bluuebunny/crossref_metadata_2025 huggingface dataset:
Contrasting behaviour in the photosolvolysis of 1- and 2-hydroxy-9-fluorenols in aqueous solution
Authors: Fischer Maike , Shi Yijian , Zhao Bao-ping , Snieckus Victor , Wan Peter
Published: April 2011

The photosolvolysis of 1- and 2-hydroxy-9-fluorenols 4-6 has been studied in aqueous solution. All of these 9-fluorenols photosolvolyze efficiently in 1:1 H2O-CH3OH, to give the corresponding methyl ether products in high chemical and quantum yields. Whereas the photosolvolysis of the parent 9-fluorenol (2, R = H) is known to proceed via the very short-lived and formally ground-state antiaromatic 9-fluorenyl cation (1, R = H), the photosolvolysis of 1-hydroxy-9-fluorenol (4) proceeds via a much longer-lived (approximately = 5-10 s) fluorenyl quinone methide 9, which is trappable by ethyl vinyl ether via a [4+2] cycloaddition reaction to give a chroman derivative. Interestingly, 2-hydroxy-9-fluorenol (5) photosolvolyzes via a very short-lived intermediate with similar lifetimes as observed for the 9-fluorenyl cation (1, R = H), although a corresponding fluorenyl quinone methide intermediate is accessible for this compound. This study demonstrates that the mechanism of photosolvolysis of these types of compounds can be dramatically altered when an aryl hydroxy group is present.Key words: photosolvolysis, 9-fluorenyl cation, quinone methide, carbocation, antiaromatic.
Reactivity in acid-catalyzed carbon–carbon heterolysis
Authors: Cao Weiguo , Erden Ihsan , Grow Richard H , Keeffe James R , Song Jiangao , Trudell Mary B , Wadsworth Teri L , Xu Fu-Pei , Zheng Ji-Bin
Published: April 2011

Equilibrium and rate constants have been determined for the acid-catalyzed heterolysis of two alcohols, 9-xanthydrol and p-anisyldiphenylmethanol, and two sulfides, (9-xanthyl) methyl sulfide and (7-tropyl) methyl sulfide. These data together with literature information are compared with rate constants for acid-catalyzed C-C heterolysis of several (9-xanthyl) compounds, (7-tropyl) compounds, a set of 3-arylcyclobutanones, and two 2-arylnitrocyclopropanes, all of which fragment to carbocations plus a carbon-centered nucleofuge. The fragmentation mechanisms are shown to be A1 or A1(ion pair) except for the 2-arylnitrocyclopropanes which cleave in trifluoroacetic acid by a concerted mechanism. Rate comparisons among several unstrained substrate sets indicate that O-centered nucleofuges undergo acid-catalyzed heterolysis ca. 103-104 faster than S-centered nucleofuges and ca. 109-1014 faster than the C-centered nucleofuges used here. Factors assisting C-C heterolysis (and their effectiveness) include the acidity of the medium (strong); the basicity and nucleofugality of the nucleofuge (moderate); the stability of the electrofugic carbocation (strong); and relief of ring strain (enormous). Compared with acyclic cleavages, rate accelerations worth ca. 15 kcal/mol (for cyclobutanones) and ca. 27 kcal/mol (for nitrocyclopropanes) are found. These effects are discussed in terms of transition-state structure, aided by computational evidence.Key words: C-C heterolysis, fragmentation, acid catalysis, carbocation, ring strain.
The methylgallium-2-carboxylatobenzimidazolato hexamer, [MeGa·C8H4N2O2]6·C6H6·(Me2C6H4)2; synthesis and X-ray crystal structure
Authors: Rettig Steven J , Storr Alan , Trotter James
Published: April 2011

The room-temperature reaction of Me3Ga with benzimidazole 2-carboxylic acid in xylene solvent has yielded a novel crystalline hexameric gallium compound with "MeGa" moieties bridged by the doubly depronotated ligand precursor. Crystals of [MeGa(4,5-benzimidazolato-2-carboxylato)]6·(C6H6)·(m-Me2C6H4)2 are monoclinic, a = 18.091(2), b = 17.094(2), c = 13.2215(5) Å, Z = 2, space group C2/m. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R (F, I [Formula: see text] 3σ(I)) = 0.064 (Rw (F2, all data) = 0.134). The hexameric Ga complex contains a six-membered ring of Ga atoms, bridged by the benzimidazolate ligands with the benzo rings projecting alternately above and below the Ga plane, thus forming a ball-shaped molecule. The complex could have ideal D3d symmetry, but it contains an encapsulated molecule of benzene, which distorts the regularity of the Ga6 hexagon, and reduces the symmetry of the complex to the crystallographically observed C2h. The coordination geometry at each of the two independent GaO2N2C centres approximates a trigonal bipyramid, with a N2C trigonal plane, and the O atoms above and below; average dimensions are Ga-O = 2.176(2), Ga-N = 1.973(3), Ga-C = 1.927(5) Å, O-Ga-O = 165°. The unit cell also contains four m-xylene solvent molecules (outside the molecular cage).Key words: gallium, crystal structure, benzene intercalate, benzimidazolecarboxylic acid.
Vicinal-diaryl interactions in stilbenoid hydrocarbons as observed in the through-space charge delocalization of their cation radicals
Authors: Rathore Rajendra , Kochi Jay K
Published: April 2011

The conformational preference of vicinal or 1,2-phenyl groups is probed in two classes of ring-substituted 1,2-diphenylbicyclooctene (stilbenoid) hydrocarbons 1a-1d and 2a-2c. UV-vis spectroscopy reveals, and X-ray crystallography verifies, the intramolecular (edge-to-face) orientation for the phenyl-phenyl interaction in stilbenoids 1a-1d. Most importantly, when two pairs of ortho-methyl substituents are present, the cofacial phenyl groups in the stilbenoid donors are established by X-ray crystallography and spectrally observed in the cation radicals (2a+.-2c+.) by the appearance of new bands with strong absorptions in the near IR with λmax = 1100-1315 nm, analogous to those previously observed in intermolecular (aromatic) interactions of aromatic cation radicals.Key words: stilbenoid hydrocarbon, cation radical, aryl-aryl interaction.
Effect of cyclodextrin on elimination reactions
Authors: Viola Luis , de Rossi Rita H
Published: April 2011

