bluuebunny/crossref_metadata_2025 huggingface dataset:
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

A 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

The 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

A 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

The 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

Carbon‐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

Solvent 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

The 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

The 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

The 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

LL‐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

The 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

The 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

Transfer 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

The 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

The 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

The 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

Some 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

13C 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

The 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

The 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

The1H 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

Chemically 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

The 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

1H 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.
NMR shift reagents for organic salts: Shift mechanism, bound shifts and structural analysis
Authors: Wenzel Thomas J. , Cameron Kirk
Published: May 2005

Lanthanide tetrakis(β‐diketonate) anions are effective NMR shift reagents for organic salts. The shifts in the 1H NMR spectra were analyzed and explained without invoking a contact shift mechanism. The equilibria of the shift reagent‐substrate complex was examined. No evidence was found for a 1:2 shift reagent‐substrate complex. Bound shifts and association constants were determined for 1‐methyltetrahydrothiophenium iodide (1) and 1‐ethylquinolinium (3) iodide. The observed shifts for these salts were fitted to calculated shifts using the simplified dipolar shift equation. A single lanthanide location was identified with 3. A time average of at least two configurations of shift reagent‐substrate complex was required with 1.
Full assignments of the 1H and 13C NMR spectra of sodium fusidate in organic and aqueous media
Authors: Barber Jill , Lian Luyun , Morris Gareth A. , Tehrani M. Hassan
Published: May 2005

The 1H and 13C nuclear magnetic resonance spectra of sodium fusidate in methanol‐d4 and in D2O have been assigned using a range of one‐ and two‐dimensional NMR techniques.
EPR study of radicals derived from fumaronitrile and related carboxylic acids
Authors: Smith Peter , Donovan William H. , Parker Julie K.
Published: May 2005

Using aqueous TiCl3‐based continuous‐flow, radical‐generating methods, a comprehensive investigation was made of radicals derived from fumaronitrile and cis‐ and trans‐carboxylic acids of general structure CY(COOH)CH(COOH), where Y is H, CH3 or CH2COOH. All substrates were examined at 25°C but, where appropriate, the temperature was also varied over the range 10–40°C. The radicals observed from these compounds were of two main types, addition and electron transfer. This investigation emphasized the study of the addition radicals formed from fumaronitrile and maleic acid and of the electron‐transfer radicals formed from the carboxylic acids. Several of the addition radicals showed long‐range splittings. Among these, the radicals magnified image and magnified image were given particular attention, thus showing that they exist in axial and equatorial conformations—a situation not commonly encountered. All of the acids yielded one‐electron transfer radicals, some of which had interesting stereochemical properties, including intramolecular hydrogen bonding.
1H and 13C NMR analysis of three quinone‐methide triterpenoids
Authors: Gunatilaka A. A. Leslie , Fernando H. Chandrasiri , Kikuchi Tohru , Tezuka Yasuhiro
Published: May 2005

Detailed analysis of the 1H and 13C NMR spectra of three natural quinone‐methide triterpenoids, pristimerin, tingenone and 20‐hydroxytingenone, was carried out by the application of two‐dimensional 1H1H and 1H13C shift correlation techniques. Some previous 1H and 13C assignments have been revised.
13C chemical shift assignments for isomers of some 1,2‐diaroyl‐3‐aryl‐cyclopropanes
Authors: Reddy D. Bhaskar , Reddy B. Venkataramana , Seshamma T. , Sankaraiah B. , Reddy M. V. Ramana
Published: May 2005

The 13C chemical shifts of some isomeric 1,2‐diaroyl‐3‐arylcyclopropanes have been determined. The δc, values of C‐1 and C‐2 of the cyclopropyl ring carbons and those of the C‐4 and C‐5 carbonyl carbons are used as a criterion for distinguishing between the stereoisomeric forms.
Structure of and electronic interactions in aromatic polyvalent iodine compounds: A 13C NMR study
Authors: Katritzky Alan R. , Gallos John K. , Durst H. Dupont
Published: May 2005

