bluuebunny/crossref_metadata_2025 huggingface dataset:
Origins of thermodynamic stability of urea:alkane inclusion compounds
Authors: White Mary Anne
Published: April 2011

Using existing thermodynamic data, the thermodynamic stabilities of urea inclusion compounds with three different alkane guests (decane, dodecane, and hexadecane) have been determined. In all cases the inclusion compounds are found to be thermodynamically stable with respect to their component species up to their melting points. From the present analysis, the solid-solid phase transitions are found to play only a minor role in stabilization, and the thermodynamic stability is found to be predominantly enthalpically driven. At a given temperature, the stability increases in the order urea:decane < urea:dodecane < urea:hexadecane. The present analysis shows that above the melting point of the guest species the inclusion compounds become much less stable because of the competition of the liquid guest. This destabilization eventually leads to incongruent melting.Key words: urea inclusion compounds, thermodynamic stability.
The secondary α-deuterium isotope effect at the site of the enzyme-catalyzed cis-trans isomerization of maleylacetone. Mechanistic implications
Authors: Finnin Michael S , Seltzer Stanley
Published: April 2011

An equal mixture of monodeuterated isotopomers of maleylacetone, viz., 4-hydroxy-6-oxo-2,4-heptadienoic-2-d1 acid (1) and 4-hydroxy-6-oxo-2,4-heptadienoic-3-d1 acid (2) was synthesized and subjected to catalyzed cis-trans isomerization by the maleylacetoacetate cis-trans isomerase-glutathione system. These studies were designed to probe the detailed mechanism of enzyme-catalyzed cis-trans isomerization about the C-2C-3 bond of the substrate. In the competitive reaction, 2 accumulates with respect to 1 at high extent of reaction. 1H NMR analysis indicates that 1 reacts 15% faster than 2 in line with a nucleophilic addition mechanism at C-2 rather than a conjugate addition mechanism at C-2 and C-3, to convert the C-2C-3 double bond to a single bond to allow facile internal rotation.Key words: enzyme-catalyzed cis-trans isomerization, secondary α-deuterium isotope effect, maleylacetoacetate cis-trans isomerase, cis-trans isomerization mechanism.
The absolute kinetics for radical addition and electronic energy transfer to [1.1.1]propellane
Authors: McGarry P F , Scaiano J C
Published: April 2011

Free radicals react more readily with [1.1.1]propellane, 1, than with styrene. For example Et3Si· reacts with 1 and styrene with rate constants of 6 × 108 M-1 s-1 and 2 × 108 M-1 s-1, respectively. Fluorenone, phenanthrene, triphenylene, benzophenone, and pyrene transfer electronic energy to 1 with rate constants well below the diffusion-controlled limit. For example, triplet benzophenone is quenched by 1 with a bimolecular rate constant of 9.9 × 106 M-1 s-1. A linear dependence of the log of the quenching rate constants, log kq, upon the excited-state energy of the donors is found.Key words: propellane, laser flash photolysis, free radicals, triplet states.
Article
Authors: Shimizu K , Flinn B J , Wong P C , Mitchell KAR
Published: April 2011

Secondary ion mass spectrometry (SIMS) has been used to guide the search for an oxidation procedure that can produce a thin and relatively sharp oxide layer on macroscopic zirconium. A new preparation based on dip coating in H2O2 solution is indicated to be suitable for this purpose. SIMS further indicates that the oxide interface, for such a prepared sample, shows substantial degradation when it is heated in H2 gas at 300°C. The presence of H appears to facilitate O migration into the metallic region, an observation that supplements those made previously on oxidized thin-film samples of zirconium prepared by deposition under ultrahigh-vacuum conditions.Key words: secondary ion mass spectrometry, interfacial reactivity, oxidized zirconium.
Mixed anion effect in sodium thiocyanate + sodium thiosulfate + water systems
Authors: Kalita G , Dass N N , Mahiuddin S
Published: April 2011

Densities and viscosities of R[xNaSCN + (1 - x)Na2S2O3] + (1 - R)H2O systems with R = 0.05, 0.10, 0.14, and 0.18 were measured as functions of mole fraction, x (= 0.0-1.0), and temperature (293.15 <= T/K <= 323.15). A significant mixed anion effect has been observed within the temperature range of the study. The normalized viscosity isotherms were found to detect the mixed anion effect ~1.2 to 3.7 times more than the simple viscosity isotherms. The progressive replacement of S2O32- ions by SCN- ions causes the systems to be less structured and S2O32- ions polarized Na+ ions more towards themselves than towards the SCN- ions. Both these effects govern the variation of the viscosity with mole fraction, x. The mixed anion effect was found to vanish at around R = 0.0106.Key words: mixed anion effect, sodium thiocyanate, sodium thiosulfate, viscosity.
DISQUAC structure-dependent interaction parameters for mixtures containing sec-alkanols and benzene, toluene, or n-alkanones
Authors: González Juan Antonio , de la Fuente I García , Cobos J C
Published: April 2011

Mixtures containing sec-alkanols and benzene, toluene, or n-alkanones are characterized in terms of DISQUAC by means of structure-dependent interaction parameters. The quasichemical (QUAC) interchange coefficients are independent of the size alcohol for a given organic solvent; while the dispersive (DIS) ones change with the size of the alcohol. This behaviour has been found for other many alcoholic solutions. DISQUAC represents well the thermodynamic properties of these mixtures: vapor-liquid equilibria, VLE, molar excess Gibbs energies, GE, and molar excess enthalpies, HE. So, the mean deviations between experimental and calculated results are about 2% for pressure, and 8% for HE. A short comparison between DISQUAC results and those given by the Dortmund version of UNIFAC, the UNIQUAC association theory and other similar model is also presented. In the case of the Dortmund version of UNIFAC, poorer predictions are obtained for HE, as this property is more sensitive to molecular structure. Association theories provide better results because are used as correlation equations.Key words: thermodynamics, 2-alkanols, group contributions, interactions.
Spectroscopic studies of triphenyltin azide and of its adducts with O- and N-donor ligands; the crystal and molecular structures of triphenyltin azide and of azido(hexamethylphosphoramide)triphenyltin(IV)
Authors: Wharf Ivor , Wojtowski Ryszard , Bowes Carol , Lebuis Anne-Marie , Onyszchuk Mario
Published: April 2011

