Published: May 2006

5-Amino-4,4-dibenzyl-3-one-2H-1,2,6-thiadiazine-1,1-dioxide was structurally characterized in the solid state by X-ray diffraction. The structure shows that the title compound exists in the solid in a sheet-packed form and as the amino tautomer. The unconjugated heterocyclic ring, which in the structures of related compounds is usually folded, is planar in the present structure. 1H nuclear magnetic resonance and dipole moment data indicate that the title compound has the same structure in the solid state and in solution.
Kinetics of hydride transfer between nitrogen heteroaromatic cations
Authors: Bunting John W. , Luscher Mark A.
Published: May 2006

The kinetics of the reduction of the 3-cyano-1-methylquinolinium, 4-cyano-2-methylisoquinolinium, and 2-methyl-5-nitro-isoquinolinium cations by 9,10-dihydro-10-methylacridine, and also the reduction of these same three cations as well as the 10-methylacridinium cation by 5,6-dihydro-5-methylphenanthridine, have been investigated in 20% acetonitrile – 80% water, ionic strength 1.0, 25 °C. The reactions of the 2-methyl-5-nitroisoquinolinium cation with both reductants, and also of the 4-cyano-2-methylisoquinolinium cation with 9,10-dihydro-10-methylacridine, display kinetic saturation effects in the pseudo-first-order rate constants as a function of heterocyclic cation concentration. These effects are consistent with the formation of 1:1 association complexes between hydride donor and acceptor prior to the rate-determining step of the reduction. The second-order rate constants for these reactions, and also those for analogous heterocyclic cation reductions by 1,4-dihydronicotinamides, show systematic variations as a function of the hydride donor and acceptor species.
Charge transfer complexation of N,N-dimethyl-p-toluidine with nitrogen heteroaromatic cations as a model for association complexes in hydride transfer reactions
Authors: Bunting John W. , Luscher Mark A.
Published: May 2006

Charge-transfer complexation of N,N-dimethyl-p-toluidine (DMT) with each of the following N-methyl cations has been investigated in acetonitrile solution at 25 °C: acridinium; phenanthridinium; 3-X-quinolinium and 4-X-isoquinolinium (X = H, Br, CONH2, CN); 5-nitroisoquinolinium; 3-X-pyridinium (X = CONH2, CN) (also the N-benzyl pyridinium cations); N,N′-dimethyl-4,4′-dipyridylium dication. Charge-transfer absorption maxima are reported, and extinction coefficients (εmax) at these maxima and association constants (K) for 1:1 complex formation have been evaluated from the dependence of absorbance upon [DMT]. In general, the longest wavelength absorption maximum increases with increasing association constant. There is a strictly linear relationship between εmax and 1/K. These observations are considered in the context of theories of charge-transfer spectra. Such charge-transfer species are considered as models for the association complexes that are believed to exist upon the reaction pathway for hydride transfer between heteroaromatic cations. Although some significant qualitative relationships are apparent, variations in susceptibility of heteroaromatic cations to charge-transfer complexation are much smaller than variations in the reactivity of these cations towards hydride donor species.
Charge distributions and chemical effects. XLV. Graphite
Authors: Peluso Andrea , Fliszár Sándor
Published: May 2006

The zero point energy of graphite, [Formula: see text], is deduced from Debye's theory by separating the lattice vibrations into two approximately independent parts, with Debye temperatures [Formula: see text] (in plane) and [Formula: see text] (perpendicular). A balanced evaluation gives [Formula: see text]. The bond energies of graphite in its potential minimum are derived from those of polynuclear benzenoїd hydrocarbons using a formula describing bond energies in terms of the charges at the bond-forming atoms. These energies plus a consideration of (i) van der Waals interactions between layers (~1.2 kcal mol−1), (ii) ZPE = 3.68, and (iii) the experimental heat content. HT − H0 = 0.25 kcal mol−1, lead to a theoretical enthalpy of atomization, ΔHa(298.15) = 174.6, which is ~2% larger than its experimental counterpart, 170.9 kcal mol−1. Exploiting the fact that the carbon atoms are electroneutral in graphite and not so in benzenoїd hydrocarbons, the results obtained for graphite support the approximate validity of bond energies deduced for polynuclear benzenoїd hydrocarbons and of the net charge, 14.8 × 10−3 e, deduced for the carbon atom of benzene.
Molecular conformation of 2′-deoxy-3′,5′-di-O-acetyl guanosine. Crystal structure and high resolution proton nuclear magnetic resonance investigations
Authors: Koole Leo H. , Buck Henk M. , Kanters Jan A. , Schouten Arie
Published: May 2006

