bluuebunny/crossref_metadata_2025_split huggingface dataset:
Investigation of hydrogen‐bonded compounds using high‐resolution NMR spectroscopy in solids. Part 3—1H chemical shifts in polycrystalline trihydrogen selenites
Authors: Rosenberger H. , Scheler G. , Moskvich Yu. N. 
Published: May 2005

Abstract1H high‐resolution NMR spectra of polycrystalline alkali trihydrogen selenites were measured at 270 MHz using the combination of a multiple‐pulse sequence and magic angle sample spinning. Good resolution of the separate 1H NMR lines belonging to different hydrogen bonds in these crystals was obtained. Correlations between the measured average values of the chemical shift tensor and the parameters characterizing hydrogen bond [R(O…O) and R(H…O)] distances and the deuteron quadrupolar coupling constant e2qQ/h are presented. These correlations are found to be linear, with high correlation coefficients for the ordered protons. The possibilities of designating disordered protons on finding large deviations from the correlations for ordered protons are discussed.
DEPT‐related sequences for selective long‐range heteronuclear chemical shift correlation spectroscopy
Authors: Kövér Katalin E. , Batta Gy. 
Published: May 2005

AbstractDEPT‐related sequences are proposed which suppress the generally unnecessary direct proton‐carbon correlations from the long‐range optimized heteronuclear chemical shift correlation spectra. One‐bond modulation of long‐range response intensities is removed through the use of a BIRD pulse train. It is demonstrated that these modified sequences are superior to the original 2D DEPT experiment for the detection of long‐range connectivities of protonated carbons.
14N NMR nuclear shielding and the electronic structure of dibenzo[1,3a,4,6a]tetrazapentalene
Authors: Witanowski M. , Sitkowski J. , Sicińska W. , Biernat S. , Webb G. A. 
Published: May 2005

Abstract14N NMR data and semiempirical molecular orbital calculations are reported for dibenzo‐[1,3a,4,6a]tetrazapentalene (1). The 14N signals were assigned by means of the results of the molecular orbital calculations and measured relative signal widths. A comparison was made with results for some azolopyridines. A linear correlation was obtained between the nitrogen shieldings, expressed with respect to the bridging nitrogen atom, and the corresponding relative charge densities. Thus nitrogen nuclear shielding results provide a good estimation of electron charge distribution in these molecules. Compound 1 provides the first case in which the shielding of a pyridine‐type nitrogen atom is greater than that of the indolizine‐type nitrogen.
Natural abundance 17O NMR spectra of chromanones, chromones, flavanones and flavones
Authors: Duddeck Helmut , Jászberényi Joseph Csaba , Lévai A. , Timár Tibor , Elgamal M. Hani A. 
Published: May 2005

Abstract17O NMR data for some chromanones, chromones, flavanones and flavones are discussed in terms of steric, mesomeric and inductive substituent interactions, intramolecular hydrogen bonding and conformational changes. It is shown that such effects can be used to rationalize 17O NMR data of compounds with larger and more complicated structures.
High‐field 2D NMR spectroscopy of amanitin isomers
Authors: Pehk T. , Haga M. , Vija H. , Lippmaa E. 
Published: May 2005

AbstractThe 1H, 13C and 15N NMR spectra of α−, β− and γ‐amanitins and amaninamide were recorded at 11.7 T and analysed by various two‐dimensional (2D) NMR spectroscopic techniques. All diastereotopic protons were assigned, and the side‐chain conformations and the conformation of the hydroxyproline residue were determined. The observed 15N chemical shifts are caused by non‐bonded interactions of the peptide nitrogens. 1H, 13C and 15N data are complementary to each other and show that the conformation of β‐amanitin is slightly different from that of the other isomers, in accordance with the lower toxicity. The NOESY and carbonyl 13C1H chemical shift diagrams confirm the closeness of the conformations of amanitins in DMSO solution to that of β‐amanitin in the crystalline state.
34S isotope effects on 31P chemical shifts in thiophosphate esters
Authors: Bjorholm Thomas , Larsson Flemming C. V. 
Published: May 2005

AbstractThe 34S isotope effect on 31P chemical shifts was measured for 15 different alkyl thiophosphates. For the doubly bonded S a linear relationship exists between the magnitude of the isotope effect and the 31P chemical shift. For the singly bonded S no such correlation was found.
Unusual behavior of isopropyl substituients in 9,10‐dihydroanthracenes: An NMR and molecular mechanics study
Authors: Rabideau Peter W. , Smith W. Kimmer , Ray Bruce D. 
Published: May 2005