The reaction of 1-bromo-2-X-2-(Y-phenyl) ethane derivatives (1: X = Y = H; 2: X = Ph, Y = H; 3: X = H, Y = 4-Ac; 4: X = H, Y = 3-NO2; 5: X = H, Y = 4-NO2; 6: X = H, Y = 3-Me; 7: X = H, Y = 4-Me) in basic solution was studied, and in most cases, only the elimination product is formed. Only (2-bromo-1-phenylethyl)benzene, 2, yielded significant substitution product, and this yield decreased with the concentration of HO-. Addition of cyclodextrin (β-CD) diminished (about half for 0.02 M cyclodextrin concentration) the reaction rate of all substrates but 4 and 5. In the latter two cases, the rate rises. The observed rate-constant value at 0.5 M NaOH is 6.78 × 10-4 s-1 (at 40°C) and 1.80 × 10-3 s-1 (at 25°C) for 4 and 5, respectively. Under the same reaction conditions but with 0.01 M β-CD, the corresponding rates were 7.70 × 10-4 s-1 and 5.20 × 10-3 s-1. The elimination yield for 2 increased from 64 to 98% when the β-CD changed from zero to 0.02 M at 0.5 M NaHO. Also, there was an increase in the relative elimination products of 20-40% for compounds 6 and 7. The Hammet ρ values were 1.3 and 2.3 for the reaction in pure solvent and in the presence of β-cyclodextrin, indicating an increase in the negative character of the transition state for the reactions in the latter conditions. The results are interpreted in terms of the formation of an inclusion complex whose structure depends on the substrate.Key words: cyclodextrin, elimination reactions, inhibition, catalysis.
Normal acid/base behaviour in proton transfer reactions to alkoxy substituted carbenes: estimates for intrinsic barriers to reaction and pKa values
Authors: Pezacki John Paul
Published: April 2011

Theoretical Eigen curves can be drawn through Brønsted data for dimethoxycarbene (1), phenyltrimethyl-siloxycarbene (3a), 4-methylphenyl(trimethylsiloxy)carbene (3b), 4-methoxyphenyl(trimethylsiloxy)carbene (3c), and β-naphthyl(trimethylsiloxy)carbene (3d). The Brønsted plots for these data are clearly curved with α values near 1 when proton transfer is thermodynamically unfavourable and α values near 0 when proton transfer is thermodynamically favourable, suggesting that these carbenes behave as "normal" Brønsted bases. Estimates of the intrinsic barriers (ΔG0‡) for proton transfer reactions and of the pKa values for the conjugate acids of the carbenes, extracted from these theoretical curves, have been made. The magnitudes of the intrinsic barriers (ΔG0‡) for these proton transfer reactions determined by Eigen and Marcus theories are all 1-5 kcal mol-1, suggesting that these reactions are intrinsically fast. Small intrinsic barriers also imply "normal" acid/base behaviour. Extrapolated pKa values are also the first estimates for the pKa values of the conjugate acids of carbenes 1 and 3a-3d. Key words: carbenes, proton transfer reactions, carbocations, Marcus theory, Brønsted plots.
Theoretical simulation of Brønsted correlations for proton transfer and methyl transfer: the significance of the Brønsted coefficient β
Authors: Williams Ian H , Austin Paul A
Published: April 2011

Brønsted correlations for proton transfer and methyl transfer between pairs of mimicked 4-substituted pyridines have been simulated by means of AM1 molecular orbital calculations. The enthalpies of activation and of reaction are well correlated by the Marcus relation. The Brønsted correlation is markedly curved for proton transfer for which the Marcus intrinsic barrier is very low (~ 5 kJ mol-1). Conversely, the Brønsted correlation is almost imperceptibly curved for methyl transfer for which the Marcus intrinsic barrier is very high (~ 152 kJ mol-1). The slope of the Brønsted correlation provides an approximate measure of the position of the transition structure along the reaction coordinate between the reactant and product encounter complexes. Primary kinetic isotope effects for proton transfer show a broad maximum, centered on ΔHrxn = 0, when plotted against reaction exo/endothermicity only if computed with respect to isolated acids and bases, but intracomplex kinetic isotope effects show a maximum displaced significantly towards the endothermic proton transfers.Key words: Brønsted correlation, rate-equilibrium relationship, transition state, isotope effect.
The hydration mechanism of ketene: 15 years later
Authors: Nguyen Minh Tho , Raspoet Greet
Published: April 2011

New insights into the detailed mechanism of the hydration of ketene yielding acetic acid (H2C=C=O + H2O →> CH3COOH) were obtained by theoretical methods in both gas phase and solution. While gas phase calculations were performed using ab initio molecular orbital theory, bulk solvent effects were included using the self-consistent reaction field method (SCRF) and the polarizable continuum model (PCM). The hydration modeled by attack of water clusters containing two, three, and four water molecules confirms that a two-step addition of water to the ketene C=O bond, yielding a 1,1-enediol intermediate as initially demonstrated in 1984, is energetically, slightly but consistently, preferred over a concerted addition across the C=C bond leading directly to the acid product. Attempts to locate a zwitterion intermediate in solution were not successful. At least a cluster of three hydrogen-bonded water molecules is present in the gas phase supersystem to facilitate the proton transfer. Further incorporation of active water molecules in the catalytic water chain induces rather minor energetic improvements on the proton relay, which indicates a certain saturation of the cluster when reaching 3-4 water molecules. Effects of the surrounding solvent bulk do not change qualitatively the facts found in gas phase. The C=O addition mechanism is in better agreement with recent experimental developments in identifying enols of carboxylic acids than other mechanisms involving either a zwitterion or a direct C=C addition, as proposed for years in the literature.Key words: ketene, ketene hydration, hydration mechanism, solvent effect, ab initio calculations.
Kinetic study of the reactions of various types of C-acids with amine bases in acetonitrile. An unusual effect of common BH+ cation on the rate constants
Authors: Galezowski Wlodzimierz , Grzeskowiak Iwona , Jarczewski Arnold
Published: April 2011

