摘要:

AbstractThe profisetinidins are the most important polyflavanoids of commerce, making up the major constituents of wattle and quebracho tannins. Even within the dimeric profisetinidins, substantial complexity exists because of stereo‐, regio‐, rotational and conformational isomers. Definition of the stereochemistry of the upper and lower flavan units, the location of the interflavanoid bond, the conformation of the heterocyclic rings in the upper and lower flavan units and the conformations of major and minor rotational isomers in a series of methyl ether acetate derivatives of dimeric profisetinidin diastereomers is possible by application of COSY and NOE experiments. All compounds studied were present in two rotameric forms with the more compact conformation favored. The heterocyclic ring in the upper chain extender unit was predominantly in an E‐conformation (i.e. half‐chair in 2R,3Sisomers and a ‘reverse’ half‐chair in the 2S,3Risomers). The heterocyclic ring conformation of the terminal cate‐chin unit, although mostly in an E‐conformation, was often represented by significant proportion

ISSN:0749-1581    

DOI:10.1002/mrc.1260330802

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA series of intramolecularly hydrogen‐bonded enamines, enols and enethiols with ester carbonylic, ketonic carbonylic, thioester carbonylic, nitro and sulphoxide acceptors were investigated to obtain13C 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 the13C 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 that4ΔC(OD) increases with increasing hydrogen bond strength and that large deviations from this relationship can be an indicator

ISSN:0749-1581    

DOI:10.1002/mrc.1260330803

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA gradient‐selectedJ‐HMBC experiment is presented. Coherence selection with magnetic field gradients results in HMBC spectra in whicht1noise is very low or completely eliminated. This makes it possible to detect unequivocally very small (<2 Hz) long‐range H,X coupling constants. With a slight modification, the HMBC experiment can be used for the quantitative measurement of these couplings. The signal amplitude of spectra with increasing coupling evolution time τ shows the characteristic sin (πJXHτ) dependence. By fitting a sine curve to the experimental data or by performing a 3DJ‐resolved HMBC experiment, accuratenJXHcoupling constants are obtained. This method is especially useful for estimating coupling constants between protons and unprotonated heteronuclei, e.g. quaternary carbons,15N nuclei in labelled prolines or31P in organo

ISSN:0749-1581    

DOI:10.1002/mrc.1260330804

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

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 foro‐,m‐ andp‐fluorines of monosubstituted pentafluorobenzenes, respectively. The significant difference forp‐fluorine of NH2is important in identifying perfluorinated diamines because they are source materials for perfluorinated polyimides. Substituent shielding parameters formetaandparasubstitution increase as the respective Hammett σ constants increase, which indicates that fluorine‐19 NMR chemical shift is primarily determined by el

ISSN:0749-1581    

DOI:10.1002/mrc.1260330805

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe spontaneous autoxidation (allomerization) of 132(R)‐chlorophyllain methanol produces seven main products containing three epimer pairs. The1H and13C NMR spectra of these products in acetone‐d6were recorded on a 500 MHz spectrometer and fully assigned using two‐dimensional HMQC and HMBC techniques. The absolute configurations of the oxidized carbons, originally C‐132, were determined using the ROESY technique. In comparison with 132(R)‐chlorophylla, the steric repulsion between the C‐17 side‐chain and the bulky substituents of the oxidized C‐132causes steric strain in the chiral part of an allomer, relieved by conformational changes in rings D and E and to a lesser extent also in the whole macrocycle. These changes were estimated for each allomer from the Δδ values of the carbon and proton resonances and from proton‐proton coupling constants. Information about the orientation of the front part of the phytyl chain in 132(R)‐chlorophyllaand its methanolic allomers was obtained by analysing the variation in the for

ISSN:0749-1581    

DOI:10.1002/mrc.1260330806

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe1H and13C 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 significa

ISSN:0749-1581    

DOI:10.1002/mrc.1260330807

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe slow inversion at pyramidal nitrogen in 2,2‐dimethylaziridine was investigated by NMR spectroscopy at 304 K using one‐dimensional spin locking methods and two‐dimensional exchange spectroscopy. The activation free energy was found to be 17.5 kcal mol−1(73.2 kJ mol−1) at this temperature. This value represents a lower limit for the unimolecular inversion barrier height as autocatalytic and water‐catalyzed inversion pathways can in principle also contribute to the observed rate. The spin locking experiments described employ non‐selective pulses and are particularly useful for the quantitative study of slow chemical exchange between nuclei of similar resonance frequency where selective magnetization transfer experiments are difficult

ISSN:0749-1581    

DOI:10.1002/mrc.1260330808

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractNatural abundance17O and13C NMR spectra for eleven diarylketenes are reported.17O shifts show invariance withparasubstitution and a shielding due toorthosubstitution. A buttressing effect is observed for bis(2,3,4,5,6‐pentamethylphenyl)ketene. A comparison between ketenes and isocyanates is made. The importance of cylindrical symmetry for half‐height linewidths is discus

ISSN:0749-1581    

DOI:10.1002/mrc.1260330809

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe nitrogen NMR shieldings (Δσ, equivalent to —Δδ on the frequency scale of chemical shifts) of the NO group in nitrosoalkanes are shown to be a sensitive probe to electron‐withdrawing effects of substituents and to steric strain effects. The former produce high‐field (low‐frequency) shifts, which can amount to about 90 ppm, while the latter result in considerable low‐field (high‐frequency) shifts of the NO nitrogen resonance. The CN nitrogen shieldings show less pronounced low‐field (high‐frequency) shifts under the influence of electron acceptors. The signs and magnitudes of the observed effects are explained in terms of semi‐empirical molecular orbital calculations of the shieldings and molecular mechanics evaluations of strain energies. The overall picture of relationships between the NO and CN nitrogen shieldings and electron charge migration within the molecules concerned is in accord with that obtained in earlier studies of solvent‐induced effects on the nitrogen shieldings in alkyl cyani

ISSN:0749-1581    

DOI:10.1002/mrc.1260330810

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

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 molecula

ISSN:0749-1581    

DOI:10.1002/mrc.1260330811

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY