bluuebunny/crossref_metadata_2025 huggingface dataset:
Ions in the terrestrial atmosphere and in interstellar clouds
Authors: Smith David , Spanel Patrik
Published: May 2005

Ions are formed in the terrestrial atmosphere (TA) and in interstellar clouds (ISC) by the action of solar/stellar UV and cosmic rays on the ambient neutral atmospheres. The precursor ions so formed develop largely via gas‐phase ion chemistry from the simple ions characteristic of the ambient atmospheres to the observed, more complex ions. In the upper TA and in ISC, only bimolecular ion chemistry occurs. In the tenuous upper TA, the ion types remain simple, O+, O, and NO+ being dominant, and similarly in the tenuous diffuse ISC, ions such as H+, H22+, C+, and CH+ are dominant. At the high pressures of the lower TA, termolecular association reactions are important, and the initial ions, e.g., O and N formed by cosmic ray ionization are quickly converted to very complex cluster ions of the type H+(H2O)n (bases)m with bases including NH3 and CH3CN, and a parallel negative ion chemistry develops from the primary negative ions O− and O producing ions like NO(H2O)n(acid)m with acids HNO3 and H2SO4. In the dense ISC, the ion chemistry proceeds to produce polyatomic ions from the most important initial ions C+, H, and CH, and the process of radiative association, the bimolecular analog of termolecular association, is important in producing, for example, ions such as CH M, where M are the common observed interstellar molecules such as the carboxy, cyano, and amino molecules, which are formed via this ion chemistry. The major ion neutralization process in the upper TA and in ISC is positive ion–electron recombination, which limits the degree of ionization in these regions, whereas in the lower TA, where negative ions balance the positive ions, the charge neutralization process is ion–ion recombinarion. © 1996 John Wiley & Sons, Inc.
Untersuchungen über den Einfluß von Prüfparametern und subjektiven Beurteilungsmaßstäben beim Nachweis von Rissen mittels Röntgendurchstrahlung
Authors: Echterhoff U.
Published: May 2005

Anhand eines praktischen Prüffalles werden die von der Bruchmechanik vorgegebenen Anforderungen an die Durchstrahlungsprüfung und deren Möglichkeiten zur Lösung der anstehenden Probleme diskutiert. Anforderungen der DIN 54 111 werden bei den Untersuchungen berücksichtigt und ihre Einflüsse auf die subjektiven Beurteilungskriterien beim Nachweis feiner Risse überprüft. Durch gezielte Variationen und kritische Einschränkung der Daten nach DIN 54 111 können bei der Durchstrahlungsprüfung ausreichende Auskünfte über das Vorhandensein von Fehlern gewonnen werden.
Investigations on Stress corrosion cracking of sensitized austenitic stainless steels and nickel‐base alloys in boiling aqueous solutions by potentiostatically controlled constant‐load experiments
Authors: Herbsleb Günter
Published: May 2005

The susceptibility of high alloyed materials to intergranular SCC is markedly influenced not only by material and environmental parameters, but also by the potential. The present paper describes in detail the influence of the potential on the susceptibility to intergranular SCC of these materials in aqueous solutions boiling under normal pressure by means of potentiostatically controlled constant load experiments. The following parameters have been varied:

Material

alloy composition: 18/10 non‐stabilized stainless steel; Incoloy 800; Inconel 600;

degree of sensitization;

pre‐damage by intergranular attack, caused in sulfuric acid/copper sulphate‐solution;

level of mechanical stresses.

Environment

sodium chloride concentration;

sodium sulphate concentration;

pH (buffer solutions of pH 2.2 and 4.5).

Potential

The test‐duration was up to several thousand hours.

The type of attack – intergranular corrosion, intergranular and/or transgranular cracking – has been established by metallografic investigations. The experimental results are presented in form of time‐to‐failure/potential curves with the above mentioned parameters. Conclusions for the in‐service behaviour of the materials investigated are derived.
Inhomogene Verformungserscheinungen bei Schwingbeanspruchung des Vergütungsstahls 42 CrMo 4
Authors: Eifler D. , Macherauch E.
Published: May 2005

Das Wechselverformungsverhalten von normalisiertem 42 CrMo 4 ist für Beanspruchungen σa < ReS durch Ent‐ und Verfestigungsvorgänge gekennzeichnet. Die Wechselentfestigung erfolgt inhomogen und ist mit der Ausbreitung sog. Ermüdungslüdersbänder verbunden, die anschließende Verfestigung erfolgt homogen.

Das Wechselverformungsverhalten von vergütetem 42 CrMo 4 ist für Beanspruchungen mit σa < Rp0,2 mit Entfestigungsvorgängen verbunden, die kontinuierlich in die Rißausbreitungsphase übergehen. Die während der Wechselverformung ablaufenden plastischen Deformationsvorgänge sind bis zum Probenbruch extrem inhomogen und auf sog. Ermüdungszonen beschränkt. Eine Erniedrigung der Anlaßtemperatur und der Spannungsamplitude verstärkt die Inhomogenität.
Entwicklung auf dem Gebiet der Lebensdauervorhersage an mehrachsig beanspruchten komplexen Bauteilen
Authors: Ott W. , Nowack H.
Published: May 2005

Im Hinblick auf die Erzielung höherer Ausnutzungsgrade technischer Strukturen ist das Vorliegen mehrachsiger Beanspruchungsbedingungen im besonderen Maße zu berücksichtigen. In der vorliegenden Arbeit wird eine Übersicht über im Schrifttum mitgeteilte Vorhersagemethoden für Belastungen in der Nähe der Dauerfestigkeit und im Zeitfestigkeitsbereich gegeben. Die Anwendung der meisten Methoden besonders für den Zeitfestigkeitsbereich ist an das Vorliegen von Beanspruchungsereignissen in Form geschlossener “Schwingspiele” gebunden. Diese Beschränkung wird mit Hilfe eines neuen Schadensberechnungsverfahrens auf der Grundlage der plastischen Arbeit überwunden. Es erfolgt eine kurze Beschreibung dieser auf beliebige dreidimensionale elastisch‐plastische Beanspruchungen anwendbaren Methode, die eine Finite Element Analyse in Verbindung mit dem Werkstoffmodell nach Mróz, weiterhin ein inkrementelles Schadensberechnungskonzept einschließt.
Einfluß der Laminatkomponenten von CFK‐Werkstoffen auf das statische und das dynamische Verhalten
Authors: Baron Ch. , Schulte K.
Published: May 2005