AbstractA collection of cis‐ and trans‐9‐isopropyl‐10‐R‐9,10‐dihydroanthracenes were investigated. The cis compounds show large H‐9‐isopropyl methine coupling constants in the series R  Me, Et, i‐Pr, t‐Bu (8.8–9.9 Hz) due to a preferred orientation of the isopropyl group(s), caused by a transannular steric effect, wherein the H‐9—isopropyl methine dihedral angle is close to 180°. These preferred conformations are predicted by molecular mechanics calculations (MM2 and MMP2) and data for the optimized geometries are presented. The trans compounds show considerably smaller coupling constants (J = 2.6, 5.0 and 7.8 Hz for R = t‐Bu, i‐Pr and Et, respectively) and these numbers are easily rationalized from the calculated geometries. However, the compound with R  Me shows an unusually high value (9.6 Hz), and calculations suggested the presence of a structure with unusually large puckering of the dihydroanthracene central ring. This was confirmed by nuclear Overhauser enhancements and by proton and carbon relaxation measurements.
Hetero‐association of caffeine and theophylline with purine and pyrimidine in aqueous solutions studied by 1H NMR chemical shift measurements
Authors: Aradi Ferenc , Földesi András 
Published: May 2005

AbstractThe interaction of caffeine and theophylline with purine and pyrimidine was studied in deuterium oxide at 35°C by measuring the concentration‐dependent selective changes in the chemical shifts of the N‐methyl protons of the xanthine derivatives. Using a competitive dimer model, the equilibrium constants give a decreasing tendency for hetero‐association within the series caffeine‐purine (2.97 ± 0.15 1 mol−1) > theophylline‐purine (2.44 ± 0.10 1 mol−1) > caffeine‐pyrimidine (1.18 ± 0.20 1 mol−1) > theophylline‐pyrimidine (0.70 ± 0.08 1 mol−1), and the upfield dimer shifts suggest a plane‐to‐plane arrangement.
Structural effects on ESR hyperfine coupling constants of some dithiazolidinyl free radicals
Authors: Krebber Angelika M. L. , Sutcliffe Leslie H. , Lubitz Wolfgang , Kurreck Harry 
Published: May 2005

AbstractA series of related dithiazolidin‐2‐yl free radicals prepared from cyclic strained alkenes and tetrasulphur dinitride, and an attempt was made to relate the nitrogen and proton hyperfine coupling constants to the radical structures. The radical made from dimethoxybenzonorbornadiene was studied by ENDOR and it was found that isotopic substitution with 15N gives well resolved ENDOR spectra; the TRIPLE measurements showed that the 14N coupling constants are positive whereas all the resolved proton coupling constants are negative.
13C NMR chemical shifts of substituted 2,2‐dimethyl‐2H‐chromenes
Authors: Tímár Tibor , Sebök Péter , Kövér Katalin E. , Jászberényi Joseph Csaba , Batta Gyula 
Published: May 2005

AbstractThe 13C NMR assignments for 35 substituted 2,2‐dimethyl‐2H‐chromene derivatives are reported. These include methyl, tert‐butyl, chloro, nitro, methoxy, ethoxy, n‐propyloxy, isopropyloxy, n‐butyloxy, sec‐butyloxy, isobutyloxy, cyclopentyloxy, benzyloxy and prop‐2‐ynyloxy groups as substituents on the aromatic ring.
Solid‐state and solution conformations of methadone hydrochloride and related compounds
Authors: Sumner Susan C. J. , Moreland Charles G. , Carroll F. Ivy , Brine George A. , Boldt Karl G. 
Published: May 2005

AbstractCarbon‐13 NMR data for salts of methadone and methadol in the solid‐state were used to elucidate the crystalline forms of the compounds and to determine if more than one conformer contributes to a single crystalline form. A comparison of 13C NMR isotropic chemical shift data for these compounds in the solid‐state and in various solvents was used to correlate solution and solid‐state conformations (determined from x‐ray data). Three‐bond 1H13C and 1H1H coupling constants, measured by various one‐ and two‐dimensional NMR techniques, were compared with those calculated from x‐ray dihedral angle data in order to confirm or dispute the conformational predictions based on chemical shift comparisons.
Proton NMR study of partially oriented benzene and chlorobenzene: Determination of the rα structure and of bond contributions to the orientation
Authors: Wasser R. , Kellerhals M. , Diehl P. 
Published: May 2005