Available data on the structures, electronic substituent effects and 13C NMR spectra of aromatic polyvalent iodine compounds, particularly iodoso and iodoxy derivatives, are summarized and discussed. A series of aromatic tri‐ and penta‐valent iodine compounds were synthesized, their 13C NMR spectra were measured and the substituent chemical shifts (SCS) for several polyvalent iodine functional groups were calculated. As the oxidation state of iodine increases (I → II → V), the strong shielding of the ipso‐carbon atom by iodine in aryl iodides due to the ‘heavy‐atom effect’ (30 ppm) decreases substantially, and this decrease is apparently general for stable hypervalent iodine compounds. Possible explanations are discussed and correlations between chemical shifts, structures and/or electronic effects are proposed.
Reaction of dibenzalacetone with hydroxylamine hydrochloride under phase‐transfer conditions. 1H and 13C 2D NMR structural elucidation of the products
Authors: Aguilera J. L. , Sosa G. , Reynolds W. F. , Diaz E. , Barrios H.
Published: May 2005

The constitution, configuration and conformation of three N‐hydroxydiphenylpiperidines were established by 1H and 13C 2D NMR spectroscopy. The products were obtained using phase‐transfer conditions.
13C NMR spectra of some halocarbons. Shift predictions in polychloro‐quinolines and ‐isoquinolines
Authors: Matthews R. S. , Jones M. , Banks J.
Published: May 2005

Three methods, two empirical and one semi‐empirical, for predicting 13C chemical shifts in six poly‐chloroheterocyclics are considered. It was found that the semi‐empirical correlations with CNDO/2 calculated charges, although useful, could not be used in isolation from the other two methods. Of these, the MSCS (multiple substituent chemical shifts) method could be used to predict all eighteen carbon shifts in perchloroquinoline and perchloroisoquinoline, whilst the NIMS method (hypothetical nitrogen insertion method) was limited to the carbons in the heterocyclic ring. In brief, it was found that the MSCS method was the most accurate (less than ± 1 ppm) for predicting chemical shifts, intrinsically compensating for some of the errors arising from mutual atom‐atom interactions.

New 13C data are reported for heptachloroquinoline, a mixture of two polychloroquinolines, hepta‐chloroisoquinoline and two, separable, new polychloroisoquinolines.
13C NMR spectra of amino‐alkylphosphonic acids
Authors: Głowacki Zdzislaw , Topolski Marek
Published: May 2005

13C chemical shifts and 31P,13C coupling constants are reported for 20 amino‐alkylphosphonic acids and two phosphonic acids. The substituent‐induced chemical shifts of the phosphonate group were calculated and their non‐additivity in 1‐aminoalkylphosphonic acids was observed. Two stable conformers of 4‐aminobutylphosphonic acid were detected by 13C NMR.
Assigned cluster11B and1H NMR properties of [3‐(η6‐C6Me6)‐closo‐3,1,2‐RuC2B9H11]
Authors: Bown Mark , Plešek Jaromír , Baše Karel , Štíbr Bohumil , Fontaine Xavier L. R. , Greenwood Norman N. , Kennedy John D.
Published: May 2005

The cluster11B chemical shifts in members of the important class ofcloso‐3,1,2‐metalladicarbaboranes were previously unassigned, and their cluster1H chemical shifts previously unmeasured. The use of [11B11B]‐COSY and1H‐{11B} NMR spectroscopy to measure and assign the cluster11B and1H chemical shifts of (previously unreported) [3‐(η6‐C6Me6)‐closo‐3,1,2‐RuC2B9H11] as a model compound is reported. The11B shielding patterns of this species and the nonmetalla simple carbaborane analogue [1,2‐C2B10H12] are compared and briefly discussed.
Interpretation of the solvent effect on the screening constant of Xe‐129
Authors: Luhmer M. , Dejaegere A. , Reisse J.
Published: May 2005

The chemical shift (δ) of a xenon monoatomic molecule is very sensitive to the environment. The major contribution to this shift arises from the van der Waals interactions between the solvent and the xenon molecules. Usually, this solvent effect is interpreted via a reaction field model. This approach leads to a search for a correlation between δ and a function of the square of the refractive index of the solvent [f(n2)]. This paper illustrates the use of a theoretical model developed in our laboratory for the interpretation of the solvent effect on the xenon chemical shift. This model allowed the calculation of the xenon‐solvent dispersion energy (Edis) for different solvents, and the correlation of Edis with δ. The Edis‐δ correlation is clearly better than the f(n2)‐δ correlation.
The influence of crown ethers on cation migration processes. Part 7.1 The 2‐pivaloyl‐1,4‐benzoquinone radical anion
Authors: Flint Nicholas J. , Tabner Brian J.
Published: May 2005