2′-Deoxy-3′,5′-di-O-acetyl guanosine crystallizes in the triclinic space group [Formula: see text], and the cell dimensions are a = 8.643(1) Å, b = 10.122(1) Å, c = 10.391(1) Å, α = 87.04(1)°, β = 73.58(1)°, γ = 72.37(1)°, V = 830.4(2) Å3; Z = 2 molecules per cell. Least-squares refinement converged at R = 0.031 for 3450 observed reflections. The asymmetric unit consists of two independent molecules. The guanine bases are linked via N(1)—H … N(7) and N(2)—H … O(6) hydrogen bonds to form a virtually planar system. Moreover, the conformational preferences of the title compound in DMSO-d6 solution have been determined with 300 MHz 1H NMR. It is found that the X-ray structure and the solution conformation are essentially similar. The C(4′)—C(5′) linkage resides in the gauche+ (g+) or trans (t) rotamers in the solid state, while an approximately equal distribution over g+, t, and gauche− is found in solution.
Intramolecular sensitization in salicylatotetraammine cobalt(III) complex
Authors: Vasquez Luis , Andrade Carlos , Crivelli Irma
Published: May 2006

Luminescence studies show that the fluorescence of salicylic acid and its anionic species is quenched when they belong to the coordination sphere of the Co(III) and no new emission appears even in glass at 77 K. The metallic center seems to provide new and efficient non-radiative deactivation paths for the ligand centered states.Irradiation in the intraligand bands of the complex, in aqueous solution, leads to photoredox reaction generating Co2+ species characteristic of the charge transfer cobalt photochemistry. Results are discussed in terms of an efficient intramolecular sensitization or intramolecular energy transfer from states centered in the ligand to LMCT state centered at the metal.
Anodic oxidation of lead in aqueous carbonate solutions. II. Film formation and dissolution in the pH range 9 to 14
Authors: Shoesmith D. W. , Bailey M. G. , Taylor P.
Published: May 2006

The anodic oxidation of lead has been studied in aqueous carbonate solutions at pH values in the range 9 ≤ pH ≤ 14. The solid films formed on the electrode surface have been identified by X-ray diffractometry. The dissolution and film formation processes have been studied by a number of electrochemical techniques at rotating disk electrodes. The anodic oxidation process can be divided into two distinct regions. In the main anodic oxidation process, a variety of surface phases is formed. The nature of the phase formed is pH-dependent. Plumbonacrite (Pb10O(OH)6(CO3)6) formation is observed at all pH values and predominates at pH ≥ 13. At pH < 11, cerussite (PbCO3) is the predominant phase, whereas at intermediate pH values, hydrocerussite (Pb3(OH)2(CO3)2) predominates. The dissolution rate of Pb2+ species from the electrode surface is directly proportional to the solubility of the predominant phase present. At more positive potentials, a reactivation, involving increased dissolution and a further stage of film formation, is observed. Litharge (PbO) is observed to grow underneath the initially formed basic lead carbonates. Dissolution occurs either by the field-assisted dissolution of the base-layer on the electrode or by metal dissolution through faults in the base-layer.
A redetermination of the ultraviolet photoelectron spectrum of (Z)-2-methyl-1,3-pentadiene; evidence that twisted conformers predominate in the gas phase
Authors: Werstiuk Nick Henry , Timmins George
Published: May 2006

A redetermination of the ultraviolet photoelectron spectrum of (Z)-2-methyl-1,3-pentadiene (1a) has resulted in a correction of the spectrum published by Masclet and co-workers. MMX and AM1 calculations on 1a predict that twisted s-cis conformations should be populated preferentially in the gas phase; this is in keeping with the experimentally determined -π−, π+ splitting of 1.55 ± 0.05 eV.
Selenium coronands. A novel conformational pair
Authors: Pinto B. Mario , Johnston Blair D. , Batchelor Raymond J. , Einstein Frederick W. B. , Gay Ian D.
Published: May 2006