Complete far-IR and Raman data (<400 cm-1) are reported for triphenyltin azide (1) as well as for adducts Ph3SnN3·L (L = hexamethylphosphoramide (HMPA), triphenylphosphine oxide, pyridine-N-oxide, 4-picoline-N-oxide, or pyridine). The small changes in v(Sn-N3) noted on going from (1) to the adducts, as well as solid-state 119Sn nmr data, indicate that (1), like the adducts, has five-coordinate tin atoms. X-ray analysis shows that (1) crystallizes with two different chains of five-coordinate Ph3Sn units joined by 1,3-azide bridges. In one unit, the geometry around tin is similar to that found for the monomeric HMPA adduct (2), which has trans-axial HMPA and azide ligands. The other unit in (1) has nonplanar -SnC3- groups connected by less symmetric 1,3-azide bridges and thus resembles the structure of isoelectronic triphenyltin isocyanate.Key words: triphenyltin azide, O- and N-donor adducts, far-IR/Raman, crystal structures.
Dioxirane oxidation of nitrosoamines. An ab initio study
Authors: Shustov Gennady V , Rauk Arvi
Published: April 2011

Pathways for oxidation of the parent nitrosoamine 2, nitrosodimethylamine 3, and anti- nitrosoethylmethylamine 4 by the parent dioxirane 1 have been explored computationally using the B3LYP hybrid density functional theoretical method in conjunction with the 6-31G* and 6-311+G** basis sets. Oxygen transfer from 1 to the nitrogen of the NO group (NO oxidation), yielding nitroamines 5,7, and 10, has the lowest activation barrier (15, 12.8, and 12 kcal mol-1 for 2, 3, and anti-4, respectively). Oxidation of the amine nitrogen (N oxidation) in 2, 3 leads to nitric oxide and nitroxyl radicals 6, 8 and is characterized by the highest activation energy: 28.5 kcal mol-1 for 2, 22.3 kcal mol-1 for 3. The potential barrier to hydroxylation of the methyl groups in 3 (CH oxidation) is intermediate - ca. 19 kcal mol-1. Introduction of a methyl group to the carbon reaction centre decreases the activation barrier of the anti-CH oxidation by 2-3 kcal mol-1. In 3, a gas-phase small preference for anti-CH oxidation over syn-CH oxidation, 0.1 kcal mol-1, is predicted to increase dramatically to 4.7 kcal mol-1 when the reaction is carried out in a dielectric medium (acetone, IPCM model). In general, polar solvents (CH2Cl2, acetone) lower the activation barriers for the NO, N, and CH oxidations, more so for the species of higher polarity than those of lower polarity (reactive complexes (1+3), (1+anti-4), transition state anti,syn-TSCH(1+4)]. However, the chemoselectivity (EaNO <Eaanti-CH <Easyn-CH <EaN) in the polar solvents is predicted to be the same as in the gas phase.Key words: dioxirane, nitrosoamines, oxidation, ab initio, omputation.
Synthesis of 5-phenyl-10-methyl-7H- pyrimido[4,5-f][1,2,4]triazolo[4,3-a][1,4]diazepine and its evaluation as an anticonvulsant agent
Authors: Phillips Oludotun A , Murthy KS Keshava , Fiakpui Charles Y , Knaus Edward E
Published: April 2011

The homolytic benzoylation (benzoyl radical) of 5-tert-butylcarbonylaminopyrimidine (6, 1 equiv.) in the presence of benzaldehyde (3 equiv.), water, sulfuric acid, and acetic acid, upon treatment with FeSO4·7H2O (3 equiv.) and 70% t-BuOOH (3 equiv.) at 5-10°C for 10 min afforded a mixture of 4-benzoyl-5-tert-butylcarbonylaminopyrimidine (7, 23%), 4,6-dibenzoyl-5-tert-butylcarbonylaminopyrimidine (8, 44%), and 4-benzoyl-5-tert-butylcarbonylamino-6- methyl pyrimidine (9, 10%). When a similar reaction was performed using 1 equiv. each of PhCHO, FeSO4·7H2O, and t-BuOOH, to prevent formation of the 4,6-dibenzoyl product 8, the monobenzoyl (7, 38%) and 4-benzoyl-6-methyl (9, 6%) products were obtained. A similar homolytic benzoylation of 5-bromopyrimidine (10) using 1.5 equiv. of reagents to generate the benzoyl radical afforded 4-benzoyl-5-bromopyrimidine (11, 61%) as the predominant product. Elaboration of 11 via a six-step reaction sequence afforded 2-hydrazino-5-phenyl-3H-pyrimido[5,4-e][1,4]diazepine (17) in 5.2% overall yield. The acid-catalyzed reaction of 17 with triethyl orthoacetate gave the title compound 5-phenyl-10-methyl-7H-pyrimido[4,5-f][1,2,4]triazolo[4,3-a][1,4]diazepine (18, 62%). The triazolo compound 18 was more potent than valproic acid (Depakene(r)) in both the subcutaneous metrazol (scMet) and maximal electroshock (MES) anticonvulsant screens, and more potent than clonazepam in the MES anticonvulsant screen but less potent than clonazepam in the scMet anticonvulsant screen.Key words: homolytic benzoylation, pyrimidines, pyrimidodiazepines, anticonvulsants.
Article
Authors: Kennedy Christopher B , Balcom Bruce J , Mastikhin Igor V
Published: April 2011

