bluuebunny/crossref_metadata_2025_split huggingface dataset:
Intramolecular van der waals interactions and chemical shifts: A model for β‐ and γ‐effects
Authors: Li S. , Chesnut D. B. 
Published: May 2005

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

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

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

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

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

AbstractThe molecular geometries and proton NMR spectra of norbornene, norbornane and adamantane are reported. The molecular geometries were refined from previous experimental determinations using MNDO and ab initio techniques to determine the positions of the hydrogen atoms. No evidence of any endo—endo interactions in norbornane was obtained from these calculations.High‐field proton NMR spectra allow the refinement of previous spectral analyses of adamantane and norbornane, and the complete analysis of the olefin decoupled eight spin system of norbornene. The chemical shifts of the parent compounds and cyclohexane under comparable conditions show minor but significant differences from previous values. The methine—methylene coupling in adamantane is 3.30 Hz. The long‐range couplings in norbornene were compared with those calculated by FPT/INDO and EHMO/VB methods. Only moderate agreement was obtained and possible reasons for this are examined.
Revision of the 13C NMR signal assignment of harman, based on one‐ and two‐dimensional INADEQUATE spectroscopy
Authors: Welti D. H. 
Published: May 2005

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

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

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

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

Abstract13C NMR shifts for ethyl (E)‐(α‐cyano)cinnamates, (E)‐(α‐cyano)cinnamamides and ethyl (α‐ethoxycarbonyl)cinnamates of general formula XC6H4CHCR1R2 (R1  CN, R2  COOEt; R1  CN, R2  CONH2; R1  R2  COOEt; X p‐NMe2, p‐OH, p‐OMe, p‐Me, H, p‐Br, p‐Cl, p‐NO2, m‐Cl, m‐NO2, o‐OMe, o‐CI, o‐NO2) in DMSO‐d6 solution are reported. Substituent chemical shift considerations allow the assignment of the aromatic carbons in the disubstituted rings. The effect of substituents on the aromatic, ethylenic, cyano and carbonyl chemical shifts is discussed in relation to that of other arylethylenes. The J(CCCH) long‐range coupling values provide evidence in favour of the E configuration for both series of cyano derivatives.
High‐resolution 13C nuclear magnetic resonance spectra of some solid biphenyl derivatives
Authors: Chippendale A. Margaret , Aujla Rajinder S. , Harris Robin K. , Packer Kenneth J. , Purser Stephen 
Published: May 2005

Abstract13C NMR spectra of the solid phase have been obtained for biphenyl and a series of its substituted derivatives. Some species give essentially similar spectra in the solid and solution phases, whereas others show clear differences. In several cases the differences are shown to be due to the locking of the molecules into a particular conformation in the solid. For some compounds different chemical shifts for the C‐1 and C‐1′; carbon atoms in the solid and solution phases are believed to reflect differences in the angle between the ring planes in the two phases.
1H and 13C assignment of cyclopenta[cd]pyrene by two‐dimensional NMR spectroscopy
Authors: Jans A. W. H. , Tintel C. , Cornelisse J. , Lugtenburg J. 
Published: May 2005

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

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

Abstract17O Chemical shifts for trimethylene sulphite and a number of substituted trimethylene sulphites are presented. Spectral values are compared by consideration of the probable conformation(s) in solution.
ESR and 1H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals
Authors: Khramtsov V. V. , Weiner L. M. , Gogolev A. Z. , Grigor'ev I. A. , Starichenko V. F. , Volodarsky L. B. 
Published: May 2005

AbstractA new synthesis of nitronylnitroxyl (NNR), iminonitroxyl (INR) and nitroxyl radicals (NR) is suggested, involving oxidation of 4H‐imidazol di‐N‐oxides with O2 or PbO2 in the presence of nucleophilic reagents. This method allowed the preparation of radicals which could not be synthesized by known procedures. These radicals were studied by ESR and 1H NMR spectroscopy. The hyperfine interaction constants of the NNR were measured and assigned (in some cases radicals containing 15N were used). The influence of protonation and deprotonation of functional groups (OH, NH2) in the NNR on their ESR spectra was investigated and the pK values of these radicals were measured by ESR.Differences between the ESR spectra of two isomeric INR were found, and the structures of these radicals were established by 1H NMR. Changes in spin density delocalization in 3‐imidazoline 3‐oxide nitroxyl radicals were studied, and the spin density was found to be ‘pushed out’ to the peripheral fragments of these radicals on increasing the number of methoxy groups in the α‐position to the N—O. fragment. Hindered rotation around the σ‐bond of a bulky substituent in the α‐position to the N—O. fragment was detected by ESR. The activation energy of this hindered rotation was ΔE = 30±1.7 kJ mol−1.
Conformational changes of amino acids monitored by deuterium nuclear relaxation rate
Authors: Van Haverbeke Yves , Muller Robert N. , Elst Luce Vander 
Published: May 2005