The synthesis and characterization of the novel selenium coronands, 1,3,7,9-tetraselenacyclododecane 1a, 1,3,7,9,13,15-hexaselenacyclooctadecane 2a, the corresponding β-gem-dimethyl derivatives 1b, 2b, and 1,5,9,13-tetraselenacyclohexadecane 3, and 1,5,9,13,17,21-hexaselenacyclotetracosane 4 are described. X-ray crystallographic analysis of 1a reveals three independent molecules that exist in two distinct conformations, one molecule having approximate two-fold symmetry together with two molecules (of similar conformation) each having crystallographic [Formula: see text] symmetry. The conformations are denoted as [3333] or [66]. Whereas one resembles that of cyclododecane and tetrathia-12-crown-4 with respect to torsion angles, the other resembles that of tetraoxa-12-crown-4. The solid state CP-MAS 77Se and 13C nmr spectra are interpreted in light of the crystallographic information. Crystal structure: formula Se4C8H16; fw = 428.05; monoclinic, P21/c; Z = 8; a = 15.823(2) Å, b = 5.534(1) Å, c = 27.962(5) Å, β = 92.26(1)°; V = 2446.6 Å3; T = 200 K; R = 0.027 for 2162 observed data (I ≥ 2.5σ(I)).
A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles
Authors: Cacace Fulvio , Crestoni M. Elisa , Petris Giulia De , Fornarini Simonetta , Grandinetti Felice
Published: May 2006

Aromatic desilylation and detertbutylation by CH5+, C2H5+, i-C3H7+, and t-C4H9+ ions have been studied in the gas phase by mass spectrometric and radiolytic techniques. The higher rate of desilylation than of dealkylation is traced to the step involving the formation of ipso arenium ions, protonated at the ring carbon bearing the SiMe3 or the CMe3 substituent. The latter has been shown by ab initio calculations at the SCF STO-3G level to selectively depress the basicity of the ipso position, hence the stability of the corresponding arenium ion relative to its protomers, e.g. the basicity of the ipso position of PhCMe3 is lower by ca. 11 kcal mol−1 than of the para position. The SiMe3 group has no comparable effect, very close basicities being computed for the para and the ipso position of PhSiMe3. The different effects of the CMe3 and SiMe3 groups are traced to electronic and steric factors.
Diastereoselective alkylation of glycinates. 4. The role of the metal cation and additives
Authors: Mcintosh John M. , Cassidy Kenneth C.
Published: May 2006

The alkylation of the sodium, potassium, zinc, and zirconium enolates of the R-camphor imine of tert-butyl glycinate have been examined and the stereochemical results compared with those obtained with the lithium enolate. No difference was noted for the sodium enolate but inferior chemical yields and stereoselectivities were obtained with the potassium enolate, perhaps due to its increased ionic character. Substituting TMEDA for HMPA in these reactions led in all but one case to reduced stereoselectivities. The results support and are interpreted in terms of the π–metal association model previously proposed.
Excess partial molar enthalpies of water in water–tert-butanol mixtures
Authors: Koga Yoshikata
Published: May 2006

The excess partial molar enthalpies of H2O, HmE(H2O) in water–tert-butanol mixtures were measured at 30.00, 36.75, and 40.45°. Using the values of HmE(TBA) of the previous work (Y. Koga, Can. J. Chem. 66, 1187 (1988)), the excess (integral) molar enthalpies of the solution, HmE, were calculated at 30.00 °C, and compared with the literature values. The comparison was satisfactory. From the literature values of the excess free energy, the chemical potentials of each component were evaluated for the range, xTBA > 0.1. With the measured values of the excess partial molar enthalpies, the excess partial molar entropies were also calculated. These partial molar thermodynamic quantities and their dependence on temperature and composition clearly support the following views: (1) in the intermediate region, 0.1 < xTBA < 0.5, the system is very close to a critical demixing, and (2) in the TBA-rich region, xTBA > 0.6, TBA molecules in the solution are in almost the same environment as in the pure liquid, while H2O molecules lose the hydrogen bond network completely and are dispersed in the TBA liquid structure.
Reactions of "cyclen-" and "cyclam-phosphorane" with [Pt2Cl4(PEt3)2]. Monodentate and bidentate complexes and the crystal and molecular structure of [PtCl(PEt3)(cyclamPH)]Cl•CH2Cl2
Authors: Berry David Eric , Browning Jane , Bushnell Gordon William , Dixon Keith Roger , Pidcock Alan
Published: May 2006