Der Einfluß auf das statische und das dynamische Verhalten eines konventionellen und eines modifizierten duktileren Matrixsystems wurde bei Verwendung zweier hochfester Kohlenstoffasertypen mit unterschiedlicher Bruchdehnung untersucht. Mit den jeweiligen Faser‐Matrix‐Kombinationen wurden drei verschiedene Laminate gebildet, ein unidirektionales Laminat [08], ein Kreuzlaminat [ 02, 902, 02, 902]sund ein annähernd quasi‐isotropes Laminat [02, ±45, 02, ±45,90]s. Es zeigt sich, daß die Verwendung eines duktilen Matrixsystems zu einer Verzögerung in der Schadensentwicklung führt. Dies wirkt sich dahingehend aus, daß sowohl die statischen als auch die dynamischen Eigenschaften verbessert werden. Bei der Verwendung des duktilen Matrixsystems zusammen mit der Faser höherer Bruchdehnung führt die Verbesserung der statischen Festigkeit nicht in gleichem Maße zu einer Verbesserung der Ermüdungsfestigkeit.
Einfluß einer Laseroberflächenhärtung auf die Dauerlestigkeit von glatten Proben mit kurzem Riß aus dem Stahl C70W2
Authors: Schädlich St. , Winderlich B. , Brenner B.
Published: May 2005

Die Laserhärtung als ein lokales Wärmebehandlungsverfahren von Stählen kann zur partiellen Wiederherstellung der Dauerfestigkeit anrißgeschädigter Bauteile eingesetzt werden. Die Untersuchungen werden an Flachproben aus dem Stahl C70W2 mit kurzem Anriß durchgeführt. Die durch die Laserhärtung erzielte Steigerung der Dauerfestigkeit gegenüber dem angerissenen Grundzustand von über 80% wird auf die Anhebung des Schwellwertes für Rißausbreitung infolge der martensitischen Umwandlungshärtung, die Wirkung von Eigenspannungen an der Rißfront und eine eventuelle Rißabstumpfung zurückgeführt.
The effect of temperature on selectivity in capillary SFC
Authors: Jones Brian A. , Shaw Timothy J. , Clark Jody
Published: May 2005

Retention indices (RI) for a variety of probe compounds were determined on 50‐μm i. d. columns coated with eight different stationary phases under density programmed SFC conditions at isothermal temperatures of 80 and 150°C. RI values were strongly dependent on temperature for polar probes on the polar phases containing high levels of cyanopropyl or polyethylene glycol (PEG) functionality. A lower apparent polarity was observed for polar columns at higher temperatures. Moderate RI differences were apparent with the same probes on columns with polarizable stationary phases of high phenyl or biphenyl content. Nonpolar alkyl substituted stationary phases showed only a slight temperature dependence on RI for all test compounds. Alkanes of lower molecular weight eluted at lower densities on all columns when the temperature was raised because of the contribution of volatility to elution in SFC. This same effect was observed for the higher alkanes on both nonpolar and PEG columns, but they eluted at similar densities on cyanopropyl columns and at higher densities on columns with polarizable phases when the temperature was raised. Temperature controlled selectivity tuning is a useful tool in optimizing separations in SFC.
On‐line coupled liquid chromatography and capillary supercritical fluid chromatography: Large‐volume injection system for capillary SFC
Authors: Cortes Hernan J. , Campbell Robert M. , Himes R. Paul , Pfeiffer Curt D.
Published: May 2005

A sample introduction system for capillary SFC was developed to allow introduction of relatively large sample volumes without significant decrease in chromatographic efficiency and resolution. The system includes a sample inlet capillary which receives the liquid aliquot from the LC, whereupon the solvent is eliminated by passage of nitrogen gas. The inlet capillary is then pressurized with CO2 and the analytes are transferred to an impactor interface where they are deposited. The analytes are then eluted through the impactor interface and the capillary SFC column by pressure programming of the CO2 mobile phase. The system was demonstrated in a multidimensional mode using microcolumn liquid chromatography in the first dimension. Narrow bands were obtained in the final SFC step for C‐36 hydrocarbon and pentaerythritol tetrastearate.
Conformational analysis of biphenyls using 13C NMR spectroscopy
Authors: Roberts R. M. G.
Published: May 2005

13C chemical shifts of biphenyl and fluorenyl derivatives have been used to estimate interplanar angles (0) for biphenyls in solution. Values of 0 for biphenyls with no ortho substituents lie in the range 26 +plusmn; 5°, which agree well with estimates using other techniques. The method also gave excellent agreement for ortho‐substituted derivatives. The value of 0 for 2,2′‐dimethylbiphenyl was calculated at 73°, close to values obtained from UV spectroscopy and adsorption studies.
NMR spectra of platinum and rhodium complexes of phenazine and related ligands
Authors: Newmark Richard A. , Siedle Allen R.
Published: May 2005

Proton and 13C NMR spectra are reported for cis‐(amine)Rh(CO)2Cl and trans‐(amine)PtCl2(C2H4), where amine = tetramethylpyrazine, phenazine, phenazine N‐oxide and quinoxaline. Relative labilities of the amine and ethylene ligands were determined.
Dynamic 13C NMR: A study of aryl‐substituted 1‐phenyl‐3,3‐tetramethylenetriazenes
Authors: Foster N. , Pestel B.
Published: May 2005

