bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of the torsional angle at the glycosidic bond in 5′‐amino‐5′‐deoxythymidine derivatives by carbon‐13 n.m.r. spectroscopy
Authors: Nottoli Eileen M. , Lambert Joseph B. , Letsinger Robert L.
Published: May 2005

The conformational preference of the thymine base ring with respect to the sugar ring in β,β,β,‐trichloroethyl 5′amino‐5′‐deoxythymidine‐5′‐phosphate has been studied by 13C n.m.r. spectroscopy. The magnitude of the three bond vicinal coupling constant, J(C‐2, H‐1′), for β,β,β‐trichloroethyl 5′‐amino‐5′‐deoxythymidine‐5′‐phosphate and the similarity between the chemical shifts for the furanose carbons C‐1′, C‐2′, and C‐3′ in β,β,β‐trichloroethyl 5‐′‐amino‐5′‐deoxythymidine‐5′‐phosphate and in β,β,β‐trichloroethyl thymidine 5′‐phosphate indicate that the amino analogue exists in aqueous solution predominantly in the anti conformation, as is the case with natural nucleotides.
RMN du 13C de Bromo et de Fluoro Cyclanones à Structure Contrainte: Variation des Effets α et γ en Fonction des Facteurs Stériques et de la Nature de l'Halogène
Authors: Reisse J. , Piccinni‐Leopardi C. , Zahra J. P. , Waegell B. , Fournier J.
Published: May 2005

On décrit les spectres de RMN du 13C de diverses halogéno bicyclo[3.2.1]octanone‐3 et diméthyl‐7,7 bicyclo[3.1.1]heptanone‐3. S'il s'avère impossible de faire des corrélations systématiques au niveau des effets β, il est par contre possible de relier semi‐quantitativement les effets α et γ aux facteurs géométriques. La RMN du 13C permet ainsi d'appréhender les effets réflexe et anti‐réflexe.
Le Spectre de L'Alcool Propargylique en Fonction du pH. Un Exemple de Transferts Protoniques Complexes sur un Diacide Organique
Authors: Bouatouch K. , Chrisment J. , Delpuech J‐J. , Nicole D.
Published: May 2005

Le diméthylsulfoxyde (DMSO) est utilisé comme solvant pour étudier l'acidité cinétique de très faibles diacides organiques par RMN du proton. Un examen du spectre de l'alcool propargylique en fonction du pH met en évidence less mécanismes de transfer suivants:magnified image.
13C nuclear magnetic resonance studies of saturated heterocycles: II—substituent effects on the 13C chemical shifts of methyl substituted 1,3‐dithianes and their application to the determination of conformational equilibria
Authors: Pihlaja Kalevi , Björkqvist Börje
Published: May 2005

The 13C chemical shifts for 1,3‐dithiane and 9 methyl substituted derivatives are reported. Only three of the methyl‐1,3‐dithianes were conformationally anancomeric and hence the conformational equilibria must be taken into account when deriving the values of the different substituent effects on the 13C chemical shifts. The best fit for each substituent effect was obtained when –ΔGθ (5a‐Me) for 5‐methyl‐1,3‐dithiane was given the value 3.8 ± 0.3 kJ mol−1 and when the difference between –ΔGθ (2a‐Me) and –ΔGθ (5a‐Me) for cis‐2,5‐dimethyl 1,3‐dithiane equalled 3.4 ± 0.4 kJ mol−1. The conformer populations chosen from our earlier paper1 were then suitable for all the other conformational equilibria in question. The magnitude of the derived substituent effects are compared with those for cyclohexane and 1,3‐dioxane.
Spectroscopic characterization of some chlorinated tricyclic hydrocarbons of the C10 series: Chlordanes, chlordenes and nonachlors
Authors: Wilson Nancy K. , Sovocool G. Wayne
Published: May 2005

Carbon‐13 and proton nuclear magnetic resonance (n.m.r.) spectra of ten chlorinated hydrocarbons, which are components of technical chlordane and one chlordane metabolite, were examined. For chlordene, dichlorochlordene, cis‐ and trans‐nonachlor, and cis‐ and trans‐chlordane, whose chemical structures are well‐established, the relationships between the n.m.r. parameters and these structures were investigated. The results allow confirmation of proposed structures for α‐, β‐, and γ‐chlordene. A structure, corresponding to 2,4,5,5,6,7,8,8‐octachloro‐2,3,3a,4,5,7a‐hexahydro‐1,4‐methano‐1‐H‐indene, is proposed for another chlordane component, previously known only as Compound K.
Arithmetic manipulation of coupling constants to obtain torsional angles
Authors: Lambert Joseph B. , Sun Hsiang‐ning
Published: May 2005

Various arithmetic combinations of Jtrans and Jcis from the AA′BB′ analysis of CH2—CH2– fragments in 6‐membered rings have been examined for their relationship to the internal torsional angle Ψ. The simple ratio R (Jtrans/Jcis) appears to offer the most complete removal of substituent electronegativity effects and hence the best values of Ψ. The Karplus equation is very inaccurate when the substituents are highly polar. The simple difference D (Jtrans–Jcis) overcompensates for electronegativity effects and thus also gives relatively inaccurate torsional angles. An alternative, artificial linear combination of the coupling constants, such as D′ (Jtrans–2Jcis), appears to be able to provide torsional angles that are almost as good as those from the R value, although the Karplus A value must be evaluated. Adjustment of the simple difference D for electronegativity is possible by including a function of the AA′ and BB′ chemical shifts. These results are fairly good, and the parameter A need not be evaluated. This approach may fail, however, with particularly anisotropic substituents, since the chemical shift may depend on factors other than electronegativity. Although all five methods give reasonably good results, the R value appears to offer the most reliable results for the full range of substituents.
Détermination Univoque des Déplacements Chimiques des Protons Axiaux et Équatoriaux dans une α‐Bromo Cyclohexanone. Une Application de la Mesure des Temps de Relaxation Longitudinaux des Protons
Authors: Pouzard G. , Hall L. D. , Zahra J. P. , Waegell B.
Published: May 2005

L'étude des constantes de couplage à longue distance entre protons et la mesure des temps de relaxation longitudinaux des protons de la bromo‐2 tétraméthyl‐3,3,5,5 cyclohexanone (BTC) permettent l'identification de tous les protons axiaux et équatoriaux. Cette attribution confirme la stéréochimie des couplages à longue distance 4J(HH) et elle est en accord avec les facteurs structuraux qui découlent de la théorie de l'Effet Réflexe.
The 13C,H coupling constants in structural and conformational analysis: II The correlation of 3J(HCCH) and 3J(CH3CCH) in ethylenes and their methyl derivatives
Authors: Äyräs Pertti
Published: May 2005

