摘要:

AbstractCarbon–carbon coupling constants in monoterpene hydrocarbons (3‐carene, α‐pinene and camphene) were investigated. A selective version of the INADEQUATE experiment was used for measurements of the long‐rangenJ(CC) values. The dependence of the vicinal13C13C coupling constants on the dihedral angles and the types of molecular framework involved within the direct coupling path is discussed. The configuration of a heteroatomic derivative from α‐pinene was determined from its

ISSN:0749-1581    

DOI:10.1002/mrc.1260300202

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractSelective INEPT (INAPT) experiments were used to correlate the nitrogen‐ and carbon‐bound hydrogens of nucleosides with13C resonances via2J(C, H) and3J(C, H). Such experiments are useful as part of a strategy to distinguish the imino hydrogens of uridine and thymidine, which cannot be accomplished on the basis of chemical shift. The strategy involves correlation of the imino hydrogen with C‐5 using INAPT and testing whether or not the C‐5 resonance is quaternary. Polarization transfer is ineffecient for most positions in nucleosides owing to the small long‐range couplings, except the H‐6C‐5 transfer in adenosine, which results in enhancement of the13C resonance over its equilibrium value. The efficiency of polarization transfer from H‐3 to C‐5 of thymidine depends on the base‐paired status of the nucleoside indicating a change in the3J(C, H) on base‐pair formation. This sensitivity to electronic reorganization on base‐pair formation makes the INAPT experiment a useful probe for the base‐paired status of nucleosides and of the electronic properties of urid

ISSN:0749-1581    

DOI:10.1002/mrc.1260300203

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractComplete and self‐consistent assignments have been achieved, through the concerted use of various 2D techniques, for the1H and13C NMR spectra of the free base and protonated forms, respectively, of apramycin (1) in D2O solution. It is shown thatJ‐relayed dipolar (orJ/R) and coherent transfer (HOHAHA) processes in ROESY experiments can hardly be eliminated with spin systems such as that studied here. True ROE cross‐peaks can, nevertheless, be identified by varying the experimental conditions. The solution conformation of 1 has been characterized based on the ROESY data, proton‐proton coupling constants and13CT1measurements. The 4‐aminoglucose unit appears to be locked together with the octodiose fragment in a conformation satisfying the requirements of theexo‐anomeric effects around the C‐1"OC‐8′ bond; the 2‐deoxystreptamine unit, on the other hand, enjoys more rotational freedom around the respective glycosidic bond. The undistorted chair conformations of all six‐membered rings in 1, and the overall conformation of the molecule, are totally independent of the pH, i.e. the protonat

ISSN:0749-1581    

DOI:10.1002/mrc.1260300204

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe results of19F,14N and15N NMR measurements for all14N/15N isotopomers of N3F in CD2Cl2solution are reported. Substantial deshielding for the N‐α and N‐γ atoms, relative to other covalent azides XN3(X  Cl, H, CF3, etc.), is observed. The coupling constants also indicate unusual bonding in N3F.1J(15N‐α, F) and3J(15N‐γ, F) are similar (58.2 and 58.6 Hz, respectively), very small for a1J(15NF) but very large for a3J(15NF) value. The observed value of 26.5 Hz for1J(15N‐α15N‐β) is the largest, and2J(15N‐α15N‐γ) (5.3 Hz) is the first reported coupling constant for an azide. The unusual N3F NMR data are discussed in terms of calculated atomic charges and the influence of the paramagnetic term which dominates the chemical shifts. This term expresses the deshielding due to the circulation of electrons in the magnetic field betwee

ISSN:0749-1581    

DOI:10.1002/mrc.1260300205

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractAs the magnetic fields used in NMR spectrometers become greater, it is expected that a field dependence of nuclear spin–spin coupling will be observed. A part of this, which would be independent of temperature, is due to the contributions of higher terms in the Hamiltonian of the molecule in the external magnetic field and the two nuclear spin operators. Perturbation theory at second, third and fourth order is used to extract the general features of these higher order terms. Contributions are due (a) to diamagnetic and paramagnetic terms of each of the four familiar parts of the spin coupling— Fermi contact, spin–dipolar, and orbital paramagnetic and orbital diamagnetic, and (b) to cross‐terms determined by operators for the diamagnetic and paramagnetic shielding at the two coupled nuclei. Explicit expressions are presented for the more important contributions. It is estimated for strongly magnetic nuclei in large molecules that the coupling varies as 0.5 × 10−5B2Hz, so that a field of about 14 T should produce a change in the coupling of 0.001 Hz. These are, respectively, the field used in high‐field NMR spectrometers and the upper limit of precision in the measurement of spin–spin coupling constants at th

ISSN:0749-1581    

DOI:10.1002/mrc.1260300206

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractLow‐temperature13C NMR spectroscopy shows perhydropyrido [1,2‐c] [1,3]oxazine to adopt a conformational equilibrium in CD2Cl2–CFCl3solution containing 98%transfused conformer in equilibrium with 2%O‐outside‐cisfused conformer at 203 K. A similar equilibrium position is adopted by thetrans‐(H‐4,H‐4a)‐4‐methylperhydropyrido[1,2‐c][1,3]oxazine, whereas the C‐4 epimer adopts exclusively the

ISSN:0749-1581    

DOI:10.1002/mrc.1260300207

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe conformational analysis of three 1,2‐3,4‐di‐O‐Methylenepyranoses was carried out using NMR (coupling constants and homonuclear Overhauser effect) and theoretical (MM2 calculations) methods. A preferred twist–boat conformation

ISSN:0749-1581    

DOI:10.1002/mrc.1260300208

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

Abstract1H and13C NMR experiments were performed to analyse the conformation and stereochemistry of diastereomeric sulphoxides of 1,3‐thiazolidine derivatives. The conformation of the thiazolidine ring was verified by an NOE study. ASIS and13C chemical shift data supported the sulphoxide configuratio

ISSN:0749-1581    

DOI:10.1002/mrc.1260300209

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractSeveral deuteriated polycyclic aromatic radical cations were studied by ESR. Their hyperfine coupling constants are reported and mechanisms for their formation are proposed.

ISSN:0749-1581    

DOI:10.1002/mrc.1260300210

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractSeveral polycyclic aromatic radical cations produced in Friedel–Crafts alkylation reactions were studied by EPR and ENDOR methods, and the following were identified: 1,2,5,6‐tetramethylanthracene, 2,3,6,7,9,10‐hexamethylanthracene, 3,6,11,14‐tetramethyldibenzo [a,c]triphenylene and 2,6,9,10‐tetramethylanthracene radica

ISSN:0749-1581    

DOI:10.1002/mrc.1260300211

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY