bluuebunny/crossref_metadata_2025_split huggingface dataset:
Proton and carbon‐13 nuclear magnetic resonance studies of substituted pyrimidines. V13C, 1H coupling contsants for some methyl‐ and aminopyrimidines
Authors: Riand Jacques , Chenon Marie‐Thérèse 
Published: May 2005

AbstractJ(13C1H) coupling constants for some methyl‐ and aminopyrimidines have been determined by 13C NMR. Both the one‐bond and long‐bond and long‐range coupling constants follow general trends which can be summarized in a few simple rules. In particular, the 3J(C‐i,H) coupling constants between a ring carbon C‐i and the ring protons are larger than the 2J(C‐i,H) coupling constants. The opposite is observed for the couplings between the ring carbons and the methyl protons: 3J(C,Me). These general rules are very useful for the assignment of resonances in complex 13C spectra of pyrimidines and seem to be valid for other 6‐membered aromatic nitrogen heterocycles. Furthermore, the additivity of substituent effects on 1J (CH) for monosubstituted pyrimidines allows the estimation of 1J (CH) for polysubstituted pyrimidines with a very good accuracy.
1H dynamic nuclear magnetic resonance study of hindered internal rotation in N‐aroyl‐ and N‐thioaroyl‐N′‐ piperonylpiperazines
Authors: Benassi Rois , Taddei Ferdinando , Leonardi Amedeo , Nardi Dante 
Published: May 2005

AbstractThe NMR variable temperature behaviour of N‐aroyl‐ and N‐thioaroyl‐N′‐piperonylpiperazines was investigated. The largest chemical shift separation between the exchanging methylene groups, and the highest energy barrier, is found for the thioamide compounds. Results from semi‐empirical calculations are qualitatively in line with the experimental findings, revealing that the higher barrier in the thiocarbonyl compounds compared with the carbonyl compounds is mainly connected with differences in the energies of ground states, rather than in those of transition states, of these molecules. The effect of the nature of the heterocyclic ring containing the amidic nitrogen atom on the barrier height is also discussed.
Nuclear magnetic resonance of psychotherapeutic agents. IV—conformational analysis of 2,3‐dihydro‐1H‐1,4‐benzodiazepines
Authors: Romeo G. , Aversa M. C. , Giannetto P. , Ficarra P. , Vigorita M. G. 
Published: May 2005

AbstractThe lanthanide shift reagent (LSR)/1H NMR study of some 7‐chloro‐5‐phenyl‐2,3‐dihydro‐1 H‐1, 4‐benzodiazepines shows that these compounds exist in CDCI3 solution as two pseudo‐boat conformers, rapidly interconverting at room temperature. Computer simulated lanthanide induced shifts (LIS) are consistent with LSR complexation by the imine nitrogen atom. The lanthanide shifts the conformational equilibrium towards conformer (a), with a pseudoequatorial 1‐substituent: this effect may be the result of the greater basicity of 4‐N when the 1‐N lone pair is pseudoaxially directed, thus permitting an extended electron delocalization from 1‐ to 4‐N through the fused benzene ring. The detection of H‐9/1‐Me through space spin‐spin coupling in medazepam (7‐chloro‐1‐methyl‐5‐phenyl‐2,3‐dihydro‐1H‐1,4‐benzodiazepine) supports the observed higher availability of conformer (a) to LSR. A LIS computer calculation to predict the population ratio results in reasonable agreement with the conformational changes based on the response of 3J(HH) to lanthanide addition. The electronic and steric effects which determine the greater stability of conformer (a) may be effective in the drug‐receptor interaction.
1H and 13C NMR study of dihydro derivatives of methylphenobarbital
Authors: Rautio Marjatta , Rahkamaa Erkki 
Published: May 2005