The vicinal 3J(CH3CCH) coupling constants were determined for a number of propylene derivatives and compared with the 3J(HCCH) couplings of the corresponding ethylenes. A linear regression analysis yielded the correlation 3J(CH) = 0.46 ×3 J(HH)+1.58 Hz, the correlation coefficient being 0.956.
Progress in nuclear quadrupole resonance investigations of halogen‐containing organic and organometallic compounds
Authors: Voronkov M. G. , Feshin V. P.
Published: May 2005

Recent nuclear quadrupole resonance (n.q.r.) investigations of halogen‐containing organic and organometallic compounds, covering the period between 1970–1976, are reviewed. Special attention is paid to n.q.r. studies of the stereochemistry of organic molecules with particular emphasis on the interactions of geminal atoms, and the application of Stark effects to the study of the coordination and hybridization of atoms.
Three nuclei n.m.r. spectroscopy of dimethoate compounds. A large solvent effect on the 31PSC1H vicinal coupling
Authors: Paasivirta Jaakko , Simanainen Jarmo , Vesterinen Raili , Virkki Liisa
Published: May 2005

Proton, phosphorus and carbon magnetic resonance spectra of dimethoate, dimethoxon, des‐N‐methyldimethoate, ω‐hydroxydimethoate, trimethyldithiophosphate and O,O‐dimethyldithiophosphate in different solvents have been measured. Most of the n.m.r. parameters were characteristic of the structural environment of the corresponding nucleus and solvent‐independent. However, in the amide structures dimethoate, dimethoxon, des‐N‐methyldimethoate and ω‐hydroxydimethoate the 31PSC1H vicinal coupling showed an unusually large solvent effect of a 2.3 to 6.5 Hz decrease when CDCI3 was replaced by acetone‐d6 or DMSO‐d6.
Carbon‐13 n.m.r. studies: 68—diastereomeric nonequivalence and the characterization of some bisnorbornylpinacols
Authors: Stothers J. B. , Teo K. C.
Published: May 2005

The sodium‐ammonia reduction of racemic norcamphor, exo‐3‐methylnorcamphor and 3,3‐dimethyl‐norcamphor (camphenilone) leads to substantial yields (60–90%) of the corresponding 2,2′‐bisnorbornyl‐2,2′‐diols as diastereomeric mixtures. A combination of their 1H, 13C and infrared spectra together with the results of periodic acid and lead tetraacetate oxidations permits stereochemical assignments for these six pinacols.
Etude en RMN 13C de Spiropyrannes Hétérocycliques
Authors: Appriou P. Y. , Samat A. M. , Guglielmetti R. J. , Le Gall J. Y.
Published: May 2005

Une étude en RMN du carbone‐13 d'une série de spiropyrannes qui diffèrent par leur partie hétérocyclique a été réalisée. Nous avons tenté d'évaluer l'influence spécifique des divers hétéroatomes sur le déplacement chimique des atomes voisins. Les déplacements chimiques et les constantes de couplage J(CH) ont également été déterminés pour les composés analogues à chacune des deux parties constitutives de la molécule de spiropyranne: hétérocycle et benzopyranne.
Use of lanthanide shift reagents for the determination of the alkyl conformation in 2‐alkyl‐2‐adamantanols
Authors: Hájek M. , Trška P. , Vodička L. , Hlavatý J.
Published: May 2005

The position of the lanthanide atom calculated from the pseudocontact shift equation was studied for 2‐adamantanol and 2‐methyl‐, 2‐ethyl‐, 2‐propyl‐, 2‐butyl‐2‐adamantanol. The possible methods of calculation considering rotation of the methyl group were ascertained in the case of 2‐methyl‐ and 2‐ethyl‐2‐adamantanol. It follows from these results that a 2‐alkyl group affects the effective position of the lanthanide atom. The conformational analysis of 2‐ethyl‐2‐adamantanol demonstrates a difference among conformations obtained with various shift reagents. A comparison with extended Huckel theory calculations shows that the shift reagent method can be used only for a qualitative determination of the most advantageous conformation of the ethyl group.
Etude en Résonance Magnétique Nucléaire et Stéréochimie d'un Composé Cyclopropanique d'Intérět Thérapeutique. Résonance du Proton à 250 MHz avec Triple Irradiation
Authors: Gey C. , Perraud R. , Pierre J. L. , Cousse H. , D'Hinterland L. Dussourd , Mouzin G.
Published: May 2005

Le pivaloyl‐1 hydroxyméthyl‐2 cyclopropane est étudié en résonance magnétique nucléaire. La configuration relative au niveau C‐1—C‐2 est établie à partir de l'analyse du spectre de résonance du proton à 250 MHz, analyse qui nécessite des expériences de triple irradiation. La conformation autour de la liaison cycle‐carbonyle est examinée à partir de l'effet de solvant ASIS. La conformation autour de la liaison cycle‐hydroxyméthyle est étudiée à partir de l'effet de température sur les constantes de couplage vicinales. La conformation de l'hydroxyle est abordée (dans CCl4) à partir de J(HOCH). La conformation globale du composé peut ětre décrite.
Etude du Groupe Azomethine par RMN du Carbone‐13. Cétimines et Dibenzo[b, f] diazocines[1,4]
Authors: Ruxer J. M. , Solladié‐Cavallo A. , Solladié G. , Olliero D.
Published: May 2005

L'étude des déplacements chimiques 13C (effets de substituants, variation du blindage ou du déblindage dǔ à l'intervention d'interactions n.π ou π.π) confirme la structure non plane des cétimines diàromatiques étudiées et permet d'évaluer les angles de torsion des cycles aromatiques hors du plan de la double liaison CN. L'étude des déplacements chimiques 13C de dibenzo[b,f] diazocines‐1,4 confirme la structure ‘tub’ et la présence d'interactions n.π et π.π.
Zur Anwendung von NMR‐Verschiebungsreagenzien: Die Komplexbildungskonstante als Strukturparameter
Authors: Große M. , Roth K. , Rewicki D.
Published: May 2005