Reaction of "cyclamphosphorane" (cyclamPH) with [Pt2Cl4(PEt3)2] yields [PtCl(PEt3)(cyclamPH)]Cl. The complex crystallizes as a dichloromethane solvate in the monoclinic space group P21/n, with a = 13.877(3), b = 23.231(7), c = 8.295(2)Å, β = 91.86(4)°, and an X-ray diffraction study shows square planar platinum coordination in which the labile proton of cyclamPH has transferred from phosphorus to nitrogen and the ligand is attached via simple [Formula: see text] chelation. The phosphorus is trans to chlorine in the platinum coordination plane.The corresponding product, trans-[PtCl2(PEt3)(cyclenPH2)]Cl, derived from reaction of "cyclenphosphorane" (cyclenPH) with [Pt2Cl4(PEt3)2], is shown by NMR studies to have a quite different structure in which the ligand is protonated at two nitrogen sites but not at phosphorus. The phosphorus is pentacoordinate with four attachments to nitrogen atoms and one to platinum. The two chlorine atoms are mutually trans in the platinum coordination plane. Keywords: crystal structure, cyclenphosphorane reaction, cyclamphosphorane reaction, X-ray diffraction.
Preparation and absolute stereochemistry of (−)-acenaphthenol
Authors: Hu Yulin , Ziffer Herman , Sllverton J. V.
Published: May 2006

1-Acenaphthenol was resolved by fractional crystallization of its camphanate ester followed by hydrolysis. The absolute stereochemistry of the (+)-camphanate and (−)-alcohol, derived therefrom, was established as (R) by X-ray crystallography. Keywords: (−)-acenaphthenol, stereochemistry, synthesis, X-ray crystallography.
Isomerization, fluorine exchange, and stereoselective synthesis of cis- and trans-F2TePh3Cl: application of the coordination model of reaction mechanisms
Authors: Janzen Alexander F. , Jang Meehae
Published: May 2006

The stereoselective synthesis of cis- and trans-F2TePh3Cl (phenyl substituents adopt a mer arrangement) via 5-coordinate cations Ph3TeFCl+ and [Formula: see text] is described. trans-F2TcPh3Cl isomerizes to the thermodynamically more stable cis isomer, and cis -F2TePh3Cl has non-equivalent fluorines which undergo stereoselective fluorine exchange, as confirmed by 19F and l25Te nmr spectroscopy. These results are consistent with an intermolecular mechanism of isomerization and fluorine exchange involving Te—F bond cleavage.The coordination model of reaction mechanisms provides a unified description of stereochemical rearrangements, ligand exchange processes, and chemical reactions in these, and related, octahedral and trigonal bipyramidal compounds. Keywords: synthesis of cis- and trans-F2TcPh3Cl, isomerization of octahedral Te compounds, fluorine exchange in cis-F2TePh3Cl, synthesis of Ph3TeF, coordination model of reaction mechanisms.
Stability relationships between solid cesium aluminosilicates in aqueous solutions at 200 °C
Authors: Taylor Peter , DeVaal Sheila D. , Owen Derrek G.
Published: May 2006

Interconversion reactions among the solids SiO2 (quartz and fused), γ-AlOOH (boehmite), CsAlSiO4, CsAlSi2O6 (pollucite), and CsAlSi5O12, in contact with aqueous solutions at 200 °C, have been investigated. Results support earlier suggestions that CsAlSi5O12 is not stable with respect to CsAlSi2O6 + SiO2. CsAlSiO4 is only stable, if at all, when the activity product of dissolved Cs+ and OH−, with respect to unit molality standard states, exceeds about 10−2. CsAlSi2O6 is stable with respect to boehmite and quartz when the cesium hydroxide activity product exceeds 10−7.8±1.5. The significance of these results to the possible immobilization of radioactive cesium as a solid aluminosilicate is discussed. Based on a comparison with published estimates of enthalpies of formation, the following revised values are suggested:[Formula: see text]Keywords: cesium aluminosilicates, enthalpy of formation, hydrothermal reactions, pollucite, thermodynamic stability.
The photochemistry of 5-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene
Authors: Bender Christopher Owen , Dolman Douglas
Published: May 2006

