bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation of the hydrogénation catalytic activity of dinuclear complexes of rhodium(I) with the formation of carbonyl-bridged complexes
Authors: Sanger Alan R.
Published: May 2006

Methanolic solutions of the dinuclear cationic complexes of rhodium(I), [Rh2(μ-X)(CO)2(Ph2ECH2EPh2)2]+ (X = Cl, Br; E = P, As) and iridium(I), [Ir2Cl(CO)3(Ph2PCH2PPh2)2], are each active catalysts for the hydrogenation of alkynes to alkenes, and alkenes to alkanes. Neutral complexes with pseudohalide ligands have also been investigated but only the cyano-complex [Rh2(CN)2(CO)2(Ph2PCH2PPh2)2], and the arsine analog, are of significant catalytic activity. Only those complexes which reversibly form bridged complexes with ligands such as CO are also catalytically active.Similar complexes with dithioether ligands are not efficient catalysts under the conditions tested.
Isolation and structure of combretastatin
Authors: Pettit George R. , Cragg Gordon M. , Herald Delbert L. , Schmidt Jean M. , Lohavanijaya Prasert
Published: May 2006

The South African tree Combretumcaffrum has been shown to contain a constituent capable of significantly reversing astrocyte formation employing the National Cancer Institute's 9ASK system. The constituent responsible for astrocyte reversal was isolated and designated combretastatin (1). Structural elucidation was initiated employing spectral methods and completed by X-ray crystallographic analysis. By this means combretastatin was assigned structure 1. Further biological evaluation and a total synthesis are now in progress.
Radical reactions in aqueous solution: use of the aci-anion of nitromethane as a spin trap
Authors: Gilbert Bruce C. , Norman Richard O. C.
Published: May 2006

The type of radical detected by esr when first-formed short-lived radicals are generated by photolytic, radiolytic, or flow techniques in the presence of nitromethane is shown to depend upon the nature and structure of the transient radical and on the pH. Thus electron-donating radicals may react with CH3NO2 itself under certain conditions to give rise to CH3NO2−• and the alkoxy nitroxides CH3N(OR)O•. At higher pH, a wide variety of radicals (X) react very readily with the aci-anion CH2:NO2− to give adducts XCH2NO2−• (and, from radicals capable of one-electron transfer, O2NCH2CH2NO2−•). Structural characteristics of adducts XCH2NO2−• are described and it is shown how radicals which are normally undetectable in fluid solution can be trapped and recognized in this way. Mechanistic pathways diagnosed with this trap (involving, e.g. alkoxyl radicals, aroyloxyl radicals, aromatic carboxylate cation-radicals) are exemplified.
Metal-nitroxyl interactions. 25. Exchange interactions via saturated linkages in spin-labeled porphyrins
Authors: More Kundalika M. , Eaton Gareth R. , Eaton Sandra S.
Published: May 2006

A series of spin-labeled copper porphyrins has been prepared in which nitroxyl radicals are attached to a porphyrin pyrrole ring by propionamide and propionate ester linkages. The solution epr spectra at room temperature indicate that the values of the electron spin – electron spin coupling constant, J, for these compounds are in the range 4 to 18 G, depending on the solvent and the size of the nitroxyl ring. Below −45 °C two isomers with different values of J were observed in the epr spectra of two of the compounds. Variable temperature epr spectra were also analyzed for analogous spin-labeled copper porphyrins which have cis and trans acrylic ester and acrylamide linkages between the porphyrin and the nitroxyl ring. Comparison of the values of J for the complexes with saturated and unsaturated linkages suggests that σ-bonding pathways dominate the copper–nitroxyl interaction in these compounds.
Nitroxides. IX. Synthesis of nitroxide free radical α-amino acids
Authors: Lex László , Hideg Kálmán , Hankovszky H. Olga
Published: May 2006

Three methods are described for preparation of a new stable nitroxide free radical amino acid, 2-amino-3-(1-oxyl-2,2,5,5-tetramethyl-3-pyrrolin-3-yl) propionic acid and its derivates. This amino acid may be used as a paramagnetic amino acid synthon in studies of peptides.
Nitroxide destruction and flavin-photosensitized damage in inner mitochondrial membranes
Authors: Mehlhorn Rolf Joachim , Packer Lester
Published: May 2006

Nitroxide free radicals lost their paramagnetic absorption spectrum when they were illuminated with visible light in aged but not freshly isolated inner mitochondrial membranes (SMP's). The action spectrum of spin loss rates coincided with a flavin absorption spectrum. Spin loss consisted of one-electron reduction and "destruction", the latter being defined as spin loss that cannot be reversed by ferricyanide oxidation. By placing SMP's in gas permeable tubing and illuminating alternately under nitrogen and air, it was possible to discriminate qualitatively between spin reduction and destruction. Aerobic spin loss consisted entirely of destruction. When aged SMP's were centrifuged, spin loss was observed in supernatants but not pellets. Flavin fluorescence was observed in the supernatants, suggesting that free flavins catalyzed spin loss. However, addition of exogenous flavins to fresh SMP's did not cause spin loss; hence some other factor was required to cause nitroxide destruction. This factor accumulates during either aerobic or anaerobic aging of SMP's and may be present in mitochondria. Supernatants of aged SMP's, when added to freshly isolated SMP's, induced rapid nitroxide destruction, slightly accelerated photodamage to succinate oxidase, and considerably increased photo-induced lipid peroxidation.
Electron spin resonance of spin trapped radicals in γ-irradiated polycrystalline dipeptides. Chromatographic separation of radicals
Authors: Makino Keisuke , Riesz Peter
Published: May 2006