The rates of proton transfer reactions between C-acids of different types such as 1-(4-nitrophenyl)-1-nitroalkanes, 4-nitrophenylcyanomethanes, and 2,4,6-trinitrotoluene, and organic bases such as 1,1,3,3-tetrametylguanidine, 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), and tri-n-butylamine have been measured in acetonitrile at pseudo-first-order conditions. A general equation for the rates of proton transfer reactions between C-acids and bases with product existing in two forms, ions and ion pairs, has been derived and its applicability tested. The equation works well except for reactions of 1-(4-nitrophenyl)-1-nitroalkanes with guanidines for which the second-order rate constant is diminished with concentration of guanidinium cation, while tetrabutylammonium salts accelerate the reactions. Possible reasons for this are discussed.Key words: proton transfer, kinetic study, ion pairs, C-acids, organic bases, acetonitrile, salt effect.
Picosecond radical kinetics. Rate constants for ring openings of 2-aryl-substituted cyclopropylcarbinyl radicals
Authors: Newcomb Martin , Choi Seung-Yong , Toy Patrick H
Published: April 2011

The kinetics of ring openings of a series of eight (trans-2-arylcyclopropyl)methyl radicals (1) were studied by indirect kinetic methods using Barton's PTOC esters as radical precursors and reaction with PhSeH as the competition reaction. The substituents were CF3, F, Me, and OMe located on both the para and meta positions of the aromatic ring. Syntheses of the radical precursors and the products of the radical reactions are described. Kinetics were determined between -43 and 25°C in four cases (CF3 and OMe substituents) and at 0 and 25°C in the other four cases. The rate constants at 25°C ranged from 1.0 × 1011 s-1 (p-CH3) to 4.1 × 1011 s-1 (p-CF3). The relatively large acceleration of the p-CF3 group, ca. 2.5 times as fast as the parent system with Ar = Ph, correlates well with Adam's ΔD substituent parameters but not with other radical substituent parameters. These calibrated radical rearrangements provide a new set of ultrafast reactions that can be applied in mechanistic probe studies.Key words: cyclopropylcarbinyl radical, kinetics, PTOC esters, benzeneselenol.
The hydration of ketene and carbon dioxide: an examination of the kinetics in terms of No Barrier Theory
Authors: Guthrie J Peter
Published: April 2011

Rate constants for hydration of carbon dioxide and ketene can be calculated by applying No Barrier Theory, which needs only equilibrium constants and distortion energies, the latter calculated using molecular orbital theory. The calculated free energies of activation are in satisfactory agreement with experiment: the rms error in free energy of activation is 2.38 kcal/mol. These compounds can also be described using Marcus Theory or Multidimensional Marcus Theory using the transferable intrinsic barrier appropriate to simple carbonyl compounds; in this case the rms error in free energy of activation is 2.19 kcal/mol. The two methods agree on preferred mechanistic path except for uncatalyzed hydration of ketene where Multidimensional Marcus Theory leads to a lower activation free energy for addition to the C=O, while No Barrier Theory leads to a lower free energy of activation for addition to the C=CH2. A rate constant for hydroxide ion catalyzed hydration of ketene can be calculated and is in accord with preliminary experimental results.Key words: ketene, carbon dioxide, hydration, Marcus Theory, No Barrier Theory.
A 2-D NMR investigation of the micellar solubilization site in ionic micellar solutions
Authors: Hawrylak Brent E , Marangoni D Gerrard
Published: April 2011

Two-dimensional NOE spectroscopy (NOESY) experiments have been used to examine the dissolution of two typical solubilizates (1-butanol and benzene) in anionic sodium dodecyl sulfate (SDS) micelles and of benzene in cationic dodecyltrimethyammonium (DTAB) micelles. In all cases, significant, positive NOESY cross peaks were obtained that appeared to correlate well with the expected locus of solubilization of these solubilizates in these surfactant systems. The NOESY technique, which is a noninvasive experiment, is found to provide significant insight in terms of the solubilization of these molecules in the interior of ionic micelles.Key words: solubilization, surfactants, NMR spectroscopy, 2-D NOESY experiments.
Surface tension studies of lauryl sulfobetaine - β-cyclodextrin and dodecyltrimethylammonium bromide - β-cyclodextrin inclusion complexes in aqueous solution
Authors: Göktürk S , Mahramanlloglu M , Tunçay M
Published: April 2011

β-Cyclodextrin (β-CD) is a cyclic oligosaccaride with an apolar cavity that shows a certain degree of selectivity in binding organic and inorganic compounds. Lauryl sulfobetaine (LSB) and dodecyltrimethylammonium bromide (DTAB), having the same hydrophobic and different polar groups are used in order to compare the influence of the polar group on complex formation with β-CD. The surface tension increased as β-CD is added to premicellar region of amphoteric (LSB) and cationic surfactant (DTAB) solutions. The relation between the surface tension and surfactant concentration can be expressed by the Szyszkowski equation, which combines the Langmuir adsorption model and the Gibbs equation. Binding constants were determined from the effect of added β-CD on the surface tension vs. concentration curves for LSB and DTAB aqueous solutions.Key words: β-cyclodextrin, surfactant, surface tension method, complex formation, binding constant.
Thermodynamics of substituted pyrazolone. V: Potentiometric and conductometric studies of complexes of some transition metals with 4-(4-acetophenyl)hydrazono-3-methyl-2-pyrazolin-5-one
Authors: El-Bindary Ashraf A , El-Sonbati Adel Z , Kera Hanan M
Published: April 2011