Die Struktur eines Tetra‐tert‐butyfluoren‐9‐ols wird mit Hilfe der Komplexbildungskonstanten tert‐Butyl‐substituierter Fluorenole bekannter Struktur mit Eu(fod)3, ermittelt.
La Vitamine B6 et ses Dérivés. I—Attribution des Spectres de Résonance 13C du Pyridoxol, du Pyridoxal et du Phosphate de Pyridoxal. Etude de la Protonation
Authors: Haran R. , Nepveu‐Juras F. , Laurent J.‐P.
Published: May 2005

L'existence de deux sites de protonation dans les molécules de pyridoxol et de pyridoxal se traduit au niveau de la résonance de chaque noyau 13C par des effects caractéristiques dont la résultante acquiert ainsi un caractère spécifique. L'analyse de ces effects permet de confirmer les attributions proposées par ailleurs et d'établir la nature des formes intervenant à chaque valeur du pH dans la structure de ces molécules. Il apparaǐt, en particulier, qu'aux pH voisins de 7, le pyridoxol et le pyridoxal existent essentiellement sous une forme zwitterionique, le pyridoxal étant par ailleurs sous une forme de type hémi‐acétal.
Conformational study of diastereomers. V—assignment of RR and RS relative configurations to 2,2,6,6‐tetramethyl‐5‐phenyl‐3‐heptanol by proton n.m.r.
Authors: Ibarra C. Alvarez , Perez M. S. Arias , Gonzalez F. Fernandez , Sarompas M. Rico
Published: May 2005

Assignment of RR and RS configurations to 2,2,6,6‐tetramethyl‐5‐phenyl‐3‐heptanol diastereomers has been carried out through an analysis of the observed values for vicinal coupling constants of both isomers and their conformational distribution. The latter was previously estimated from steric interaction energies between groups. Since the RR isomer is monoconformational its magnetic parameters can be used to carry out conformational analysis of related acyclic compounds.
1H and 13C n.m.r. spectra of dichloro(trans‐2‐chlorovinyl)arsine
Authors: Paasivirta Jaakko , Versterinen Raili , Virkki Liisa , Pyykkö Pekka
Published: May 2005

Proton and carbon magnetic resonance spectra of Lewisite or dichloro(trans‐2‐chlorovinyl)arsine have been measured and the results are compared with the n.m.r. spectral parameters of other trans‐1,2‐substituted ethylenes. The coupling constants can be rationalized by substituent electronegativity. The chemical shifts show an unusually large paramagnetic effect from the AsCl2 group.
An investigation of the utility of using n.m.r. lanthanide shift reagents for 13C assignments in selected aromatic ethers and ketones. A detailed description of the lanthanide induced shift ratio method for lanthanide shift reagent–substrate geometry analysis
Authors: Ammon Herman L. , Mazzocchi Paul H. , Colicelli Elena J.
Published: May 2005

The 13C and 1H spectra of a series of aromatic ketones, ethers and keto‐ethers were studied using the lanthanide shift reagent (LSR) Yb(fod)3 and the data analyzed using the lanthanide induced shift (LIS) ratio method. Studies were carried out on phenalenone, 6‐methoxyphenalenone, 1‐ and 2‐methoxynaphthalene, 1‐indanone, 5‐ and 6‐methoxy‐1‐indanone and flavone and the data obtained subjected to LSR‐substrate geometry analysis as a means of making 13C assignments. The utility and limitations of the LSR method for making 13C assignments is discussed and the LIS ratio method for LSR‐substrate geometry analysis is described in detail.
N.m.r. studies on aromatic compounds. I—13C n.m.r. spectra of some mono‐ and disulpho‐substituted hydroxycarboxylic acids
Authors: Räisänen Kauko , Lajunen Lauri H. J.
Published: May 2005

Carbon‐13 n.m.r. spectra of 3‐hydroxy‐4‐sulpho‐2‐naphthoic, 3‐hydroxy‐5‐sulpho‐2‐naphthoic, 3‐hydroxy‐7‐sulpho‐2‐naphthoic, 5‐sulphosalicylic, 3‐hydroxy‐5,7‐disulpho‐2‐naphthoic, 1‐hydroxy‐4,7‐disulpho‐2‐naphthoic, and 3,5‐disulphosalicylic acids were recorded with and without proton noise‐decoupling. Analyses of the spectra were carried out for all compounds except 3‐hydroxy‐5‐sulpho‐2‐naphthoic acid which dimerized. The fine splitting caused by long‐range coupling was used in identifying the lines of the 13C n.m.r. spectra.
Carbon‐13 NMR spectra of a series of para‐substituted N,N‐dimethylbenzamides. Comparisons of linear free energy relationships for carbon‐13 and nitrogen‐15 chemical shifts and rotation barriers
Authors: Jones Ronald G. , Wilkins Joyce M.
Published: May 2005

All carbon‐13 chemical shifts for 11 para‐substituted N,N‐dimethylbenzamides in 1 mole % chloroform solution are reported, with assignments based upon double resonance experiments, analogy to chemical shifts of benzamide, and self‐consistency between experimental and calculated values using recognized substituent parameters. In contrast to earlier reports, the aryl carbon chemical shift assignments for N,N‐dimethylbenzamide are C‐2, 127.0; C‐3, 128.7; C‐4, 129.4, and for p‐chloro‐N,N‐dimethylbenzamide are C‐1, 134.6; C‐4, 135.5 ppm, relative to internal TMS. Good Hammett correlations (σp) are reported for 13C chemical shifts of C‐1 (σ = 11.9 ppm) and even for the carbonyl group (σ = −2.3 ppm) but are markedly improved if correlated with σp+ (σ = 9.5 ppm) and Dewar's F (σ = −1.9 ppm), respectively. Excellent Swain–Lupton F and R correlations were found for some of the 13C chemical shifts and yielded values for percent resonance contributions to transmission of substituent effects as follows, C‐1, 75 ± 4%; C‐2, 51 ± 3%; CO, 31±2%. These are compared to similar values calculated from the CO of benzoic acids of 34±10%, and from the nitrogen‐15 chemical shifts of benzamides of 56±2%. Correlations of these 13C δ values and 15N δ values with rotation barriers (ΔG) for N,N‐dimethylbenzamides were examined, and it was found that while CO δ values correlated only poorly the C‐1 δ values correlated very well, but the best correlation was for 15N δ values of benzamides. It is suggested that Δ G and δ 15N are intrinsically related due to their numerical correlation, and the close similarity in percent resonance contribution of substituent influence on these parameters.
Nuclear magnetic resonance spectroscopy nitrogen‐15—hydrogen coupling constants in oximes
Authors: Oatis John E. , Schultz Harry P.
Published: May 2005

