bluuebunny/crossref_metadata_2025_split huggingface dataset:
Oxidation of 4‐methylcatechol by dioxygen studied by ESR spectroscopy. The different regioselectivity of OH− and MeO− nucleophilic attack and kinetic deuterium isotope effects
Authors: Davies Martin S. , Mile Brynmor , Rowlands Christopher C. , Barratt Martin D. 
Published: May 2005

AbstractThe oxidation of 4‐methylcatechol by dioxygen and the subsequent reactions of the two nucleophiles OH− and MeO− with the oxidation products were studied by electron spin resonance (ESR) spectroscopy. These reactions are models for those of skin proteins with substituted 1,2‐dihydroxybenzenes (catechols) which produce the severe allergic contact dermatitis associated with plants such as poison ivy, poison oakd and the Japanese lac tree. The rates of formation of the primary and secondary semiquinone radical anions show a large kinetic deuterium isotope effect (ca 10) at 20°C, which is ascribed to a slow deprotonation of the hydrogen bridged 4‐methylcatechol monoanion. There is a marked difference in regioselectivity for OH− and MeO− substitution of the 4‐methylcatechol monoanione; OH− reacts exclusively at the 3‐position whereas MeO− attacks at the 5‐position. This dissimilarity between two normally similar nucleophiles is also ascribed to the occurrence of hydrogen bridging between the substituting OH− at the 3‐position and the adjacent quinone oxygen at the 2‐position. The expected attack at the more electropositive 5‐position occurs with MeO− because no such hydrogen bonding can occur. Disubstitution occurs for OH− but not for MeO−, probably reflecting the greater nucleophilicity of the former.
ID and 2D 1H NMR studies of iron(III) complexes on geoporphyrins of the deoxophylloerythroetio structural type derived from oil shale
Authors: Wototwiec Stanisaw , Latos‐Gražyński Lechostaw , Serebrennikova Olga V. , Czechowski Franciszek 
Published: May 2005

AbstractOne‐ and two‐dimensional 1H NMR studies of low‐spin iron(III) geoporphyrins derived from the deoxophylloery‐throetioporphyrin series (the oil shale source) resulted in an unambiguous assignment of all resonances. Deoxophylloerythroetioporphyrin (DPEP) and its 17‐desethyl homologue were isolated from dictyonema shale as vanadyl complexes. They were demetallated and subsequently converted into iron(III) porphyrins under mild conditions. The 17‐propionic methyl ester analogue, i.e. deoxophylloerythrin methyl ester, was synthesized for comparative studies. A spectroscopic probe to identify iron(III) isocyclic porphyrins was established. The five‐membered isocyclic ring induced considerable asymmetry in the spin density distribution at the 3eπ bonding orbitals, placing the unpaired electron on the dxz orbital located along the N‐21—N–23 axis. The temperature dependence of the hyperfine shift is consistent with the thermal equilibrium between two non‐degenerated electronic states (dxz)2(dyz)1↔(dxz)1(dyz)2. The new structural factor generating the asymmetric spin density distribution in low‐spin iron(III) porphyrin complex was determined. The hyperfine shift pattern of high‐spin iron(III) isocyclic geoporphyrins provided useful probes for their detection in iron geoporphyrin mixtures by 1H NMR prior to their separation. Valuable information is derived from the number of observed signals of meso‐protons and characteristic, extremely downfield resonances of isocyclic ring hydrogens. Both features may be useful for the spectral identification of iron(III) porphyrins of the DPEP series from geological materials of relatively low purity.
MBOHO calculations of carbon‐nitrogen nuclear spin‐spin coupling constants
Authors: Hu Zhen‐Min , Zhan Chang‐Guo 
Published: May 2005

AbstractThe novel generalized correlation of the nuclear spin‐spin coupling constants with the atomic hybrids and net charges was employed to give two new relationships for calculating the carbon‐nitrogen coupling constants by use of the maximum bond order hybrid orbital (MBOHO) procedure together with the simple CNDO/2 calculation. The calculated coupling constants of carbon‐nitrogen single and triple bonds are all in good agreement with the experimental data, which shows that the new relationships obtained are satisfactory for calculation of carbonnitrogen coupling constants.
Conformations and conformational processes of pelargolactam
Authors: Borgen Gerd , Dale Johannes , Rise Frode , Gundersen Lise‐Lotte 
Published: May 2005