AbstractSodium borohydride reduction of methylphenobarbital leads to the formation of two different dihydroderivatives, reduced either in position 4 or position 6. The structures of the derivatives have been determined through analysis of the 1H and 13C NMR spectra of phenobarbital, methylphenobarbital and their derivatives. Detailed interpretation of the spectra and the resulting spectral parameters indicate conformations where the hydroxyl groups are axial and trans to the ethyl group. In these configurations the phenyl ring becomes equatorial. The results also allow an unambiguous assignment of the resonances of the phenyl carbons 2′(6′) and 3′(5′) of phenobarbital and the carbonyl carbons 4 and 6 of methylphenobarbital, differing interpretations having been previously advanced in the literature.
13C chemical shifts and conformational analysis of S‐methylthiolanium cations
Authors: Barbarella G. , Dembech P. 
Published: May 2005

AbstractThe 13C NMR spectra of all cations obtained by methylation at sulphur of the mono‐and dimethylthiolanes are reported. The methyl substituent on sulphur affects the shieldings of the adjacent carbons in a manner which allows easy identification of the cis and trans isomers. For most compounds the 13C pattern is consistent with a half‐chair ring conformation with maximum staggering at C‐3, C‐4. Only with methyl groups at the 1,2‐or 1,2,3‐positions is the half‐chair appreciably deformed. It is suggested that in these cases the preferred conformation is a quasi‐envelope with C‐3 at the top.
8 and 16‐Membered ring phosphorus compounds. Ring size dependence of the NMR parameters
Authors: Martin J. , Robert J. B. 
Published: May 2005

AbstractThe 1H NMR spectral analysis of four 8‐membered rings, 2‐thioxo‐ (or 2‐oxo‐) 2‐R‐1,3,6,2‐ trithiaphocane, is reported. The stereochemistry of the 8‐membered ring is discussed. The 31P NMR spectral parameters [δ31P, 1J(PC)] obtained on several cyclic 1,3,2‐dithiaphospha compounds of variable size (5, 6, 8, 12 and 16‐membered rings) are discussed as a function of the ring size and of the geometry of the molecule.
15N and 13C satellites in proton spectra of nitromethane oriented in a lyotropic solvent
Authors: Diehl P. , Bjorholm T. , Amrein J. 
Published: May 2005

Abstract15N and 13C satellites in the Proton spectrum of nitromethane oriented in a lyotropic solvent have been observed. Indirect as well as direct (13CH) and (15NH) couplings have been measured and the molecular structure has been derived.
15N NMR substituent effects in pyridines and pyrimidines
Authors: Städeli W. , von Philipsborn W. 
Published: May 2005

Abstract15N chemical shifts of 32 substituted pyridines and 19 substituted pyrimidines, together with additional data from the literature, are used to evalute substituent increments, Ai and Aik, in the respective series. Differential chemical shifts, Δδ(N), correlate with corresponding Δδ(C) values whereby, on the ppm scale, nitrogen shifts are approximately three times more sensitive towards substituents than carbon shifts. The 15N increments have proven additive and useful for assignment purposes.
Some quantum chemical aspects of solvent effects on NMR parameters
Authors: Ando I. , Webb G. A. 
Published: May 2005

AbstractThe non‐bonded interactions which occur between solute and solvent molecules are discussed and the various models which describe the interactions are presented. The models are employed, within a quantum chemical framework, to estimate the extent of medium effects on chemical shifts and spin‐spin couplings. Comparison between the calculated and available experimental data is provided for some first row nuclei.
Dynamic NMR study of hindered rotation around the CN bond in thiopiperidides
Authors: Ciureanu Mariana , Sahini Victor E. , Cornea Felicia , Cercasov Cornelia , Contineanu Mihai 
Published: May 2005

AbstractThe barriers to internal rotation about the CN bond in several thiopiperidides have been determined by the NMR technique. It was found that the barrier height (ΔG‡) increases in the series cinnamoylthiopiperidide<thiobenzoylpiperidide<phenylthioacetylpiperidide. This trend was discussed in terms of the electronic structure, using HMO calculations. For m‐and p‐substituted cinnamoylthiopiperidides the barriers were shown to increase with the electron withdrawing character of the substituent in the aromatic ring.
Examination of the aspirin acetylation site of human serum albumin by 13C NMR spectroscopy
Authors: Gerig J. T. , Katz K. E. , Reinheimer J. D. , Sullivan Glenn R. , Roberts John D. 
Published: May 2005