Nitrogen‐15—hydrogen coupling constants between 15N‐labeled oximes and α‐C‐protons, 3J(15NH), of two series of E/Z ketoximes were studied in CCl4, CDCl3, CF3COOH, and DMSO‐d6. 15N‐substituted‐methyl phenyl ketoximes constituted the first series and 15N‐substituted‐methyl 2‐quinoxalinyl ketoximes the second series studied. Substituents in the first group were H, Me, t‐Bu, NMe2, OMe, Cl, Br and I; in the second they were H, NEt2, and Cl. |3J(15NH)| values for the isomers wherein the aliphatic substituent was anti to the oximino hydroxy group were always larger then those for the syn related isomers, permitting assignment of oxime stereochemistry to even a lone oxime diastereomer. Additionally, geminal nitrogen‐15—hydrogen coupling, 2J(15NH) effects were observed in some compounds.
Bredt's rule. IV—the structure of an alleged bicyclo[3.3.1]nonan‐2‐ol‐9‐one
Authors: Buchanan G. L.
Published: May 2005

Claims that the condensation product of (−)‐menthone with benzylideneacetophenone is a bridged bicyclic ketol are shown to be erroneous. The true structure is shown by 13C n.m.r. to be 2‐(1′,3′‐diphenyl‐3′‐oxopropyl)‐3‐methyl‐6‐isopropylcyclohexanone.
RMN 13C d'Alcynes Aliphatiques: Analyse Topologique des Effets de Substituants Alkyles sur les Déplacements Chimiques des Carbones sp par la Méthode DARC PELCO
Authors: Dubois Jacques‐Emile , Doucet Jean‐Pierre
Published: May 2005

Les effets de substitution alkyle sur les déplacements chimiques des carbones sp d'alcynes aliphatiques sont analysés par la méthode DARC PELCO estimant les perturbations liées à l'introduction de sites localisés dans l'Environnement Limité, Concentrique, Ordonné des carbones sp, décrit par génération progressive à partir du foyer origine –CC..

Une corrélation de site ‘topologie‐δ’ est proposée et son aptitude à la prévision des déplacements chimiques est vérifiée sur une population de 55 alcynes linéaires ou fortement ramifiés correspondant à un domaine expérimental de 25,2 ppm.

Cette étude met aussi en éevidence (i) l'indépendance des effets des groupes alkyles portés par la triple liaison; (ii) leur atténuation lorsque croǐt la connectivité des sites substitués; (iii) la faible importance des effets γπ; (iv) l'additivité des incréments de perturbation de site—sans termes d'interaction—jusqu'à une frontière correspondant aux groupes –C(H,Me,tBu) et –C(Me2,tBu).

Un modèle simplifié est proposé à partir d'un graphe partiellement ordonné et à des fins de calcul rapide des δ, les effets globaux des radicaux alkyles sont caractérisés et exprimés en paramètres de groupe par sommation de leurs paramétres de site.
NMR studies on some 1,3,5‐triazaadamantane derivatives
Authors: Edwards A. , Webb G. A.
Published: May 2005

1H and 13C data are presented for eleven 1,3,5‐triazaadamantane derivatives. Ring protonation produces a decrease in 1H screening and the opposite effect in the corresponding 13C spectrum. The trisaryl substituted compounds appear to exist as a single isomer with diequatorial and monoaxial substituents.
Carbon‐13 NMR spectra of all the isomeric methyl hydroxy‐ and acetoxyoctadecanoates. Determination of chemical shifts by deuterium isotope effects
Authors: Tulloch A. P.
Published: May 2005

Carbon‐13 NMR spectra of the 17 isomeric methyl hydroxyoctadecanoates and the corresponding acetate derivatives have been measured and chemical shifts assigned to most carbons. Sixteen specifically deuterated hydroxy esters, and their acetates, were employed to make unambiguous assignments from the deuterium isotope effects on the spectra. When substituents are separated from the ends of the chain by 2–3 methylene groups their effects are largely additive. Long range effects of the hydroxyl group were γ, +0.01; δ, −0.09; ε, −0.11; ζ, −0.06; η, −0.05; and θ, −0.04 ppm, and of the acetate group were γ, −0.20; δ, −0.20; ε, −0.16; ζ, −0.11; η, −0.08 and θ, −0.07 ppm, showing that they extend across seven methylene groups.
Carbon‐13 NMR chemical shifts in some substituted 1,2,4‐triazol‐3‐ones
Authors: Litchman William M. , Hollstein Ulrich , Papadopoulos E. Paul
Published: May 2005

Carbon‐13 NMR data for 15 substituted 1,2,4‐triazol‐3‐ones are presented and discussed with regard to enolization in the neutral molecules. The coupling constant and chemical shift data show that the proton at N‐2 is not exchanging rapidly in the DMSO‐d6 solvent. Using D2OOD− as a solvent, it is found that the C‐3 and C‐5 resonances are shifted downfield by nearly the same amount, suggesting that the proton at N‐4 is being removed. Enolization in the neutral molecules does not occur to any significant extent.
Etude par RMN de 29Si de Sila‐1 Cyclopentènes‐2 et ‐3
Authors: Filleux‐Blanchard Marie‐Louise , Nguyen‐Dinh‐An  , Manuel Georges
Published: May 2005

Les mesures des déplacements chimiques de 29Si dans les silacyclopentènes vérifient les conclusions obtenues par RMN de 1H et 13C.
13C13C spin–spin coupling constants and 13C isotope effects on 13C chemical shifts in some 4‐membered rings
Authors: Jokisaari Jukka
Published: May 2005

The 13C13C spin–spin coupling constants in natural abundance oxetane, thietane, cyclobutanone, bromo‐and chlorocyclobutane have been measured. Furthermore, the 13C isotope‐induced changes in the chemical shifts of the different 13C nuclei in the molecules mentioned above are reported. These shifts are normally to higher magnetic field; in cyclobutanone, however, the resonance of the carbonyl carbon has shifted to lower field because of the substitution of 13C−3 for 12C−3.
Dynamic proton magnetic resonance studies on complex spin systems. Non‐mutual three‐spin exchange in N‐(trideuteriomethyl)‐2‐cyanoaziridine
Authors: Höfner Dieter , Tamir Ilana , Binsch Gerhard
Published: May 2005