The temperature‐dependent 13C NMR spectra of a series of para‐substituted 1‐phenyl‐3,3‐tetramethylenetriazenes have been analyzed by the band width method. Electron‐donating substituents clearly accelerate the rate of rotation about the N‐2N‐3 bond in the triazenes; electron withdrawers retard that motion. The exchange averaging process observed in the carbon spectra obeys a linear free energy relationship when Hammett σ* parameters are used in the correlation. These results corroborate those already obtained by analysis of 1H NMR spectra of similar triazenes, regarding the likely contribution of a 1,3‐dipolar resonance form to the ground state of the molecule.
1H NMR analysis of the methoxy group conformation in methoxychromones
Authors: Kövér Katalim E. , Borbély J.
Published: May 2005

The methoxy group conformation of chromone derivatives was studied by 1H NMR methods. For the qualitative determination of the conformational preferences of the model compounds, proton relaxation rates, spin‐spin decoupling difference spectroscopy and 2D COSY experiments were applied. NOE data were used in the calculations of conforrmer population ratios in these six‐spin (ABCX3) systems.
13C NMR Study of 3,9‐di(alkylphenoxy) ‐2,4,8,10‐tetraoxa‐3,9‐diphosphaspiro[5.5]undecanes
Authors: Pätoprstý V. , Malík Ľ. , Goljer I. , Göghová M. , Karvaš M. , Durmis J.
Published: May 2005

13C NMR chemical shifts and 13C‐31P coupling constants of fourteen derivatives from 3,9‐di(alkylphenoxy)‐2,4,8,10‐tetraoxa‐3,9‐diphosphaspiro[5.5]undecane and of eight derivatives of the 3,9‐dioxo type are reported. All investigated compounds possess a chair conformation with the lone pair or the phosphoryl oxygen in the equatorial position. The alkylphenoxy groups are in a cis position with respect to the lone pair or the phosphoryl oxygen. Conformational changes about the arylO bond can be monitored via 31POC13C couplings and their angular dependence was found to be non‐symmetric.
Carbon‐13 NMR study of some new macrocycles. 13C chemical shifts of nactin antibiotic models
Authors: El Malouli Bibout M. , Samat A. , Faure R. , Elguero J.
Published: May 2005

Thirty two‐linked macrocycle models of natural ionophorus nactin antibiotics were studied by 13C NMR spectroscopy and their chemical shifts are reported.
Applications of 15N NMR to a study of tautomerism in some monocyclic azoles
Authors: Bojarska‐Olejnik E. , Stefaniak L. , Witanowski M. , Hamdi B. T. , Webb G. A.
Published: May 2005

15N NMR shielding measurements reveal that thiolo‐imidazole and ‐tetrazole exist almost entirely as the thione form. Quantitative estimates of the position of tautomerism in 1,2,3‐triazole and tetrazole are obtained. The reported assignments are supported by means of INDO/S‐SOS shielding calculations.
Two‐dimensional NMR studies of polyene systems. II. β‐carotene
Authors: Wernly Jacques , Lauterwein JÜRgen
Published: May 2005

Two‐dimensional NMR results are presented for β‐carotene in chloroform. J‐resolved 1H NMR spectroscopy allows the accurate determination of the chemical shifts and coupling constants for the olefinic protons. In order to obtain sufficient signal intensity in the projection spectrum for the HC‐7, HC‐10 and HC‐14 protons, spin decoupling of the long‐range coupled methyl and methylene protons was necessary. 1H,13C chemical shift‐correlated NMR spectroscopy gave a relationship between the directly bonded olefinic CH pairs. Cross‐sections through each 13C chemical shift allowed the assignment of the carbon resonances. Simulation of the cross‐sections was possible when the 180° carbon pulse was removed in the standard pulse sequence. It is shown that, under these conditions, the cross‐sections correspond to partial 13CH satellite spectra.
Carbon‐13 NMR lanthanide induced shifts and T1 studies of the conformation of friedelin
Authors: Wazeer Mohammed I. M.
Published: May 2005

By measuring the shifts of the 13C NMR signals of 29 carbons of friedelin, induced by the lanthanide shift reagent Pr(fod)3, and using a computer program, it is shown that friedelin exists in a chair‐chair‐chair‐twist boat‐boat conformation in solution. Lanthanide metal binds exclusively to one site on the carbonyl group of friedelin, away from the nearest methyl group. 13C T1 data are consistent with the suggested conformation of friedelin.
Conformational analysis and 13C NMR spectra of some 2,6‐diarylpiperidin‐4‐ones
Authors: Hasan Misbah Ul , Arab Mohammad , Pandia Rajan K. , Sekar R. , Marko Dale
Published: May 2005

The 13C NMR spectra of several methyl substituted 2,6‐diaryl‐4‐piperidones (aryl = phenyl, o‐chlorophenyl) have been measured. Substituent parameters for methyl and chloro groups have also been derived and are discussed in terms of their steric and electronic interactions. Both the methyl substitution at C‐3 and chlorine substitution in the ortho position of the phenyl ring cause significant changes in the 13C chemical shifts, which can be satisfactorily explained on the basis of steric interactions taking place in the preferred conformations of these molecules. In compounds with o‐chloro groups, the aryl ring is oriented such that the chlorine is syn to the axial hydrogen on the aryl‐bearing carbon. The shielding effect of the carbonyl group is found to be more pronounced on an adjacent equatorial methyl than on an adjacent axial methyl group.
Proton deshielding by spatial proximity: An experimental and theoretical study for ortho‐substituted anisoles
Authors: Contreras R. H. , Ferraro M. B. , de Kowalewski D. G. , Díaz L.
Published: May 2005

A quantitative estimation of the proton deshielding effect produced by a sterically crowded environment was obtained experimentally, based on departures from the chemical shift additivity rule of protons ortho to OCH3 and NO2 groups in benzene derivatives, where a conformational change from these side‐chain groups is expected. The influences of these conformational changes were also analysed from a theoretical point of view. Calculations of the magnetic shielding tensor were carried out at the INDO approximation with gauge invariant atomic orbitals, together with the uncoupled Hartree‐Fock approach. Theoretical results for these proton deshielding effects are in very good agreement with those determined experimentally.
Investigation of Benzimidazoles. 6—transmission of substituent effects in dianion radicals of 2‐substituted 5(6)‐nitrobenzimidazoles studied by ESR spectroscopy
Authors: Lopyrev V. A. , Larina L. I. , Vakul'skaya T. I. , Shibanova E. F. , Titova I. A. , Voronkov M. G.
Published: May 2005