AbstractHuman serum albumin has been specifically acetylated using aspirin in which the methyl carbon of the acetyl group was enriched to 90% 13C. A single resonance at 23.13 ppm downfield from tetramethylsilane was observed in 13C differences spectra obtained at both 25.2 and 45.3 MHz. Chemical shift studies of several model compounds suggest that this is the resonance position to be expected for an acetamide group exposed to solvent. The line width observed for the enriched methyl resonance is consistent with free rotation of the methyl group.
Fast searching for identical 13C NMR spectra via inverted files
Authors: Bremser W. , Wagner H. , Franke B. 
Published: May 2005

AbstractInverted files of spectral features should be employed when searching for identical or similar spectra. The efficiency of interescting the entries for chemical shifts with a small deviation is very high for larger molecules and incomplete spectra. A novel spectral code describing the spectral appearance in hierarchical order (SAHO) facilitates the search for complete spectra with a relatively high tolerance of the chemical shifts. A vailable information on the chemical composition of the molecule, as well as the incusion of SFOR‐multiplicities, will further narrow down the number of suitable reference compounds.
Diastereomeric non‐equivalence of (−)‐ω‐camphanic esters of chiral secondary alcohols in 13C NMR spectroscopy
Authors: Jurczak Janusz , Konowal Andrzej , Krawczyk Zbigniew , Ejchart Andrzej 
Published: May 2005

AbstractThe diastereomeric differences (Δδ) were measured by 13C NMR spectroscopy for a series of 11 (−)‐ω‐camphanic esters of secondary alcohols of known absolute configuration. This method cannot be used for the determination of the absolute configuration, but it can be applied to the estimation of the enantiomeric purity of chiral alcohols.
Carbon‐13 NMR spectra of 3‐aryl‐2‐thioxo‐4‐imidazolidinones. Conformational isomerism and substituent effects
Authors: Khadim Mohammad A. , Colebrook Lawrence D. 
Published: May 2005

AbstractA 13C NMR study of a series of 3‐aryl‐2‐thioxo‐4‐imidazolidinones, which are capable of existing as enantiomeric or diastereomeric rotational isomers about the aryl CN bond, shows that rates of internal rotation are slow at the probe temperature. The effects of hetero‐ and aryl‐ring substituents on hetero‐ring carbon atoms have been established.
Conformational preferences of some substituted methyl groups in cyclohexanes as studied by carbon‐13 nuclear magnetic resonance
Authors: Kitching William , Olszowy Henry , Adcock William 
Published: May 2005

AbstractLow temperature 13C NMR spectra of 80:20 mixtures of cis and trans‐4CH3CH3C6H10CH2X, where C6H10‐is 1, 4‐disubtituted cyclohexyl and X=Br, CN, OH, OCH3, Si(CH3)3, Sn(CH3)3, Pb(CH3)3 and HgOCOCH3 have been recorded. The signals of the trans (e, e) components were assigned from the ambient temperature spectra of C6H11CH2X and the established substituent effects of an equatorial methyl group in cyclohexane. Conformational equilibria of the cis (e, a⇄a, e) components were then computed from the intensities of the (remaining) signals (∼180 K) of the two conformational isomers. From these equilibria A values of CH2X were calculated, assuming additivity of conformational energies of CH3 and CH2X (the counter‐poise approach). In general, these values are very similar to the value of the CH3, although some trends do emerge. This study provides α, β, γ, and δ effects for a wide range of axial and equatorial CH2X groups.
The relationship between 19F and 13C substituent chemical shifts and electron densities. 2—p‐phenylacetyl fluorides
Authors: Brownlee Robert T. C. , Craik David J. 
Published: May 2005