At room temperature and below, the proton NMR spectrum of N‐(trideuteriomethyl)‐2‐cyanoaziridine consists of two superimposed ABC patterns assignable to two N‐invertomers; a single time‐averaged ABC pattern is observed at 158.9°C. The static parameters extracted from the spectra in the temperature range from –40.3 to 23.2°C and from the high‐temperature spectrum permit the calculation of the thermodynamic quantities ΔH0 = −475±20 cal mol−1 (−1.987 ± 0.084 kJ mol−1) and ΔS0 = 0.43±0.08 cal mol−1 K−1 (1.80±0.33 J mol−1 K−1) for the cis ⇌ trans equilibrium. Bandshape analysis of the spectra broadened by non‐mutual three‐spin exchange in the temperature range from 39.4–137.8°C yields the activation parameters ΔHtc≠ = 17.52±0.18 kcal mol−1 (73.30±0.75 kJ mol−1), ΔStc≠ = −2.08±0.50 cal mol−1 K−1 (−8.70±2.09 J mol−1 K−1) and ΔGtc≠ (300 K) = 18.14±0.03 kcal mol−1 (75.90±0.13 kJ mol−1) for the trans → cis isomerization. An attempt is made to rationalize the observed entropy data in terms of the principles of statistical thermodynamics.
Proton‐coupled 13C NMR and {14N}—1H‐INDOR of highly‐substituted 6‐membered heterocycles. I—trisubstituted pyridones and isomeric aminopyrones; tetrasubstituted pyridines
Authors: Cavagna F. , Pietsch H.
Published: May 2005

The structural elucidation by NMR spectroscopy of trisubstituted α‐pyridones and the isomeric 2‐amino‐γ‐pyrones as well as their internal and external pyrylium salts is described. The most useful parameter for the differentiation between α‐pyridones and isomeric γ‐pyrones is the geminal coupling constant 2J(C‐6, H‐5) which changes from ∼2.5 Hz to ∼7 Hz whenever the cyclic amide group is replaced by an oxa‐function; this applies to both the γ‐pyrones and their pyrylium salts. The value of J(C‐6, H‐5) in the pyridones resembles that of the analogous coupling in N‐vinylacetamide, whose sign determination by the selective population inversion (SPI) technique is reported. The 13C chemical shifts of seven pyridones, pyrones and pyrylium salts are reported and their structural correlations are discussed. Quick structural assignments in these classes of compounds may also be performed by evaluating the 14N chemical shifts, which often are accessible by the {14N}—1H‐INDOR technique. The proton coupled 13C NMR spectra of two tetrasubstituted pyridines are also reported, and empirical correlations between long range CH coupling constants and substituent electronegativity are discussed.
An unusual long‐range proton hyperfine coupling in a nitroxyl radical
Authors: Heinig Marsha J. , Eaton Gareth R. , Eaton Sandra S.
Published: May 2005

The EPR spectrum of doxyl cyclododecane (3,3‐dimethyl‐1‐oxa‐4‐azaspiro[4.11] hexadec‐4‐yloxy) exhibits a large (0.195 mT at −39°C) temperature‐dependent proton hyperfine coupling. Selective deuteration demonstrates that the coupling is to one of the protons in the CH2 groups adjacent to the point of attachment of the doxyl ring.
Carbon‐13 NMR studies of substituted naphthalenes. II—rotational barriers and conformations in some naphthaldehydes and azulenaldehydes
Authors: Drakenberg Torbjörn , Sandström Jan , Seita José
Published: May 2005

Proton‐coupled and noise‐decoupled 13C NMR spectra of 1‐azulenecarbaldehyde, 1‐acetylazulene and 1,3‐azulenedicarbaldehyde have been studied, and complete assignments have been made based on the 13C1H coupling constants, additivity of substituent effects (SIS), and previous assignment for the parent hydrocarbon. The barriers to rotation of the aldehyde group in the above azulenecarbaldehydes and in some naphthaldehydes have been determined by 13C dynamic NMR, (DNMR), resulting in free energies of activation of 42.7, 26.8 and 34.4 kJ mol−1 for 1‐azulenecarbaldehyde, 1‐naphthaldehyde and 2‐naphthaldehyde respectively. The same order of barriers is obtained by CNDO/2 calculations. A 4‐methoxy substituent in 1‐naphthaldehyde and a 6‐methoxy substituent in 2‐naphthaldehyde increases the rotational barrier by 4.6 and 2.9 kJ mol−1, respectively, whereas a 3‐methoxy substituent in 2‐naphthaldehyde reduces the barrier by 6.7 kJ mol−1. The conformations of the dominant rotamers are deduced from 13C chemical shifts to be Z for 1‐azulenecarbaldehyde and 1‐naphthaldehyde, and E for 2‐naphthaldehyde.
Optimisation et Calcul d'Erreurs des Equations du Type Karplus
Authors: Jankowski K. , Söler F.
Published: May 2005

On analyse trois méthodes pour obtenir des solutions des équations du type Karplus et le calcul d'erreurs. Une interprétation géométrique du problème permet de le résoudre par une méthode itérative.
Conformational study of phosphothreonine in aqueous solution. H and 13C NMR results
Authors: Pogliani L. , Ziessow D.
Published: May 2005

The 270 MHz 1H and 22.6 MHz 13C NMR spectra of DL‐phosphothreonine in D2O have been measured and analysed as a function of pD. The trans‐trans conformation of the fragment H‐αC‐αC‐βOP predominates at all pD values. The C‐β—O gauche contribution is notably larger for pD values in the range 7≤pD<10 than for acidic or more basic solutions which is in accordance with earlier results for phosphoserine (PSer).
Carbon‐13 NMR spectra of monosulphones and disulphones: Substitution rules and conformational effects
Authors: Fawcett A. H. , Ivin K. J. , Stewart C. D.
Published: May 2005

Carbon‐13 NMR spectra have been obtained for 27 aliphatic monosulphones and 13 aliphatic disulphones. Substitution rules have been formulated which allow the prediction of chemical shifts when a CH2 group in a hydrocarbon is replaced by an SO2 group, or when an H atom in a sulphone is replaced by an alkyl group. Carbon atoms β to a sulphone group show a marked upfield shift effect which is sensitive to the conformation about the CS bond; atoms in the trans position experience an upfield shift (−9.39±1.64 ppm) compared with those in the gauche position. An assignment of the meso (m) and racemic (r) β‐carbon peaks in di‐sec‐butyl sulphone was made by synthesis of the optically active compound. |δm–δLr| decreases with increasing temperature for both β–carbon atoms; this effect was interpreted in terms of two available conformers for each of the m and r compounds, those for the m form having the same energy and entropy, and those for the r form having ΔS=0.6 J K−1 mol−1 and ΔH=0.8 kJ mol−1. Observed chemical shifts for the disulphones were compared with those estimated by the application of the first set of substitution rules.
Differentiation between isomeric structures of benzazoles by 14N, 13C and 1H NMR
Authors: Stefaniak Lech
Published: May 2005