The substituent effects in dianion radicals of fourteen 2‐substituted 5(6)‐nitrobenzimidazoles, electrochemically generated in acetonitrile, were studied by ESR. It was found that the substituent influence on the nitro group is transmitted with approximately equal contributions from the inductive and resonance components, independently of the pairs of the σIσR, σIσRo,σIσR+ and ℱℛ substituent constants used. The radical stabilization constant σ contributes much to the correlation of the hyperfine splitting constants of H‐6 and the heterocyclic nitrogen atoms. The transmission through the benzimidazole ring in dianion radicals of nitrobenzimidazoles is half that of the benzene ring in anion radicals of para‐substituted nitrobenzenes.
Structure at the anomeric centre in 2‐alkoxytetrahydropyrans and their radicals, studied by 13C coupling in NMR and ESR spectra
Authors: McKelvey Ronald D. , Sugawara Tadashi , Iwamura Hiizu
Published: May 2005

Several 2‐alkoxytetrahydropyrans labelled with 13C in the 2‐position were prepared by hydroboration‐carbonylation (with 13CO) of the protected but‐3‐en‐1‐ol derivatives. The ESR spectra were measured for the corresponding 2‐alkoxytetrahydropyran‐2‐yl radicals produced by the reaction with the tert‐butoxy radical. The hyperfine coupling constant to carbon was 99±0.5 G. The deviation from planarity for these radicals was estimated to be approximately 11°, with sigma bond angles of 116°. The s character of the singly occupied orbital was approximately 8.1%. Anomeric, one‐bond 13CH NMR coupling constants were also determined. Axial hydrogens had coupling constants of approximately 155 Hz, while equatorial hydrogen coupling constants were ca 164 Hz. These values were interpreted in terms of greater s character in the equatorial CH bonds. This could be caused by greater ring CO double bond character when the alkoxy group is axial, through no‐bond resonance to the exocyclic alkoxy group.
On the Chemistry of Pyrrole Pigments. 60—Natural isotopic abundance 15N NMR spectra of verdinoid bile pigments and their partial structures
Authors: Falk Heinz , Müller Norbert
Published: May 2005

15N NMR spectra, at natural isotopic abundance, of synthetic bilatrienes‐abc and 2,3‐dihydrobilatrienes‐abc and also those of partial structures are presented. The implications of the 15N chemical shifts on structural features, especially tautomerism, are discussed and compared with results obtained by x‐ray photoelectron spectroscopy.
NMR studies in the heterocyclic series. 27—Carbon‐13 NMR determination of the protonation site of aminopyrazoles in trifluoroacetic acid
Authors: Bruix Marta , De Mendoza Javier , Claramunt Rosa Maria , Elguero José
Published: May 2005

The 13C NMR data for 3‐aminoindazole and 14 aminopyrazoles were obtained in neutral and acidic media (trifluoroacetic acid) and compared with those for a series of pyrazoles, aminothiophenes and anilines as models for protonation. From the chemical shifts and coupling constants it is possible to conclude that all 3‐aminopyrazoles, 3‐aminoindazole and 5‐aminopyrazoles protonate on the heterocyclic ring nitrogen atom. This first protonation strongly deactivates the exocyclic nitrogen atom, so that no double protonation is observed, even in sulphuric acid. On the other hand, 4‐aminopyrazoles are stronger bases, which are protonated first at the amino group and then on both nitrogen atoms.
13C NMR Spectroscopy of lignan and neolignan derivatives
Authors: Agrawal P. K. , Thakur R. S.
Published: May 2005

13C NMR shielding data for about 300 compounds are tabulated, and critical spectral features are discussed as a guide to the use of the technique in structure elucidation and stereochemical allocations of lignan, neolignan and their derivatives. Unanalysed reported data have been analysed and, wherever necessary, 13C signal assignments have been revised on the basis of more recent evidence and, to obtain consistency throughout the series.
Quaternization of pyrido[2,3‐b]pyrazines: 1H and 13C NMR Study
Authors: Chupakhin O. N. , Charushin V. N. , Chernyshev A. I. , Esipov S. E.
Published: May 2005

The structure of N‐methylpyrido[2,3‐b]pyrazinium salts has been examined by 1H and 13C NMR. 6‐Dimethylamino‐ and 6‐morpholino‐pyrido[2,3‐b]pyrazines were shown to undergo quaternization by methyl iodide at N‐4 of the pyrazine ring, whereas the pyridine ring N‐5 was the N‐alkylation site in the reaction of unsubstituted pyrido[2,3‐b]pyrazine with methyl iodide under the same conditions. The effects of quaternization on the 1H and 13C chemical shifts and the nJ(CH) values are discussed.
Rotational Dynamics by NMR to Probe Solvation of 2‐Pyridone in Hydroxylic Solvents
Authors: Tiffon Bernard , Guillerez Jean , Ancian Bernard
Published: May 2005

The dynamics of the molecular rotation of 2(1H)‐pyridone and 1‐methyl‐2(1H)‐pyridone in toluene, carbon tetrachloride, methanol and water have been investigated at 305 K by13C and2H NMR. Both chemical shifts and relaxation times show that 2‐pyridonc forms stable hydrogen‐bonded complexes in methanol and in water, reorienting as a complete unit and taking with it two solvent molecules. These solvated species are stable within the liquid‐state temperature range, and reorient according to the hydrodynamic law as indicated by the14N line width measurements. It follows that the lifetimes of these complexes are always greater by at least an order of magnitude than the rotational correlation time scale. Analysis of the14N quadrupole coupling constants and the rotational anisotropy support the structure and stability of these complexes.
Observation of long‐range 1H‐1H coupling correlation by the two‐dimensional SUPERCOSY technique as an aid in resonance assignment
Authors: Mayor S. , Hosur R. V.
Published: May 2005