AbstractThe 19F substituent chemical shifts (SCS) of a series of para‐phenylacetyl fluorides (XPhCH2COF) are reported and compared with the related benzoyl fluoride series (XPhCOF). A dual substituent parameter analysis of the results for the new series shows that both inductive and resonance effects are reduced by one third when compared with the benzoyl fluorides. 13C shifts for the side chain carbonyl were also measured and found to follow a reversed trend in substituent effects, consistent with a pi polarization mechanism. SCS values for the fluorine and its adjacent carbon are not directly related. Ab initio (STO‐3G) calculations of the carbon and fluorine electron density for this series have been compared with the appropriate SCS values. From the electron densities and the observation that the fluorine SCS values follow a normal direction, whilst those for electron densities and the observation that the fluorine SCS values follow a normal direction, whilst those for the adjacent carbon are reversed, it is concluded that fluorine SCS values (and Δqπ values) result from polarization of the CF pi bond and do not merely monitor changes in electron density of the adjacent carbon.
Studies in tertiary amine oxides. III—carbon‐13 NMR assignment of N‐alkynyl cyclic amines
Authors: Al‐Rawi Jasim M. A. , Khuthier Abdul‐Hussain 
Published: May 2005

AbstractCarbon‐13 NMR spectra of 24 N‐alkynyl cyclic amines were analysed. The effect of substituting the acetylenic proton by n‐butyl, tert‐butyl and phenyl groups was determined.
Carbon‐13 nuclear magnetic resonance studies of anthraquinones. IV—correlation of substituent effects for 1‐substituted anthraquinones
Authors: Berger Y. , Berger‐Deguée M. , Castonguay A. 
Published: May 2005

AbstractThe spectra of various 1‐substituted and 1,5‐disubstituted anthraquinones have been studied and the chemical shifts of the different carbon nuclei determined. The chemical shifts previously reported for 1‐Me‐anthraquinone have been corrected and reassigned. The C‐1, C‐4 and C‐13 shifts of 1‐substituted derivatives are correlated with the chemical shifts of monosubstituted benzenes. Deviations from the regression lines can be explained by the existence of steric factors and hydrogen bonding. A three‐parameter correlation with Swain and Lupton's ℱ and ℛ and Schaefer's Q provides relationships for the prediction of all chemical shifts of 1‐substituted anthraquinones when the substituents have a cylindrical symmetry.
The use of 13C satellites in proton spectra for a study of the rα structure and of the internal rotation of p‐chlorotoluene partially oriented in nematic phases
Authors: Diehl P. , Moia F. 
Published: May 2005

AbstractThe proton magnetic resonance spectra with natural abundance 13C satellites of p‐chlorotoluene in isotropic as well as in oriented solvents are analyzed. The indirect CH and HH couplings are determined from the isotropic sample and one bond 13C isotope effects on the proton chemical shifts are measured. The direct couplings derived from the oriented samples are corrected for harmonic vibration and used to determine the molecular rα structure with a 6‐fold hindering potential for the methl group. The resulting CH and CC bond lengths have errors which are smaller than 0.002Å. They deviate from the unsubstituted benzene molecule by between −1.1 and −0.5% for the CH and between +0.3 and +0.5% for the CC distances. It is not possible to obtain conclusive information about the hindering potential.
1H NMR spectra of benzo[b]furan, benzo[b]thiophene and benzo[b]selenophene oriented in a lyotropic mesophase
Authors: Bechtold W. E. , Goldstein J. H. 
Published: May 2005

AbstractThe proton nuclear magnetic resonance spectra of benzo[b]‐furan, ‐thiophene and ‐selenophene oriented in a potassium laurate lyotropic liquid crystal have been analyzed. Geometrical information has been obtained and compared to previous thermotropic results and various hypothetical models.
13C nuclear magnetic resonance spectra. XV—the γ‐anti substituent effect and its dependence on substitution at the α‐ and γ‐carbon atom
Authors: Duddeck Helmut , Islam Md. Rabiul 
Published: May 2005