A new MRI method, recently employed in porous-media studies, is introduced for the imaging of rigid polymeric materials. The method, Single-Point Ramped Imaging with T1 Enhancement (SPRITE), is designed to spatially encode short lifetime, T2*, signals characteristic of rigid materials. We investigate the advantages of the SPRITE technique over conventional imaging methods by comparing a 2D SPRITE image of four common polymers to a 2D spin-echo image of the same substances. We demonstrate the general applicability of SPRITE by imaging two commercial products, a stop-valve made of chlorinated polyvinyl chloride and a solid-core, multilayer golf ball, both containing synthetic materials with effective 1H spin-spin relaxation times, T2*, well under 300 µs. Three-dimensional images of millimeter resolution reveal internal physical differentiation and chemical inhomogeneities that cannot be observed using conventional MRI methods. We also consider the incorporation of active spoiler gradients in SPRITE for visualization of samples with long spin-spin relaxation times, T2.Key words: MRI, NMR, SPI, SPRITE, polymers, imaging.
The problem of distinguishability of rate constants in the two-step consecutive sequence A → B → C
Authors: Andraos John
Published: April 2011

A detailed analysis of the absorbance versus time functional forms of the chemical species in the consecutive two-step unimolecular reaction scheme is presented. Distinguishability of rate constants is discussed with respect to the characterization of growth-decay and decay-growth curves and the rate constant - reaction step ambiguity. A new method based on the functional form of the absorption extremum involving the intermediate B with catalyst concentration is proposed to overcome the problem of rate constant assignments.Key words: reaction mechanism, kinetics analysis, physical organic chemistry.
1998 Alfred Bader Award Lecture Tangents and targets: The synthetic highway from natural products to medicine
Authors: Fallis Alex G
Published: April 2011

Synthetic studies directed toward various classes of compounds including taxoids and enediyne systems are described. These investigations include tether-controlled imtramolecular Diels-Alder reactions, carbometallation routes to dienes and furans, the use of carbohydrate building blocks, pentadienyl indium reagents for tandem cycloadditions to steroid and triterpenoid skeletons, and the synthesis of taxamycins and revolveneynes and related cyclophanes.Key words: cycloaddition, taxoids, enediyne, cyclophanes, carbometallation.
1998 Hoffmann La Roche Award Lecture Understanding and exploiting glycosidases
Authors: Withers Stephen G
Published: April 2011

Glycosidases fall into two major mechanistic classes; those that hydrolyse the glycosidic bond with retention of anomeric configuration and those that do so with inversion. Retaining glycosidases employ a mechanism involving a covalent glycosyl-enzyme intermediate formed and hydrolysed with acid-base catalytic assistance via oxocarbenium ion-like transition states. This intermediate has been trapped in two distinct ways, either by modification of the substrate through fluorination, or of the enzyme through mutation of key residues. This allows the amino acid residue to which this sugar is attached to be identified through LC/MS-MS analysis of peptic digests. Three-dimensional structures of several of these glycosyl-enzyme complexes, along with those of Michaelis complexes, have been determined through X-ray crystallographic analysis, revealing the identities of important amino acid residues involved in catalysis, particularly the involvement of the catalytic nucleophile in strong hydrogen bonding to the sugar 2-hydroxyl. They have also revealed evidence of substantial substrate distortion upon binding. Insight into the function of the acid-base catalyst Glu172 in Bacillus circulans xylanase has been obtained through NMR titration of side chain 13C-labelled glutamic acid enzyme both free and in the 2-fluoroxylobiosyl-enzyme complex. The pKa of Glu172 is relatively high in the free enzyme, consistent with its role as an acid catalyst, but drops 2.5 units upon formation of the intermediate, consistent with its new role as a base catalyst. This "cycling" of the pKa is shown to be a direct consequence of the change in charge of the nucleophile, Glu78. Finally, an efficient catalyst for synthesis, but not hydrolysis, of glycosidic bonds has been generated by mutation of the glutamic acid catalytic nucleophile of a β-glucosidase to an alanine. When used with alpha-glucosyl fluoride as a glycosyl donor, along with a suitable acceptor, oligosaccharides up to five sugars in length have been made with yields of up to 90% on individual steps. These new enzymes have been named glycosynthases.Key words: enzymatic mechanism, glycoside hydrolysis, inhibitors
Thermodynamics of the interaction of HCl with D-xylose in water at 278.15-318.15 K
Authors: Wang Jianji , Zhuo Kelei , Zhang Qiufen , Lu Jinsuo
Published: April 2011

The thermodynamic parameters of interaction of HCl with D-xylose in water were determined from electromotive force measurements from 278.15 to 318.15 K. The correlation between the pair interaction Gibbs free energy parameters for HCl-sugar (gHCl-S) and the mean number (n(e-OH)) of equatorial hydroxy groups in sugar molecules was found to be gHCl-S/J kg mol-2 = (41.2 ± 2.1) + (16.88 ± 0.52)n(e-OH). Refractive indices of aqueous D-xylose solutions were measured and used to evaluate the polarizability of D-xylose molecule in water. Several theoretical models were used to calculate the salting constant of D-xylose. The results show that the calculated values are reasonably in agreement with the experimental ones.Key words: thermodynamics, HCl, D-xylose, interaction parameter, salting constant.
Accurate density-functional calculation of core-electron binding energies with a scaled polarized triple-zeta basis set. VI: Extension to boron-containing molecules.
Authors: Cavigliasso Germán , Chong Delano P
Published: April 2011

Our procedure for calculating core-electron binding energies (CEBEs), based on the unrestricted generalized transition state model using B88/P86 functional, was extended to boron-containing molecules. Both unscaled (cc-pVTZ, cc-pVQZ, cc-pV5Z) and scaled (scaled-pVTZ and scaled-pVQZ) basis sets were used. The average absolute deviation from experiment for boron CEBEs with the scaled-pVTZ basis set was found to be 0.24 eV, compared to 0.23 eV for the much larger cc-pV5Z basis set.Keywords: DFT, boron, core-electron binding energies.
1998 W.A.E. McBryde Medal Lecture: Searching for the Holy Grail in analytical separations
Authors: Lucy Charles A , Yeung Ken K. -C , Fu Shilin , Li Dongmei , Henselwood Tracey L , Underhill Royale S
Published: April 2011