Nitrogen chemical shifts constitute an effective means of distinguishing between isomeric benzazole ring systems. There is a large difference in nitrogen shielding, usually more than 20 ppm, between isomeric benzenoid‐ and quinonoid‐like structures. 13C and 1H NMR is shown to provide unambiguous structure assignment only in cases of symmetric quinonoid ring systems of azoles.
13C‐NMR‐Spektren von Schwefelorganischen Verbindungen mit zwei vicinalen oder geminalen Sulfidgruppen
Authors: Petrova R. G. , Kandror I. I. , Dostovalova V. I. , Churkina T. D. , Freidlina R. Kh.
Published: May 2005

Es wurden die 13C‐NMR‐Spektren von 39 vicinalen und geminalen Bis‐sulfiden sowie einigen verwandten Verbindungen gemessen und die Nichtadditivität des Einflusses von zwei naheliegenden Sulfidgruppen auf die chemischen Verschiebungen von benachbarten Kohlenstoffatomen festgestellt. Die chemischen Ver‐schiebungen von 13C in den Bis‐sulfiden wurden mit denen der isostrukturen Kohlenwasserstoffe sowie der sauerstoff‐ und chlorhaltigen Verbindungen verglichen.
An 1H and 13C NMR study of pentane‐2,4‐dione tellurium(II) compounds; chemical shifts, 125Te1H and 125Te13C couplings
Authors: Dewan John C. , Jennings W. Brian , Silver Jack , Tolley Malcolm S.
Published: May 2005

1H and 13C NMR data are reported for pentane‐2,4‐dione tellurium(II) compounds which contain a 6‐membered tellurane ring bearing methyl or ethyl substituents. The 125TeCH coupling constants are particularly interesting in that they are highly stereospecific, viz. ∼60 Hz and 0–8 Hz for equatorial and axial protons, respectively, in the chair conformation. 1J(TeC) and 2J(TeCC) couplings lie in the ranges 118–146 Hz and 44–49 Hz, respectively. Geminal HCH couplings in these compounds (9–11 Hz) are lower than analogous values in tellurane or in cyclohexane‐1,3‐diones.
CIDNP‐Effekte bei Photolyse und Thermolyse von substituierten Phenyldiazoniumsalzen
Authors: Sterk H. , Lechner G.
Published: May 2005

Mit Hilfe des CIDNP‐Effekts wird die Existenz von Radikalpaaren bei der Photolyse und Thermolyse von substituierten Phenyldiazoniumtetrafluoroboraten und Hydrochinon in Azetonitril aufgezeigt. Die Reaktion wird als Einelektronenübergang unter Ausbildung von Phenyldiazen und p‐Benzosemichinon formuliert. Dieses Radikalpaar verliert, entweder durch Photolyse oder Thermolyse, Stickstoff und kann eine Disproportionierung oder verschiedene Rekombinationen eingehen.
Temperature dependence of the carbon‐13 chemical shifts of paraffinic hydrocarbons
Authors: Cheng H. N. , Bovey F. A.
Published: May 2005

A study of several linear and branched alkanes indicates that the temperature dependence of 13C chemical shifts is a complex phenomenon in which several non‐additive effects may be operative. The chemical shift temperature coefficients dδ/dT do, however, reveal some systematic trends which could be helpful in the assignments of 13C resonances. An empirical equation is proposed (akin to that of Grant and Paul for 13C chemical shifts) which accurately correlates all the data obtained near ambient temperatures.
Substituent effects on 1H NMR spectra of N,N‐dialkylanilines
Authors: Koleva V. , Galabov B. , Simov D.
Published: May 2005

Correlations with reactivity constants and quantum chemical parameters were used to characterize the substituent effects on the 1H NMR spectra of a series of N,N‐dimethylanilines and N,N‐diethylanilines. The results indicate that the variations of the chemical shifts of the methyl and methylene protons are largely determined by resonance effects.
Applications of NMR spectroscopy of chiral association complexes 5. Stereodynamics and diastereotopism in chiral 1,3‐dienes \documentclass{article}\pagestyle{empty}\begin{document}${\rm HOCMe}_{\rm 2} \rlap{--} ({\rm CCl =\!= CCl\rlap{--})}_{\rm 2} {\rm X}$\end{document}
Authors: Becher Georg , Burgemeister Thomas , Henschel Hans‐Helmut , Mannschreck Albrecht
Published: May 2005

The barriers to partial rotation around the central single bond in chiral dienes \documentclass{article}\pagestyle{empty}\begin{document}${\rm HOCMe}_{\rm 2} \rlap{--} ({\rm CCl =\!= CCl\rlap{--})}_{\rm 2} {\rm X}$\end{document} have been determined by coalescence of either 1H NMR signals (X = CH2OCH3) or 13C NMR signals (X = H). In the presence of the optically active shift reagent (+) − Eu(hfbc)3 all 1H signals were split at temperatures where the interconversion of enantiomers is slow. The temperature dependence of these spectra also yielded free activation enthalpies for the enantiomerizations which were in agreement with the ones obtained without Eu(hfbc)3. The assignment of the four methyl resonances appearing in the presence of (+) − Eu(hfbc)3 at low temperature was possible by gradually increasing the rate of enantiomerization or gradually replacing the optically active auxiliary compound by the racemic one.
Cage compounds IV. 1H and 13C NMR study of 3,6‐epoxypentacyclo‐[6.2.2.02,7.04,10.05,9]dodecane and 3,6‐epoxypentacyclo‐[6.2.1.02,7.04,10.05,9]undecane
Authors: Dekker T. G. , Pachler K. G. R. , Wessels P. L. , Dekker J. , Dekker J. J. , Fourie L.
Published: May 2005

The complete 1H and 13C NMR assignments for 3,6‐epoxypentacyclo[6.2.2.02,7.04,10.05,9]dodecane and 3,6‐epoxypentacyclo[6.2.1.02,7.04,10.05,9]undecane are reported. The difference between the 1H and 13C chemical shifts and one‐bond proton‐carbon coupling constants of these two compounds are adequately explained by the difference in the hybridization of the CH bonds.
Effects of solute–solvent interactions and acid catalysis on the rotational barriers in polarized ethylenes. The significance of large entropies of activation as measured by dynamic NMR
Authors: Berg Ulf , Sjöstrand Ulf
Published: May 2005