The 2‐pivaloyl‐1,4‐benzoquinone radical anion has been prepared by alkali metal reduction in tetrahydrofuran in the presence of dibenzo‐18‐crown‐6, 15‐crown‐5, or 12‐crown‐4, and its ESR spectrum in each system has been recorded over a range of temperatures. Line‐width variation in the ESR spectrum is observed in some systems. This line‐width variation has been attributed to cation migration between sites adjacent to the non‐equivalent carbonyl groups rather than to restricted rotation of the pivaloyl group. In some systems, however, the rate of migration is too fast and in others too slow, compared with the time‐scale of the ESR experiment, for line‐width variation to be observed. The influence of each crown ether upon the rate of cation migration has been found to depend upon the size of the crown ether ‘cavity’ compared with the size of the alkali metal counter‐ion.
Stereochemical studies of the 4‐alkyl‐4‐arylpiperidine class of opioid ligand
Authors: Casy A. F. , Dewar G. H. , Al‐Deeb Omar A. A.
Published: May 2005

The 1H (270, 400 MHz) and 13C (67.5 MHz) NMR spectra of some 4‐methyl‐ (also 4‐n‐propyl‐ and ‐isobutyl)‐4‐(3‐hydroxy‐ and 3‐methoxy‐phenyl)piperidines and their 3‐methyl diastereoisomers are reported. Many of the compounds had opioid ligand activities. The data were analysed in terms of preferred conformation and configuration (3‐methyl derivatives). Only compounds with preference for axial 4‐aryl chair conformations displayed marked agonist properties and the one potent antagonist, cis‐1,3,4‐trimethyl‐4‐(3‐hydroxyphenyl)piperidine, favoured an equatorial 4‐aryl chair.
High resolution NMR study of some chromium compounds in solution
Authors: Dove Michael F. A. , Jones Eleanor M. Lloyd , Clark Ronald J.
Published: May 2005

A study was made of some pseudo‐tetrahedral CrVI and octahedral Cr0 compounds by high‐resolution 53Cr NMR spectroscopy. Cr(PF3)6 and fac‐Cr(CO)3(PF3)3 provide the first examples of compounds showing a one‐bond coupling, 1J(53Cr, 31P), in a 53Cr NMR spectrum. This technique also provides proof of the formation of CrO2ClF in a mixture of the dichloride and difluoride.
1H, 13C 2D NMR experiments for the identification of an impurity in lauryl lactam from cyclododecane
Authors: Huvenne Jean‐Pierre , Atochem Bernard Lamy , Mouchel Bernard
Published: May 2005

The main impurity in lauryl lactam from cyclododecane has been identified as 1‐azabicyclo[8.3.0]tridecan‐13‐one by means of several two‐dimensional NMR techniques.
Two‐dimensional NMR study of ganwuweizic acid
Authors: Cui Yu‐Xin , Hua Su‐Ming , Cheng Jin‐Long , Yue Jian‐Min , Chen Yao‐Zu
Published: May 2005

Several two‐dimensional NMR techniques were applied to assign completely the 1H and 13C chemical shifts and to study the configuration and conformation of a new triterpenoid, 3‐keto‐9,10,19‐cyclopropyllanost‐24‐en‐26‐oic acid (ganwuweizic acid). Some 13C NMR signals previously reported for the triterpenoid skeleton were corrected.
Suppression of artefacts in nuclear overhauser effect difference spectroscopy by reference deconvolution
Authors: Morris Gareth A. , Cowburn David
Published: May 2005