This paper describes the chemistry presented during the W.A.E. McBryde Medal address given at the 81st Chemistry in Canada Conference held in Whistler. The narrative chronicles our Quest to perform isotopic separations in the solution phase using as our Excalibur, capillary zone electrophoresis. The narrative takes you through the highs of our early success in separating 35Cl- and 37Cl-. This separation was achieved by adjusting the electroosmotic flow to be equal in magnitude but opposite in direction to the chloride mobility. The narrative then takes you through the dark days, when we could not extend the isotopic separations to cationic species or even explain why there was an isotopic effect on mobility. Since those dark days, we have made numerous discoveries that have aided our Quest. Firstly, the development of mixed surfactant wall coating procedures yielded control of the reversed electroosmotic flow. This control enabled us to perform isotopic separations of systems such as 15N-/14N-aniline and 15NH4+/14NH4+. In terms of understanding electrophoretic mobility, we demonstrate the importance of dielectric friction to mobility. Further, the effect of ionic strength in capillary zone electrophoresis is explained using the Pitts treatment, which is analogous to the extended Debye-Hückel equation for ionic activity. So, have we completed our Quest? Read on.Key words: capillary zone electrophoresis, isotopic, electroosmotic flow, mobility modeling, ionic strength.
Detection and kinetic characterization of SNV intermediates. Reactions of thiomethoxybenzylidene Meldrum's acid with thiolate ions, alkoxide ions, OH-, and water in aqueous DMSO
Authors: Bernasconi Claude F , Ketner Rodney J , Chen Xin , Rappoport Zvi
Published: April 2011

The reaction of thiomethoxybenzylidene Meldrum's acid (5-SMe) with thiolate and alkoxide ion nucleophiles is shown to proceed by the two-step addition-elimination SNV mechanism in which the tetrahedral intermediate accumulates to detectable levels. For the reactions with thiolate ions, rate constants for nucleophilic addition (k1RX), its reverse (k-1RX), and for conversion of the intermediate to products (k2RX) were determined. For the reactions with alkoxide ions, only k1RXand k-1RXcould be obtained; the intermediate in these reactions did not yield the expected substitution products, and hence no k2RXvalues could be determined. The reactions with OH-and water are believed to follow the same mechanism, but the respective intermediates remain at steady-state levels, and only k1OHand k1H²Ofor nucleophilic attack on 5-SMe were measurable. New insights regarding structure-reactivity behavior in SNV reactions are gained from comparisons of rate and equilibrium constants in the reactions of 5-SMe with the corresponding parameters in the reactions of methoxybenzylidene Meldrum's acid (5-OMe) and benzylidene Meldrum's acid (5-H). In particular, the relative importance of steric and pi-donor effects of the MeS vs. MeO group in 5-SMe and 5-OMe, respectively, and their role in affecting the intrinsic rate constants for nucleophilic addition, has been clarified by these comparisons. Our results also add support to a previous suggestion that soft-soft type interactions tend to increase intrinsic rate constants for thiolate ion addition to vinylic substrates, especially 5-SMe with the soft MeS group.Key words: nucleophilic vinylic substitution, intrinsic rate constants, transition state imbalances, steric/pi-donor/anomeric effects.
Oxidation of secondary alcohols and ethers by dimethyldioxirane
Authors: Baumstark Alfons L , Kovac Franci , Vasquez Pedro C
Published: April 2011

The oxidation of several series of secondary alcohols 2-9, ethers 10-17, and related derivatives 18 and19, by dimethyldioxirane, 1, in acetone at 25°C produced the corresponding ketones in good to excellent yields for all but two cases. (The exceptions: oxidation of 1-methoxy-2-methyl-1-phenylpropane (48%) and 1-methoxy-2,2-dimethyl-1-phenylpropane (24%).) The oxidation of the secondary alcohols was found to yield k2 values that were roughly 10-fold greater than those of the corresponding methyl ethers. The rate constant for oxidation of a silyl ether was slightly lower than that for the corresponding methyl ether while that for the ester derivative was roughly half the value. For oxidation of alcohols and methyl ethers, the k2 values became smaller as the R " series (Me, Et, nPr, iPr, and tBu) increased in steric bulk (ρ* = 1.7; r = 0.998 and ρ* = 3.2; r = 0.95, respectively). The Hammett study for the oxidation of the methyl ethers of α-methyl-p-benzyl alcohols (10, 20-25) yielded a ρ value of -0.74. The activation parameters for oxidation of the parent compound of the ether series (1-methoxy-1-phenylethane) were ΔH = 14.8 ± 0.5 kcal/mol, ΔS = -21.9 eu, ΔG = 21.3 kcal/mol, k2 (25°C) = 1.6 × 10-3 M-1 s-1. The mechanistic aspects of the oxidation are discussed in relation to two mechanistic extremes: (a) direct insertion of the oxygen atom into the CH bond and (b) direct abstraction of the H by dimethyldioxirane to yield a caged-radical pair, with subsequent coupling to hemi-ketal intermediates that fragment to yield acetone, alcohol or water, and ketone as the final products.Key words: dimethyldioxirane, oxidation.
Electrophilic aromatic substitution in substituted anilines; kinetics of the reaction with 4,6-dinitrobenzofuroxan
Authors: Crampton Michael R , Rabbitt Lynsey C , Terrier François
Published: April 2011

Kinetic results are reported for reaction of aniline and six of its N- and ring-substituted derivatives with 4,6-dinitrobenzofuroxan (DNBF) in water - dimethyl sulfoxide mixtures. In acidic solution σ-adducts, the result of electrophilic substitution in the anilines, are formed with bonding between a ring-carbon atom of the anilines and the 7-position of DNBF. Reaction normally occurs at the 4-position of the aniline unless this carries a substituent, when reaction occurs at the 2-position. A value of 2.0 for kH/kD, the kinetic isotope effect, indicates that in the reaction with aniline bond formation is largely rate determining in the substitution pathway. The results allow estimates to be made for the pKa values relating to carbon protonation of the anilines.Key words: electrophilic substitution, σ-adducts, 4,6-dinitrobenzofuroxan, ambident reactivity of anilines.
Reaction of diphenylborinic acid with pyridinium and phosphonium bisacyl methylides. Crystal and molecular structures of resulting B,N- and B,P-betaines
Authors: Grünefeld Johann , Kliegel Wolfgang , Rettig Steven J , Trotter James
Published: April 2011