AbstractSolvent effects on the structures of benzene and chlorobenzene were studied by NMR of partially oriented molecules in six liquid crystals. Allowing for the correlation between vibration and rotation, a unique solvent‐independent rα structure could be determined for both molecules. The contributions of equivalent bonds to the orientation energy of the two molecules were found to differ slightly.
13C NMR study of ortho‐, meta‐ and para‐alkoxycarbonylalkyloxychalcones and their α,β‐dibrome derivatives
Authors: Szajda Maria 
Published: May 2005

AbstractThe 13C NMR spectra of eighteen new derivatives of chalcone have been recorded, calculated and fully assigned. The Ai, empirical parameters of the styrylketone, p‐alkoxycarbonylalkyloxystyrylketone and benzoylethylene group were calculated for the prediction of the chemical shifts of substituted benzenes and chalcones.
Effect of electron lone‐pairs on nuclear spin–spin coupling constants
Authors: Gil Victor M. S. , Von Philipsborn Wolfgang 
Published: May 2005

AbstractThe nature and types of lone‐pair effects on nuclear spin coupling constants are reviewed in the context of a localized bond description of molecular electronic structure. Emphasis is placed on the importance of residual delocalization involving the otherwise lone‐pair orbital, in terms of which the effect of an X lone‐pair, when compared with an isoelectronic YH or XH+ group or with an XR group, and its orientational dependence can be interpreted. One‐, two‐ and three‐bond coupling constants are considered and the importance of lone‐pair effects for configurational and conformational information is stressed and illustrated. They also serve for a better understanding of substituent inductive effects on coupling constants.A large collection of illustrative examples are presented, with particular attention paid to couplings involving H, C, N, F and P nuclei, organized in a systematic manner into nine categories. The signs of the lone‐pair effects on the reduced coupling constants are found to be independent of the actual nuclei under study in the same category. This is taken as an indication that the electron lone‐pairs mainly affect the Fermi contact contribution to the coupling and, accordingly, an interpretation is given in terms of simple sum‐over‐states models. In addition, symmetry‐based relationships are established involving the sign of lone‐pair effects in coupling constants between nuclei which are a different number of bonds apart.
Chemistry of the SO bond. 11—carbon‐13 and oxygen‐17 nuclear magnetic resonance studies of stereoisomerism in 1,3,2‐dioxathiepanes
Authors: Hellier Desmond G. , Liddy H. Glendon 
Published: May 2005

AbstractThe stereoisomerism of some methyl‐substituted 1,3,2‐dioxathiepanes has been investigated by 13C and 17O NMR spectroscopy. The effects of substituents on the conformational equilibria of the seven‐membered rings are discussed and compared with those for six‐membered ring sulphites.
Two‐dimensional NMR spectroscopy of LL‐28249‐α, a potent macrolide anthelmintic: 13C, 1H chemical shift assignments and solution conformation
Authors: Rajan S. , Stockton G. W. 
Published: May 2005

AbstractLL‐F28249‐α is a potent antiparasitic antibiotic produced by Streptomyces cyaneogriseus ssp. noncyanogenus. Its complete 13C NMR spectral assignment based on two‐dimensional double‐quantum experiments in a sample at natural isotopic abundance is described. The proton NMR assignments were made from a combination of homonuclear and heteronuclear two‐dimensional correlation experiments. Proton‐proton scalar couplings, nuclear Over‐hauser enhancements and molecular mechanics energy minimization calculations are also reported, permitting a comparison of solution conformation with the known x‐ray structure adapted from a closely related analogue.
1H NMR studies of proton transfer in ortho‐mannich bases
Authors: Rospenk M. , Sobczyk L. 
Published: May 2005

AbstractThe 1H chemical shifts of the active bridge protons and their temperature dependences were measured in various solvents for a number of ortho‐Mannich bases containing proton‐donor (OH) and proton‐acceptor (N) centres of varying strength. It was found that a maximum on the plot of δH versus ΔpKa occurs, similar to that in intermolecular hydrogen‐bonded systems, although the shape of the plot is different. The maximum is located in the critical (inversion) region where the proton transfer equilibrium takes place. The contribution of the proton transfer form increases substantially on cooling, so that one observes a dramatic temperature effect on the chemical shift for some derivatives. The line width of the bridge protons shows a sharp narrowing in the critical region, which can be interpreted as being due to the fast jumping of protons between the donor and acceptor atoms.
Observation of a tight‐turn conformation in a proline‐containing inhibitor of renin angiotensin
Authors: Srivastava Sudha , Phadke Ratna S. , Govil Girjesh 
Published: May 2005