Proton-ligand dissociation constants of 4-(4-acetophenyl)hydrazono-3-methyl-2-pyrazolin-5-one (AHMP) and metal-ligand stability constants of its complexes with some transition metal ions were calculated potentiometrically in 0.1 M KCl and 50% (v/v) ethanol-water mixture. The order of stability constants was found to be Th4+ > UO22+ > Ce3+ > La3+ > Mn2+ < Co2+ < Ni2+ < Cu2+ > Zn2+. The dissociation constant, pK1H, of AHMP and the stability constants, log K, of their complexes were determined at different temperatures (298, 308, and 318 K). The corresponding thermodynamic parameters (ΔG, ΔH, and ΔS) were derived and discussed. The dissociation process is nonspontaneous, endothermic, and entropically unfavourable. The formation of the metal complexes has been found to be spontaneous, exothermic, or endothermic (depending the metal), and entropically favourable. The stoichiometries of these complexes were determined conductometrically and indicated the formation of 1:1 and 1:2 (metal:ligand) complexes. Key words: 4-(4-acetophenyl)hydrazono-3-methyl-2-pyrazolin-5-one, dissociation and stability constants, thermodynamic parameters, stoichiometries.
Chiral recognition in solution. Interactions of α-amino acids in concentrated aqueous solutions of urea or ethanol
Authors: Castronuovo Giuseppina , Elia Vittorio , Pierro Anna , Velleca Filomena
Published: April 2011

Enthalpies of dilution of the L and D forms of glutamine, citrulline, and phenylalanine in concentrated aqueous solutions of urea or ethanol were measured calorimetrically at 298 K. Glycine, urea, formamide, and phenol were also studied under the same experimental conditions, to get information about the behaviour of the zwitterion and of the functional group in the side chain of the cited amino acids when the concentration of the cosolvent changes. The derived pairwise enthalpic interaction coefficients for the three amino acids were rationalized according to the preferential configuration model. Indications are that, in concentrated urea, the coefficients for citrulline and glutamine are determined mainly by the interactions between the cosolvent and the hydrophilic groups in the molecule of the amino acids. For phenylalanine, coefficients are less positive than in water, because the presence of urea, which solvates preferentially the zwitterions, attenuates hydrophobic interactions between the benzene rings. In ethanol, coefficients for the three amino acid become negative or more negative than in water, because in this medium hydrophilic interactions are enhanced. Chiral recognition, namely the difference in the values of homo- and heterochiral interaction coefficients, was detected only for phenylalanine in urea. Hence, the nature of the cosolvent, influencing differently hydrophilic and hydrophobic interactions, can lead to the detection of chiral recognition also for those systems that, as phenylalanine, do not present this effect in pure water.Key words: α-amino acids, excess functions, molecular interactions, preferential configuration.
1998 E.W.R. Steacie Award Lecture Asymmetric amphiphilic block copolymers in solution: a morphological wonderland
Authors: Cameron Neil S , Corbierre Muriel K , Eisenberg Adi
Published: April 2011

Asymmetric amphiphilic diblock copolymers self-assemble in selective solvents. Since 1995, when we first reported the systematic preparation of a sequence of various "crew-cut" aggregate morphologies from this class of copolymer in solution (1), we have identified a vast array of structures and have begun a detailed investigation of the thermodynamic and kinetic parameters that induce morphogenesis. Not only spheres, rods, bilayer and bicontinuous architectures, as well as inverted structures are observed, but also a selection of mixed, combined and much more complex aggregates is documented. All of these aggregates have a phase-separated insoluble core and a crew-cut soluble corona. Thus, all parameters that permit selective modification of the component of either phase or of the interface provide a window for morphological control. By carefully adjusting the polymer chain environment, it has been possible to develop a systematic understanding of morphogenic parameters, which include, among others, polymer composition, common solvent, initial concentration, temperature, type and concentration of added ions, method of preparation, and added homopolymer. To date, more than 30 publications have appeared in the literature from our group alone on this subject. One of the problems inherent with such a complicated system is the taxonomy or classification: which morphologies correspond to equilibrium positions and which are intermediate or trapped? An attempt at a logical presentation of the observed aggregates is given, preceded by a qualitative discussion of the thermodynamic framework for this system. Where possible, the transitions between morphologies are explained in the context of the thermodynamic parameters. Finally, parallels are drawn between the copolymer aggregates and biological architectures.Key words: crew-cut, morphology, block copolymer, self-assembly, amphiphile.
NMR and molecular structure of partially oriented mono and para methyl- and chlorobenzenes dissolved in nematic liquid crystals
Authors: Syvitski Raymond T , Burnell E Elliott
Published: April 2011

Spectral parameters, order parameters, and structural parameters, including the vibrationally corrected ralpha structure of the partially oriented solutes p-xylene, p-chlorotoluene, p-dichlorobenzene, toluene, and chlorobenzene dissolved in three liquid crystal mixtures, are reported. For samples containing the three solutes p-xylene, p-dichlorobenzene, and 1,3,5-trichlorobenzene, multiple quantum (MQ) nuclear magnetic resonance was used to aid in the analysis of the complex high-resolution spectra of the p-xylene. The high-resolution spectra of 1,3,5-trichlorobenzene and p-dichlorobenzene were easily identified and analyzed once the calculated p-xylene spectrum was subtracted from the experimental one. The methyl groups of p-xylene, p-chlorotoluene, and toluene have similar geometries when the structure is determined from dipolar couplings corrected for harmonic vibrations.Key words: nematic liquid crystal, NMR, molecular structure, order parameter.
Stereochemical rearrangements in tricarbonylrhenium(I) halide complexes of the nonracemic chiral ligand 2-[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L): a dynamic NMR study
Authors: Heard Peter J , Bain Alex D , Hazendonk Paul
Published: April 2011