The direct irradiation of 5-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (18) led to 6-cyanobenzocyclooctatetraene (17; Φ = 0.00075), 6-cyanobenzosemibullvalene (14; Φ = 0.0003), and 2-cyanonaphthalene (Φ = 0.00015). The triplet excited state of 18, generated by sensitization with a variety of ketone sensitizers (e.g., Michler's ketone and acetophenone), showed no unimolecular reactivity. Compound 18 does not participate in Zimmerman di-π-methane rearrangement; the potential product from such a transformation, 8-cyanobenzosemibullvalene (19), was, however, isolated from direct (Corex filter; Φ = 0,0010) irradiation of the semibullvalene 14. Deuterium labelling experiments suggest that 14 derives from a mechanism involving initial C4—C8 bonding of 18, and that cyclooctatetraene 17 is probably formed by simple electrocyclic opening of the cyclobutene ring of 18. Keywords: photochemistry, rearrangement mechanism, di-π-methane, semibullvalenes.
High temperature–pressure aqueous oxidations. IV. A kinetic study of the oxidation and enolization of cyclohexanone in the presence of catalytic metal ions
Authors: Thomas John W. , Taylor Jay E.
Published: May 2006

The aqueous high pressure–temperature oxidation of cyclohexanone was found to be strongly catalyzed by silver, iron(III), and copper ions but not by aluminum, cobalt, manganese(II), or nickel ions. The reactivities of the catalytic ions were inhibited by the acidic oxidation products. Accordingly, initial reaction velocities were determined and used to correlate the kinetic data. A detailed kinetic study of copper ion catalysis has shown that in the presence of that ion the reaction was first order in cyclohexanone, one-half order in copper ion, and zero order in oxygen. The rate of enolization of cyclohexanone appeared to be unaffected by the presence of copper ion under conditions of constant acidity but was strongly catalyzed by both deuterium ion and acetate ion. Further, the rate of oxidation exceeded the rate of enolization at higher copper ion concentrations. These data contrast with the uncatalyzed oxidation, for which the kinetic reaction was first order in cyclohexanone, one-half order in oxygen, and the rate of oxidation was much less than the rate of enolization. The activation energies for the catalyzed and uncatalyzed oxidations were about the same, but the entropies of activation were quite different. From these data it is concluded that the enol is not a major intermediate for the catalyzed reaction. It is suggested that the copper ion may form an active peroxy complex with the oxygen molecule to serve as the reactive oxidant. Keywords: metal ion catalysis, kinetics, cyclohexanone, high temperature oxidation, high pressure oxidation.
An acidity scale, [H+]hv, for proton quenching of excited states
Authors: Pincock J. A. , Redden P. R.
Published: May 2006

An acidity scale for excited state protonation kinetics in 20% ethanol:80% aqueous sulfuric has been developed using 1-cyano-naphthalene as a fluorescent indicator. The utility of this new scale is demonstrated using the proton quenching of fluorescence of a variety of chromophores. These include 1- and 2-cyanonaphthalenes, 1- and 2-methoxynaphthalenes, benzyl alcohols, toluenes, and 2-vinylnaphthalene. Keywords: acidity scale, fluorescence quenching, excited state basicity.
Some physicochemical studies on organic eutectics and 1:1 addition compound; p-phenylenediamine – benzoic acid system
Authors: Rai U. S. , Mandal K. D.
Published: May 2006

The phase diagram of p-phenylenediamine – benzoic acid system, determined by the thaw–melt method, shows the formation of two eutectics and a 1:1 addition compound. The linear velocity of crystallization of pure components, eutectics and addition compound, determined by measuring the movement of growth front in a capillary, suggests that crystallization data obey the Hillig–Turnbull equation. Using experimental values of heats of fusion, entropy of fusion and excess thermodynamic functions were calculated and the results are explained on the basis of cluster formation in the melts. X-ray diffraction data infer that these eutectics are not simply the mechanical mixture of the two components and there is preferential ordering of atomic planes during their formation. The infrared spectral studies suggest the formation of intermolecular hydrogen bonding between the components forming the molecular complex. Keywords: organic eutectics, growth kinetics, phase diagram, thermochemistry, X-ray diffraction studies.
Deprotonation and transfer energetics of glycine in aqueous mixtures of urea and glycerol from emf measurements at different temperatures
Authors: Talukdar Himansu , Rudra Sibaprasad , Kundu Kiron K.
Published: May 2006