Long‐range four‐ and five‐bond 1H‐1H coupling correlations have been observed in the dipeptide L‐Lys‐L‐Phe by using the two‐dimensional SUPERCOSY technique. Complete resonance assignments of all the non‐exchangeable protons in the dipeptide have been obtained.
Applicability of vicinal heteronuclear (13C‐1H, 15N‐1H, 13C‐15N) coupling constants for conformational analysis of amino acids: Phenylalanine, α‐aminobutyric acid and β‐alanine
Authors: Mikhova Bozhana P. , Simeonov Mario F. , Spassov Stefan L.
Published: May 2005

The vicinal 13C1H, 15N1H and 13C15N spin coupling constants were obtained from the 1H and 13C NMR spectra of various ionic forms of the amino acids phenylalanine, α‐aminobutyric acid and β‐alanine, and also from their 15N isotopomers. It is concluded that the 13C1H and 15N1H coupling constants are reliable parameters for conformational studies of amino acids, whereas the practical applicability of the 13C15N constants for the same purpose is doubtful.
Carbon‐13 nuclear magnetic resonance spectra of the hydrate, keto and enol forms of oxalacetic acid
Authors: Buldain Graciela , Santos Carlos De Los , Frydman Benjamín
Published: May 2005

The 13C resonances of the hydrate, keto and enol forms of oxaloacetic acid in aqueous solutions at low pH, at pH 4.3 and at pH 7.3 were assigned on the basis of the J(CH) values and the chemical shifts of reference compounds. At low pH the hydrate form predominated, while only small amounts of the keto and enol forms were detected. At pH 4.3 and 7.3 the keto form was the major form. In anhydrous dimethyl sulphoxide solution only the keto and enol forms were detected, whereas on addition of water the hydrate form became predominant. On prolonged recording of the spectra at the probe temperature (30°C) citroylformic acid was formed either at low pH or at pH 7–12, whereas only traces of pyruvic acid were detected. The aldol condensation product of oxalacetic acid was detected when the spectra were recorded at 7°C before its decarboxylation to citroylformic acid. In the spectra recorded at pH 3–7 pyruvic acid and its aldol condensation product, 4‐hydroxy‐4‐methyl‐2‐ketoglutaric acid, were detected together with citroylformic acid. A complete assignment of the 13C resonances of these derivatives was made.
Molecular conformation of N,N′‐diarylthioureas: An assessment by 1H NMR and infrared spectroscopy
Authors: Sudha L. V. , Manogaran S. , Sathyanarayana D. N.
Published: May 2005

Several N,N′‐dipyridyl‐ and N‐phenyl‐N′‐pyridyl‐thioureas were examined in different solvents at various temperatures by 1H NMR in order to study their conformational properties. The influence of concentration and the methyl substituent in the pyridine ring on the chemical shifts of the NH and pyridine groups was investigated. The observed chemical shifts are analysed in terms of the conformational properties of the molecules. Free energy barriers to the internal rotation about the CN bonds have been determined. Infrared spectra have been measured to supplement the NMR studies. Intramolecular hydrogen bonding played a major role in the preferred conformation of pyridylthioureas. The data further revealed an interesting dynamic exchange phenomenon occurring in symmetric N,N′‐dipyridylthioureas between two intramolecularly hydrogen bonded conformers.
13C NMR signal assignment of friedelin and 3α‐hydroxyfriedelan‐2‐one
Authors: Gottlieb Hugo E. , Ramaiah Parimi A. , Lavie David
Published: May 2005

The 13C NMR spectrum of the pentacyclic triterpene friedelin was fully assigned, taking into account signal multiplicities, correlations of the carbon lines with known proton resonances and Eu‐induced shifts. This work clarifies the situation prevalent in the literature, in which three separate groups have published contradictory information. In addition, the carbon and proton spectra of the naturally occurring 3α‐hydroxyfriedelan‐2‐one and its synthetic acetate were analysed. The different preferred conformations of the D/E rings for friedelin (boat‐boat) and polpunonic acid (chair‐chair) are rationalized.
Intramolecular van der waals interactions and chemical shifts: A model for β‐ and γ‐effects
Authors: Li S. , Chesnut D. B.
Published: May 2005

A correlation has been discovered between the local steric van der Waals energy as calculated by the MM2 force field and the chemical shift of a resonant nucleus within classes of saturated aliphatic phosphines, amines, hydrocarbons and monofunctional alcohols. Root‐mean‐square errors found for 37 phosphorus, 35 nitrogen and 164 carbon nuclei are 10.9, 5.9 and 2.8 ppm, respectively. It is found that repulsive van der Waals interactions are associated with deshielding effects in the observed chemical shifts, and attractive interactions with smaller shielding effects. A simple physical picture emerges which describes both β‐ and γ‐effects under one common model. It is found that β‐effects are associated with repulsive interactions while, contrary to earlier approaches γ‐effects are due basically to attractive interactions.
17O NMR chemical shifts of the twenty protein amino acids in aqueous solution
Authors: Gerothanassis Ioannis P. , Hunston Roger N. , Lauterwein JÜRgen
Published: May 2005

The 17O chemical shifts of the α‐carboxyl groups of the protein amino acids, including 4‐hydroxyproline, sarcosine and N,N‐dimethylglycine, were measured at 40°C in aqueous solution at variable pH (17O enrichment 10 atom‐%). The chemical shifts of the amino acids were found to be between 250 and 257 ppm at low pH and between 266 and 273 ppm at neutral pH. For the purpose of comparison, several aliphatic and halogen‐containing carboxylic acids have also been measured under identical conditions. The high‐frequency titration shifts observed on deprotonation of the α‐carboxyl oxygens of the amino acids (15‐17 ppm) are similar to those of α‐haloalkanoic acids. A linear correlation of the titration shifts and the pKa values was found for the carboxylic acids, including the amino acids. The differences in the chemical shifts of the amino acids in either the cationic or zwitterionic state can be explained by the substituent effects of the side‐chains, introducing γ shielding and δ deshielding values of approximately half the size of those for aldehydes and ketones. Deprotonation of the primary α‐amino group of the amino acids induced low‐frequency shifts between ‐2.3 and ‐3.9 ppm, with the exception of serine and threonine (‐1.3 and ‐1.7 ppm, respectively). On deprotonation of the secondary α‐amino group of proline, 4‐hydroxyproline and sarcosine, a decrease in the titration shifts was observed (‐0.9, ‐0.2 and ‐1.1 ppm, respectively) and on deprotonation of the tertiary α‐amino group of N,N‐dimethylglycine the sign was reversed (+3.9 ppm). The titration shifts can be explained by an inductive shift to high frequency, which is counterbalanced by an electric field shift of variable magnitude to low frequency.
An NMR study of the conformation of covalently linked porphyrin dimers
Authors: Benthem Lambertus , Koehorst Rob B. M. , Schaafsma Tjeerd J.
Published: May 2005

