bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dynamic proton magnetic resonance studies on complex spin systems. Non‐mutual three‐spin exchange in four 1‐substituted cyclohexanes‐2,2,3,3,4,4,5,5‐d8 and mutual four‐spin exchange in cyclohexane‐1,1,2,2,3,3,4,4‐d8
Authors: Höfner Dieter , Lesko Susan A. , Binsch Gerhard 
Published: May 2005

AbstractThe intrinsic advantage proffered by coupled three‐proton spin systems in the quantitative study of conformational equilibria and equilibrations, as elaborated and exemplified in the preceding publication, is exploited for the determination of precise thermodynamic and dynamic parameters for octadeuteriocyclohexanes containing a cyano, chloro, bromo or deuteriomethoxy substituent and hydrogen atoms on the substituted and on one contiguous carbon. Dynamic quantities for the ring reversal of cyclohexane‐1,1,2,2,3,3,4,4‐d8, obtained by bandshape analysis of the deuterium‐decoupled proton NMR spectra broadened by mutual four‐spin exchange, are also reported. The measured standard entropy differences between the axial and equatorial conformational isomers of the monosubstituted cyclohexanes show that, in addition to the rotational entropy, there must be a significant contribution from the vibrational partition functions. The experimental activation entropies for the cyclohexane chair‐to‐twist‐boat conversion are at variance with conclusions drawn from naive symmetry considerations, but agree with the more sophisticated treatment of Pickett and Strauss; the data establish that there is virtually free pseudorotation in the transition states of the ring reversal for cyclohexane itself as well as for the monosubstituted derivatives studied in this investigation.
High resolution 13C NMR spectroscopy. IV—stereochemical assignments in butenedioic acids and 3‐pentene‐2‐ones
Authors: Braun S. 
Published: May 2005

AbstractVicinal 13C, H coupling constants 3J(CO, H) for butenedioic acids and 3J(CH3, H) for 3‐pentene‐2‐ones have been determined and are correlated with the configuration of the corresponding CC double bond. For both types the relationship 3J(CH) trans > 3J(CH)cis holds; in the case of the CH3, H couplings, however, the 3J(CH3, H) trans values are reduced because of steric reasons, so that configurational assignments seem possible only when both isomers are present. Additionally, the coupling constants 3J(COCH3,H) and the chemical shifts δ have been evaluated for the pentenones and it is shown that these parameters give information about the predominating conformation of α, β‐unsaturated methyl ketones.
Carbon‐13 NMR studies of substituted naphthalenes. I—complete assignments of the 13C chemical shifts with the aid of deuterated derivatives
Authors: Seita José , Sandström Jan , Drakenberg Torbjörn 
Published: May 2005

AbstractProton‐coupled as well as noise‐decoupled 13C NMR spectra of several substituted naphthalenes have been studied. Complete assignments of the 13C signals based on selectively deuterated derivatives and on the 13C1H coupling pattern have been made. For the methoxynaphthalenes, acetylnaphthalenes and naphthaldehydes the dominant conformations of the substituents have been deduced from the 13C chemical shifts.
Methyl substituent effects in the 13C spectra of cyclohexanone O‐methyl oximes and N,N‐dimethylhydrazones
Authors: Fraser Robert R. , Dhawan Kasturi L. , Taymaz Kevser 
Published: May 2005

AbstractThe 13C NMR spectra of a series of 4‐t‐butylcyclohexane‐O‐methyl oximes and N,N‐dimethylhydrazones containing α‐methyl substituents have been measured. The effects of α‐methyl substitution (syn‐axial, anti‐axial and anti‐equatorial) on the carbons α, β and γ to the substitution site are similar to those previously found in cyclohexanones. Use has been made of these substituent parameters to determine the conformational equilibria of the mobile 2‐methylcyclohexanone derivatives.
Ring conformations of 2‐substituted 2‐oxo‐4‐methyl‐1,3,2‐dioxaphosphorinanes. Evidence for a boat conformation in equilibrium with two interconverting chairs
Authors: Mosbo John A. 
Published: May 2005