Reaction of pyridinium and phosphonium 2-carboxy-1-formyl-2-oxoethylides with oxybis(diphenylborane) yields 5-(2-oxo-1-(pyridinio or phosphonio)ethylidene)-2,2-diphenyl-1,3-dioxa-2-borata-4-cyclopentanones, which represent novel types of boron nitrogen and boron phosphorus betaines, respectively. Crystals of 5-(2-oxo-1-pyridinioethylidene)-2,2-diphenyl-1,3-dioxa-2-borata-4-cyclopentanone,1a, are monoclinic, a = 15.589(3), b = 12.366(2), c = 20.1869(11) Å, β = 111.227(2)°, Z = 8 (two independent molecules), space group P21/a; and those of 5-[2-oxo-1-(triphenylphosphonio)ethylidene]-2,2-diphenyl-1,3-dioxa-2-borata-4-cyclo-pentanone,11, are monoclinic, a = 9.6922(7), b = 25.027(2), c = 11.3041(4) Å, β= 92.1418(13)°, Z = 4 (two independent molecules), space group P21. The structures were solved by direct methods and refined by full-matrix least-squares procedures to R (F, I [Formula: see text] 3σ(I)) = 0.047 and 0.042 (Rw (F2, all data) = 0.096 and 0.088), respectively, for 1a and 11.Key words: pyridinium ylides, phosphonium ylides, bisacyl methylide diphenylboron chelates, boron nitrogen betaines, boron phosphorus betaines, organoboron compounds, crystal structures.
Calculation of atomic recombination in a decaying high-temperature hydrogen gas column
Authors: Stevefelt Jörgen
Published: April 2011

The temporal evolution of the concentration of atomic radicals [H] in a high-temperature hydrogen gas column has been evaluated, assuming an exponentially decaying temperature from 5000 K to 1000 K, and for constant pressures in the range from 0.125 to 2.0 bar (1 bar = 105 Pa). The initial concentration [H]t=0, assumed to correspond to thermal equilibrium, would increase with pressure, but the termolecular recombination processes also become faster with increasing pressure. As a result, high concentration of atomic radicals rapidly becomes favoured by lower gas pressure after the temperature has decayed for a certain time. Also, operation at lower pressures would make the decay of [H] radicals less dependent upon the cooling rate of the gas.Key words: high-temperature gas, atomic recombination, hydrogen radicals.
The surprising formation of structurally distorted ap-9-(o-tert-butylphenyl)-9-methylthiofluorene and its facile homolysis into sp-9-(o-tert-butylphenyl)-3-methylthiofluorene and sp-9-(o-tert-butylphenyl)fluorene
Authors: Hou Yuqing , Meyers Cal Y
Published: April 2011

The continued study of rotationally restricted 9-(o-tert-butylphenyl)fluorenes has provided surprising results. Treatment of sp-9-(o-tert-butylphenyl)-9-fluorenol (1) with ethanol or methanol under acidic conditions affords sp-9-(o-tert-butylphenyl)-9-ethoxyfluorene (2b) and sp-9-(o-tert-butylphenyl)-9-methoxyfluorene (3b), respectively, but similar treatment with methanethiol converts 1 into the rotamerically opposite ap-9-(o-tert-butylphenyl)-9-methylthiofluorene (4a). While all three products reflect reaction with inversion at C-9, 2b and 3b reflect subsequent rotation, which is not the case with 4a. X-ray diffraction shows 4a to be highly distorted and strained, but apparently favored thermodynamically over its sp rotamer. Homolysis of 4a is observed at room temperature, and at elevated temperatures accounts for the formation of sp-9-(o-tert-butylphenyl)fluorene (6b) and sp-9-(o-tert-butylphenyl)-3-methylthiofluorene (7c) as major products. X-ray diffraction shows 6b and 7c to be virtually devoid of distortion. Methylation of 6b via its anion also proceeds with inversion without rotation to form ap-9-(o-tert-butylphenyl)-9-methylfluorene (4c). Dynamic NMR unexpectedly showed that in these ap configurations 9-CH3 (of 4c) has a greater bulk effect than 9-CH3S (of 4a) in forcing the o-tert-butyl group into the fluorene plane.Key words: fluorene, rotamer, distortion, homolysis, carbanion, carbocation, radical.
Strong Zn2+ and Co2+ catalysis of the methanolysis of acetyl imidazole and acetyl pyrazole
Authors: Khan Ayla , Neverov Alexei A , Yatsimirsky Anatoly K , Brown R S
Published: April 2011

The kinetics of methanolysis of acetyl imidazole (1) and acetyl pyrazole (2) have been investigated under anhydrous conditions in the presence of Zn(ClO4)2, Co(ClO4)2, and HClO4 at 25°C. In all cases, the plots of the pseudo-first-order rate constant for methanolysis (kobs) vs. [metal ion] or [HClO4] show saturation behavior indicative of equilibrium binding of the M2+ or H+ to the amide. Relative to the spontaneous methanolysis rate constant (ko), the catalytic rate constant obtained at saturation, kcat, is larger for metal-ion catalysis than for H+ catalysis. The (kcatH+/ko) ratio is 10.7 and 1.25 for 1 and 2, respectively, while the (kcatM2+/ko) for these divalent metals varies from 150-fold for 1 to between 700 and 5700-fold for 2. By contrast, in water, proton is far more effective at promoting the hydrolysis of 1 than are metals, the aqueous (kcatH+/ko) ratio being 560, while the (kcatZn2+ /ko) and (kcatNi2+/ko) ratios are 15 and 3.2, respectively.Key words: methanolysis, kinetics, metal-ion catalysis, acetyl imidazole, acetyl pyrazole.
The decomposition of methyl hemiacetals of benzaldehyde in aqueous solution: a study of the effect of aromatic substitution
Authors: Engell Karen M , McClelland Robert A , Sørensen Poul E
Published: April 2011