AbstractThe solution conformation of renin inhibitor peptide, Pro‐His‐Pro‐Phe‐His‐Phe‐Phe‐Val‐Tyr‐Lys, was established using proton magnetic resonance techniques. As a first step a complete resonance assignment was made in DMSO‐d6 using 2D NMR techniques. Some of the backbone NH protons show a very low temperature coefficient, indicating their involvement in intramolecular hydrogen bonding. The 3J(NH–Cα, H) values the inter‐residue NOEs, the temperature coefficient of the backbone NH protons and the chemical shift positions indicate the presence of a tight turn in the molecule. A model is proposed, using computer graphics, based on the experimental results.
Transfer of polarization in the rotating frame. Application to drug—nucleotide complexes
Authors: Belleney Joél , Delepierre Muriel 
Published: May 2005

AbstractTransfer of polarization experiments in the rotating frame are used to assign proton resonances in drug‐nucleotide complexes. Transfer by an incoherent process (CAMELSPIN, ROESY) is used to separate contributions from chemical exchange and dipolar interactions, and transfer by a coherent process (HOHAHA, TOCSY) is used to assign scalar coupled protons when lines are broad.
Proton nuclear magnetic resonance study of the biologically active peptide analog [Pro2]thymopentin
Authors: Griffith Guy K. , Huang Dee‐Hua , Nettesheim David G. , Heavner George A. , Krishna N. Rama 
Published: May 2005

AbstractThe solution conformation of the pentapeptide Arg‐Pro‐Asp‐Val‐Tyr ([Pro2]TP5), a biologically active analog of the immuno‐regulatory peptide thymopentin, Arg‐Lys‐Asp‐Val‐Tyr (TP5), was investigated by proton nuclear magnetic resonance spectroscopy. The chemical shift variations with pH, the vicinal coupling constants and the amide hydrogen exchange rates were measured. The hydrogen exchange rates for Val4 NH and Tyr5 NH are more than six times slower than the predicted values for the corresponding solyated peptides. The guanidino NaH proton of the Arg1 residue shows a downfield shift with a pKa of 3.5 as the pH is increased. The conformational properties of [Pro2]‐thymopentin were compared with those of two other closely related immunoregulatory peptides, thymopentin and [Glu3]TP5 (SP5).
Inter‐ and intra‐molecular hydrogen bonding effects on geminal 15N, 1H spin coupling and 15N chemical shifts in pyridine derivatives
Authors: Holzer W. , Von Philipsborn W. 
Published: May 2005

AbstractThe two‐bond 15N,H coupling constant in 3,5‐dimethylpyridine was determined in 30 solvents and shown to vary between −11 and −3 Hz. An increasing ability of the solvent to form hydrogen bonds causes a steady decrease in ∣ 2J(15N,H)∣, which is also observed with some selected pyridine derivatives with intramolecular hydrogen bonds. A linear correlation is found between the positive lone‐pair effect on ∣2J(15N,H)∣ and the decrease in nitrogen shielding.
Simple model for NMR relaxation times of flexible molecules in solution
Authors: Baldo M. , Grassi A. 
Published: May 2005

AbstractThe motion of a flexible molecule in solution can be described as a Brownian motion between the different conformations that the molecule can assume. Each conformation corresponds to a local minimum of the energy surface. A simple model is given for this type of motion in relation to nuclear magnetic resonance (NMR) relaxation measurements. This is based only on the assumption that the diffusion tensor for the overall tumbling is not appreciably affected by jumping between the different conformations. The limits of the model are discussed and an illustrative application to 2‐fluorobutane is given.
Relationship between proton—proton NMR coupling constants and substituent electronegativities. IV—An extended karplus equation accounting for interactions between substituents and its application to coupling constant data calculated by the Extended Hückel method
Authors: Donders Lambertus A. , De Leeuw Franciscus A. A. M. , Altona Cornelis 
Published: May 2005

AbstractSome well known Karplus equations are discussed and their shortcomings are indicated. To remedy the problems associated with the experimentally observed non‐additive substituent effects on vicinal proton‐proton coupling constants, an extension given recently is discussed; this accounts for pairwise interactions between substituents through specific quadratic cross terms used to describe the electronegativity dependence of the coefficients in a truncated Fourier series. Since this formulation will be used in combination with simultaneous least‐squares optimization of both the Fourier coefficients and the electronegativity values, an invariance property of this formulation, and of similar ones, is discussed in order to elucidate problems associated with parameter redundancy. The procedure is then applied in a stepwise manner to the analysis of a set of 1404 coupling constants calculated by a repara‐meterized version of the Extended Hückel method for ethanes singly or multiply substituted with Cl, F, Me and OH, and ethane itself. This clearly reveals its essential features, and leads to a markedly improved description.
13C and 1H NMR studies of some aminoarsonium chlorides
Authors: Watkins C. L. , Krannich L. K. , Kanjolia R. K. , Srivastava D. K. 
Published: May 2005