AbstractThe 220 MHz proton NMR spectra of three isomeric pairs of 2‐R‐2‐oxo‐4‐methyl‐1,3,2‐dioxaphosphorinanes, where R = methoxy (1a, b), methyl (2a, b) and dimethylamino (3a, b) (a represents the trans and b the cis arrangement of R and the 4‐methyl group) were analyzed by iterative computer techniques. Ten ring conformations, two chairs, two half‐chairs and six boats were initially considered as being possible contributors to the overall solution conformations. Compounds 1a, 2a, 2b and 3b were all concluded to exist as single chair conformations with the 4‐methyl group equatorially oriented (eqch). In addition to 68% of compound 3a being in that eqch conformation, however, significant concentrations of the chair form containing an axially oriented 4‐methyl group (axch, 16%) and a boat conformation containing an equatorially oriented 4‐methyl group with phosphorus and C‐5 serving as the bow and stern (eq25, 16%) were also postulated. Similarly, it was suggested that 1b contained 60% eqch, 20% axch and 20% of the boat eq25. From the data for compounds 2a and b it was concluded that in the chair conformations of 2‐oxo species the phosphorus substituent orientation has little effect upon the 3J(POCH) coupling constants.
An application of 1H nuclear magnetic resonance to the determination of hydrophobic interactions in peptides
Authors: Kumar N. G. , Roeske R. W. 
Published: May 2005

AbstractIntermolecular hydrophobic interactions between the indole or phenyl moieties of the peptides containing L‐tryptophan (L‐Trp) or L‐phenylalanine (L‐Phe) residues and the apolar isopropyl groups of the peptides containing L‐leucine (L‐Leu) or L‐valine (L‐Val) in aqueous solutions have been detected by 1H NMR by monitoring the observed changes in the proton magnetic resonance parameters of the methyl resonances of the peptides containing L‐Leu or L‐Val residues. The 1H NMR data indicate that intermolecular hydrophobic interactions are responsible for the observed changes in the proton magnetic resonance parameters of the methyl resonances. For example, when a solution of glycylglycylleucine (Gly‐Gly‐L‐Leu) in deuterium oxide was mixed with glycylglycyltryptophan (Gly‐Gly‐L‐Trp), the methyl protons of Gly‐Gly‐L‐Leu exhibited large diamagnetic ring current shifts. However, when glycylglycylglycine (Gly‐Gly‐Gly) was substituted for Gly‐Gly‐L‐Trp in the NMR experiment, the methyl resonances did not show any upfield or downfield shift, thereby demonstrating that the observed upfield shifts are not due to bulk susceptibility differences between solutions. The C‐terminus peptides containing L‐Leu or L‐Val residues bind to the aromatic L‐Trp or L‐Phe peptides better than the N‐terminus L‐Leu or L‐Val peptides. The C‐terminus Gly‐Gly‐L‐Leu binds better than the C‐terminus glycylglycylvaline (Gly‐Gly‐L‐Val). The strength and specificity of hydrophobic interactions in several small peptides are discussed.
Etudes Conformationnelles par Résonance Magnétique Nucléaire d'Ethanes Disubstitués par un Hétéroatome appartenant aux Colonnes IV, V et VI et par un Groupement Fonctionnel
Authors: Greiner J. , Rouvier E. , Pastor R. , Cambon A. 
Published: May 2005

AbstractDans ce travail, nous présentons les résultats obtenus dans l'analyse conformationnelle d'ethers, de sulfures, de sélénoéthers, d'amines, d'arsine, de germanes et de stannanes du type RXCH2CH2Y. La partie AA′BB′ (–CH2CH2–) des spectres a été analysée par une méthode basée sur les déplacements chimiques. Nous avons pu d'abord relier l'électronégativité de l'hétéroatome à la différence des déplacements chimiques des protons AA′BB′. Ensuite, une analyse conformationnelle par variation de température nous a permis de montrer que, dans le domaine étudié (230 K à 320 K), le triphénylstannyl‐3 propionitrile possède une structure ‘bloquée’.
Effect of paramagnetic anions on cations possessing stereochemically non‐equivalent methylene groups
Authors: Rosenthal Leslie , Walker Ian M. , Wiggins Don 
Published: May 2005