The acid-base catalysed decomposition of hydrates and hemiacetals of carbonyl compounds are classical examples of reactions where (slow) proton transfer is coupled with heavy atom reorganization, i.e., CO bond breaking and solvent reorganization. We have studied the influence of m- and p-substitution in the carbonyl electrophile on the kinetics of the acid and base catalysis of the decomposition of methyl hemiacetals of benzaldehyde. The experimental data are well described by three-dimensional More O'Ferrall - Jencks energy contour diagrams according to principles developed by Jencks (the BEMA HAPOTHLE). Thus, for acid catalysis, a Cordes cross-interaction coefficient pxy' = δρ/δpKa = 0.15 indicates the coupled nature of the rate-limiting step in a class e mechanism, similar to conclusions reached from systematic substitution in the nucleophile. Our more extensive set of data for base catalysis permits a more rigorous analysis according to the BEMA HAPOTHLE. The data are consistent with a class n mechanism as also suggested earlier on the basis of substitution in the nucleophile. A slight upward curvature observed in the Hammett plots for the various catalysts is described by the direct correlation parameter py = -δρ/δσ = -0.11. This second derivative demonstrates the concerted nature of the CO bond cleavage and O-H formation in the transition state, which changes with changing substituent. A class n mechanism for base catalysis is also supported by the observation of a Cordes cross-interaction parameter pxy = -δρ/δpKa = -δβ/δσ = 0.03, which describes the experimentally observed decrease in Hammett ρ with increasing pKa of the catalyst. This change may be rationalized by the movement of a saddle point on a diagonal reaction coordinate in the energy contour diagram, as a resultant of shifts parallel and perpendicular to the coordinate, when the energy along one side of the diagram is changed. It is concluded that observed rate changes as a result of substitution in the electrophile are consistent with and present further confirmation of earlier suggested mechanisms of hemiacetal decomposition reactions.Key words: methyl hemiacetals of benzaldehydes, acid-base catalysed breakdown, Hammett plots, More O'Ferrall - Jencks diagrams.
Successive photocyanation of highly chlorinated aromatic compounds
Authors: Konstantinov Alexandre D , Johnston Andrea N , Bunce Nigel J
Published: April 2011

Successive photocyanation was found to be a general reaction when chlorinated aromatic compounds were photolyzed with sodium cyanide. The products were polycyanated hydroxychloro compounds with various degrees of chlorine replacement. Although the products from some substrates could be isolated, identified, and characterized, most reactions proceeded with low regioselectivity, which limits their synthetic potential. Quantum yields of substrate disappearance increased with the number of chlorine substituents on a substrate, and followed the expected relationship ϕ-1 is proportional to [CN-]-1. In some cases, ϕ depended also on the concentration of the chloro compound, indicating the involvement of excimers, although the major reaction channel appears to be SN2Ar*. Sensitization and quenching experiments established the triplet excited state to be reactive for all substrates tested.Key words: photocyanation, chlorinated aromatic compounds.
Butenolide annelation using a manganese(III) oxidation. A synthesis of chromolaenin (Laevigatin)
Authors: Demir Ayhan S , Gercek Zuhal , Duygu Nese , Igdir A Cigdem , Reis Omer
Published: April 2011

A procedure was developed for the annelation of a butenolide to an aromatic ketone that highlighted a manganese(III) oxidation of an aromatic ketone. The merit of this procedure was illustrated in a synthesis of Chromolaenin (Laevigatin) (1) from 4,7-dimethyl-1-tetralone (2c). Oxidation of tetralone and indanone with manganese(III) acetate in the presence of chloro- or bromopropionic acid or their Mn(II) salts furnished a α-haloesters 3a-d. An Arbuzov reaction of haloesters with triethylphosphite and an intramolecular Horner-Watsworth-Emmons cyclization of the resulting phosphonate gave butenolide 5a, b in 77-79% yield. This methodology was also applied to 4,7-dimethyl-1-tetralone (2c), and the corresponding butenolide 6 is synthesized in 85% yield, which was then converted to Chromolaenin (1) using DIBAL-H.Key words: chromolaenin (Laevigatin), manganese(III) acetate oxidation, aromatic ketones, butenolide annelation, lactone, Horner-Watsworth-Emmons cyclization, Arbuzov reaction.
A reinterpretation of the substituent effect on the amide barrier. An experimental and theoretical study
Authors: Bain Alex D , Hazendonk P , Couture P
Published: April 2011

Recent theoretical work on amide systems has brought into question the application of the concept of resonance. In particular, the role of the oxygen atom was questioned, since the calculations showed little change in its properties when the amide bond was rotated. This paper investigates, both experimentally and computationally, the effect of a substituent on the carbonyl carbon on the amide barrier, in order to test this view. The barriers to the amide rotation in seven spiro-fused oxazolidines were measured by NMR, to within 1 kJ mol-1. A subset of three of them was modelled to the 6-31G** level. For all three substituents the computed and measured barriers corresponded to within 7 kJ mol-1. The electron densities were analyzed using the Atoms in Molecules (AIM) theory. The AIM analysis revealed that the oxazolidines behaved similarly to formamide. The substituent effect was described in terms of the atomic populations and energies of the amide C, O, and N. A substituent on the carbonyl carbon caused electron redistributions between N and C, changing their basin attractive energies. Neither the population nor the energy of oxygen changed significantly. When interactions outside the basin of interest were considered, the energy of C was seen to be more sensitive to changing the substituent than the energy of N. However, the atomic parameters from the AIM analysis did not fully reflect the substituent effects observed. For these molecules, the barrier includes contributions from several sources - there is no single, dominant contribution.Key words: amides, barrier, substituent, nuclear magnetic resonance spectroscopy, chemical exchange, atoms in molecules, slow exchange, NMR lineshape analysis.
Solubility of phenanthrene in organic nonelectrolyte solvents. Comparison of observed versus predicted values based upon Mobile Order theory
Authors: Hernández Carmen E , De Fina Karina M , Roy Lindsay E , Sharp Tina L , Acree, Jr. William E
Published: April 2011