Abstract13C and 1H NMR spectral data for a homologous series of tertiary arsines (R3As, where R = Me, Et, n‐Pr and Ph), aminoarsines and arsonium and aminoarsonium salts are reported and discussed. In nearly all cases, quaternization of the tertiary arsine to its corresponding arsonium or aminoarsonium salt causes a decrease in the magnitude of δc. These results are compared with those from an analogous study on the quaternization of tertiary phosphines, R3P. A discussion of the factors that affect the relative magnitudes of δc and R3As and R3P is given. Hammett‐Taft dual substituent parameter analysis was used to show that the positive charge on the arsenic in the atomatic quaternary salts is delocalized by an inductive mechanism. The 1H NMR spectra of the n‐propyl‐ and phenyl‐containing compounds are non‐first order. The δH and nJ(HH) values were determined by iterative computer analysis. Likewise, the nJ(CH) values for the phenyl carbons of [Ph3AsNH2]Cl and [Ph3AsNHMe]Cl were determined from the coupled 13C NMR spectra.
NMR spectral assignment and conformation of macrocyclic biaryl ethers from Garuga gamblei
Authors: Kalchhauser H. , Krishnamurty H. G. 
Published: May 2005

AbstractThe assignment of the 1H and 13C NMR spectra of macrocyclic biphenyl ethers isolated from Garuga gamblei King was performed by 2D NMR techniques. An erroneous assignment given in the literature for one of the compounds was corrected. The use of shift increment systems and a 13C NMR data base demonstrated the applicability of these methods even in the case of very unusual structural elements. The conformation of the compounds in solution is discussed with the help of arguments derived from coupling constants and intramolecular anisotropy effects.
Determination of the 13C chemical shift tensors in isotactic polypropylene via the two‐dimensional powder pattern in rotating solids
Authors: Nakai T. , Ashida J. , Terao T. 
Published: May 2005

AbstractThe two‐dimensional powder pattern for isotactic polypropylene in a sample rotating at an off‐magic angle has been measured and the chemical shift principal values and principal axis directions for each of the CH, CH2 and CH3 carbons, together with their CH bond lengths, have been determined.
The stereochemistry of crown ethers. III—Conformational variations during alkali complexation
Authors: Kleinpeter E. , Stoss S. , Gäbler M. , Schroth W. 
Published: May 2005

AbstractThe1H and 13C NMR spectra of a series of structurally related crown ethers were recorded, and alkali metal complexation of the crown ethers was studied for different Li, Na and K salts. The alkali metal salt‐induced 1H and 13C chemical shifts are discussed in terms of conformational changes within the macrocyclic ring, and the results obtained were proved to be general by means of NOE enhancement factors and spin‐lattice relaxation time measurements.
Solid‐state 13C chemical shift calculations in aryl ether compounds
Authors: Clayden N. J. 
Published: May 2005

AbstractChemically inequivalent carbons ortho to ether substituents are often seen in the solid‐state 13C NMR spectra of aryl ether compounds. Calculation of the shielding caused by the steric, magnetic anisotropy, electric field and ring‐current mechanisms demonstrates the importance of the electric field contribution for small phenyl ring— ether dihedral angles. For large dihedral angles, steric and ring‐current mechanisms must also be taken into account.
13C NMR study of tetrahydrothiazolo‐[3,2‐d][1,4]benzodiazepin‐3[2H]‐ones
Authors: Zappalá M. , Romeo G. , Grasso S. , Chimirri A. , Mon‐Forte A. M. 
Published: May 2005

AbstractThe complete assignments of the 13C NMR resonances for eight tetra‐hydrothiazolo [3,2‐d] [1,4] benzodiazepin ‐3(2H)‐ones are reported. The data obtained account for a conformational rigidity of the heptatomic ring as a consequence of the annelation of the thiazolidinone nucleus.
Assignment of 1H and 13C NMR spectra of retinoic acid ester isomers observed in the long‐wavelength UV‐induced photostationary state
Authors: Curley Robert W. , Fowble John W. 
Published: May 2005

Abstract1H and 13C chemical shift assignments, obtained with the aid of two‐dimensional heteronuclear shift correlation and double‐quantum coherence 13C‐INADEQUATE experiments, are reported for four retinoic acid ester isomers observed at the photo‐stationary state. Also detailed are 1H assignments for three additiona isomers obtained in insufficient amounts for 13C measurements.