AbstractIt is shown that the γα effects of hetero substituents on the 13C chemical shift (γα SCS) are enlarged by +2 to +5 ppm by substituting the α‐hydrogen atom by any group or atom (e.g. CH3, OR, F, Cl, Br). The same is encountered when the axial γ‐hydrogen is replaced by CH3, OH or F. If, however, the substituting atom at the γα‐carbon atom is a higher‐row halogen (Cl or Br), diamagnetic γα SCS for this signal are observed which may even exceed those for unsubstituted γα‐carbon atoms. The removal of a 1,3‐diaxial hydrogen‐hydrogen interaction and the existence of a still unspecified ‘heavy halogen effect’–both diamagnetic contributions to the γγ SCS of hetero substituents–are responsible for these findings. Methyl groups do not behave like hetero substituents with respect to the γα SCS.
Analysis of the 13C NMR spectra of mono‐ and dimethoxy‐β‐methyl‐β‐nitrostyrenes: Spectral assignments and conformational effects
Authors: Bailey Keith , Legault Donald 
Published: May 2005

AbstractThe 13C NMR spectra of β‐methyl‐β‐nitrostyrene and its nine mono‐ and dimethoxy‐derivatives have been determined. Chemical shift data are presented and analysed in a self‐consistent manner which allows most of the signals to be unambiguously assigned. Trends in the data can be interpreted in terms of the conformational properties of the variously‐substituted compounds.
Application of 13C and 15N NMR spectroscopy to structural studies on nitramines
Authors: Bulusu Suryanarayana , Axenrod Theodore , Autera Joseph R. 
Published: May 2005

Abstract15N and 13C chemical shifts and the 1J(15N15N), 1J(15N13C) and 1J(13CH) coupling constants have been determined for a number of 15N‐enriched cyclic and acyclic secondary nitramines. The results are interpreted in terms of both electronegativity effects and conformational factors.
Solvent effects on the nitrogen screening of some nitroalkanes
Authors: Witanowski M. , Stefaniak L. , Lamphun B. Na , Webb G. A. 
Published: May 2005

AbstractHigh precision 14N screening measurements are presented for some nitroalkanes. The screening increases as the dielectric constant of the solvent decreases in all cases considered. The β effect on the nitrogen screening is found to be largely independent of solvent, and is thus an intrinsic property of the nitroalkanes. Good agreement is obtained between the observed solvent effects on nitrogen screening and those calculated by the SOS procedure using INDO/S parameters. It is concluded that the measured nitrogen screening changes monitor the electronic changes which occur in the nitroalkanes as the medium is changed.
Saturated 5‐membered ring conformation. 1—1H NMR study of protonated methyltetrahydrofurans in superacid medium
Authors: Infarnet Y. , Duplan J. C. , Huet J. 
Published: May 2005

AbstractSeveral methyltetrahydrofurans have been studied in HSO3FSbF5SO2 solution. O‐Protonation was observed from 1H NMR measurements at low temperature, and the coupling constants between +OH and the CH protons α to oxygen were measured from the CH signals. A conformational interpretation could be carried out from these results.
On the nature of haloform–aromatic complexes
Authors: Fernández Bertrán J. , Rodríguez M. 
Published: May 2005

AbstractA study of the correlation of haloform proton solvent shifts in 13 aromatichydrocarbons gives clear evidence that the nature of the interactions is predominantly dipole‐induced dipole and not specific acid‐base interactions. The presence of halogen bonding interactions between the halogen atoms and the π electrons are not detected for CHCl3 and CHBr3, and can only be present to a marginal extent for CHI3. The ASIS effect is not related to the molar concentration of the aromatic solvents, and the effect of alkyl substituents is explained in terms of exctuded zones around the aromatic ring.
Preferential site of solvation of the furan ring in acidic solvents
Authors: Bertrán José Fernández , Rodríguez Mario 
Published: May 2005