Proton NMR spectra of covalently linked tetraphenyl‐ or tetra‐p‐tolyl‐porphyrin dimers of three different types (para‐para, ortho‐para and ortho‐ortho) can be interpreted using a ring current model. For the assignment of the protons of the linking chain a tetraaryl porphyrin monomer has been used, containing this chain at the same position as in the corresponding dimer but without the presence of a second porphyrin group. The porphyin ring current shift of the para‐para dimer was used as a reference for the calculation of the resonance positions of the aromatic propyloxyphenyl protons. Using these NMR results, the dimer conformation in the ground state was determined for the various dimers. From space‐filling models of the dimers in different conformations, the polar coordinates of the various protons of the propyloxyphenyl group of one porphyrin ring with respect to the other ring can be obtained, and the ring current shifts due to the presence of the latter can be calculated for these protons. It emerges that the para‐para dimer exists mainly in a stretched conformation, and that the ortho‐ortho dimer has an exo conformation (unfolded). It is likely that in the ortho‐para dimer, the para‐linked porphyrin group has a rotation around the linking chain axis which is fast on the NMR time scale. The degree of folding of the ortho‐para dimer is shown to be influenced by the solvent polarity.
Studies in the synthesis of C‐ring bridged morphinans 3—13C NMR analysis of some simple and complex bicyclooctane derivatives
Authors: Roche Victoria F. , Nagel Donald L. , Roche Edward B.
Published: May 2005

The 13C NMR spectra of eight novel derivatives of bicyclo[2.2.2]‐ and ‐[3.2.1]octane have been recorded and analysed. These compounds were isolated and characterized as intermediates in the synthesis of a unique derivative of 1,2,3,3a,11b,11c‐hexahydroaporphine. Carbon chemical shift assignments of the molecules under investigation were based on 13C NMR theory including splitting patterns, differences in chemical shift values of carbons α and β to amino nitrogens in their protonated and non‐protonated forms, and through the anticipated shielding effects of γ‐syn and γ‐anti heteroatoms. This latter method of analysis was particularly useful in assigning the chemical shift values of carbons in the bicyclooctane region of the simpler molecules.
An ESR study of the polymerization of tetrahydrofuran with simple inorganic cations containing sulphur and nitrogen
Authors: Fairhurst Shirley A. , Hulme‐Lowe Alan , Johnson Karen M. , Sutcliffe Leslie H. , Passmore Jack , Schriver Melbourne J.
Published: May 2005

The cations of NS2+·AsF6−, N(SCI)2+·AsF6− and NS+·AsF6− have been shown to be efficient initiators for the polymerization of tetrahydrofuran. The former two salts were found, by electron spin resonance spectroscopy, to produce remarkably high concentrations of persistent free radicals which have been identified as being entirely inorganic, namely N2S3+˙ and, more tentatively, N2S2+˙. A mechanism for generation of the radicals is proposed.
Substituent chemical shifts in NMR. 1—Proton resonance spectra and geometries of norbornene, norbornane and adamantane
Authors: Abraham Raymond J. , Fisher Julie
Published: May 2005

The molecular geometries and proton NMR spectra of norbornene, norbornane and adamantane are reported. The molecular geometries were refined from previous experimental determinations using MNDO and ab initio techniques to determine the positions of the hydrogen atoms. No evidence of any endo—endo interactions in norbornane was obtained from these calculations.

High‐field proton NMR spectra allow the refinement of previous spectral analyses of adamantane and norbornane, and the complete analysis of the olefin decoupled eight spin system of norbornene. The chemical shifts of the parent compounds and cyclohexane under comparable conditions show minor but significant differences from previous values. The methine—methylene coupling in adamantane is 3.30 Hz. The long‐range couplings in norbornene were compared with those calculated by FPT/INDO and EHMO/VB methods. Only moderate agreement was obtained and possible reasons for this are examined.
Revision of the 13C NMR signal assignment of harman, based on one‐ and two‐dimensional INADEQUATE spectroscopy
Authors: Welti D. H.
Published: May 2005

For harman, a key molecule in the structure elucidation of β‐carboline‐type plant alkaloids, an unambiguous assignment of the 13C NMR spectrum is given, based on two‐dimensional INADEQUATE spectroscopy. One‐bond carbon—carbon coupling constants have been obtained from one‐dimensional INADEQUATE spectra. The unequivocal results obtained by these methods allow the correction of all earlier, controversial resonance assignments for harman in the literature. The assignments for a number of related molecules will, in turn, have to be corrected.
1H NMR spectra of variously annulated furocoumarins
Authors: Manzini Paolo , Rodighiero Paolo , Chilin Adriana , Guiotto Adriano
Published: May 2005

The 1H NMR spectra of variously annulated methylfurocoumarins are reported. The assignment of the chemical shifts for all the protons was achieved by using coupling constants, and shift effects induced by the introduction of methyl groups at various positions on the furocoumarin nucleus.
17O NMR of nucleosides. 3—Chemical shifts of substituted uridines and ribothymidines
Authors: Schwartz Herbert M. , Maccoss Malcolm , Danyluk Steven S.
Published: May 2005