AbstractA series of cations based on substituted pyrrolidines and piperidines has been investigated by NMR in low dielectric solvents. Non‐equivalent cation methylene protons are selectively shifted by paramagnetic anions; the size of the shift depends directly on the ability of the anion to approach closely the group in question. The assignments resulting from the shift data have been compared with the work of other authors.
A carbon‐13 NMR study of benzonorbornene isomers
Authors: Burn Peter K. , Crooks Peter A. , Meth‐Cohn Otto 
Published: May 2005

AbstractThe carbon‐13 NMR spectra of a series of exo‐and endo‐epimers of 2‐substituted benzonorbornene (2‐substituent = OH, OCHO, Br, NH2, NHMe or NMe2) have been examined. The spectra are readily assigned by comparison with the coupled and off‐resonance proton decoupled spectra as well as by the use of shift reagents. Interesting and diagnostic features of the spectra allow the distinction between epimers.
The carbon‐13 nuclear magnetic resonance spectra of anthraquinone, eight polyhydroxyanthraquinones and eight polymethoxyanthraquinones
Authors: Berger Y. , Castonguay A. 
Published: May 2005

AbstractThe 13C NMR spectra of anthraquinone, eight hydroxyanthraquinones and eight methoxyanthraquinones are reported. Peak assignment for the carbon atoms of these compounds is achieved by using decoupling techniques and by intercomparison of the variously‐substituted derivatives. Carbonyl chemical shifts in the hydroxyanthraquinones can be rationalized in terms of cross conjugation and intramolecular hydrogen bonding and in terms of cross conjugation in the methoxyanthraquinones. Hydroxy and methoxy substituent chemical shifts are also reported.
Elektronenstruktur und NMR‐Parameter von substituierten 5‐Phenyl‐2,4‐pentadiensäuren
Authors: Radeglia R. , Wolff R. , Spassov S. L. , Anghelova Y. , Ivanov Ch. 
Published: May 2005

Abstract1H‐ und 13C‐NMR‐chemische Verschiebungen sowie vicinale HH‐Kopplungskonstanten von verschieden substituierten 5‐Phenyl‐2,4‐pentadiensäuren ArCHCHCRCRCOOH werden mitgeteilt und im Zusammenhang mit der Molekülstruktur diskutiert. Die entlang der Molekülkette alternierenden 13C‐chemischen Verschiebungen können linear mit den entsprechenden in PPP‐Näherung berechneten π‐Elektronenladungsdichten korreliert werden. Die ebenfalls alternierenden Einflüsse der paraständigen Substituenten bzw. der Lösungsmittel auf die 13C‐chemischen Verschiebungen sind mittels Atom–Atom‐Polarisierbarkeiten beschreibbar.
Carbon‐13 NMR spectra of some 8,13‐diazasteroids and related model isoquinolines
Authors: Verchère Catherine , Rousselle Dominique , Viel Claude 
Published: May 2005

AbstractCarbon‐13 NMR spectra of some dihydro‐ and tetrahydro‐isoquinolines were studied and applied to the assignments of 13C NMR signals of some 8,13‐diazasteroids related to the estrone series. Some deuterated compounds were also examined. All signals could be assigned and an important long‐range isotopic effect was observed which seems to be due to an interaction between the CH(D) bond and the nitrogen lone pairs.
Lanthanide shift reagents. IX. Proton and carbon magnetic resonance study of specific contact interactions of a bifunctional oxazole amide with fluorinated lanthanide shift reagents
Authors: Rackham David M. , Cockerill Anthony F. , Harrison Roger G. 
Published: May 2005

AbstractHighly specific proton line‐broadening and paramagnetic 13C shifts observed for N‐butyl‐N(4‐methyloxazol‐2‐yl)‐2‐methylpropionamide in the presence of fluorinated lanthanide shift reagents, are absent when non‐fluorinated reagents are used. This unusual behaviour is rationalized in terms of possible chelation of the N‐butyl‐N(4‐methyloxazol‐2‐yl)‐2‐methylpropionamide to the lanthanide reagent, causing transfer of the unpaired electron into the heterocyclic ring.
1H and 13C NMR spectra of new 2‐thiabicyclo[3.2.0]hept‐3‐enes
Authors: Kalabin G. A. , Trofimov B. A. 
Published: May 2005