Experimental solubilities are reported at 25.0°C for phenanthrene dissolved in 41 different organic nonelectrolyte solvents containing ether, carbonyl, hydroxy, chloro, amino, ester, methyl and tert-butyl functional groups. Results of these measurements are used to test the applications and limitations of expressions derived from Mobile Order theory. For the 31 solvents for which predictions could be made, computations show that Mobile Order theory does provide fairly reasonable estimates of the saturation mole fraction solubilities. Average absolute deviation between predicted and observed values is 41.3% (excluding acetonitrile). In comparison, the average absolute deviation is 1610% when ideal solution behavior is assumed. Also reported are experimental solubilities at 26°C for pyrene dissolved in 1-hexanol, 1-heptanol, ethylbenzene, aniline, and 2-butanone.Key words: phenanthrene solubilities, organic nonelectrolyte solvents, solubility predictions.
Zinc-67 NMR study of tetrahedral and octahedral zinc sites with symmetrical oxygen, nitrogen, and sulfur ligands
Authors: Sham Simon , Wu Gang
Published: April 2011

We report a 67Zn nuclear magnetic resonance (NMR) study of compounds containing zinc ions coordinated by oxygen, nitrogen, and sulfur ligands. New information concerning 67Zn nuclear quadrupole coupling constants (NQCC) and chemical shift was obtained from magic-angle spinning (MAS) spectra of solid compounds containing both natural abundance and enriched 67Zn isotopes. Rapid ligand exchange processes of [Zn(thiourea)4]2+ in aqueous solutions were also observed.Key words: 67Zn NMR, nuclear quadrupolar coupling constant, ligand exchange.
Solubility of acenaphthene in organic nonelectrolyte solvents. Comparison of observed versus predicted values based upon Mobile Order theory
Authors: De Fina Karina M , Sharp Tina L , Acree, Jr. William E
Published: April 2011

Experimental solubilities are reported at 25.0°C for acenaphthene dissolved in 36 different organic nonelectrolyte solvents containing ether-, carbonyl-, hydroxy-, ester, methyl-, and tert-butyl functional groups. Results of these measurements are used to test the applications and limitations of expressions derived from Mobile Order theory. For the 29 solvents for which predictions could be made computations show that Mobile Order theory does provide fairly reasonable estimates of the saturation mole fraction solubilities. Average absolute deviation between predicted and observed values is 37.8%. In comparison, the average absolute deviation is 1080% when ideal solution behavior is assumed.Key words: acenaphthene solubilities, organic nonelectrolyte solvents, solubility predictions.
Carboxylic acid amide enols: keto-enol/enolate interconversion of N-methylindoline-2-one and its 2-thione and 2-selone analogs in aqueous solution
Authors: Kresge A Jerry , Meng Qingshui
Published: April 2011

Carbon-acid ionization constants, QaK (concentration quotient at ionic strength = 0.10 M), were determined by spectrophotometric titration in aqueous solution for N-methylindoline-2-one, pQaK = 15.70, N-methylindoline-2-thione, pQaK = 8.93, and N-methylindoline-2-selone, pQaK = 7.25. Rate profiles were also constructed for the thione and selone. These were interpreted, with support from the form of acid-base catalysis as well as solvent and substrate isotope effects, as representing keto - enol/enolate ion interconversion. That led to the enol acidity constant pQaE = 4.05 and the keto-enol equilibrium constant pKE = 4.88 for the thione and estimates of the limits on these quantities, pQaE< 3 and pKE > 4, for the selone.Key words: lactam, thiolactam, selenolactam, keto-enol equilibria, carbon-acid ionization constants.
Solubility of biphenyl in organic nonelectrolyte solvents. Comparison of observed versus predicted values based upon Mobile Order theory
Authors: De Fina Karina M , Sharp Tina L , Acree, Jr. William E
Published: April 2011

Experimental solubilities are reported at 25.0°C for biphenyl dissolved in 26 different alkane, dialkyl ether, and alcohol solvents. Results of these measurements, combined with previously published biphenyl solubility data, are used to test the applications and limitations of expressions derived from Mobile Order theory. For the 40 solvents for which predictions could be made computations show that Mobile Order theory does provide fairly reasonable estimates of the saturation mole fraction solubilities. Average absolute deviation between predicted and observed values is 13.0%.Key words: biphenyl solubilities, organic nonelectrolyte solvents, solubility predictions.
Excess molar volumes of methanol or ethanol + n-polyethers at 298.15 K
Authors: Carmona F J , Arroyo F J , de la Fuente I García , González J A , Cobos J C
Published: April 2011

Excess molar volumes VmE at 298.15 K and atmospheric pressure for methanol and ethanol + 2,5-dioxahexane, + 3,6-dioxaoctane, + 2,5,8-trioxanonane, + 3,6,9-trioxaundecane, + 5,8,11-trioxapentadecane, + 2,5,8,11-tetraoxadodecane, and + 2,5,8,11,14-pentaoxapentadecane have been obtained from densities measured with an Anton-Paar DMA 602 vibrating-tube densimeter. All the excess volumes are negative over the whole mole fraction range.The VmE curves are shifted to the region rich in the alkanol, increasing their asymmetry with the number of oxygen groups in the polyethers. Results seem to remark the predominant contribution of free volume effects on interactional effects, particularly when the ethers are of the type CH3-(O-CH2CH2)m-CH3. In the case of polyethers with longer n-alkyl chain ends, self-association of methanol and ethanol is more relevant. A short comparative study with results for mixtures with higher 1-alkanols and polyethers is also presented.Key words: experimental, excess volumes, 1-alkanols, polyethers, self-association, free volume effects.
Gas-phase luminescence of aromatic carbonyl compounds in excited nitrogen at atmospheric pressure
Authors: Lin Zhong-Ping , Aue Walter A
Published: April 2011