Polycrystalline dipeptides (glycyl-glycine, glycyl-L-valine, glycyl-L-leucine, L-alanyl-glycine, and L-prolyl-L-alanine) were γ-irradiated at room temperature in the absence of air. Subsequently they were dissolved in aqueous solutions containing 2-methyl-2-nitrosopropane as the spin trap. From the esr spectra of the nitroxide radicals separated by high-performance liquid chromatography, structural assignments of the radicals were made. For glycyl peptides, H-abstraction from the α-carbon atoms of the carboxyl terminal residues and from the side-chains were observed. For L-alanyl-glycine, H-abstraction from the glycyl residue and the formation of the deamination radical could be shown to occur. For L-prolyl-L-alanine, the ring opening (deamination) reaction, decarboxylation and H-abstraction from the C-terminal a-carbon were seen.
Do spin-adducts of 5,5-dimethylpyrroline-N-oxide dimerize?
Authors: Castelhano A. L. , Griller D. , Ingold K. U.
Published: May 2006

The conclusion by Bullock etal. that aminoxyls formed by radical addition to 5,5-dimethylpyrroline-N-oxide are extensively dimerized in aqueous solution is shown to be incorrect. Aminoxyl dimerization is not a serious limitation to the use of this spin trap in quantitative studies of radical production.
Synthesis of new radical-cations with 2,2,6,6-tetramethylpiperidine-N-oxyl and pyridinium moieties
Authors: Dragutzan Ileana , Balaban Alexandru T.
Published: May 2006

Two new radical-cations (as crystalline perchlorates) 1 and 2 were synthesized from the respective amines 3 and 4, and 2,4,6-trimethylpyrylium perchlorate 5. The properties and spectra of the new radical-cation perchlorates (which may be recrystallized from ethanol or from water) are described.
Synthesis and spin trapping kinetics of new alkyl substituted cyclic nitrones
Authors: Haire D. L. , Janzen Edward G.
Published: May 2006

Three new nitrones, 5-n-propyl-5-methyl-1-pyrroline-N-oxide (PMPO), 5-n-hexyl-5-methyl-1-pyrroline-N-oxide (HMPO), and 5-n-decyl-5-methyl-1-pyrroline-N-oxide (DeMPO), amphiphilic versions of the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), have been synthesized. A comparative study of the absolute rate constants for formation and decay of the tert-butoxyl spin adducts to these cyclic nitrones as well as 3,3,5,5-tetramethyl-1-pyrroline-N-oxide (M4PO) has been undertaken using esr kinetic techniques.
Specific adsorption of sulfamate ions at the mercury/solution interface
Authors: Gonzalez Ernesto R.
Published: May 2006

The adsorption of sulfamate ions on mercury electrodes has been studied by measuring the double layer capacity in constant ionic strength solutions of composition xM NH4SO3NH2 + (1−x)M NH4F. It was found that sulfamate ions are weakly adsorbed on mercury, the amount adsorbed being significant only for x > 0.1 at positive charges on the electrode. Because of this, the properties of the inner layer were found to be strongly dependent on the adsorbed water molecules and the structure of the diffuse layer. It was determined that the adsorption of sulfamate ions can be described by an isotherm based on the electrostatic model of the double layer. The possible effects of the activity coefficient variation with solution composition are discussed.
Photorearrangement of 2-cyanobicyclo[2.2.1]hept-2-ene. Observation of 1,2- and 1,3-sigmatropic shifts
Authors: Akhtar Ikbal A. , McCullough John J. , Vaitekunas Susan , Faggiani Romolo , Lock Colin J. L.
Published: May 2006

Irradiation of 2-cyanobicyclo[2.2.1]hept-2-ene (2-cyanonorbornene, 4) in hexane, with the full arc of a mercury vapour lamp, gives the rearrangement products 1-cyanobicyclo[4.1.0]hept-2-ene 5 and 7-cyanotricyclo[4.1.0.03.7]heptane 6 in the ratio 20:1. These products were separated by preparative vpc. The structure of the major product 5 was determined by single crystal X-ray analysis. Reduction of 5 with lithium aluminum hydride gave the corresponding primary amine, which was converted to the p-bromobenzenesulfonamide 9, mp 150–151 °C, which gave single crystals from ethanol–water. The crystal and molecular structures are described. The minor product 6 was hydrogenated to give 7-cyanobicyclo[2.2. 1]heptane. Formation of 5 and 6 may involve concerted σ2s + π2s and σ2a + π2a processes respectively, which are photochemically allowed.
Electrochemical oxidation of trifluoroacetic acid anion. V. Formation of bis(trifluoromethyl) alicyclic compounds from activated dienes
Authors: Renaud Roger N. , Stephens Campbell J. , Bérubé Denis
Published: May 2006

The electrochemical oxidation of trifluoroacetate anion in the presence of activated dienes, ROOCCH=CH(CH2)nCH=CHCOOR, gave bis(trifluoromethyl) alicyclic products. The ring size of the products was determined by comparing the fluorine nuclear magnetic resonance spectra of the protonated with the corresponding α,α′-dideuterated compounds. The results can be explained by an intramolecular radical addition and by an intramolecular coupling of a biradical intermediate.
The reinvestigation of the kinetics of the reactions of fluorodi(4-nitrophenyl)ethanes with sodium methoxide in methanol
Authors: Leffek Kenneth T. , Schroeder Grzegorz
Published: May 2006

The procedure previously described for the preparation of 1-fluoro-2,2,-di(4-nitrophenyl)ethane actually yields 1,1,2-tri-(4-nitrophenyl)ethane. 1-Fluoro-2,2-di(4-nitrophenyl)ethane has been prepared and rate constants, isotope effects, and activation parameters for the β-elimination reaction with methoxide ion in methanol are reported. These parameters indicate a concerted E2 mechanism, with a fairly symmetrical transition state. The subsequent dimerization reaction of the olefin product to yield 1,1,3,3-tetra(4-nitrophenyl)butene-1 is described.The reaction of 1,1,1-trifluoro-2,2-di(4-nitrophenyl)ethane with methoxide ion in methanol has been reinvestigated and the reaction of the first product 1,1-difluoro-2,2-di(4-nitrophenyl)ethylene with excess methoxide, to give di(4-nitrophenyl)ketene dimethylacetal in a multistep reaction, is reported.
Étude des bases de Schiff dérivées de la salicylaldehyde et de la 2-aminopyridine. Propriétés et capacité de complexation
Authors: Capitán-Vallvey L. F. , Salinas F. , Jiménez C. , Cuadros L.
Published: May 2006

The spectroscopic (ir, uv, nmr, and ms) and thermal properties of N-(4-methyl-2-pyridyl)benzaldimine, N-(6-methyl-2-pyridyl)benzaldimine, N-(4-methyl-2-pyridyl)salicylaldimine and N-(6-methyl-2-pyridyl)salicylaldimine have been studied. Their acidity constants are discussed in relationship to their stereochemistry. Finally, reaction of these compounds with metal salts has been studied in a qualitative manner.