Deprotonation constants, Ka(RH2+) and Ka(RH±), of glycine (RH±) have been determined at five equidistant temperatures ranging from 15 to 35 °C by measuring the emf of galvanic cells comprising Pt/H2 and Ag–AgCl electrodes in aqueous mixtures of protophilic protic urea (UH) and protophobic protic glycerol (GL). Medium effects on deprotonation of the acid: [Formula: see text] have been dissected into transfer free energies [Formula: see text] and entropies [Formula: see text] of the species involved as obtained by measuring the transfer energetics [Formula: see text] of RH± from solubility measurements at different temperatures and of H+ based on tetraphenylarsonium tetraphenylborate (TATB) reference electrolyte assumption determined earlier. The [Formula: see text] values obtained after due correction from the cavity effect based on scaled particle theory (SPT) and electrostatic effects including Born and ion–dipole effects for the charged species involved in the two deprotonation equilibria enable better understanding of the solvent effect on the deprotonation constants. Moreover, the [Formula: see text]–composition profiles are found to exhibit similar characteristic maxima and minima as for simple cations and anions in these solvent systems, thus providing useful information on the structural characteristic of these cosolvents. Keywords: deprotonation energetics, glycine, aqueous urea, aqueous glycerol, EMF measurements.
Bromophloroglucinols and their methyl ethers
Authors: Kiehlmann E. , Lauener R. W.
Published: May 2006

All 16 bromination products of phloroglucinol and its methyl ethers, as well as five bromoresorcinols and three of their dimethyl ethers, were synthesized and analyzed by nuclear magnetic resonance spectroscopy. Two or three equivalents of bromine convert phloroglucinol to di- and tribromophloroglucinol, 5-methoxyresorcinol to the tri- and 2,4-dibromo, 3,5-dimethoxyphenol to the tri- and 2,6-dibromo, and 1,3,5-trimethoxybenzene to the dibromo compound. With one equivalent of bromine, 3,5-dimethoxyphenol reacts preferentially at C-2 while 5-methoxyresorcinol gives both monobromo isomers. Partial debromination with sodium sulfite yields successively 2,4-dibromo- and 2-bromo-5-methoxyresorcinol from the tribromo compound but fails with brominated 3,5-dimethoxyphenol. In the resorcinol series, C-2 is invariably the least reactive position. 4,6-Dibromo-5-methoxyresorcinol and 2,4-dibromo-3,5-dimethoxyphenol are accessible by methylation of dibromophloroglucinol, obtained from 3,5-dibromo-2,4,6-trihydroxybenzoic acid by decarboxylation. In contrast to resorcinol and tribromoresorcinol, the partial bromination of phloroglucinol and debromination of tribromophloroglucinol are not selective. The 13Cmr spectra of bromophloroglucinol methyl ethers show characteristic downfield shifts for methoxy groups orthogonal to the aromatic ring plane. Keywords: bromophloroglucinols, synthesis, nmr, and debromination; bromo-5-methoxyresorcinols, synthesis, nmr, and debromination; bromo-3,5-dimethoxyphenols, synthesis and nmr.
Hydration studies on some polyhydroxy non-electrolytes and non-ionic surfactants
Authors: Moulik S. P. , Gupta Syamasri , Das A. R.
Published: May 2006

The hydration numbers of some polyhydroxy non-electrolytes and polyoxyethylenated non-ionic surfactants have been determined and found to depend considerably on the method used, the general order being conductometry < ultrasonic interferometry < ir, spectrophotometry. PEG and Tx-100 are exceptions to this trend. The results from ultrasonic interferometry study strongly depend on the data treatment. The maximum hydration obtained using the ir method shows fair agreement with recent nmr and low angle light scattering results on similar compounds. The results indicate the existence of both strongly and loosely bound water molecules with the micelles as well as the penetration of water in the non-polar micellar region. Keywords: hydration, non-electrolytes, non-ionic surfactants, conductometry, ultrasonic, near infrared.