Reference deconvolution allows the effects of common instrumental imperfections in Fourier transform NMR, including field inhomogeneity, field/frequency ratio instability and pulse phase and amplitude irreproducibility, to be removed from spectra containing a well‐resolved reference signal. By applying reference deconvolution to the two component spectra, perturbed and unperturbed, of a nuclear Overhauser effect difference spectrum, the artefacts that instrumental imperfections normally cause in the difference spectrum may be suppressed. A small penalty in signal‐to‐noise ratio is paid for an improvement of several orders of magnitude in artefact suppression, allowing even very small Overhauser enhancements to be detected with confidence. Results are presented showing the reduction of difference artefacts for methyl singlets, normally the most severe test for NOE difference spectra, to below 1 part in 50 000.
Proton and carbon‐13 NMR studies of 1‐substituted pyridinium salts
Authors: Katritzky Alan R. , Dega‐Szafran Zofia
Published: May 2005

1H and 13C NMR chemical shifts of a series of 1‐substituted pyridinium salts were measured in dimethyl sulfoxide. The effect of the quaternary pyridinium ring on the resonance of the methylene group of the substituent is reported. Increments of δ1H = 3.89 ± 0.16 ppm and δ13C = 47.5 ± 3.7 ppm and are estimated for replacing YCH2H with YCH2Py+. The effects of 1‐substituents on the pyridinium ring chemical shifts are discussed.
Natural abundance one‐bond 13C13C coupling constants and unambiguous assignments of 13C NMR spectra of thieno[c]quinolines and thieno[c]isoquinolines
Authors: Gronowitz Salo , Servin Rolf , Yang Youhua
Published: May 2005

The one‐bond 13C13C coupling constants [1J(CC)] in all structurally possible thieno[c]quinolines, thieno[c]isoquinolines, phenanthridine, quinoline and iso‐quinoline were measured at natural abundance by using the INADEQUATE pulse sequence technique. The unambiguous assignments of the 13C NMR spectra of thieno[c]quinolines and thieno[c]isoquinolines are reported.
1H and 13C NMR studies of some O‐carbamoyloximes
Authors: Wazeer Mohammed I. M. , Ali Sk. A. , Arab Mohammed
Published: May 2005

The 1H and 13C NMR spectra were assigned for a series of O‐carbamoyloximes of ortho‐ and para‐substituted benzaldehyde. These compounds exist exclusively in the E configuration. The aromatic protons and carbons show correlations with the appropriate substituent‐induced shifts and with Hammett parameters.
1H and 13C NMR studies on the oxydimethyleneaminomethyl fragment in six‐ and eight‐membered rings. The solution stereochemistry of N‐methyl diastereomers of phendimetrazine (an oxazine anorexic drug) versus nefopam (a benzoxazocine analgesic drug)
Authors: Glaser Robert
Published: May 2005

The 1H NMR spectrum (CD2Cl2) of the mesylate analogue of phendimetrazine bitartarate, a central nervous system stimulant and an anorexic drug, showed two isomers (ratio ≈︁ 17:1) differing in the stereochemistry of the N+HCH3 moiety. A similar diastereomeric ratio was also noted in 13C{1H} NMR spectra recorded in either CD2Cl2 or in acidic D2O (pD ≈︁ 1). For the major species in CD2Cl2 solution, four antiperiplanar vicinal 1H1H coupling constants indicated a chair‐conformation 2,3‐trans‐1,4‐oxazine ring with equatorially oriented 2‐phenyl, 3‐methyl and N‐methyl substituents. The minor species also has the same chair conformation with equatorially oriented 2‐phenyl and 3‐methyl substituents, since the coupling constant for the minor species vicinal OCH(Ph)CH(Me)N methine protons was also antiperiplanar in magnitude. Axial N‐methyl stereochemistry was assigned to the minor species, since chemical shifts of the ring carbons gauche to N‐methyl (and of the N‐methyl carbon atom itself) were shifted characteristically upfield relative to absorbances for the corresponding carbons in the major [equatorial N‐methyl] species. The R‐factor method was used to estimate an OC6C5N dihedral angle of 56.0(7)° for the major equatorial N‐methyl species in CD2Cl2 solution. CP‐MAS 13C NMR chemical shifts for solid‐state phendimetrazine bitartarate are the same as those for the solution‐state equatorial N‐methyl diastereomer. The oxydimethyleneaminomethyl fragment in the equatorial N‐methyl phendimetrazine chair conformation was used as a model for the corresponding fragment in the eight‐membered ring of nefopam. Comparison of OCH2CH2N+H(equatorial CH3)— and OCH2CH2N+H vicinal coupling constants in the two rings suggests dynamic weighted averaging for those in the octagonal ring of the nefopam equatorial NCH3 diastereomer.
NMR self‐diffusion measurements: A different approach to complexation phenomena
Authors: Geringer M. , Sterk H.
Published: May 2005