AbstractPelargolactam (1‐azacyclodecan‐2‐one) was studied by 1H and 13C NMR spectroscopy in solution between 25 and −145°C. Equilibrium solutions of pelargolactam at 25°C consist mainly of the trans isomer together with 5–10% of the cis isomer. Solid pelargolactam consists of a mixture of cis and trans isomers, the cis form dissolving more rapidly than the trans form at −95°C. At temperatures above 90°C the 13C NMR resonances for the cis and trans amides coalesce to one averaged set. A conformational process with a coalescence temperature of −30 to −50°C and a free energy of activation of ΔG≠ = 46 kJ mol−1 observed in the 1H spectrum of the trans isomer is interpreted as geminal exchange in the methylene groups. A lower barrier process was also observed, with a coalescence temperature of ca −115°C and ΔG≠ = 32 kJ mol−1. Only the latter process was observed in the 13C spectrum, and below −120°C all 13C lines split in two in the proportion 1:2. This process is interpreted as the interconversion between two trans conformers. Four conformational candidates were considered and their relative energies calculated by molecular mechanics.
Dynamic behaviour of fluoro‐substituted 1,8‐diarylnaphthalenes: A 19F,19F and 1,1H 2D EXSY NMR and AM1 computational study
Authors: Annunziata Rita , Ponzini Francesco , Raimondi Laura 
Published: May 2005

AbstractInteractions between polar groups and π systems were studied for stacked aryls in 1‐fluorophenyl‐8‐arylnaphthalenes, in which an ‘electron‐rich’ aryl ring is forced in a stacked geometry with an ‘electron‐poor’ fluorophenyl ring; the rotation process was examined using different NMR techniques (lineshape analysis, 2D EXSY). The ΔG≠S obtained when considering 1H and 19F nuclei were perfectly comparable. Semi‐empirical calculations (AM1) were performed to evaluate ΔG≠S for the rotation process, and the results are compared with the experimental NMR values.
Selective homonuclear Hartmann—Hahn transfer using two coherent r.f. channels: Application to relaxation time measurements
Authors: Yang Daiwen , Nagayama Kuniaki 
Published: May 2005

AbstractA method to implement selective homonuclear Hartmann—Hahn transfer by using two coherent r.f. channels has been developed. This new method does not require complex pulse sequences. When this approach is combined with selective relaxation measurements, the proton relaxation times (T1, T2 and T1p) of individual protons can be determined even when the signals are hidden under other resonances. An example of the use of this method for the analysis of ribonuclease H is given.
125Te, 13C and 1H NMR characterization of a series of diorganotellurium(II) and tetraorganotellurium(IV) compounds
Authors: Gedridge Robert W. , Higa Kelvin T. , Nissan Robin A. 
Published: May 2005

Abstract125Te, 13C and 1H NMR studies on a series of symmetric diorganotellurium(II) [R2 Te, where R = Me, Et, n‐Bu, CH2SiMe3, allyl, vinyl, CCMe, CCEt, CC(n‐Pr), CC(t‐Bu), CC(SiMe3) and CCPh], unsymmetric diorganotellurium(II) (RTeR′) (where R = allyl, R′ = Me, Et, i‐Pr, t‐Bu; R = Me and R′ = benzyl), symmetric bis(alkyltelluro)ethyne (RTeC=CTeR, where R = Me and Et) and symmetric tetraorganotellurium(IV) (R4Te, where R = Me, n‐Bu, CH2SiMe3 and vinyl) compounds are presented. 125Te was acquired under 1H‐decoupled and 1H‐coupled conditions. Whereas the 1H‐decoupled 125Te NMR spectra are extremely useful in identifying low levels of tellurium containing impurities, the 1H‐coupled 125Te experiments allow for structure determination and identification of long‐ and short‐range coupling pathways. 1H and 13C NMR were used to assess purity in terms of organic materials which can include solvents, decomposition byproducts and unwanted organometallic compounds from side reactions occurring during synthesis. 125Te satellites are observed in the 1H and 13C NMR spectra and are useful in resonance assignments and in identification of long‐ and short‐range coupling pathways. The effects of substituents on chemical shifts and coupling constants are discussed.
Use of shaped pulses for semi‐selective excitation of imino 1H resonances in duplex DNA and RNA
Authors: Gmeiner William H. , Sahasrabudhe Parag , Pon Richard T. 
Published: May 2005