[Journal Translation]
An infrared study of the photoisomerization of N-benzylideneaniline
Authors: Lewis J. W. , Sandorfy C.
Published: May 2006

The infrared spectra of Nujol solutions and thin polycrystalline films of N-benzylideneaniline have been investigated. The 313 nm irradiation of ambient temperature samples produces no observable changes in the infrared spectra, however, similar irradiation of samples maintained at 77 K results in spectral changes that are thermally and photochemically reversible. The infrared spectra of low temperature irradiated samples are interpreted in terms of trans → cis photoisomerization.
A spectroscopic study of the solvent dependent processes of the anils of benzaldehyde and salicylaldehyde
Authors: Lewis Jerry W. , Sandorfy Camille
Published: May 2006

The ultraviolet–visible, infrared, and Raman spectral characteristics of some anils of benzaldehyde and salicylaldehyde in several solvents have been investigated. The ultraviolet–visible absorption data indicate that in solvents with a proton donating ability less than or equal to trifluoroethanol a single equilibrium exists in solutions of N-(2-hydroxybenzylidene)aniline, whereas in solvents with a proton donating ability equal to or greater than hexafluoroisopropanol more than one equilibrium occurs. The infrared spectra of this compound dissolved in trifluoroethanol do not show new absorption bands in the 1700–1500 cm−1 region; however, new absorption bands in this region do appear when hexafluoroisopropanol is employed as solvent. From these data it is inferred that the first equilibrium involves the breaking of the chelate hydrogen bond and the second equilibrium involves actual protonation of N-(2-hydroxybenzylidene)aniline in the electronic ground state. A comparison of the infrared and Raman spectra of the title compounds is also made and alternate assignments for several observed bands in the 1700–1500 cm−1 region are proposed.
Osmotic and activity coefficients of sodium sulphate in water from 150 to 250 °C
Authors: Bhatnagar Om N. , Campbell A. N.
Published: May 2006

Osmotic and activity coefficients of sodium sulphate in aqueous solution have been determined at temperatures up to 250 °C, at solute concentration from 0.3 m to saturation, by measurements of vapour pressure using a newly designed high pressure cell. A precision pressure transducer is used in this cell for pressure measurement. Certain thermodynamic quantities have been calculated from the activity coefficients.
Quenching of the singlet and triplet state of benzene by halogenated alkanes in the vapor phase
Authors: Khwaja H. A. , Semeluk G. P. , Unger I.
Published: May 2006

The second order rate constants (kq) for the quenching of the singlet state of benzene by several halogenated alkanes (viz. CH2Br2, CCl4, CH3CCl3, CHCl3, CHF3, CH3Cl, CH3F, CH2F2, CH2Cl2, CFCl3, CF2Cl2, CF3Cl, CF4) in the vapor phase have been measured. For CCl4, CHCl3, CH2Cl2, and CH3Cl, which were previously studied by Das Gupta and Phillips, there is satisfactory agreement between the kq values obtained in the separate investigations. The linear dependence of the electron affinity of quencher with the corresponding quenching rate constants supports an exciplex mechanism in accordance with Klein's model. Charge transfer occurs from benzene to halogenated alkane and is greater for the chloro alkanes than for mixed fluoro–chloro alkanes. In the case of the triplet state of benzene, whose behaviour was monitored using the sensitized emission of biacetyl technique, quenching effects were at best one order of magnitude smaller and probably involve more than one bimolecular process.
Synthesis and spectroscopic study of aminals derived from 8-aminoquinoline
Authors: Bellanato Juana , Galvez Enrique , Escobar Rosario , Cano-Pavon Jose Manuel
Published: May 2006

Treating 8-aminoquinoline with di-2-pyridyl ketone, pyridine-2-carboxaldehyde, or 6-methylpyridine-2-carboxaldehyde leads to the formation of the corresponding aminal instead of the expected Schiff base. The ir, 1H nmr, and mass spectra of these compounds are described and discussed.Infrared and 1H nmr analyses indicate that these aminals enjoy intramolecular hydrogen bonding. This effect is considered to contribute significantly to their unusual stability.
Asymmetric homogeneous hydrogenation of olefins catalyzed by alkylphosphine complexes of rhodium(I)
Authors: Cullen William R. , Woollins J. Derek
Published: May 2006

The cationic rhodium(I) complexes of the optically active ferrocenes, C5H5FeC5H3[CH(CH3)N(CH3)2][ER2]-1,2, catalyze the asymmetric hydrogenation of acetamidoacrylic acid derivatives, itaconic acid and styrene, when ER2 is P(C6H5)2 or P(C(CH3)3)2. The configuration of the product is reversed on substituting C6H5 for C(CH3)3 groups. The catalyst with the tert-butyl groups can afford higher optical yields and is faster overall. The rhodium(I) complexes of the arsenic derivatives ER2 = As(C6H5)2 or As(CH3)2 do not catalyze the hydrogenation reactions. The nmr spectrum of the arsenic complex (ER2 = As(C6H5)2) is temperature dependent which seems to be due to a process involving the making and breaking of the Rh—N bond. The unsuccessful use of the MEM group (β-methoxyethoxymethyl) to protect an alcohol function α to a ferrocene ring is described. The hydrogenation results are discussed in the light of models which are currently used to predict the results of rhodium(I) catalyzed reactions but which are inapplicable to this work.
The mass spectra of pyrimidines: 2(1H)-pyrimidinone and some N(1)-substituted derivatives
Authors: Kazdan Eva M. , Rye Robin T. B. , Tee Oswald S.
Published: May 2006