Complete 1H NMR bandshape analysis has been used to study the rotational barrier around the carbon–carbon double bond in two types of polarized ethylenes; on the one hand for planar ketene mercaptals (A), for which the hindered rotation over the 90 degrees twisted transition state is measured and, on the other hand, for sterically hindered ketene aminals (B and C) which are twisted in the ground state, and have measurable barriers over a planar, sterically crowded transition state. For compounds A activation entropies of the order of −45 to −76 J mol−1 K−1 (−11 to −8 cal mol−1 K−1), and for compounds B and C +28 to +42 J mol−1 K−1 (+7 to +10 cal mol−1 K−1) were obtained. This important difference was interpreted in terms of opposite charge development during the course of the rotation in the two cases, with concomitant reversed changes in the orientations of the solvent. The effects of solvent polarity and the presence of acid are shown to be reversed for the two types of compounds. A more polar solvent, and, in particular, addition of small amounts of acid decrease the barrier in system A but increase the barrier in systems B and C. A large negative entropy of activation was found for the acid‐catalyzed process, ΔS≠ = −107 J mol−1 K−1 (−25.5 cal mol−1 K−1). This low value is briefly discussed in relation to the mechanism of the acid‐catalyzed rotation.
Konformationsuntersuchungen mit Hilfe der 13C‐NMR‐Spektroskopie. III. 13C‐NMR‐spektroskopische Verschiebungen durch sterisch gehinderte Methylgruppen
Authors: Mann G. , Kleinpeter E. , Werner H.
Published: May 2005

Die 13C‐NMR‐Spektren einer Reihe von Cyclohexanen mit 1,3‐diaxialen Methylgruppen wurden bestimmt. Die Resonanzsignale der Methyl‐Kohlenstoffatome 1 und 3 sind in diesen Verbindungen um durchschnittlich 4,5 ppm nach tieferem Feld verschoben (δ‐Effekt). Auch die Ringkohlenstoffatome 1 und 3 zeigen Verschiebungen um durchschnittlich 0,7 ppm nach tieferem Feld (γ‐Effekt). In offenkettigen Kohlenwasser‐stoffen werden analoge Verschiebungseffekte beobachtet, wenn in den betreffenden Verbindungen die Geometrie des gPgM‐Konformeren des n‐Pentans vorkommt.
Dynamique Moléculaire des Chloro‐4 et Bromo‐4 Méthylènecyclohexanes par Résonance Magnétique Nucléaire du 13C
Authors: Iratçlabal P. , Grenier‐Loustalot M. F. , Lichanot A. , Métras F.
Published: May 2005

L'étude RMN 13C et 1H à basse température des chloro‐4 et bromo‐4 méthylènecyclohexanes nous a permis de déterminer les paramètres d'activation associés aux équilibres conformationnels de ces composés. Les résultats obtenus montrent que d'un point de vue dynamique les substituants Cl et Br ont la měme influence en séries cyclohexanique et méthylènecyclohexanique.
Effects of impurity on 35Cl and 14N NQR in p‐chloraniline
Authors: Buck K. R.
Published: May 2005

The effect of impurity (p‐toluidine) on the nuclear quadrupole resonance of two different nuclei (35Cl and 14N) in the same molecule has been studied. The 14N resonance is unaffected whereas the 35Cl resonance is greatly reduced.
13C NMR of methyl N‐methylthiobenzimidates. A novel approach to the interpretation of the relative 13C‐E/Z shifts in amides and related compounds
Authors: Walter Wolfgang , Ruback Wulf , Meese Claus O.
Published: May 2005

The 13C NMR shifts of the E/Z mixtures of twelve, mainly ortho substituted, methyl N‐methylthio‐benzimidates have been completely assigned. The angles of twist about the aryl‐imino‐carbon bond were determined by means of the C‐1 aryl shifts. Depending on the bulk of the ortho substituents the values vary from 18° in methyl 2‐fluoro‐N‐methylthiobenzibenzimidate (Z) to 73° in methyl 4‐bromo‐N,2,6‐trimethylthio‐benzimidate (E). An explanation for the 13C‐E/Z‐N‐alkyl and ‐S‐alkyl resonances is facilitated by a new model which is initially based on the influence of electric fields. As this model is derived from amides, thioamides etc. it is possible to interpret the 13C‐E/Z shifts in all related classes of compounds. The role of steric compression does not seem to be as important as was previously assumed.
A multinuclear NMR (13C, 15N and 29Si) study of the SiN bond in silylamines: (p – d) π interaction
Authors: Filleux‐Blanchard M. L. , An Nguyen Dinh
Published: May 2005

A series of variously substituted aminosilanes was investigated by 15N NMR spectroscopy to obtain further information on the controversial problem of pπ‐dπ interaction in these systems. The 15N NMR data are consistent with the 13C and 29Si results and suggest that the (p‐d)π backbonding is not negligible in these systems. The values of the 15N chemical shifts and the 13C parameters [δ13C and J(13CH)] are discussed in terms of nitrogen lone‐pair delocalization and provide a good basis for explaining the variations of the 29Si chemical shifts with the nature of the nitrogen atom substituents.
A 13C NMR study of tautomerism in phenoxymethylisoxazol‐5‐ones
Authors: Auzou Gilles , Rips Richard , Likforman Joseph , Hazebroucq Georges , Aji Marc
Published: May 2005

A 13C NMR study of new isoxazol‐5‐ones has been carried out in different solvents. Mathematical analysis of the tautomeric ring proton exchange in pyridine solvent indicates that the relative contributions of the NH, CH and OH tautomers are respectively 10 ± 5, 15 ± 3, 10 ± 8%, while the anionic form represents the remaining 65 ± 8%.
195Pt NMR studies on monomer and dimer complexes. Observation of PtPt coupling constants
Authors: Soulié J. , Lallemand J. Y. , Rao R. C.
Published: May 2005

Various platinum complexes have been studied by 195Pt FT NMR. Long range J(31P195Pt) and J(195Pt195Pt) are observed in dinuclear complexes. The value of 1J(31P195Pt) in monomeric and dimeric complexes is shown to depend mainly upon the Pt—P bond length.
Sels de Pyrylium. XVIII. Etude par RMN du Carbone‐13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N‐Diméthylaminopyrylium
Authors: Chenon M. Th. , Sib S. , Simalty M.
Published: May 2005