Tricarbonylrhenium(I) halide complexes of the nonracemic chiral ligand 2-[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L), namely fac-[ReX(CO)3L] (X = Cl, Br, or I), have been prepared and their latent fluxionality studied by dynamic NMR techniques in the slow and intermediate exchange regimes. In solution, these complexes give rise to four diastereoisomers, depending on the configuration at the metal and at the acetal-carbon atom, respectively; the relative populations are in the order SR > RR >>; RS > SS. At moderate temperatures, a reversible "acetal ring flip" leads to formal inversion of configuration at the acetal-carbon atom; the free energies of activation are in the range 84-88 kJ mol-1 at 298 K. Above ca. 370 K, reversible ligand dissociation also occurs, leading to an exchange of all four diastereoisomers on the NMR chemical shift time-scale.Key words: dynamic NMR, fluxionality, tricarbonylrhenium(I) complexes, chiral acetal ligands.
The second dielectric virial coefficient of the dipolar Gay–Berne fluid
Authors: Buckingham A D , Tantirungrotechai Y
Published: April 2011

We report the second dielectric virial coefficient Bε of a nonpolarizable dipolar Gay-Berne fluid. The second dielectric virial coefficient is evaluated for several sets of shape parameters and temperature. The second dielectric virial coefficient for rodlike molecules is negative while it is large and positive for disklike molecules. It decreases to zero on increasing the temperature.Key words: dipolar fluids, dielectric constant, intermolecular forces, shape parameters.
A solid state NMR and molecular orbital study of hydroxylammonium chloride
Authors: Penner Glenn H , Chang YC Phillis , Grandin H Michelle
Published: April 2011

Deuterium and nitrogen-15 NMR spectroscopy has been used to measure the 2H quadrupolar coupling and 15N chemical shift tensors in solid hydroxylammonium chloride, NH3OH+Cl-, (HAC). In addition, the NH3 and OH dynamics have been investigated by variable temperature 2H line shapes and T1 measurements. The Arrhenius activation energy for NH3 rotation is 22.5 ± 1.8 kJ/mol with a pre-exponential factor of 8 ± 3 × 1012 s-1 from line shapes and 21.3 ± 2 kJ/mol with an infinite temperature correlation time, τinf,, of 5.0 ± 0.4 × 10-14 s from the T1 analysis. The latter value corresponds to a pre-exponential factor of 6.7 ± 0.5 × 1012 s-1, if a three-site exchange is assumed. There was no evidence for OH reorientation up to 405 K, indicating a rather strong OH···Cl hydrogen bond. Previously reported inconsistencies between crystal structure and molecular orbital derived N-O bond lengths are cleared up by performing geometry optimizations with large basis sets and taking electron correlation into account. The internal rotational potential for the isolated HA cation is calculated to be 5.8 kJ/mol at the MP2/6-31G** level, with the trans geometry preferred. Calculations that employ the neutron diffraction geometry and include the Cl- anions that surround the HA+ cation yield an upper limit for the activation energy for NH3 group rotation of 62 kJ/mol. Analysis of the deuterium spectrum and T1 data yield nuclear quadrupolar coupling constants of 160 ± 5 kHz and 194 ± 5 kHz (η = 0.50 ± 0.05) for the ND3 and OD deuterons, respectively. Density functional calculations of the deuterium and nitrogen-14 nuclear quadrupolar coupling constants at the B3LYP level show that it is necessary to include the influence of the surrounding chloride anions. We have also shown that it is possible to obtain accurate proton chemical shifts from the deuterium MAS spectrum of solid HAC-d4.Key words: solid state NMR, molecular dynamics, nitrogen 15 chemical shift anisotropy.
3-Deoxy-D-erythro-hexos-2-ulose bis(thiosemicarbazone) copper(II) chelate. Studies in solution and in the solid state
Authors: Signorella Sandra , Palopoli Claudia , Frutos Alejandro , Escandar Graciela , Tanase Tomoaki , Sala Luis F
Published: April 2011

The 3-deoxy-D-erythro-hexos-2-ulose bis(thiosemicarbazone) (LH2) acts as a tetradentate ligand with S2N2 donor sites to form a copper(II) chelate (Cu(II)L). The Cu K-edge X-ray absorption spectra analysis of a powdered sample revealed a square-planar S2N2 local coordination geometry with average CuN and CuS distances of 1.92 and 2.33 Å. The EXAFS and XANES spectra and the structural parameters of a DMF solution sample indicate the retention of the square-planar local structure of the Cu(II)S2N2 chromophore upon dissolution. Semiempirical PM3 calculations indicate that the structure of Cu(II)L with the copper(II) ion bound to the S2N2 donor set disposed in a square-planar geometry is the most favorable one (with the highest |ΔHf |). The formation constant (log Kf = 20.65 ± 0.06) of the neutral [CuL] species was calculated in 30% v/v dioxane-water and the protonation and deprotonation constants determined.Key words: bis(thiosemicarbazone), copper(II), chelate, structure, stability constant.
Annotinine revisited. A new pentacyclo[7.3.3.01,13.02,12.05,13]pentadecane ester and other products derived from annotinine
Authors: Ayer William A , Wichlacz Monika , Trifonov Latchezar S
Published: April 2011