AbstractThree new compounds, one with the molecular formula C6H8S and two corresponding to C7H10S have been isolated together with the main product, divinyl sulphide, from the reaction products of acetylene with sodium sulphide. Structure determination through 1H and 13C NMR with selective 13C{1H} decoupling, showed C6H8S to be a new heterocyclic compound, 2‐thiabicyclo[3.2.0]hept‐3‐ene, and the two C7H10S compounds its exo‐ and endo‐6‐methyl derivatives. Since the spectral parameters of these heterocycles were not known, all possible structures corresponding to the molecular formulae were considered.
The torsional barrier in aromatic carbonyl compounds. VI. Carbon‐13 and 1H DNMR study of protonated alkyl and haloalkyl p‐tolyl ketones
Authors: Barthelemy Jean‐Françlois , Jost Roland , Sommer Jean 
Published: May 2005

AbstractThe free energy of activation of the torsional barrier around the phenyl–carbonyl bond in a series of stable protonated α‐substituted p‐methylacetophenones (p‐tolyl‐ where R = CH3, C2H5, i‐C3H7, t‐C4H9, CH2Cl, CHCl2, CCl3, CH2F, CHF2, CF3 and CH2Br) has been evaluated by complete lineshape analysis of 13C or proton low temperature NMR spectra. α‐Substitution by an alkyl group leads to a decrease of the barrier height due to both steric and electronic effects. For the halo compounds, steric and electronic effects of the substituents are in competition, leading to a very high ΔG≠ value in protonated α,α,α‐trifluoro‐p‐methylacetophenone (63.2 kJ mol−1). In addition, the twist angle of the protonated carbonyl group has been tentatively evaluated by the use of the (modified) Stothers's method.
Application of selective 13C{1H} nuclear overhauser effects with low‐power 1H irradiation in carbon‐13 NMR spectroscopy
Authors: Uzawa Jun , Takeuchi Setsuo 
Published: May 2005

AbstractThe feasibility of the application of selective 13C{1H} nuclear Overhauser effect (NOE) measurements to organic structural analysis in 13C NMR spectroscopy has been investigated at 25 MHz using citraconic anhydride and the methyl ester of pentalenolactone as model compounds. This selective 13C{1H} NOE technique with single frequency low‐power 1H irradiation should find wide application in the structural elucidation of organic compounds, especially for the reliable assignment of quaternary carbon signals.
INDO/S parameterized calculations of some 13C nuclear screening data
Authors: Jallali‐Heravi M. , Webb G. A. 
Published: May 2005

AbstractNuclear screening tensors, their anisotropies and 13C chemical shifts are calculated for a number of carbon environments and compared with the corresponding experimental data. The agreement found with the experimental results is an improvement over that obtained from CNDO/S parameterized calculations. The improvement stems from the inclusion of the one‐centre exchange integrals, which results in smaller values for the excitation energies from the INDO/S calculations. It is demonstrated that a linear relationship between 13C chemical shift and the lowest energy transition is not to be expected. The average values of the excitation energies of the various transitions contributing to the 13C screening are found to have significant variations.
Carbon‐13 NMR spectra of some chlorinated polycyclodiene pesticides
Authors: Cox Richard H. , McKinney James D. 
Published: May 2005

AbstractCarbon‐13 NMR spectra have been obtained for aldrin, dieldrin, isodrin, endrin, heptachlor, heptachlor oxide and 24 of their degradation/conversion products. Chemical shifts were assigned on the basis of (1) comparison among the spectra, (2) single frequency, off‐resonance decoupling experiments, (3) substituent effects derived for norbornanes and (4) spectra obtained with an added lanthanide shift reagent. The substituent effects previously reported for norbornanes were found to be applicable for establishing trends in the chemical shifts for many of the compounds studied.
Stereochemical assignment at the C‐13 carbon of pimaradienes by 13C NMR. A reassessment
Authors: Beier Ross 
Published: May 2005