Uridine and ribothymidine derivatives, bearing different substituents‐at C‐5 and enriched (Ca50%) with 17O in the O‐4 and O‐2 carbonyls, have been studied via 17O NMR in both acetonitrile and aqueous solvents. The solvent shift differences between acetonitrile and water at O‐4 (30–42 ppm) and O‐2 (13–16 ppm) vary significantly from each other, but the chemical shift changes induced by changing the substituent at C‐5 correlated well only with the O‐4 shifts and the electron‐withdrawing ability of the substituent. Examination of the 17O shifts of model compounds reconfirms the predominance of keto tautomers for both carbonyls. The significance of the solvent shifts and substituent shifts are discussed with respect to the electronie structure of the nucleoside base kings, and with respect to the hydrogen‐bonding abilities of the carbonyl groups. Other nucleoside derivatives studied include those in which the 17O enrichment is in the ring linking the base to the sugar moiety in a pyrimidine cyclonucleoside, in the sugar hydroxy groups and in the phosphodiester linkage of a highly strained ring system in a nucleoside cyclic monophosphate.
17O NMR spectra of 2‐substituted and 2,5‐di‐substituted furans. The inapplicability of the hammett LFER to correlate chemical shifts
Authors: Clennan Edward L. , Mehrsheikh‐Mohammadi M. E.
Published: May 2005

The 17O NMR spectra of 27 2‐substituted and 2,5‐disubstituted furans are reported. The additivity of the 17O chemical shifts but the inapplicability of Hammett substituent constants to predict the observed shifts is demonstrated.
Studies of substituent effects by carbon‐13 NMR spectroscopy.V—Ethyl (E)‐(α‐cyano)cinnamates, (E)‐(α‐cyano)cinnamamides and ethyl (α‐ethoxycarbonyl)cinnamates
Authors: Bottino Francesco A. , Musumarra Giuseppe , Rappoport Zvi
Published: May 2005

13C NMR shifts for ethyl (E)‐(α‐cyano)cinnamates, (E)‐(α‐cyano)cinnamamides and ethyl (α‐ethoxycarbonyl)cinnamates of general formula XC6H4CHCR1R2 (R1  CN, R2  COOEt; R1  CN, R2  CONH2; R1  R2  COOEt; X p‐NMe2, p‐OH, p‐OMe, p‐Me, H, p‐Br, p‐Cl, p‐NO2, m‐Cl, m‐NO2, o‐OMe, o‐CI, o‐NO2) in DMSO‐d6 solution are reported. Substituent chemical shift considerations allow the assignment of the aromatic carbons in the disubstituted rings. The effect of substituents on the aromatic, ethylenic, cyano and carbonyl chemical shifts is discussed in relation to that of other arylethylenes. The J(CCCH) long‐range coupling values provide evidence in favour of the E configuration for both series of cyano derivatives.
High‐resolution 13C nuclear magnetic resonance spectra of some solid biphenyl derivatives
Authors: Chippendale A. Margaret , Aujla Rajinder S. , Harris Robin K. , Packer Kenneth J. , Purser Stephen
Published: May 2005

13C NMR spectra of the solid phase have been obtained for biphenyl and a series of its substituted derivatives. Some species give essentially similar spectra in the solid and solution phases, whereas others show clear differences. In several cases the differences are shown to be due to the locking of the molecules into a particular conformation in the solid. For some compounds different chemical shifts for the C‐1 and C‐1′; carbon atoms in the solid and solution phases are believed to reflect differences in the angle between the ring planes in the two phases.
1H and 13C assignment of cyclopenta[cd]pyrene by two‐dimensional NMR spectroscopy
Authors: Jans A. W. H. , Tintel C. , Cornelisse J. , Lugtenburg J.
Published: May 2005

The 1H and 13C NMR spectra of cyclopenta[cd]pyrene have been assigned completely by a COSY experiment and 13C1H shift correlation diagrams. Both the 1H and 13C assignments were confirmed by the observation of long‐range couplings.
Condensation of 1,3‐trimethinecyanines with arylamines. Structural assignment of the products by NOE difference spectroscopy
Authors: Tóth Gábor , Bitter István , Bigam Glen , Strausz Ottó
Published: May 2005

The condensation of 1,3‐dichlorotrimethinecyanines and arylamines was found to be highly regioselective. Nuclear Overhauser effect difference experiments proved a linear structure for the formed perchlorates of 1‐aza‐9‐azonia‐1H‐benz[f]indene, 1‐aza‐9‐azoniaanthracene, 1,10‐diaza‐4a‐azonia‐9‐thiacyclopenta[b]fluorene, 6‐aza‐7‐azoniabenz[a]anthracene and 6,7‐diaza‐11a‐azoniabenz[a]anthracene derivatives. NOE difference experiments, together with 2D proton‐proton and proton‐carbon correlation measurements, allowed the complete 1H and 13C assignment.
Chemistry of the SO bond. 9—Conformational analysis of some substituted trimethylene sulphites (1,3,2‐dioxathiane 2‐oxides) via17O NMR spectroscopy
Authors: Hellier Desmond G.
Published: May 2005

17O Chemical shifts for trimethylene sulphite and a number of substituted trimethylene sulphites are presented. Spectral values are compared by consideration of the probable conformation(s) in solution.
ESR and 1H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals
Authors: Khramtsov V. V. , Weiner L. M. , Gogolev A. Z. , Grigor'ev I. A. , Starichenko V. F. , Volodarsky L. B.
Published: May 2005

A new synthesis of nitronylnitroxyl (NNR), iminonitroxyl (INR) and nitroxyl radicals (NR) is suggested, involving oxidation of 4H‐imidazol di‐N‐oxides with O2 or PbO2 in the presence of nucleophilic reagents. This method allowed the preparation of radicals which could not be synthesized by known procedures. These radicals were studied by ESR and 1H NMR spectroscopy. The hyperfine interaction constants of the NNR were measured and assigned (in some cases radicals containing 15N were used). The influence of protonation and deprotonation of functional groups (OH, NH2) in the NNR on their ESR spectra was investigated and the pK values of these radicals were measured by ESR.