The EI induced fragmentation of 2-pyrimidinone (1) and several N(1)-substituted derivatives has been studied. Principal fragmentation pathways have been identified using 2H labelling, metastable defocussing, and exact mass measurements.The fragmentation of 1 parallels that of cytosine. The hydrogen atom at C(4), and not the tautomeric hydrogen, is involved in the formation of the prominent [M – H]+ peak in the spectrum of 1; subsequent fragmentations of the [M – H]+ moiety contribute significantly to the spectrum.Side chain eliminations predominate in the mass spectra of the N(1)-substituted derivatives, but peaks characteristic of the pyrimidinone nucleus are observable in every case. Alternative structures for the [M – H]+ entity formed by H• loss from the substituent are proposed for the N-phenyl and N-benzyl compounds.
Effect of alkylation of ammonia on the optical band shape of solvated electrons
Authors: Jou Fang-Yuan , Freeman Gordon R.
Published: May 2006

At 200 K the width at half height, W1/2, of the e−solv optical absorption band in n-propyl amine is 2.1-fold greater than that in ammonia. Three quarters of the broadening occurs on the high energy side of the band. The energy Er at half height on the low energy side of the band is nearly the same in the amine as in ammonia, while Eb, the energy at half height on the high energy side, is 42% greater in the amine. The temperature coefficient dEAmax/dT is 1.8-fold greater in the amine than in ammonia. The larger width is consistent with there being a less uniform distribution of localization sites in the system, and the larger temperature coefficient implies that the sites are more easily disturbed by thermal agitation. A quantum statistical mechanical model, such as the one begun by Simons, is needed to extend the theoretical treatment of e−solv spectra. The correlation between optical absorption energies of e−solv and the structure of the solvent, as partially reflected in the Kirkwood correlation factor, is re-emphasized.
Spectroscopic and chemical reactivity manifestations of strong hydrogen bonding: HCNH proton–proton coupling and alkylation reactions of amines
Authors: Clark James H. , Pile James D. , Miller Jack M. , Paone Sergio , Tan Shwe Y.
Published: May 2006

Two useful but quite different manifestations of strong hydrogen bonding in systems containing NH groups are reported and discussed. The pKa limit for observation of proton–proton HNCH spin-spin coupling in N-methylanilines is shown to be related to the strength of added H-bond electron donors so that Ph3PO, Ph3AsO, and F− in particular are useful reagents for resolving this type of coupling. The mechanism of hydrogen exchange between anilines and water and the role of the added electron donors are discussed. Strong H-bonding between anilines or benzylamines and F− causes pronounced changes in the chemical reactivity of the amine. Results of experiments on the alkylation of these amines using potassium and tetra-n-butylammonium fluorides demonstrate the synthetic utility of this effect.
The chemistry of thujone. V. Thujone as a chiral synthon for the preparation of sesquiterpenes. Synthesis of (+)-carissone
Authors: Kutney James P. , Singh Ashok K.
Published: May 2006

An efficient synthesis of (+)-carissone (4) from (+)-β-cyperone (2) is described. The latter sesquiterpene is readily available from thujone (1), via a previously described synthesis.
Single crystal Raman spectra of an organic semiconductor: KTCNQ
Authors: Bandrauk André D. , Truong K. D. , Jandl S.
Published: May 2006

Raman spectra are reported for single crystals of the organic salt KTCNQ (potassium tetracyanoquinodimethane) and its deuterated analogue. Intensities of intramolecular and intermolecular (phonons) vibrations exhibit strong variations at the phase transition temperature associated with a dimmer–monomer transition. A simple model is presented to interpret and correlate these results with previously observed ir intensity anomalies in this and other organic semiconductors.
Scaled particle theory of gas solubility — inclusion of the temperature dependent hard sphere term
Authors: Cosgrove Bruce A. , Walkley John
Published: May 2006

The limiting behaviour of the scaled particle theory (spt) of gas solubility has been examined for the inert gases in a range of solvents. The hard sphere limit is shown to exhibit thermodynamic self-consistency only with the inclusion of an effective hard sphere diameter temperature dependence whereas the original spt (exclusion of a temperature dependence) fails to extrapolate to the correct hard sphere limit. Furthermore, inclusion of the temperature dependence yields correct thermodynamic properties for gases of small atomic diameter (and well depth potential) as would be expected from a hard sphere based theory.
Syntheses of N-substituted 3,3,4-triphenyl-4-benzoylazetidin-2-ones and 3′,3′-diphenyl spiro(acenaphthen-1-one-2,4′-azetidin-2′-ones); novel examples exhibiting a high reactivity of imino group in ketoimines
Authors: Singh S. B. , Mehrotra K. N.
Published: May 2006

The reaction of 2-diazo-1,2-diphenylethanone (1) with 2-imino-1,2-diphenylethanones 2a–e gave new N-substituted 3,3,4-triphenyl-4-benzoylazetidin-2-ones 3a–e, together with 1,1′,2,2′-tetraphenyl-2,2′-azinodiethanone (4). Similar reaction of 1 with 2-iminoacenaphthenones 6a, b yielded N-substituted 3′,3′-diphenyl spiro(acenaphthen-1-one-2,4′-azetidin-2′-ones) 7a, b and ketazine 4. Lithium aluminium hydride reduction of azetidinones 3a, b gave 4-α-hydroxybenzylazetidin-2-ones 5a, b. The spiro azetidinones 7a, b on reduction with sodium borohydride yielded spiro(1-hydroxyacenaphthene-2,4′-azetidin-2-ones) 8a, b. The azetidinones 3a–e and 7a, b were found to be unaffected by either acid or base hydrolysis. The marked selective reactivity of the imino group as compared to the carbonyl group towards diphenylketene has been observed in the present studies. The comparative reactivity of carbonyl groups present in azetidinones 3a, b and 7a, b has been presented.
Hydrogen bonding of O—H and C—H hydrogen donors to Cl−. Results from mass spectrometric measurements of the ion–molecule equilibria RH + Cl− = RHCl−
Authors: French M. A. , Ikuta S. , Kebarle P.
Published: May 2006