Several annotinine derivatives were prepared and their conformations were determined on the basis of 1D NMR and 2D NMR spectra (GCOSY, HMQC, HMBC, TROESY). One of these compounds has new unique 5-ring skeleton.Key words: alkaloids, chemical tranformations, structure elucidation, conformation.
Probing the motion of the η2-dideuterium ligand by solution and solid-state 2H NMR spectroscopy
Authors: Facey Glenn A , Fong Tina P , Gusev Dmitry , Macdonald Peter M , Morris Robert H , Schlaf Marcel , Xu Wei
Published: April 2011

Variable temperature 2H NMR is used to measure the T1min values of the η2-D2 and D ligands in trans-[M(η2-D2)(D)(dppe)2]+, M = Ru (1) and Os (2) in solution. The rapid spinning motion of the η2-D2 ligand results in a much longer T1min than that of the terminal deuteride. The quadrupole coupling constant (CQ) for the terminal deuteride is calculated to be 79 kHz for 1 and 81 kHz for 2 while motion-reduced coupling constant (CQeff) for the D in the η2-D2 ligands in 1 and 2 are in the range 19-22 and 27-31 kHz, respectively. The actual CQ for these ligands with short D-D distances (<1 Å) should be at least 2CQeff and probably greater than that of the terminal deuteride but less than that of D2(g), 227 kHz. A fast spinning (>>61 MHz) and tilting of the primary electric field gradient component of between 90° and 60° or between 50° and 40° with respect to the axis of D2 rotation is an explanation for the small CQeff. Therefore neither D-D nor M-D bonding dominates the electric field gradient direction in these M(η2-D2) bonds. The complex [RuD2(C5Me5)(dppm)]+ 3, which exists in solution as a 3.3:1 mixture of Ru(D···D) (3a) and Ru(D)2 (3b) tautomers, has T1min(D) values that provide CQeff values of 66 kHz for 3a and 71 kHz for 3b. The elongated D···D ligand in 3a with d(D···D) ~ 1.10 Å may be "static" compared to the 61.4 MHz 2H spectrometer frequency and therefore have "compressed dihydride" character and display similar Ru-D bonds as in 3b. However, it is more likely that the static CQ for the D of 3a is much larger than that of a terminal Ru-D but averaged to 66 kHz by a 180° flip of the D2 as observed in 7. The complexes trans-[M(D···D)(Cl)(dppe)2]PF6, M = Ru (4), Os (5), and Ru(η2-D2)(dppb)(µ-Cl)3RuCl(dppb) (6) also have motion-reduced CQ values. Some 2H NMR quadrupole echo wide-line spectra of 1, 4, and Os(D···D)(Cl)2(CO)(PiPr3)2 (7) (contaminated with 20% Os(H···D)) were recorded in the solid phase from 293 to 123 K. These also indicate that the CQeff of the D in the D2 ligands are motion-reduced. Simulation of the spectra of 7 are suggestive of a twofold reorientation of a D2 ligand with a static CQ value in the range of 120-167 kHz, a tilt angle of the electric field gradient of about 50°, and an asymmetry parameter near to zero. Significantly, the MAS 2H spectrum of 7 has a broad doublet, possibly due to a non-averaged D-D dipolar/quadrupolar interference phenomenon; this is first time this has been observed. Variable temperature T1 data for solid 7 are also reported, which allow the evaluation of the activation barrier to the twofold flipping motion of the D2; the only other way of quantifying such an energy barrier is by use of an inelastic neutron scattering method.Key words: dihydrogen, deuterium, NMR, ruthenium, osmium, complexes, dynamics, hydride, bonding.
The photochemistry of 4-halobenzonitriles and 4-haloanisoles with 1,1-diphenylethene in methanol. Homolytic cleavage versus electrontransfer pathways
Authors: Mangion Dino , Arnold Donald R
Published: April 2011

The photochemical reactivity of a series of 4-halobenzonitriles and 4-haloanisoles with 1,1-diphenylethene in a nucleophilic solvent (methanol) has been investigated. Analysis of the photochemical reactions involving the 4-halobenzonitriles revealed formation of alkene-methanol adducts, such as 1-methoxy-2,2-diphenylethane, 1-methoxy-2,2-diphenylethene, and 1,1-dimethoxy-2,2-diphenylethane, indicative of a photochemical electron-transfer mechanism. These products were not significant in the photochemical reactions involving the 4-haloanisoles. Both the 4-halobenzonitriles and the 4-haloanisoles produced an arene-alkene-methanol Markovnikov adduct, 1-aryl-2-methoxy-2,2-diphenylethane (aryl = 4-cyanophenyl or 4-methoxyphenyl). This compound was shown to undergo an acid-catalysed elimination to 1-aryl-2,2-diphenylethene under the reaction conditions, which subsequently underwent a 6pi-electrocyclization to the 3-substituted(cyano or methoxy)-9-phenylphenanthrene. Possible mechanisms for the observed reactivity are discussed and evaluated.Key words: photochemistry, photoinduced electron transfer, bond homolysis, radical ions, radicals, exciplexes.
Thermodynamic calculations for reactions involving hydrogen halide polymers, ions, and Lewis acid adducts. 4. The temperature dependence of some reactions of HF and HCl
Authors: Rankin Kathryn N , Chandler W David , Johnson Keith E
Published: April 2011

Ab initio MP2/6-311++G(d,p) structure optimizations and frequency calculations have been carried out on 10 polymers, (HX)n; on eight polyhalohydrogenate(1-) anions, HnXn+1-, and on four polyhalohydrogen(I) cations, Hn+1Xn+, (X = F and Cl) at a number of temperatures from 188 to 673 K, and at various total pressures. From the results, ΔG° and K values have been calculated for some reactions involving these species, and the relative amounts determined under a variety of conditions:

nHX –> (HX)n (n = 2-6) HX + HnXn+1- àHn+1Xn+2- (n = 0-3) HX + Hn+1Xn+ àHn+2Xn+1+ (n = 0, 1) There are some marked differences between the fluorine and chlorine series, but generally, the relative amounts of the large molecules and ions decrease as the temperature increases. For all reactions, ln K can be expressed as function to the fourth power in 1/T indicating that ΔH° is more or less temperature dependent over a range of 400-500°.Key words: ab initio, thermodynamics, HCl, HF.
Thiol functionalization of surfaces for biosensor development
Authors: McGovern Mark E , Thompson Michael
Published: April 2011