Differences between the ESR spectra of two isomeric INR were found, and the structures of these radicals were established by 1H NMR. Changes in spin density delocalization in 3‐imidazoline 3‐oxide nitroxyl radicals were studied, and the spin density was found to be ‘pushed out’ to the peripheral fragments of these radicals on increasing the number of methoxy groups in the α‐position to the N—O. fragment. Hindered rotation around the σ‐bond of a bulky substituent in the α‐position to the N—O. fragment was detected by ESR. The activation energy of this hindered rotation was ΔE = 30±1.7 kJ mol−1.
Conformational changes of amino acids monitored by deuterium nuclear relaxation rate
Authors: Van Haverbeke Yves , Muller Robert N. , Elst Luce Vander
Published: May 2005

As an extension of previous work on simple amino acids, deuterium nuclear relaxation rates were used to study the molecular dynamics of γ‐2,2‐[2H2]aminobutyric acid, d,l‐2‐[2H1]glutamic acid and d,l‐2‐[2H1]lysine, and also model compounds 2,2,2‐[2H3]acetic acid and 1,1‐[2H2]butylamine. This study was carried out in aqueous solution in the pH range ca 0–14. From the observed rotational correlation times and solution viscosities, molecular volumes of various ionic forms were calculated using the Gierer and Wirtz model. The results obtained are in good agreement with conformational changes expected in the pH range investigated.
A 13C NMR investigation of [5]metacyclophanes
Authors: Jenneskens L. W. , de Kanter F. J. J. , de Wolf W. H. , Bickelhaupt F.
Published: May 2005

Carbon‐13 NMR data of [5]metacyclophane (1a), its 8,11‐dihalosubstituted derivatives 1b‐1d and of some model compounds are reported and discussed. A remarkable downfield shift is observed for the aromatic carbon between the bridging carbons; this is tentatively ascribed to long‐range effects of the bridge and to the bending of the aromatic ring.
Modern NMR techniques for structure elucidation
Authors: Morris G. A.
Published: May 2005

The rapid growth during the last decade of multiple pulse methods in one (1D) and two‐dimensional (2D) NMR has produced over 100 new techniques of potential use in high‐resolution NMR. This review sets out to identify and to illustrate those new methods which are most useful in the determination of molecular structure. Ten basic techniques are considered, including 2D homo‐ and hetero‐nuclear chemical shift correlation by direct and by relayed coherence transfer, and 1D polarization transfer. Numerous variations on the basic techniques, for example multiple quantum filtration, are also discussed. Particular attention is paid to the problems of sensitivity and of the optimum choice of experimental techniques for different classes of structural problem.
Carbon‐13 NMR of β‐lactam antibiotics and related compounds
Authors: Branch S. K. , Casy A. F. , Lipczynski A. , Ominde E. M. A.
Published: May 2005

The 13C NMR features of most β‐lactam antibiotics of the penicillin and cephalosporin groups in clinical use are presented and reviewed. Data on precursor and breakdown products are included in the survey, with particular reference to benzylpenicillin. Penicillin and cephalosporin spectra are discussed, and points of similarity and difference emphasized. The potential of the data for solving problems of analysis, stability, degradation and biological structure–activity relationships is summarized.
Stereochemistry of nitrogen‐containing heterocycles. IV—1H and 13C NMR spectra, ASIS effects, nitrogen inversion, configuration and conformations of substituted tetrahydro‐1,3‐oxazines
Authors: Samitov Yu. Yu. , Danilova O. I. , Unkovsky B. V. , Boiko I. P.
Published: May 2005

The 1H and 13C NMR spectra of 50 methyl‐ and phenyl‐substituted tetrahydro‐1,3‐oxazines are presented and discussed. Conformational analysis based on the model of atom‐atom interactions in 3,5‐dimethyltetra‐hydro‐1,3‐oxazine shows that the free energy difference between the chair and twist forms is 21 kJ mol−1. The distortion of the ring, which adopts a chair conformation, is analysed by the R value method and by the ASIS effect. The use of geminal 2J(HH′) values in the heteroatomic moieties as a criterion of the ring conformation has been demonstrated. The equilibrium and activation parameters of the heterocycle and nitrogen inversion have been determined. A good correlation between the predicted and experimental chemical shift values, δc, has been obtained, and this is shown to depend on the spatial structure of the substituted tetrahydro‐1,3‐oxazine molecule; moreover, the influence of intermolecular associations on δc has been demonstrated. Conformational preferences and some peculiarities in the chemical shifts are explained in terms of interactions with the endocyclic β‐oxygen atom and the N‐methyl group.
Asymmetric peak heights in the 13C triplet of the C‐α nucleus in single crystals of L‐alanine
Authors: Takegoshi K. , Naito A. , McDowell C. A.
Published: May 2005

The asymmetric peak heights observed in the 13C triplet signals, caused by the 13C‐14N dipolar interaction, of the C‐α nucleus in L‐alanine can be explained by the fact that a given 1H decoupling frequency cannot simultaneously fulfill the ‘on‐resonance’ condition for each of the three 1H resonances of the H‐α nucleus bonded to C‐α. Therefore, the off‐resonance 1H decoupling causes an excess line broadening, to a different extent, for each of the 13C triplet lines, with concomitant changes of the line heights.
The static and Dynamic NMR spectra of 4,4′‐bis(N‐isopropyl‐N‐nitrosoamino)biphenyl, a four‐site exchange problem
Authors: Grindley T. Bruce , Lu Mu‐Jian , Colpa Johannes P. , Wu Shi‐Ming , Buncel Erwin
Published: May 2005

The 1H and 13C NMR spectra of 4,4′‐bis(N‐isopropyl‐N‐nitrosoamino)biphenyl at 22°C were consistent with the presence of three isomers due to slow rotation about the N—N bonds of the two N‐nitroso groups. The structures of the three isomers were assigned by means of their symmetries and from the known effects of the two orientations of the N‐nitroso group on 1H and 13C NMR chemical shifts. Each conversion of an N‐nitroso group from anti to syn to an aryl group was associated with the same free energy difference (0.26 kcal mol−1) after symmetry corrections were made. The changes in the 13C NMR spectra recorded from 95 to 150°C were simulated using a program for classical four‐site exchange. The potential barrier to rotation was calculated to be 20.76±0.06 kcal mol−1 at 400 K.