1H and 133Cs NMR self‐diffusion measurements were performed by means of the pulsed gradient spin‐echo method (PGSE) to evaluate the complex formation constants of Cs and 18‐crown‐6 and those of Cs and some ethylene glycols in D2O. The calculated complex formation constants were checked as far as possible by using similar calculations based on relaxation data. Chemical shift measurements do not allow the evaluation of complex formation constants in these systems. A cross‐check was introduced to verify the calculations based on the diffusion data. Similar investigations were performed on Na, K, Cs complexes and N‐phenylaza‐15‐crown‐5 in methanol. In this case, analysis of the diffusion data and as the relaxation rate and chemical shift data led to complex formation constants for 1:1 complexes, which is in contrast to a previous report suggesting the formation of 2:1 complexes.
Revised 13C NMR signal assignments of 2‐(bromomethyl) naphthalene
Authors: Ernst Ludger
Published: May 2005

The assignments of the 13C NMR signals of 2‐(bromomethyl)naphthalene (1) were derived by comparison of its spectrum with those of the α,α‐D2 and 1‐D derivatives, by determination of relative spin‐lattice relaxation rates and by inspection of the 13C,1H spin coupling patterns. Literature assignments of 1 and of 2‐(bromomethyl)‐6‐methylnaphthalene are corrected. The procedure by which these assignments has been obtained is critically evaluated. Very good additivity of substituent effects on chemical shifts is observed in the 13C NMR spectra of 2,6‐ and 2,7‐bis(bromomethyl)naphthalene.
13C NMR data for isopimarane and 8‐epi‐isopimarane diterpenoids
Authors: Feliciano A. San , Medarde M. , Tomé F. , Hebrero B. , Caballero E.
Published: May 2005

13C NMR spectra of several isopimarane and 8‐epi‐isopimarane diterpenoids without double bonds have been recorded. The most significant effects caused by the change in the stereochemistry of the B/C rings junction are discussed.
NMR coupling of 13C and 19F with 14N in nitro compounds
Authors: Coburn Michael D. , Storm Carlyle B. , Moore Donald W. , Archibald Thomas G.
Published: May 2005

The 13C and 14N NMR spectra of a variety of highly nitrated aliphatic, aromatic and heterocyclic compounds were obtained. The proton decoupled 13C peaks of the nitro‐substituted carbons in most of these compounds were split into the multiplets expected for coupling with spin 1 nuclei with negligible quadrupolar broadening. In addition, the 19F spectra of some fluoronitroaliphatic compounds showed that the 19F signals were also split into the expected multiplets from coupling with the 14N of the nitro groups.
Effect of the orientation of an α‐substituent on vicinal 13C–;1H spin–spin coupling constants
Authors: van Beuzekom Aart A. , de Leeuw Frank A. A. M. , Altona Cornelis
Published: May 2005

The magnitude of the NMR spin‐spin coupling constant, 3J(CH), between a vicinal 13C–1H pair depends, inter alia, on the value of the torsion angle ΦCH(13CCCH) and is influenced by the presence of an electronegative substituent located on the coupling 13C nucleus. The form and magnitude of the effect of the orientation ΨXC of such an α‐substituent were examined. The coupling constant between C‐1 and a hydrogen atom located on C‐3 in a series of α‐substituted propanes were studied by means of the semi‐empirical INDO method. In the calculations both Φ and Ψ(X13CCC) were systematically varied in steps of 30°. These calculations reveal that the variation of Ψ at a constant Φ has a pronounced effect on the calculated coupling constant Jcalc. The magnitude of this effect is shown to be strongly dependent on the electronegativity χ of the α‐substituent. Thus, it is shown that Jcalc depends on Φ and Ψ, in addition to χ. The resulting set of two‐dimensional Karplus‐type surfaces can be described by an equation that contains only nine adjustable parameters. Measurement of 3J(CH) in cis‐ and trans‐2,2,6,6‐tetradeuterio‐4‐tert‐butylcyclohexanol confirmed some of the theoretical predictions. In the cis compound (ΦCH = 180°, ΨOC = 60°) 3J(C‐1,H‐3eq) is 7.1 Hz, whereas in the trans compound (ΦCH = 180°, ΨOC = 180°) 3J(CH) equals 10.4 Hz, in qualitative agreement with the INDO calculations.
Applications of two‐dimensional NMR in spectral assignments of some friedelanes and secofriendelanes
Authors: Patra Amarendra , Chaudhuri Swapan K. , Acharyya Aditi K.
Published: May 2005