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

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

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

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

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

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

AbstractThe 1H and 13C NMR spectra of 4,4′‐bis(N‐isopropyl‐N‐nitrosoamino)biphenyl at 22°C were consistent with the presence of three isomers due to slow rotation about the N—N bonds of the two N‐nitroso groups. The structures of the three isomers were assigned by means of their symmetries and from the known effects of the two orientations of the N‐nitroso group on 1H and 13C NMR chemical shifts. Each conversion of an N‐nitroso group from anti to syn to an aryl group was associated with the same free energy difference (0.26 kcal mol−1) after symmetry corrections were made. The changes in the 13C NMR spectra recorded from 95 to 150°C were simulated using a program for classical four‐site exchange. The potential barrier to rotation was calculated to be 20.76±0.06 kcal mol−1 at 400 K.
A molecular dynamics approach concerning the structures of 2‐hydroxy‐ and 2‐methoxy‐benzyl methyl ethers based on thenJ(CH2,H) long‐range spin–spin couplings
Authors: Laatikainen Reino , Tuppurainen Kari 
Published: May 2005

AbstractA procedure for combining experimental NMR information and, for example, molecular mechanics is described. An iterative computer program and a strategy based on the approach are applied for the determination of the torsional behaviour of the CH2group in 2‐hydroxy‐ and 2‐methoxy‐benzyl methyl ethers. The analysis gives fairly well defined potentials of the torsion and angular density functions. The least‐squares model obtained for the5J(CH2,H) coupling is in accordance with previous observations. STO‐3G levelab initiocalculations for 2‐hydroxybenzyl methyl ether are reported and are in reasonable agreement with the dynamic model.
Stereochemical assignment of the two diastereomeric 4‐tert‐butyl‐7,11‐bisarylspiro[5.5]undecane‐1,9‐diones using two‐dimensional nuclear magnetic resonance spectroscopy
Authors: Rele Dinesh H. , Mathur Hari H. , Trivedi Girish K. , Chary K. V. R. 
Published: May 2005

AbstractPhase‐transfer catalysed condensation between 4‐tert‐butylcyclohexanone and bis(benzal)acetone yielded two 4‐tert‐butyl‐7,11‐bisarylspiro[5.5]undecane‐1,9‐dione diastereomers. The stereochemical assignment of these molecules was carried out using two‐dimensional (2D) NMR spectroscopy. Combined use of COSY and NOESY spectra of these diastereomers helped in unambiguous and complete proton spin system assignment. In addition, from the application of 2D J‐resolved spectra together with the Karplus equation, the exact conformation of the molecules could be established.
Spin delocalization from paramagnetic metal complexes [Cr(acac)3 and Fe(acac)3] to saturated hydrocarbons in CHCl3 and CCl4 solutions
Authors: Kitaigorodskii Alexander N. , Stepanov Alexander G. , Nekipelov Vyatcheslav M. 
Published: May 2005

AbstractParamagnetic 13C NMR shifts induced by tris(acetylacetonato)chromium(III) [Cr(acac)3] and tris(acetylacetonato)iron(III) [Fe(acac)3] in various saturated hydrocarbons were determined for the first time using a special technique.The observed shifts, Δδspec, are the result of partial delocalization of unpaired electrons from the paramagnetic complexes on the alkanes. The value of Δδspec depends strongly on the position of a carbon atom in the hydrocarbon molecule, and tends to decrease on passing from primary to quaternary carbon atoms. The observed spin transfer seems to reflect the ‘contact charge transfer.’ The Δδspec shifts in the unsaturated hex‐1‐ene only slightly exceed those in alkanes.