AbstractThe application of shaped pulses, e.g. TOPHAT and hyperbolic secant, for the semi‐selective excitation of the imino 1H resonances of duplex DNA and RNA in aqueous solution is explored. The excitation profiles for the shaped pulses are calibrated on a sample containing a mixture of A:U base pairs in deuteriochloroform solution. The data obtained by performing a semi‐selective inversion‐recovery pulse sequence using a TOPHAT waveform for the 90° pulse and a hyperbolic secant waveform for the 180° pulse are fitted well by a single exponential function, are independent of transmitter offset over a 1 kHz bandwidth and are in good agreement with the value obtained with hard rectangular pulses. This approach for measuring T1 values was applied to a 2 mM sample of a self‐complementary oligoribonucleotide containing the nucleoside analogue 5‐fluorouridine in 90% H2O. The T1, values were calculated as a function of Tris concentration, a catalyst added to promote imino hydrogen exchange. The T1, values were obtained by good fits of single exponential functions to the inversion‐recovery data. The T1, values depend on the concentration of added Tris. Shaped pulses provide a convenient alternative to long rectangular pulses and hard pulse trains for the semi‐selective excitation of imino 1H resonances.
Carbon‐13 NMR study of the B9(Asp) mutant of human insulin: Sequence‐specific carbon‐13 assignments and structural information
Authors: Kristensen Søren M. , Led Jens J. 
Published: May 2005

AbstractThe resonances of the proton‐bearing 13C nuclei at natural abundance were assigned for the solution dimer of the B9(Asp) mutant of human insulin. The assignments are based on a combination of direct and relayed 1H13C correlated spectra, i.e. SQC, SQC‐HOHAHA and HMQC‐COSY, and the previously assigned 1H spectrum. The study proves valuable not only because of the 13C assignments, but also because it provides a verification, a completion and a few corrections of the previous 1H assignments. Finally, the obtained 13C assignments support the previously reported correlation between the secondary 13Cα and 13Cβ chemical shifts and the secondary structure of proteins.
Structural elucidation of rokitamycin, midecamycin and erythromycin metabolites formed by pathogenic Nocardia
Authors: Morisaki Naoko , Iwasaki Shigeo , Furihata Kazuo , Yazawa Katsukiyo , Mikami Yuzuru 
Published: May 2005

AbstractPathogenic Nocardia spp. converted rokitamycin, midecamycin and erythromycin into inactive metabolites, the structures of which were determined from NMR and mass spectral data. Depending on the species of Nocardia, these macrolides underwent phosphorylation at 2′‐OH, hydrolysis of the 4″‐O‐acyl group, glycosylation at 2′‐OH and/or reduction of the 18‐formyl group. Full 1H and 13C assignments are presented and some solvent effects are described.
Carbon‐13 NMR investigations of three fluorinated steroids
Authors: Carss Steven A. , Harris Robin K. , Fletton Richard A. 
Published: May 2005

AbstractHigh‐resolution carbon‐13 NMR spectra were obtained for three solid fluorinated steroids. Assignments were carried out using several approaches, including two‐dimensional spectra of the compounds in solution. Solid‐state and solution‐state chemical shifts are compared. Signals from one of the fluorine‐containing carbons in each compound show the effect of interplay between shielding, dipolar and indirect scalar (J) coupling tensors. One sample is found to be a mixture of two polymorphic forms.
Hyperfine couplings of N‐alkoxy‐N‐polynitrophenylaminyl radicals determined by ENDOR and TRIPLE resonance spectroscopy
Authors: Sumi Teemu J. , Stanciuc Gabriela , Kasa Seppo , Joela Heikki 
Published: May 2005