Carbon‐13 spectral assignments of three friedelanes and nineteen secofriedelanes have been made from mutual correlations, 2D X‐H correlation analyses and comparison with the carbon shifts of friedelin, which were firmly established by an INADEQUATE experiment that required reassignment of some of its resonances. Specific methyl and methine proton resonance assignments for some friedelanes, established from X‐H correlation studies, are also reported.
Dependence of the pure quadrupole resonance frequency on temperature and molar specific heat for NaBrO3
Authors: Stahl Edward A. , Tipsword Ray F.
Published: May 2005

The molar specific heat of NaBrO3 in the temperature range 4.2–336 K was determined experimentally and compared with the theory developed by Stahl and generalized in this paper. The relationship between the molar specific heat and temperature was explicitly studied and compared with results obtained by other workers. The method developed also produces the Debye temperature, Θ the maximum angular frequency, omega;m, and the product Iωt2 of the BrO3 group, where I is the moment of inertia and ωl, is the lattice vibration. The relationships between the 79Br NQR frequency and temperature and between the NQR frequency and molar specific heat were obtained.
Conformational analysis of erythromycin A derivatives: A predictive method using NMR chemical shift and coupling constant data
Authors: Everett Jeremy R. , Hatton Ian K. , Tyler John W.
Published: May 2005

1H and 13C NMR chemical shift data and 3J(H,H) values can be used to predict the conformational blend of new members of a series of erythromycin A derivatives in CDCI3 solution.
Computer simulation of proton spin relaxation in proteins: Application to Streptomyces subtilisin inhibitor
Authors: Shibata Susumu , Akasaka Kazuyuki
Published: May 2005

A computer program has been developed which can calculate the time dependence of individual proton magnetizations in a protein, for a variety of relaxation measurements in solution, using non‐selective and selective excitation pulses. The basic design and capability of the program are described, together with its limitations. The program is used to simulate proton relaxation experiments by non‐selective and selective excitation, and to simulate spin diffusion in a typical globular protein Streptomyces subtilisin inhibitor (SSI). The mechanism of selective spin diffusion in SSI is discussed based on a comparison of the calculated results with results of cross‐saturation experiments.
Simple method for determining the parameters of a static NMR spectrum of nuclei with half‐integer spin in powder samples at a single frequency
Authors: Granger P.
Published: May 2005

A simple method, which does not use any computer calculations, is described that allows the determination of the asymmetry parameter, the chemical shift and the quadrupole coupling constant directly from the experimental NMR spectra of quadrupolar nuclei with half‐integer spin in powder samples at a single frequency.
13C NMR study of the overall and internal motions of a semi‐rigid hydrocarbon
Authors: Gillies D. G. , Matthews S. J. , Sutcliffe L. H.
Published: May 2005

An extensive investigation has been made of the 13C NMR relaxation parameters of 2,4‐dicyclohexyl‐2‐methylpentane neat and in solution. Spin‐lattice relaxation times and nuclear Overhauser enhancements were measured as a function of temperature, concentration and radiofrequency. The results for all the carbons, except the methyls, can be interpreted by using a reduced spectral density which suggests that there is an isotropically rotating semi‐rigid backbone structure whose internal librations and vibrations reduce the amplitude of the spectral density. However, interpretation of the methyl group data required the addition of a discrete correlation time characteristic of the internal motion about a C3 axis. From dilution studies, the validity of using the two‐correlation time spectral density with S2 = 0 when the rate of overall motion approaches that of the internal motion was established, for the first time.
Electron spin resonance spectroscopy of free radicals containing sulphur linked to nitrogen
Authors: Preston K. F. , Sutcliffe L. H.
Published: May 2005