The immobilization of biomolecules on substrate surfaces for biosensor development requires linking molecules that must meet a specific set of criteria. Two such agents based on bifunctional alkyltrichlorosilane structures, 1-bromo-11-(trichlorosilyl)-undecane and 1-(thiotrifluoroacetato)-11-(trichlorosilyl)-undecane, are employed to generate thiol-functionalized surfaces either by nucleophilic substitution followed by reduction (bromine-containing derivative) or deprotection (fluorine-containing compound). Both molecules have been attached to the surfaces of silicon wafers in conjunction with the diluent silane, octyltrichlorosilane. X-ray photoelectron spectroscopic analysis in the conventional and angle-resolved modes confirms that both silanization reactions were successful. The alkyl-bromine surfaces were subjected to treatment with a set of nucleophilic reactants followed by reduction and derivatization with trifluoroacetic anhydride. The latter in conjunction with surface analysis was used to estimate the level of thiol functionalization achieved. The fluorine-containing undecane surface has been studied by surface analysis both before and after deprotection of the thiol group by hydroxylamine solution. The results indicate that a high coverage of the surface was found for the protected moiety, with approximately 10% of the trifluoro acetate groups remaining after the deprotection procedure.Key words: bifunctional silanes, thiol surface functionalization, biomolecule immobilization, X-ray photoelectron spectroscopy.
A comparison of NMR distance determinations in the solid state by cross polarization, REDOR, and TEDOR techniques
Authors: Fyfe Colin A , Lewis Andrew R , Chézeau Jean-Michel
Published: April 2011

Solid-state NMR distance determinations using Hartmann-Hahn cross polarization between spin-1/2 nuclei (19F/29Si) are reported. 19F –> 29Si polarization transfer for the T-1 silicon site in the clathrasil octadecasil shows an oscillatory behaviour as a function of the contact time. These oscillations were observed for non-spinning powder samples and also for different spinning speeds at the various MAS sideband matching conditions. Three analytical functions that allow efficient nonlinear least-square regression analyses of the experimental data to determine the internuclear distances for non-spinning powder samples as well as at the ±1 and ±2 MAS sideband matching conditions are reported. Using these functions, a 19F-29Si distance of 2.53 ± 0.04 Å was determined for the T-1 silicon in octadecasil from fitting of the oscillatory behaviour. This distance is in good agreement with that known from the X-ray structure and the previous 19F/29Si REDOR and TEDOR distance measurements. The advantages and limits of the different dipolar-based NMR techniques for heteronuclear distance determinations are examined and discussed.Key words: cross polarization (CP), NMR distance measurements, octadecasil, 19F-29Si distances, REDOR, TEDOR.
Consideration of end effects of DNA hybridization in selection of fluorescent dyes for development of optical biosensors
Authors: Jakeway S C , Krull U J
Published: April 2011

Intercalating fluorescent dyes are in widespread use to detect the presence of double-stranded DNA. Applications include the development of biosensors that rely on the attachment ("tethering") of a dye molecule by a short hydrocarbon chain to the terminus of a strand of DNA so that dye is continuously available and the biosensor is fully reversible. Double strands of DNA have end effects that limit the stability of hybridization and dye intercalation near the termini of the duplexes. Therefore, the selection of the dye must be based on consideration of spectroscopic properties and also issues associated with tether length and the stoichiometry of the binding of the dye with double- and single-stranded DNA. Ethidium bromide (EB) has been used extensively to detect hybridization of DNA in applications such as electrophoresis, gene chips, and biosensors. A number of dyes with greater quantum efficiency than EB for detection of hybridization have been reported. Furthermore, other practical spectroscopic advantages can be gained in terms of improved S/N by use of dyes that have excitation that is red shifted relative to EB. Pyrilium iodide has been disclosed as an intercalator of high quantum efficiency and long excitation wavelength. This work investigates pyrilium iodide in comparison to EB as a candidate for preparation of a tethered dye for detection of hybridization of DNA 20-mers.Key words: biosensors, DNA, hybridization, fluorescence, end effects.
Influence des ions fluorure et de la silice amorphe sur la solubilité des gels de zircone et caractérisation des fluoro-complexes de zirconium en milieu moyennement acide
Authors: Valéro Rémi , Durand Bernard , Guth Jean-Louis , Chopin Thierry
Published: April 2011

The solubility of zirconia gels versus pH is studied in order to state first the influence of F- ions alone, then the influence of the simultaneous presence of F- and amorphous silica. Two concentrations of F- ions are chosen, 0.094 and 0.314 mol L-1, corresponding to initial molar ratios F/Zr = 0.6 and 2.0. Metal fluorocomplexes formed at pH 0.5 and 2.5 are characterized by 19F NMR. The influence of F- on the solubility of zircon is also studied. It is shown that the formation of silicon complexes rich in fluorine shifts the equilibriums between the zirconium fluorocomplexes toward the complexes poor in fluorine and that, in the presence of F-, the crystallization of zircon by hydrothermal synthesis can be forecasted, for pH values in the range 0-10.Key words: zirconia gels, silica, zircon, fluoride ions, solubility, 19F NMR.
Semiempirical quantum chemical PM3 computations and evaluations of redox potentials, basicities, and dipole moments of the diphenylamine series as analytical reagents
Authors: Pankratov Alexei N , Shchavlev Andrew E
Published: April 2011