Dans les cations N,N‐diméthylamino‐2 pyrylium, la conjugaison du doublet de l'azote avec le cycle confère à la liaison C‐2N un caractère partiel de double liaison. Les valeurs des paramètres ΔG≠, ΔH≠ ΔS≠ relatifs à l'empěchement de rotation correspondant ont été déterminées par analyse de la forme des bandes. Cette conjugaison diminue le caractère électrophile du carbone C‐4, ce qui rend impossible les réactions de transalcoylation. Dans les cations N,N‐diméthylamino‐4 pyrylium il existe de měme une barrière de rotation autour de la liaison C‐4N, dont les paramètres ΔG≠ ont été évalués. La comparaison de ces deux types de cations montre que l'empěchement de rotation autour de la liaison CN est plus important en position 4 qu'en position 2. Par ailleurs, plus le substituant en position 4 est électrodonneur, moins la barrière de rotation autour de la liaison C‐2N est élevée: la valeur du paramètre ΔG≠ décroǐt de 19,1 kcal mol−1 (79,9 kJ mol−1) à 12,6 kcal mol−1 (52,7 kJ mol−1) selon l'ordre suivant des substituants R‐4: ‐C6H5, ‐CH3, –OCH3, –N(CH3)2.
Conformation et Spectrometrie de RMN. III. Spectres 13C de Diméthylamino Cyclanols et Dérivés
Authors: Laffite Colette , Wylde Renée , Mavel Gérard
Published: May 2005

Les spectres RMN du 13C de diméthylamino‐2 cyclohexanols, des quatre diméthylamino‐3 décahydronaphtols‐2 trans et de leurs iodures d'ammonium quaternaires sont décrits. Le calcul des séplacements chimiques de chaque carbone en tenant compte des interactions gauches permet d'attribuer correctement chaque signal. Dans le cas des composés cis ou trans diéquatorial, on a pu non seulement vérifier des modifications conformationelles portant sur les carbones substitués, mais on a pu mettre également en évidence des effects portant sur les carbones voisins, effets que les méthodes IR et RMN du proton ne pouvaient pas permettre d'observer.
Some sum‐over‐states perturbation calculations of 19F13C coupling constants for some carbocations and related fluorobenzenes
Authors: Duangthai S. , Webb G. A.
Published: May 2005

Sum‐over‐states perturbation calculations within the INDO framework are reported for 24 1J(FC) and 34 3J(FC) couplings. In general, satisfactory agreement with the experimental data is obtained when the integral products SF2(O) SC2(O) and 〈r−3〉F 〈r−3〉C take the values of 136.543 au−6 and 58.352 au−6, respectively, for the 1J(FC) couplings. The corresponding values for the 3J(FC) couplings are 29.520 au−6 and 44.340 au−6, respectively. All of the 1J(FC) values are predicted to be negative, whereas all of the 3J(FC) values are calculated with positive signs. The results indicate the importance of including the contact, orbital and dipolar contributions in the calculations.
The 13C NMR spectrum of coumarandione
Authors: Winkler Tammo , Ferrini Pier Giorgio , Haas Georges
Published: May 2005

A re‐examination of the 13C NMR spectra of coumarandione and isatin with the aid of proton‐coupled 13C NMR spectra and selective proton decoupling experiments is reported.
Some self‐consistent perturbation calculations of 1J(CC)
Authors: Khin Tun , Webb G. A.
Published: May 2005

Self‐consistent perturbation calculations of 70 1J(CC) values are reported within the INDO framework. A least‐squares agreement between the calculated and observed couplings, for a variety of bond multiplicities, provides values of 13.503 au−6 and 5.072 au−6 for (SC2(O))2 and 〈r−3〉C2, respectively. The non‐contact terms are found to be important in cases of multiple bonding.
Carbon‐13 NMR spectra of nucleoside 3′,5′‐cyclic phosphates. Proposition for revised signal assignments
Authors: Uesugi Seiichi , Tanaka Sumie , Ikehara Morio
Published: May 2005

On the basis of the data obtained from 13C NMR spectra of 8,2′‐S‐cycloadenosine 3′,5′‐cyclic phosphate and other nucleoside 3′,5′‐cyclic phosphate analogues, it is suggested that the published assignments of the C‐3′ and C‐4′ signals in nucleoside 3′,5′‐cyclic phosphates should be reversed. According to the revised assignments, C‐4′, which is fixed very closely to the diesterified phosphate group is markedly shielded (−12.5 to −15 ppm), and the C‐3′ signal shows a downfield shift (+6 to +8 ppm) which is comparable to that for the C‐5′ signal, for all compounds so far measured when compared with the chemical shifts for the corresponding nucleosides. The 3′,5′‐cyclic phosphates of thymidine and 8,2′‐S‐cycloadenosine, which have no α‐OH group on C‐2′, show similar chemical shift changes for the corresponding sugar carbons which are different from those observed for ribonucleoside derivatives.
Les Amines Biogènes (Phénol‐ et Catéchol‐Amines): Attribution des Spectres de Résonance 13C et Etude de la Deprotonation
Authors: Haran Raymond , Nepveu‐Juras Françoise , Laurent Jean‐pierre
Published: May 2005

Un certain nombre d'amines biogènes (phénol‐ et catéchol‐ amines) ont été étudiées en solution aqueuse par RMN du 13C. Les attributions ont été réalisées sur les formes protonées et l'influence de la nature de la chaǐne latérale sur les conditions de résonance des noyaux du cycle a été étudiée. Les différences de comportement observées à la déprotonation entre phénol‐ et catéchol‐ amines sont discutées en fonction de la répartition géométrique des centres chargés. Il est établi que, dans les deux séries, le groupement amino protoné est le site le moins acide.
1H and 13C NMR studies on 3‐aryl‐5‐alkyl‐2,4‐oxazolidinediones. The magnetic non‐equivalence of isomeric nuclei
Authors: Içli Siddik
Published: May 2005

Magnetic non‐equivalence of diastereotopically related proton and 13C nuclei in enantiomeric and diastereomeric rotational isomers of 3‐aryl‐5‐alkyl‐2,4‐oxazolidinedlones has been investigated. Steric interactions between the aryl and the heterocyclic moieties of these compounds produce sufficient restriction to rotation about the aryl CN bond that the presence of torsional isomers may be detected at normal temperatures. Free energies of activation for rotation have been calculated. Chemical shift differences, associated with rotational isomerism, may be detected for both the proton and the 13C nuclei on the hetero ring, but only in the 13C spectra of the nuclei on the aryl moiety.