There is an abundance of information on closed‐shell compounds containing sulphur linked to nitrogen; much less, of course, is known about free radicals containing this moeity. This review describes the wide variety of these radicals, their remarkable stability, the characteristic values of their ESR parameters, their electronic structures and their potential applications.
Low‐temperature study of dynamic effects in tetramethyllimonene
Authors: Bigler P. , Müller C. , Bircher H.
Published: May 2005

Dynamic effects of tetramethyllimonene were studied at low temperatures. Nuclear Overhauser effects measured under optimized conditions at 223 K and the results of 2D correlation experiments allowed the identification of two conformers and the assignment of all the proton and carbon resonances. For the determination of exchange and relaxation rates the method of saturation transfer was applied. At temperatures below 223 K further dynamic effects were recognized. Structural information obtained experimentally was compared with the results of Allinger force field calculations.
1H and 13C NMR studies on the self‐association of retinoic acid
Authors: Waterhous D. Vincent , Muccio Donald D.
Published: May 2005

The monomer–dimer equilibrium of retinoic acid in carbon tetrachloride was studied by 1H and 13C NMR spectroscopy. The dimer dissociation constant at 24°C was determined to be 0.38 mM from a fit of the changes in the carboxylic acid carbon chemical shift as a function of concentration. The temperature dependence of the equilibrium constant obtained from the 13C NMR signals was used to determine the enthalpy (8.0 kcal mol−1) and entropy (12 cal mol−1 K−1) changes for the dimer dissociation process. These results were shown to be more precise than the values obtained from the carboxylic acid proton resonance.
Electron spin resonance study of polymeric charge‐transfer complexes
Authors: Palaniappan Srinivasan , Sathyanarayana Dixit N.
Published: May 2005

Polymeric charge‐transfer complexes with different polymer donors and monomer acceptors were investigated by electron spin resonance spectroscopy. The spin concentrations, g‐values and line widths in the complexes have been determined. The concentration of the paramagnetic charge‐transfer complexes generally increased with increasing strength of the donor‐acceptor tendencies, and exhibited an exponential temperature dependence of spin concentration characteristic of semiconductors. The paramagnetic charge‐transfer complexes were finally transformed into a diamagnetic product; this was confirmed by electronic spectroscopy.
Studies on the benzoxazine series. Part 3—Preparation and13C NMR structural Study of γ Effects of SomeN‐substituted 3,4‐dihydro‐2H‐1,3‐benzoxazines
Authors: Neuvonen Kari , Pihlaja Kalevi
Published: May 2005

SeventeenN‐substituted 3,4‐dihydro‐2H‐1,3‐benzoxazines [N‐substituent = Et, Pri, But, CH2C6H5or CH(CH3)C6H5] were prepared and their structures studied in the light of13C chemical shifts. The γ effects caused by C(α)‐methyl or C(α)‐phenyl substitution at the heterocyclic ring carbons were found to be valuable structural parameters. By usingN‐tert‐butyl derivatives as models, and by dividing γtoteffects into their components, the rotamer populations due to the rotation around the NC(α) bond could be evaluated. The method also allows the configurational assignment of diastereomericN‐α‐methylbenzyl derivatives. The effect of the half‐chair structure on the13C NMR parameters is discussed.
Proton and carbon NMR spectra of 4‐substituted 1‐acetylnaphthalenes
Authors: Perumal S. , Vasuki G. , Wilson D. A.
Published: May 2005

The proton and carbon spectra of ten 4‐X‐l‐acetylnaphthalenes have been assigned. Shift correlations have been made with benzene substituent‐induced chemical shifts (SCS, Lynch plots) and with dual substituent parameters (DSP) in the manner successfully done for many series of 1,4‐disubstituted benzenes. The results obtained are compared with those for 4‐X‐acetophenones and 1‐X‐naphthalenes. The main conclusions are (i) the carbon para to the variable substituent X is more sensitive to change of substituent in the naphthalenes than in the benzenes, (ii) of the carbons ortho to the variable substituent X, that at C‐3 is more sensitive and that at C‐4a is less sensitive, to change of substituent in the naphthalenes than in the benzenes, (iii) the effect of the 4‐substituents on the shift of H‐8 in 1‐acetylnaphthalenes is due mainly to changes in the carbonyl dipole affecting the diamagnetic screening of the proton.