AbstractA study of the correlation of solvent effects on the chemical shifts of the methyl protons of sylvan, toluene and α‐picoline led to the conclusion that the preferential site of solvation of the furan ring in acidic solvents is the π‐electron cloud and not the n‐electron orbital of the oxygen atom.
High resolution NMR in solids; A simple and improved ‘magic angle’ probe design for conventional small gap 13C FT spectrometers
Authors: Bayle J. P. , Courtieu J. , Jullien J. , Kan S. K. 
Published: May 2005

AbstractThe mechanical and electronic design of a ‘magic angle’ NMR probe is described. This probe can be incorporated in a conventional small gap 13C FT spectrometer, without any hardware or software change, to obtain high resolution 13C NMR spectra on organic solids. A simple way of tuning the homogeneity and adjusting the angle is also proposed. Illustrative experimental results on various types of molecular crystals with different molecular mobilities, a clathrate compound and a polymer are discussed.
Saturated 5‐membered ring conformations. 2—1H NMR study of protonated N‐methylpyrrolidines
Authors: Infarnet Y. , Duplan J. C. , Huet J. 
Published: May 2005

AbstractThe 1H NMR spectra of the protonated and unprotonated forms of cis‐ and trans‐N‐methylhexahydroisoindoline, of 1,3,3,4,4‐pentamethylpyrrolidine and of 1,3,3‐trimethylpyrrolidine were analyzed. The coupling constants between the NH⊕ and the CH protons α to the nitrogen allowed the study of the conformation of the cyclic systems in pseudorotation.
Non‐empirical calculations of the nuclear magnetic resonance spectra of imidazole and hydrated imidazole
Authors: Prado F. Ribas , Giessner‐Prettre C. , Pullman B. 
Published: May 2005

AbstractThe magnetic shielding constants of the 1H, 13C and 15N nuclei of imidazole are calculated for the isolated and hydrated molecules. The results show that the hydrogen bonds produce not only large variations of the chemical shifts for the nitrogen nuclei and the NH proton which are directly involved in the intermolecular bonding, but also measurable shifts for the carbon nuclei. The calculated shielding constants and their variation with hydration are discussed in relation to experimental results concerning imidazole, the 5‐membered ring of the purine bases and the imidazole ring of histidine. The calculated values of the spin‐spin coupling constants confirm that it is possible to study the tautomeric equilibrium of the imidazole ring from the measurement of these coupling constants and that spin‐spin coupling constants are not very sensitive to solvent effects.
13C NMR investigations of phenol–triethylamine complexes: The influence of hydrogen bond interaction on the electronic structure of phenols
Authors: Ilczyszyn Marek , Latajka Zdzisław , Ratajczak Henryk 
Published: May 2005

AbstractThe influence of hydrogen bond formation on the screening at the C‐1 carbon of phenols is investigated, both experimentally and theoretically. Qualitatively good agreement between experimental and predicted data, obtained by means of calculations based on the CNDO/2 method and the Karplus–Pople theory, is observed. The main factors determining the chemical shift changes in the series of complexes are discussed.
1H NMR studies of solvent effects on hydrogen bonding in some pyridine trifluoroacetates
Authors: Dega‐Szafran Zofia , Dulewicz Ewa 
Published: May 2005

AbstractThe chemical shifts of hydrogen bonded protons in complexes of 11 substituted pyridines with trifluoroacetic acid were examined, in five dry solvents of different activity, with respect to proton transfer and aggregation effects. The results were correlated with ΔpKa, the Kirkwood function and E T parameters. The solvent effect on the intersection point obtained from the plot of the chemical shift of the hydrogen bonded protons against ΔpKa can be used, similar to an isotopic effect, to differentiate strong hydrogen bonds. The aggregation of acid–base complexes can lead to downfield or upfield shifts; the variation of chemical shift with aggregation depends on the position of the proton in the hydrogen bridge.