By evaluating the obtained results of PM3 computations, the following correlations were found: (i) the diarylamines redox potentials (E) with the first ionization potentials (I); (ii) the pKa values characterizing both the amines nitrogen protonation and the carboxy substituted reagents dissociation via the COOH group, with the proton affinities of the corresponding diphenylamines and COO--containing anions; (iii) the experimental values of the molecules dipole moments (μ) with the theoretical ones. The feasibility of the simple semiempirical quantum chemical evaluation of E, pKa, and μ of the diphenylamine series as redox reagents was determined.Key words: diphenylamine series, redox potential, pKa, dipole moment, semiempirical quantum chemical evaluation
Reactions of monovalent ions with a versatile building block [Cr(phen)(C2O4)2]-. Molecular and crystal structures of NaCr(phen)(C2O4)2·3H2O, KCr(phen)(C2O4)2·DMF·0.5H2O, 4-NH3py[Cr(phen)(C2O4)2]·2H2O, N-(n-butyl)4[Cr(phen)(C2O4)2], and PPh4[Cr(phen)(C2O4)2]
Authors: Rochon Fernande D , Massarweh Gassan
Published: April 2011

The molecular and crystal structures of the products of the reaction between the building block [Cr(phen)2(C2O4)2)]- and several monovalent ions, two from the alkaline group (Na+, K+) and 4-NH3pyridine+, N-(n-butyl)4+, and PPh4+ were studied by crystallographic methods. Na(Cr(phen)2(C2O4)2)·3H2O (1) is monoclinic, P21/c,a = 9.129(2), b = 13.867(2), c = 17.934(5) Å, β= 117.52(2)°, Z = 4 and R = 0.0623. The sodium atom is surrounded by six oxygen atoms at distances between 2.363(6) and 2.484(8) Å. The Na···Na distance is 3.693(7) Å. The compound has an extended structure forming layers, held together by H bonds. K(Cr(phen)2(C2O4)2)·DMF·0.5H2O (2) is monoclinic, P21/n,a = 11.298(2), b = 14.048(5), c = 14.240(5) Å, β= 108.19(2)°, Z = 4 and R = 0.0647. The material also possesses an extended 2D structure, but quite different from the one of the sodium compound. The coordination number around the K atom is 6 with K-O distances between 2.663(13) and 2.920(9) Å. The K···K distance is 3.939(9) Å. The other crystals 4-NH3py[Cr(phen)2(C2O4)2]·2H2O (3), N-(n-butyl)4[Cr(phen)2(C2O4)2] (4), and PPh4[Cr(phen)(C2O4)2] (5) have simple discrete ionic structures. Crystal 3 is monoclinic, Cc,a = 17.282(5), b = 15.037(3), c = 9.563(2) Å, β= 115.25(2)°, Z = 4, and R = 0.0386. The ions are held together by an extensive hydrogen bonding system involving the two molecules of water located in the lattice. Crystal 4 is monoclinic, P21/c, a = 12.329(3), b = 14.471(3), c = 18.181(3) Å, β= 97.75(2)°, Z = 4, and R1 = 0.076 and finally, 5 is monoclinic, P21/c,a = 13.231(2), b = 14.410(2), c = 18.758(4) Å, β= 106.00(1)°, Z = 4, and R1 = 0.0517.Key words: crystal structure, chromium, oxalato, phenanthroline, sodium, potassium, 4-aminopyridinium, tetrabutylammonium, tetraphenylphosphonium.
Lifetimes and UV-visible absorption spectra of benzyl, phenethyl, and cumyl carbocations and corresponding vinyl cations. A laser flash photolysis study
Authors: Cozens Frances L , Kanagasabapathy V M , McClelland Robert A , Steenken Steen
Published: April 2011

Benzyl (4-MeO, 4-Me, and 4-methoxy-1-naphthylmethyl), phenethyl (4-Me2N, 4-MeO, 3,4-(MeO)2, 4-Me, 3-Me, 4-F, 3-MeO, 2,6-Me2, parent, and 4-methoxy-1-naphthylethyl) and cumyl (4-Me2N, 4-MeO, 4-Me, parent) cations have been studied by laser flash photolysis (LFP) in 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). In most cases styrene or α-methylstyrene precursors were employed for the phenethyl and cumyl ions, the intermediate being obtained by solvent protonation of the excited state. Benzyl cations were generated by photoheterolysis of trimethylammonium and chloride precursors. While a 4-MeO substituent provides sufficient stabilization to permit observation of cations in TFE, cations with less stabilizing substituents usually require the less nucleophilic HFIP. Even in this solvent, the parent benzyl cation is too short-lived (lifetime <20 ns) to be observed. When generated in HFIP, phenethyl cations can be seen to react with unphotolyzed styrene, giving rise to dimer cations that are observed to grow in as the initial phenethyl cation decays. The dimer cations, in common with the oligomer cations seen in cationic styrene polymerization, have a λmax15-20 nm higher than the monomer and react with both solvent and styrene several orders of magnitude more slowly. This stabilization relative to the phenethyl may reflect an interaction with the aryl group present at the gamma-carbon. Cations 4-MeOC6H4C+(R)-CH3(R = Me, Et, i-Pr, t-Bu, cyclopropyl, C6H5, 4-MeOC6H4) were generated in TFE via the photoprotonation route. The alkyl series shows that steric effects are important in the decay reaction. The cation with R = cyclopropyl is a factor of 1.5 less reactive than the cation where R = phenyl. Several vinyl cations have also been generated by photoprotonation of phenylacetylenes. ArC+=CH2has a reactivity very similar to that of its analog ArC+H-CH3, the vinyl cation being slightly (factors of 2-5) shorter-lived. For the various series of cations, including vinyl, substituents in the aryl ring have a consistent effect on the λmax, a shift to higher wavelength relative to hydrogen of 15 nm for 4-Me, 30 nm for 4-MeO, and 50 nm for 4-Me2N.Key words: photogenerated carbocations, carbocation lifetime, styrene, photoprotonation.