AbstractLead tetraacetate one‐electron oxidation of nine different N‐alkoxy‐2,6‐dinitroanilines substituted with trifluoromethyl, methyl and nitro groups in position 4 yielded aminyl radicals for which hyperfine couplings were measured by ENDOR and TRIPLE resonance spectroscopy. The optimum temperature range for proton ENDOR and general TRIPLE resonance measurements of aminyl radicals was 210–250 K and for nitrogen ENDOR 260 K in toluene. Further lowering for the temperature rapidly decreased the EPR intensity. The concentration of the sample and the amount of oxidant were optimized for obtaining ENDOR spectra. The relative signs of the hyperfine couplings of nitrogens, fluorines and protons were determined in the basis of general TRIPLE resonance experiments. The oxidation of N‐methoxy‐N‐2,6‐dinitrophenylamine and N‐ethoxy‐N‐2,6‐dinitrophenylamine with lead tetraacetate produced first the aminyl radical of the respective 2,6‐dinitro compound at low temperature (below 260 K) and very soon afterwards the aminyl radical of the respective 2,4,6‐trinitro compound, by a route in which hydrogen has to be removed from the system. Rotational correlation times were estimated for N‐methoxy‐and N‐ethoxy‐N‐2,4,6‐trinitrophenylaminyl radicals. Nitroxyl radicals were detected only under conditions where oxygen was present in the solvent.
Multiple‐field carbon‐13 NMR relaxation investigation on melezitose
Authors: Mäler Lena , Lang Jan , Widmalm Göran , Kowalewski Jozef 
Published: May 2005

AbstractMultiple‐field carbon‐13 relaxation data are reported for melezitose, a trisaccharide consisting of two α‐linked glucose residues attached at positions 2 and 3 to a β‐linked fructofuranose residue. Values for T1 and NOE were measured at 4.7, 9.4, 11.8 and 14.1 T and transverse relaxation rates at 4.7 and 9.4 T. The measurements were carried out by means of standard techniques and by polarization transfer (INEPT) enhanced methods. The relaxation data were interpreted with the Lipari‐Szabo ‘model‐free’ approach. It was found that the relaxation data of the ring carbons can be described by the use of the truncated form of the Lipari‐Szabo equation. The global correlation time and order parameter for melizitose were calculated at two temperatures. For the exocyclic CH2 carbons, fits to obtain a local correlation time are reported. For three out of the four exocyclic carbons, a fast correlation time is obtained and their order parameter is lower than that for the ring carbons. The fructose C‐1 exocyclic carbon does not seem to display the same local motions as the other CH2 groups, and a higher order parameter is obtained indicating hindered local motion of that CH2 group.
Dynamic NMR study of the factors governing nitroso group rotation in p‐nitrosoanilines in the solution and solid states
Authors: Fletcher Daniel A. , Gowenlock Brian G. , Orrell Keith G. , Šik Vladimir 
Published: May 2005

AbstractThe energy barriers associated with NO rotation in a series of p‐nitrosoanilines were measured by the total bandshape NMR method. Energies [ΔG‡ (298.15 K)] are in the range 38–57 kJ mol−1, the magnitudes being lowered appreciably by the presence of methyl substituents flanking the nitroso group on either side. In the case of p‐nitroso‐N,N‐dimethylaniline, NO rotation has been shown to occur also in the solid state with a slightly higher energy barrier to that in the dissolved state. For p‐nitroso‐N‐methylaniline, a combination of 2D‐exchange spectroscopy (EXSY) and 1D‐bandshape analysis NMR methods enabled the barriers to NO rotation and NHMe rotation to be measured separately.
Deuterium isotope effects on 13C chemical shifts of intramolecularly hydrogen‐bonded olefins
Authors: Hansen P. E. , Bolvig S. , Duus F. , Petrova M. V. , Kawecki R. , Krajewski R. , Kozerski L. 
Published: May 2005