Certain types of aroyl compounds such as benzaldehyde, acetophenone, benzophenone, and anthraquinone produce intense gas-phase luminescence in excited nitrogen at atmospheric pressure. This luminescence was measured in pressure ranges of 1.001.67 atm and temperature ranges of 343473 K. A novel, radioactively (Ni-63) stimulated, high-voltage( [Formula: see text]1750 V/mm), low-current ( [Formula: see text]35 nA) discharge in high-purity nitrogen was used for gas chromatographic detection and spectral excitation. The gas-phase luminescence spectra of about sixty aroyl compounds - introduced as gas chromatographic peaks - could thus be measured and compared with literature spectra obtained by conventional excitation in condensed phases. Only a few gas-phase spectra are available from the literature, and these did agree well with the spectra of this study. A speculative luminescence mechanism is proposed, in which ground-state N2 becomes excited by collision with fast electrons. This is followed by efficient triplet-triplet energy transfer from N2(A 3Σu+) to the n>π* excited aroyl compound. Key words: aroyl phosphorescence, excited-nitrogen excitation, aroyl triplet-triplet excitation, aroyl luminescence detection, gas-phase phosphorescence spectra, gas-phase aroyl detection.
Schiff base insertion in titanium alkyls; reduction of imine functions by benzyl addition
Authors: Coles Simon J , Hursthouse Michael B , Kelly David G , Toner Andrew J , Walker Neil M
Published: April 2011

TiBz4 reacts with N-2-fluorenyl(salicylideneimine) to afford a crystallographically characterized titanium(IV) complex containing two conventional bidentate Schiff base ligands and two O-bound ligands in which the imine function has been reduced by the addition of benzyl and hydrogen moieties.Key words: insertion, nucleophilic additions, Schiff bases, titanium
Preparation of N-aryl-substituted spiro-β-lactams via Staudinger cycloaddition
Authors: Barba Victor , Hernández Cecilia , Rojas-Lima Susana , Farfán Norberto , Santillan Rosa
Published: April 2011

The interest in the study of β-lactams continues due to their therapeutic importance as antibiotics. In this work, six spiro-β-lactams (7a-7c, 8a-8c) have been prepared using the [2+2] cycloaddition of isomaleimides to acid chlorides. The heterobicyclic structures obtained have been characterized by mass spectrometry, IR, NMR spectroscopy, and for compounds 7a, 7b, and 8b the X-ray crystallographic study showed a nearly planar arrangement for the β-lactam ring.Key words: β-lactams, azetidinone, isomaleimides, ketenes, X-ray crystallography.
A kinetic modeling study of ethylene pyrolysis
Authors: Roscoe John M , Bossard Alain R , Back Margaret H
Published: April 2011

A kinetic model is presented for the pyrolysis of ethylene at pressures ranging from 0.8 to 27 kPa and temperatures from 774 to 1023 K. The model is based on experimental measurements of C2H2, C2H6, C3H6, 1-C4H8, and 1,3-C4H6. In this temperature range the reaction is initiated by the disproportionation of C2H4 and the observed products result from reactions of the C2H3 and C2H5 radicals produced in this process. The C2H2 and 1,3-C4H6 result from reactions of C2H3 while C2H6, C3H6, and 1-C4H8 result from reactions of C2H5. C2H2 is produced exclusively by the decomposition of the C2H3 radical. This process is in its falloff region throughout the range of experimental conditions examined and the yield of C2H2 provides a measure of the degree of falloff. The production of 1,3-C4H6 is controlled by the reaction C4H7 –> C4H6 + H. The rate constants for this reaction were independent of pressure and are given as a function of temperature by k = 2.2 × 1013 exp (-19.6 × 103/T). Production of C2H6 is controlled by the reaction C2H5 + C2H4 –> C2H6 + C2H3. The rate constant for this reaction is given as a function of temperature by k = 5.83 × 1011 exp (-14.6 × 103/T). C3H6 is produced by decomposition of 2-C4H9 and is controlled kinetically by the isomerization reaction 1-C4H9 –> 2-C4H9. The temperature dependence of the rate constants obtained for this reaction leads to a preexponential factor of approximately 3 × 1016 and an activation energy of approximately 200 kJ mol-1. The yield of 1-C4H8 is controlled by 1-C4H9 –> 1-C4H8 + H. The rate constants for this reaction were independent of pressure and are given as a function of temperature by k = 2.97 × 1012 exp (-17.1 × 103/T).

Key words: kinetic modeling, ethylene pyrolysis.
A performance comparison between the CEP effective core potential/triple-split basis set approach and an all-electron computational method with emphasis on small Ti and V alkoxide complexes
Authors: Larsen Gustavo
Published: April 2011

The structures of 30 neutral and positively charged small clusters were optimized and a number of hypothetical reaction pathways were investigated to compare the performance of the B3LYP/CEP-121G(d) Effective Core Potential/triple-split basis set method (ECP, method 1) and the B3LYP/6-311G(d) approach (method 2) for geometry optimizations. In addition, single point calculations at the B3LYP/CEP-121+G(d,p) (on geometries optimized by method 1) and at the B3LYP/6-311+G(d,p) (on optimized structures from methods 1 and 2) levels were also carried out. The augmented CEP-121G scheme is roughly 1.4 to 2.0 times computationally cheaper than full electron calculations, and is able to reproduce structures, infrared frequencies, thermochemical parameters and dipole moments predicted by the all-electron basis set employed in this study. From a practical viewpoint, the results confirm that ECPs can be used in combination with density functional methods, despite the fact that the former are typically generated via a Hartree-Fock approach.Key words: CEP, Effective Core Potentials, alkoxides, DFT, Ti.