AbstractAccording to reported results by Kato et al. in their 1977 paper, 13C data can not be used for assignment of stereochemistry at carbon‐13 in pimaradiene‐type compounds. By comparing well established 13C trends to the appropriate conformations, one can indeed predict the correct stereochemistry at C‐13 of pimaradienes. The result by this method correlates exactly with crystal structure data. 13C NMR is reliable for determination of C‐13 stereochemistry in the pimaradienes.
Additivity of proton and carbon nmr chemical shifts in protonated benzene carboxylic acids
Authors: Bruck Daniel , Rabinovitz Mordecai 
Published: May 2005

AbstractThe additivity of the proton and carbon chemical shift increments due to structural changes in a series of conjugate acids derived from benzene polycarboxylic acids is reported. In the acids, two sets of increments had been applied, one to the ortho diacids and one to acids which do not bear ortho substituents. In the conjugate acids only one set of increments is required. The disappearance of the ‘ortho’ effect may be the outcome of properties inherent to these conjugate acids.
Nitrogen‐15 nuclear magnetic resonance spectroscopy. Pyridine N‐oxides and quinoline N‐oxides
Authors: Yavari Issa , Roberts John D. 
Published: May 2005

AbstractThe noise‐decoupled nitrogen‐15 NMR spectra of ten pyridine N‐oxides and two quinoline N‐oxides have been obtained at the natural‐abundance level by high‐resolution NMR spectroscopy. Substituents at the 4‐ring position of pyridine N‐oxide, capable of resonance interaction with the N‐O moiety, give fairly large shifts in the expected directions. Spectra taken in dimethyl sulfoxide solution give 5–20 ppm and 33–55 ppm downfield shifts with respect to the solutions of the same substances in 2,2,2‐trifluoroethanol and trifluoroacetic acid. Solvent influences are discussed in terms of hydrogen bonding and protonation of the N‐oxide oxygen. Carbon‐13 chemical shifts and one‐bond carbon‐hydrogen coupling constants of some substituted pyridine N‐oxides are reported and discussed.
Detection of halogen bond formation by correlation of proton solvent shifts. 1. Haloforms in n‐electron donor solvents
Authors: Bertrán José Fernández , Rodríguez Mario 
Published: May 2005

AbstractThe solvent shifts of haloformic protons, (Cl3CH, Br3CH, I3CH), have been measured in 24 n‐electron donor solvents consisting of halogenated hydrocarbons, esters, ketones, ethers and amines. Deviations of ΔBr and Δ1 from linear dependence with ΔCl are indicative of the presence of halogen bond formation competitive with hydrogen bonding interactions. Bromoform interacts predominantly by hydrogen bonding, halogen bonding being detected to a small extent in chlorinated hydrocarbons and amines. Iodoform shows halogen bonding interactions which increase in relative importance to hydrogen bonding with solvent basicity. Halogen bonding is predominant for solutions of iodoform in amines.
Über Zuordnungsprobleme von 13C‐Signalen in den NMR‐Spektren von Benzheteroazolen
Authors: Gründemann E. , Martin D. , Wenzel A. 
Published: May 2005

AbstractFür verschiedene Benzimidazol‐ und Benzoxazolderivate wurden die chemischen Verschiebungen im 13C‐NMR‐Spektrum bestimmt. Mit Hilfe von Additivitätsparametern lassen sich die 13C‐Signale der Benzokohlenstoffatome in unsymmetrischen Benzheteroazolen bestimmen. Die Struktur der Reaktionsprodukte aus Benzimidazolin‐ und Benzoxazolin‐thion‐(2) und Arylcyanaten werden abgeleitet.
Substituent Conformational effects in vicinal 13C13C spin–spin coupling constants
Authors: Marshall James L. , Faehl Larry G. , Kattner Richard , Hansen Poul Erik 
Published: May 2005