AbstractA series of intramolecularly hydrogen‐bonded enamines, enols and enethiols with ester carbonylic, ketonic carbonylic, thioester carbonylic, nitro and sulphoxide acceptors were investigated to obtain 13C chemical shifts and deuterium isotope effects. Results from 33 new compounds and six remeasurements are compared with already existing data. An important aim was to show that isotope effects on chemical shifts are useful descriptors of hydrogen‐bonded systems and not only a parameter proportional to the 13C chemical shifts. Substituent effects were studied and the donors and acceptors ranked according to their abilities to support hydrogen bonding. Steric effects strengthen the hydrogen bonding in cyclic five‐membered β‐diketones. Plots of two‐bond [2ΔC(OD)] vs. four‐bond isotope effects [4ΔC(OD)] show that 4ΔC(OD) increases with increasing hydrogen bond strength and that large deviations from this relationship can be an indicator of tautomerism.
Substituent shielding parameters of fluorine‐19 NMR on polyfluoroaromatic compounds dissolved in dimethyl sulphoxide‐d6
Authors: Ando Shinji , Matsuura Tohru 
Published: May 2005

AbstractHigh resolution fluorine‐19 NMR spectra of polyfluoroaromatic compounds dissolved in deuterated dimethyl sulphoxide were measured and substituent shielding parameters were derived. These parameters were compared with the values observed in deuterated chloroform solutions and used to identify fluoroaromatic compounds related to perfluorinated polyimides and poly(amic acids)s. Average differences of Δδ between the solvents are 1.1, 1.0 and 2.1 ppm for o‐, m‐ and p‐fluorines of monosubstituted pentafluorobenzenes, respectively. The significant difference for p‐fluorine of NH2 is important in identifying perfluorinated diamines because they are source materials for perfluorinated polyimides. Substituent shielding parameters for meta and para substitution increase as the respective Hammett σ constants increase, which indicates that fluorine‐19 NMR chemical shift is primarily determined by electron density.
Complete assignment of the aliphatic chains in dimers, trimers and polymer of 3‐hexylthiophene through 2D‐NMR spectroscopy
Authors: Ferrari M. , Mucci A. , Schenetti L. , Malmusi L. 
Published: May 2005

AbstractThe 1H and 13C signals of the hexyl chains of the title compounds were fully assigned. The regiochemical features of the aliphatic region of proton and carbon spectra were analysed in dimers and trimers. Dimers and trimers proved to be reliable model compounds in the regiochemical assignment of poly(3‐hexylthiophene) even when the aliphatic region of proton and carbon spectra was involved. The chemical shifts of the protons of CH2(α)s and of the hexyl chain as a whole and those of the first three aliphatic carbons appear to be the most significant.
Electron spin resonance studies of the anions of some azulene derivatives: 1‐tert‐butyl‐, 1,3‐di‐tert‐butyl‐ and 4,6,8‐trimethylazulene
Authors: Waltman R. J. , Bargon J. 
Published: May 2005

AbstractHyperfine splitting constants are reported for radical anions of the azulene derivatives 1‐tert‐butyl‐, 1,3‐di‐tert‐butyl‐ and 4,6,8‐trimethylazulene. Radical anion geometries are optimized at the Hartree‐Fock level of theory using 3–21G and 3–21 + G* basis sets. The perimeter bond lengths are all approximately 1.40–1.44 Å except for the C‐4C‐5 (and C‐7C‐8) bond which, at 1.36 Å, is best described as a double bond. The singly occupied antibonding molecular orbitals are computed to be symmetric with respect to the mirror plane perpendicular to the molecular plane, consistent with the ESR data.
61Ni NMR spectroscopy of di‐ and tricarbonylnickel complexes
Authors: Behringer Klaus D. , Blümel Janet 
Published: May 2005

AbstractDi‐ and tricarbonylnickel phosphine complexes (22) were prepared and their liquid‐state 61Ni NMR spectra were recorded with the use of special solenoid glass sample tubes and a solenoid probehead, which are described. The acquired chemical shifts δ(61Ni), linewidths v1/2(Ni), relaxation times T1 and coupling constants 1J(61Ni,31P) are discussed with reference to the electronegativity of the substituents at the phosphine ligands and the Tolman angle θ of the latter.
Dynamic NMR: A new procedure for the estimation of mixing times in the 2D EXSY experiments. A four‐site exchange system studied by 1D and 2D EXSY spectroscopy
Authors: Dimitrov Valentin S. , Vassilev Nikolay G. 
Published: May 2005