Equilibrium constants K1 for reaction [1] RH + Cl− = RHCl− in the gas phase were measured with a high pressure mass spectrometer under chemical ionization conditions. Data for some 40 compounds RH are presented. It is found that the binding free energies [Formula: see text] for RH = oxygen acids increase with the gas phase acidity of RH. The strongest bonds are formed with strong acids like HCO2H, CH3CO2H, and phenol. Water and alkyl alcohols give much weaker interactions. A simple relationship between gas phase acidity and binding free energy does not occur for RH = carbon acids. Carbon acids like cyclopentadiene, whose high gas phase acidity is largely due to charge derealization by conjugation in the completed anion, do not give Cl− adducts with stability commensurate with the acidity. A relationship between gas phase acidity and binding energy is found for carbon acids with carbonyl groups and for the substituted toluenes. Molecular orbital calculations with the STO-3G basis set provide insights to the bonding occurring in RHCl−. For all cases investigated, hydrogen bonding to Cl− provides the most stable structure. Generally the hydrogen bond occurs through the hydrogen which has the highest net positive charge. The hydrogen bond strength is found approximately proportional to this positive charge. Another proportionality is found between the charge transferred from Cl− to RH, on formation of RHCl−, and the strength of the hydrogen bond.
Infrared spectra of the ammonium ion in crystals. Part XII. Low-temperature transitions in ammonium dichromate, (NH4)2Cr2O7
Authors: Keresztury Gábor , Knop Osvald , Falk Michael
Published: May 2006

Examination of the infrared spectra of the probe ions NH3D+ and NHD3+ in ammonium dichromate confirms the existence of the lowest (Ttr ~ 125 K) of the three transitions that are known, from nonspectroscopic evidence, to occur in this crystal below room temperature. Below Ttr the ammonium ions are of two types, in an abundance ratio of 1:1 and both of symmetry C1. Above Ttr the probe ion spectra are difficult to interpret in detail. The strength of the hydrogen-bonding interactions covers a wide range, as indicated by the difference between the highest and the lowest values of the isotopically isolated ND stretching frequencies at 10 K, 2392 and 2234 cm−1.
Xanthocyclines: 5-oxaanthracyclines (7,8,9,10-tetrahydrobenzo[b]xanthen-12-one)
Authors: Charlton James L. , Sayeed Vilayat A. , Lypka Gerald N.
Published: May 2006

The anthracycline antibiotics daunorubicin and adriamycin are potent antitumor agents. It has recently been suggested that 5-oxaanthracycline analogs of these compounds might show reduced cardiotoxicity, a property that limits the clinical use of the anthracyclines. In this paper we describe the condensation–cyclization reactions of 2-methylchromone-3-carboxylate with cyclohexanones, a reaction that leads ultimately to the xanthocycline skeleton.
Enzymes in organic synthesis. 26.Synthesis of enantiomerically pure grandisol from an enzyme-generated chiral synthon
Authors: Jones J. Bryan , Finch Mark A. W. , Jakovac Ignac J.
Published: May 2006

The synthesis of enantiomerically pure (+)-grandisol has been achieved from an enzymically generated chiral synthon. The synthesis is an efficient one, the yield of optically pure material being 12% up to the penultimate step and 3% overall.
On the synthesis of cryptosporiopsin: stereochemistry of early intermediates
Authors: Strunz George M. , White Peter S.
Published: May 2006

The relative configurations of two intermediates in the synthetic route to cryptosporiopsin are clarified through the structure of 3,5-dichloro-1,4-dihydroxy-2-methylcyclopent-2-ene-1-carboxylic acid methyl ester, which was determined by single crystal X-ray techniques. The crystal was monoclinic, space group P21/c, a = 7.352(1), b = 8.868 (3), c = 16.300(7) Å; β = 96.49(2)°. The structure was solved by direct methods and refined to a final R of 0.057.
The molecular structure of 1-(2′-triphenylphosphonio-3′,3′,4′,4′-tetrafluorocyclo-butenyl) (3,3-difluorocyclobutene-2,4-dionate)•chloroform solvate
Authors: Einstein F. W. B. , Jones T.
Published: May 2006

The title compound, C26F15O2F6P(0.5CHCl3) (A), crystallizes as pale yellow crystals in the monoclinic system C2/c with a = 29.624(6), b = 10.224(3), c = 18.576(4) Å; β = 116.26(2)°, Z = 8. On the basis of 1652 observed reflections (I ≥ 2.3σ(I)) in the range 3° ≤ 2θ ≤ 42° the structure was solved and refined by least-squares methods to R = 0.042 and shown to be a zwitterion consisting of a phosphonium cation and a delocalised anion. The bond parameters of the cation are normal; P—C distances range from 1.773(5)–1.786(5) Å and C—P—C angles range from 106.4(2)–111.7(2)°. Within the delocalised anionic fragment of the molecule C—C distances range from 1.388(7)–1.440(8) Å and C—O distances are 1.216(7) and 1.208(7) Å.
Préparation stéréosélective de méthyle cétones α-bromées: stéréochimie de l'addition d'anions en α d'un groupe sulfone sur des cyclohexanones monocycliques et stéroïdiques
Authors: Bégué Jean-Pierre , Bonnet-Delpon Danièle , Charpentier-Morize Micheline , Sansoulet Jean
Published: May 2006