AbstractA series of 7‐13C‐labeled o‐substituted toluene derivatives and carboxyl‐13C‐isocrotonic and crotonic acid were synthesized and studied by 13C NMR spectroscopy to obtain 13C13C spin‐spin coupling constants involving the labeled carbon. The cis 3J(CC) values were different from those in previous studies in that these J(CC) values were relatively small and the usual dependence of 3J(CC) on the s‐character of the terminal carbon was reversed. Further, a strong dependence of 3J(CC) on the conformational orientation of a terminal carbonyl group was shown to exist. Through‐space interactions of the two coupling carbons were shown to contribute to these ‘anomalous’ results, and thus it was shown the cis carbon‐carbon couplings may not be directly related to geometrically equivalent proton‐proton couplings, as are other carbon‐carbon couplings.
A study of the use of MINDO/3 parameters in calculations of carbon and nitrogen screening constants
Authors: Jallali‐Heravi M. , Webb G. A. 
Published: May 2005

AbstractThe calculation of some carbon and nitrogen screening constants and chemical shifts is reported. The independent electron theory developed by Pople is used as the basis of the calculations. Within this framework the MINDO/3 parameterization scheme is employed and shown to provide quantitatively unreliable results. Improved agreement with the experimental data is found when the electronic excitation energies are taken from INDO/S parameterized calculations. The relative lack of success achieved by the MINDO/3 calculations is probably a reflection on the inability to reproduce the electronic excited states satisfactorily.
Comparison of steric effects on 31P chemical shifts in some 7‐norbornyl, 7‐norbornenyl and cyclohexyl phosphorus compounds
Authors: Littlefield Lory B. , Quin Louis D. 
Published: May 2005

AbstractPhosphorus‐31 NMR spectra were obtained on both syn and anti isomers of norbornenes with the 7‐position bearing Cl2P‐, Me2P‐, Me2(S)P‐ and Me3P as substituents. Norbornanes with 7‐Cl2P‐ and 7‐Me2P‐ were also studied. Just as is true for 13C shifts for 7‐Me in these bicyclo[2.2.1]heptane derivatives, the 31P shifts all fall noticeably upfield of published values for comparable cyclohexane derivatives. Consistent also with the 13C effect, a slight shielding of 31P in Me2P‐ (1.1 ppm) was noted when this group was syn to the double bond of the norbornene system, relative to the value for the anti compound, suggesting the sensitivity of 31P to a diamagnetic effect from the double bond. However, the more space‐demanding Cl2P group when moved from the crowded anti to the less crowded syn position experienced a strong deshielding effect, as did the Me2(S)P‐ and Me3P‐ groups. The opposite is seen when these same groups are moved from the crowded axial to the less crowded equatorial position of cyclohexane, and a new structural influence appears to be operating in this system. A recent proposal that steric compression causes bond angles about phosphorus to increase and that this is the cause of the γ‐shielding effect is not supported by published NMR and X‐ray structural data for cis‐ and trans‐1‐methyl‐4‐tert‐butyl‐4‐phosphorinanol. Bond angles about phosphorus are virtually identical in these isomers, yet 31P shifts differ by 6.9 ppm.
Specific deuteration effects on carbon‐13 NMR relaxation data of amido‐type systems
Authors: Shapiro M. J. , Kahle A. D. 
Published: May 2005

AbstractThe effect of deuterium substitution in amido‐systems is explored. For benzamide, both positive and negative changes in TDD1 are observed. A method for structure elucidation of these and related systems is presented.
A 13C{1H} double resonance study of the signs of 1H19F and 13C19F spin coupling constants in fluorobenzenes and 2‐fluoropyridine
Authors: Brey W. S. , Jaques L. W. , Jakobsen H. J. 
Published: May 2005

AbstractThe signs of all 13C19F and 1H–19F coupling constants in fluorobenzene, some substituted derivatives, and in 2‐fluoropyridine have been related using single‐frequency 13C{1H} double resonance techniques. All 13C19F couplings in these compounds are shown to be positive. Direct experimental proof is given for a positive sign for the small five‐bond 1H19F coupling constant (=0.2—0.3 Hz) in fluorobenzene. The positive sign for 4J(C‐4, F) in fluorobenzene is in accord with a positive π‐electron contribution to this coupling.