AbstractA procedure for the estimation of the mixing time between the last two 90° pulses in the classic three‐pulse sequence NOESY/EXSY is proposed and tested and some considerations for the treatment of the two‐dimensional (2D) 1H NMR exchange spectra are given. The rate constants are thus obtained with reasonable precision. This procedure was followed to obtain the 2D spectra of the model compound α‐[bis(dimethylamino)methylene]‐4‐nitrophenylacetonitrile, which represents a four‐site exchange system. The barriers to restricted rotations found in this compound were also determined from one‐dimensional (1D) 1H NMR spectra, which were processed with the iterative complete lineshape analysis (CLSA) method. The double‐fit approach was incorporated in the CLSA method. It is shown that the results from the 2D dynamic NMR spectral studies corroborate those obtained by the CLSA double‐fit method.
Preparation of exo‐ and endocyclic aminoxyl radicals of 2‐Amino‐substituted quinazolin‐4(3H)‐ones
Authors: Omelka Ladislav , Světlik Jan 
Published: May 2005

Abstract2‐Amino‐substituted quinazolin‐4(3H)‐ones were oxidized with RO2· radicals and 4‐nitroperoxybenzoic acid. Two types of radical products, i.e. exo‐ and endocyclic aminoxyl radicals, were observed. Their differentiation was carried out on the basis of ESR parameters.
33S NMR spectroscopy: Substituent γ‐effect. Halogen derivatives of tetrahydrothiophene 1,1‐dioxide
Authors: Rozhenko A. B. , Bzhezovsky V. M. , Polovinko V. V. , Makarenko A. G. 
Published: May 2005

AbstractA series of halogen‐substituted tetrahydrothiophene 1,1‐dioxides (sulpholanes) were investigated by 13C, 17O and 33S NMR spectroscopy. It was shown that successive replacement of hydrogen atoms in positions 3 and 4 of the sulpholane ring led to an increase in 33S nuclei shielding. The 33S NMR chemical shift range exceeded 39 ppm. The variation of the 33S nuclei shielding with increasing number of chlorine or bromine atoms was caused by an electrostatic through‐space effect of the halogen atoms and the strongly polarized CHal bonds.
17O NMR studies of diaryl α‐diketones. The case of 2,2′,4,4′,6,6′‐hexa‐tert‐butylbenzil
Authors: Cerioni Giovanni , Plumitallo Antonio , Frey Joseph , Rappoport Zvi 
Published: May 2005

AbstractNatural abundance 17O NMR spectra of eleven diaryl α‐diketones are reported. An excellent correlation was found between 17O shifts and σp+. Conformations and π character of the intercarbonyl CC bond are discussed.
Differentiation of meso isomers from racemic mixtures with the combined use of chiral shift reagents and two‐dimensional heteronuclear correlation NMR spectroscopy
Authors: Henrichs P. Mark , Rodger Charles A. , Caulfield Thomas , Guo Peng 
Published: May 2005

AbstractIn the presence of chiral shift reagents the enantiotopic nuclei of a pair of enantiomers become diastereotopic and have the potential to give resolved NMR signals. Similarly, the enantiotopic nuclei within a meso isomer become diastereotopic in the presence of a chiral shift reagent and may give resolved NMR signals. However, the diastereotopic nuclei of a meso isomer mixed with a chiral shift reagent, unlike those of a racemic mixture, are located in the same molecule. Their intramolecular character can be established experimentally by detection of spin‐spin splitting between them or to a common third nucleus. Comparison of the correlation peaks in a two‐dimensional, heteronuclear, multiple‐quantum correlation (HMQC) spectrum with those of a heteronuclear multiple‐bond correlation (HMBC) spectrum is an effective means of detection of coupling to a third nucleus. Two‐dimensional NMR spectroscopy was used to identify the meso form of di‐(trans‐2‐aminocyclohexyl)amine.