This work describes stereochemical results obtained for the preparation of α-bromomethyl ketones or α-bromoaldehydes starting from monocyclic and steroid cyclohexanones, using the Durst method. In all the cases studied here, the bromine is introduced selectively in the equatorial position. However, for monocyclic compounds, the yields of α-bromocarbonyl compounds are limited by the competitive formation of α-ethylenic carbonyl compounds.
Formation of acid–water hydrogen bonds with large proton polarizability and molecular processes with dissociation of acids in the pKa range 0–4
Authors: Leuchs Martin , Zundel Georg
Published: May 2006

Aqueous solutions of fourteen acids in the pKa region 0–4 were studied by ir spectroscopy as a function of concentration. All data are summarized in a table. The degree of dissociation was determined from bands of the anions. The removal of the protons from the anions decreases in the following series of the acids CF3COOH, H2CrO4, (HIO3), CF2HCOOH, HPO(OH)2, HSO4−, (HSeO4−), H2POOH, PO(OH)3, AsO(OH)3 (Table 1, columns 3 and 4). Continua indicate the formation of hydrogen bonds with large proton polarizabilities. In the case of H3PO3, H3PO4, and H3AsO4, the acid–acid hydrogen bonds are still polarizable although they have relatively asymmetrical proton potentials. The acid–water hydrogen bonds (I) [Formula: see text] (II) may show considerable proton polarizability, as indicated by strong continuous absorptions. The proton potential, and thus the proton polarizability of the acid–water hydrogen bonds, are discussed taking into consideration the intensity and the wave number range of the continuum (column 7), as well as the line width of the vibration with δ OH character (column 9). With increasing asymmetry of the hydrogen bonds, the continuum changes to the bands observed with asymmetrical hydrogen bonds. The sequence of acids in the series arranged according to the degree of dissociation is not the same as the sequence according to the intensity of the continuum and proton polarizability. This is explained by the fact that the proton polarizability is mainly determined by the shape of the proton potential, whereas the dissociation equilibrium is also influenced by entropy effects associated with hydrate structure formation.It is shown that the following molecular processes are of significance for the dissociation of the acids: the formation of acid–water hydrogen bonds with proton polarizability, whereby the weight of the polar proton-limiting structure [Formula: see text] increases with increasing addition of water molecules. Besides the shape of the proton potential, the entropy effects mentioned influence the degree of dissociation, too. Finally, the positive charge transfers from the acid–water into water–water hydrogen bonds, i.e., H5O2+ is formed.
Thermochemical study of adducts of methylurea with zinc, cadmium, and mercury halides
Authors: Airoldi Claudio , Chagas Aécio P. , Assunção Francisco P.
Published: May 2006

The adducts Zn(mu)2X2 (X = Cl, Br), M(mu)X, (M = Cd, Hg and X = Cl, Br) have been characterized. The CO and NH stretching frequencies indicate that methylurea (mu) is bound to metals through oxygen. Enthalpies of dissolution of the adducts in ethanol or methanol have been measured calorimetrically. Combination with the enthalpies of solution of metal halides and ligand in the same solvents at 298.15 K yielded the standard enthalpy [Formula: see text] for the reaction: MX,(s) + nmu(s) → M(mu)nX2, (s): Zn(mu)2Cl2, (−36.27±0.51); Zn(mu)2Br2, (−37.52±0.97); Cd(mu)Cl2, (−22.78±0.26); Cd(mu)Br2, (−9.58±0.26); Hg(mu)Cl2, (−13.91±0.41);and Hg(mu)Br2,(−13.29±0.48). The standard enthalpies of formation of the adducts in the same sequence as above, [Formula: see text] (s, 298.15 K): −1024, −938, −701, −612, −524, and −470 kJ mol−1, were determined from these values, the standard enthalpy of the ligand (−285 ± 15 kJ mol−1) being obtained via combustion calorimetry, and literature values of the standard enthalpies of metal halides being used. With the determination of the standard enthalpy of sublimation of the ligand (78.2 kJ mol−1) it was possible to determine the standard enthalpies of the reactions MX2(g) + nmu(g) → M(mu)X2 (s) and M(mu)nX2 (s) → MX2 (s) + nmu(g). The enthalpies of metal–oxygen bonds were also estimated.
Thermochemical parameters for organic radicals and radical ions. Part 1. The estimation of the pKa of radical cations based on thermochemical calculations
Authors: Nicholas A. Martin de P. , Arnold Donald R.
Published: May 2006

Three methods for estimating the pKa of a radical cation using thermochemical cycles are discussed. These methods enable the calculations of the pKa of radical cations which have not been determined experimentally. Radical cation acids are classified according to the nature of their respective conjugate bases. The application of these methods is illustrated for a σ-acid (the benzene radical cation) and π-acid (the toluene radical cation). The potential of these methods to estimate relative and absolute solvation energies is also discussed. The use of thermochemical cycles in estimating standard oxidation potentials of organic compounds is outlined.Calculations show that the toluene radical cation is an extremely strong acid and the benzene radical cation a moderately strong acid in acetonitrile solution. Difficulties in directly determining the pKa value of aromatic hydrocarbon radical cations are also discussed.
Carbon-13/carbon-12 isotope effects on nitrogen-15 chemical shielding in some diamagnetic cyano complexes
Authors: Wasylishen Roderick E.
Published: May 2006

Carbon-13/carbon-12 isotope-induced 15N chemical shifts of 0.06 to 0.10 ppm have been observed for the cyanide ion and several transition metal cyanides. In each case, the 15N resonance of the 13C labelled isotopomers is to high field of that in the corresponding carbon-12 species. The sensitivity of the 15N and 13C shielding constants in the cyanide ion to small changes in the CN bond length are evaluated using the theoretical formulation of Jameson: (∂σ(15N)/∂r)e = −872 ± 160 ppm/Å and (∂σ(13C)/∂r)e = −473 ± 90 ppm/Å. Nitrogen-15 chemical shifts in the metal complexes exhibit basically the same behaviour as do the 13C chemical shifts in these same complexes.
Detection limitations of photomultiplier tubes in pulsed-laser emission spectroscopy: time-resolved fluorescence spectra of chlorophyll solutions
Authors: de Wilton A. , Haley L. V. , Koningstein J. A.
Published: May 2006

An experimental method is described for determination of the onset of saturation effects on the performance of photomultiplier tubes (PMTs) used in time-resolved pulsed-laser spectroscopy. Changes in the dark level response in time (on the 0–100 ns time scale) of the PMT were observed to indicate saturation of the tube. At low repetition rates conditions were found for which an RCA 8850 PMT and an RCA 7102 PMT can be used to obtain reliable fluorescence decays on the 0–100 ns time scale while operating at high anode currents, without modification of interdynode voltages.Results for time-resolved fluorescence studies of Chl a in solution, in benzene and in pyridine, are discussed. The fluorescence decays at ~680 nm indicate that several Chi a species are present in each solution. In pyridine, the main fluorescence process at 677 nm, characterised by a lifetime of ~7 ns, is assigned as arising from ligated monomeric species of Chi a. A second longer-lived and weaker component (τ = 75 ns) is assigned to the ligated monomeric species of Chl a′. A third, and even weaker process characterised by a lifetime τ = 145 ns is also observed. The time-resolved fluorescence spectral band shape of this component has a unique narrow profile, and is assigned to a dimeric or more highly aggregated species of Chl a. The time-resolved measurements also indicate that another emission band occurs at 740 nm, and that this process has a lifetime 75 ns < τ < 145 ns.
Synthesis of metal-chelates of amino sugars: Schiff's base complexes
Authors: Adam Michael J. , Hall Laurance D.
Published: May 2006

Metal-complexing, salicylaldimine ligands 4, 7, 8, and 9 have been synthesized from 2-amino-2-deoxy-D-glucopyranose and some of its derivatives with salicylaldehyde and 3-formyl-2-hydroxybenzoic acid. Copper(II), zinc, and cobalt(II) complexes were formed from ligand 7, whereas only copper(II) would complex with ligand 8. The sugar ligand 4 forms a water-soluble complex with copper(II) and ligand 9 forms a binuclear copper(II) complex. These products have been studied by 1H nmr, visible absorption, and esr spectroscopy, mass spectrometry, and by their magnetic moment data.
Synthèse de nucleosides pyrimidiques à partir de bromures de désoxy-4-pentopyranosyles non totalement protégés
Authors: Gagnieu Christian H. , Grouiller Annie V. , Pacheco Henri
Published: May 2006

The synthesis of nucleosides 15–19 has been completed in high yield following the method of Hilbert–Johnson, by condensation of 2,4-dimethoxypyrimidine with bromo-4-deoxypento pyranosides in which the C-3 hydroxyl group is free. The presence of this group augments the reactivity of the bromides but does not appear to play an important role in the stereochemistry of the reaction, which is above all influenced by steric factors and the nature of the C-2 substituent.

[Journal Translation]
Membrane transport systems. III. A mechanistic study of cation–proton coupled countertransport
Authors: Fyles Thomas M. , Malik-Diemer Virginia A. , McGavin Cynthia A. , Whitfield Dennis M.
Published: May 2006

The rate of extraction and transport of potassium ions from a basic aqueous solution into a chloroform membrane containing a macrocyclic polyether (crown ether) – carboxylic acid has been investigated as a function of the following variables: stirring rate, potassium ion concentration, crown ether concentration, proton concentration, buffer concentration and composition, and crown ether-to-potassium ion ratio. Two rate regimes are found, a "zero order" regime where transport rate depends only on the carrier concentration and a "consecutive first order" regime where transport rate depends on the concentration of a crown ether – potassium complex. The kinetic data, together with partition coefficient and interfacial surface tension data, indicate that the rate determining steps in ion extraction occur at the interface. A mechanism involving rate limiting adsorption of carrier or desorption of complex, depending on the experimental conditions, is shown to be consistent with all available data.
The electrophilic addition of dimethylbromosulfonium bromide to conjugated enones: efficient synthesis of α-bromo enones
Authors: Chow Yuan L. , Bakker Bert H.
Published: May 2006

Dimethylbromosulfonium bromide reacted readily with many enones at 0 °C or lower to precipitate a-bromo-β-sulfonium conjugated enones. These salts eliminate a proton and dimethylsulfide readily with aqueous potassium carbonate to give excellent yields of α-bromo conjugated enones cleanly. The mechanism of the addition was explained by the bromonium ion initiated 1,4-addition followed by tautomerization of enol hypobromites.
Spin–spin coupling constants as indicators of the conformation of anthrone in solution
Authors: Schaefer Ted , Sebastian Rudy , Wildman Timothy A. , Dettman Heather
Published: May 2006

The analyses of the 1H nmr spectra of fluorene, 2,7-dibromofluorene, and anthrone are reported. The benzylic couplings in these compounds are compared with those in indene and toluene. It is argued that the six-bond and five-bond couplings in anthrone are more compatible with a nonplanar than a planar central ring. The spectrum of the 2,7-dibromofluorene suggest that couplings between protons in different benzenoid rings are small, perhaps no larger than 0.05 Hz.
Electron spin resonance study of SO2 and CO on CuO–alumina
Authors: Khulbe K. C. , Mann R. S.
Published: May 2006

The intensity of the Cu2+ esr signal increased significantly on the addition of CO or SO2, especially with SO2, at room temperature to the preheated (at 400 °C) and evacuated CuO–alumina catalyst. When the CO or SO2 treated sample was evacuated at room temperature, the shape and intensity of the Cu2+ esr line returned to their original values. The intensity of the Cu2+ esr line decreased, but not to zero, on heating CuO–alumina either with CO or SO2. In the case of SO2, a new line appeared for the [Formula: see text] ion which was not affected on evacuation. It seems that the [Formula: see text] do not take part in the reduction of SO2 with CO over the CuO–alumina catalyst.
Complexes of propyl mercury thiocyanates with cobalt, nickel, and zinc thiocyanate
Authors: Singh P. P. , Singh V. P.
Published: May 2006

RHgSCN (R = n-C3H7, (iso-C3H7) react with M(NCS)2 (M = Co(II), Ni(II), Zn(II)) to form compounds of general formula (SCN)2M(NCSHgR)2. Conductance measurements and infrared and electronic spectral studies indicate that M and mercury atoms are linked by bridging thiocyanate groups. Cobalt and zinc are tetrahedrally coordinated, whereas nickel is octahedral. With the exception of zinc analogs, these compounds form complexes with Lewis bases. Pyridine and nicotinamide form adducts of general formula L2(SCN)2M(NCSHgR)2, while 2,2′-bipyridine and 1,10-phenanthroline form cationic–anionic complexes of formula [M(L—L)3][RHg(SCN)2]2.
Stereospecific and practical syntheses of both meso-cyclopentane-1,2,3-tricarboxylic acids
Authors: Camps Pelayo , Jaime Carlos
Published: May 2006

A stereospecific synthesis of both meso-cyclopentane-1,2,3-tricarboxylic acids, by using the 1,3-dimethoxytrimethylene bridge as two latent carboxyl groups, is described. A practical synthesis of these acids is also described.
The crystal structure of 4,7-oxido-7-methyl-7-hydroxymethyl-2,2,6,6-tetramethylpiperidin-1-oxyl
Authors: Cygler M.
Published: May 2006

4,7-Oxido-7-methyl-7-hydroxymethyl-2,2,6,6-tetramethylpiperidin-1-oxyl, C12H22NO3, crystallizes in the monoclinic space group P21/m with unit cell dimensions a = 8.041(1), b = 13.348(1), c = 5.945(1) Å, β = 95.90(1)°, Z = 2. Intensity data were measured on a diffractometer and the structure solved by direct methods. The least-squares refinement converged at R = 0.039 for 1304 reflections. Two enantiomeric molecules, differing in the position of the OH group, occupy the same site in the asymmetric unit (each with a probability of 0.5) giving rise to a disorder of the hydroxyl group. The piperidine ring adopts the usual chair conformation, with C—C—C—C torsion angles being intermediate between those observed for C(4) mono- and for bisubstituted nitroxypiperidine derivatives. The N—O bond is 1.282(2) Å long, and it makes an angle of 18.5(2)° with the CNC plane. Molecules are connected by O—H … O hydrogen bonds. The influence of the degree of substitution at C(4) on ring geometry is discussed.
A comparison of the relative binding energies of H+ and NO+ to aromatic and haloaromatic bases by high pressure mass spectrometry
Authors: Stone John A. , Splinter Dena E. , Kong Soon Yau
Published: May 2006

not available
BEST — A new program for rigorous calculation of equilibrium parameters of complex multicomponent systems
Authors: Motekaitis Ramunas j. , Martell Arthur E.
Published: May 2006

The FORTRAN program BEST for the refinement of stability constants from potentiometric data measured on complex systems containing any number of interacting components is described. The core algorithm consists of computation by successive approximations of the pH values at each equilibrium point from an initial set of stability constants based on the equilibrium model. Each approximation results in adjustment of the equilibrium constants being refined in order to improve the fit between the calculated and observed pH values for all the data taken for the system under investigation. Novel features of the algorithm include total generality, minimization of errors with respect to pH, and the avoidance of customary classification labels such as "metal ion", "ligand", "hydroxides", "proton", etc. Several examples of applications to real systems are provided to illustrate the versatility of the program.
Rearrangement studies with 14C. XLVII. The decomposition of chain labeled or ring labeled trianisylvinylphenyltriazene
Authors: Lee Choi Chuck , Obafemi Craig A.
Published: May 2006

Simultaneous reactions were carried out on the decomposition of the chain labeled trianisyl[2-14C]vinylphenyltriazene (1-NNNHPh-2-14C) and the ring labeled 1,2-dianisyl-2-p-[14C]methoxyphenylvinylphenyltriazene (1-NNNHPh-2-methoxy-14C) in HOAc, HOAc–NaOAc, or CF3CH2OH–HClO4. In HOAc, the presence of increasing amounts of NaOAc resulted in decreasing extents of isotopic scrambling arising from 1,2-anisyl shifts in the trianisylvinyl cation, in agreement with previous observations. For decompositions in the less nucleophilic TFE, complete equilibration was observed, giving rise to 50.0% and 33.3% scramblings, respectively, for the chain and ring labels. When the data were expressed as % rearrangement, with the assumption that 100% rearrangement would correspond to 50.0% or 33.3% scrambling, respectively, for the chain or ring label, for each experiment under a given set of conditions, the % rearrangement was found to be equal in the chain and in the ring label. These results are discussed in conjunction with earlier data from the acetolysis of doubly labeled 1,2-dianisyl-2-p-[14C]methoxyphenyl[2-13C]vinyl bromide (1-Br-2-13C-2-methoxy-14C) which gave, depending on experimental conditions, either equal or unequal values for the % rearrangement in the chain and in the ring label.
Kinetics of the intermolecular exchange reaction Cp3UIII–Cp3UIVCl
Authors: Burton M. , Folcher G. , Marquet-Ellis H. , Rigny P. , Giannotti C.
Published: May 2006

The intermolecular exchange of cyclopentadienyl groups in the system UCp3–UCp3Cl in THF solution between 170–300 K was investigated by 1H nmr. The kinetic results show a bimolecular mechanism and suggest an electron transfer via the chlorine atom. This first example of a symmetrical exchange in the UIII–UIV system illustrates the lability of actinide organometallic compounds.