Magnetic Resonance in Chemistry

Krishnamurthy, V. V.

doi: 10.1002/(SICI)1097-458X(199701)35:1<9::AID-OMR930>3.0.CO;2-Rpmid: N/A

A band‐selective homonuclear decoupled TOCSY sequence is described and compared with the standard TOCSY experiment, using a sample of a DNA 15mer. Band selection and homonuclear decoupling (BASHD) was achieved along the evolution dimension using a band‐selective double pulse field gradient spin echo (DPFGSE) technique. This semi‐selective excitation sculpting technique substantially improves the resolution and sensitivity of a crowded region in the TOCSY spectrum of the DNA 15mer chosen. The methodology described is general and can be implemented in other 2D and 3D techniques to achieve band selection and homonuclear decoupling during proton evolution. © 1997 by John Wiley & Sons, Ltd.

Gogoll, Adolf; Grennberg, Helena; Axén, Andreas

doi: 10.1002/(SICI)1097-458X(199701)35:1<13::AID-OMR27>3.0.CO;2-Mpmid: N/A

The chemical shift order of axial and equatorial methylene protons in 1,5‐disubstituted 3,7‐diazabicyclo[3.3.1]nonan‐9‐ones may be altered by substituents in the 1,5‐positions, but the corresponding alcohols behave differently. Unambiguous signal assignments for a series of the title compounds are provided, based on 3JCH coupling constants and on {1H} 13C heteronuclear Overhauser effects. Substituent anisotropy effects as a source of the chemical shift changes are discussed. © 1997 by John Wiley & Sons, Ltd.

Zhuo, Jin‐Cong

doi: 10.1002/(SICI)1097-458X(199701)35:1<21::AID-OMR28>3.0.CO;2-Ipmid: N/A

Natural abundance 17O NMR spectra of 55 primary and secondary enaminones with various substituents at the C‐1, C‐3 and N‐positions are reported. Analyses of the 17O chemical shifts of these enaminones give the values of the increments for the substituents at these positions. The influence of the substituents on the 17O chemical shifts of the carbonyl groups of the enaminones is additive. It is found that the 17O chemical shifts of the enaminones can be calculated by the sum of the substituent increments. The differences between the calculated and the experimental data are generally less than ±2 ppm. The large deviations observed for the enaminones with a primary amino group are explained in terms of the formation of intermolecular hydrogen bonds. The chemical shift differences between the E‐ and Z‐forms of enaminones is mainly attributed to intramolecular hydrogen bonding. The contribution coming from intramolecular hydrogen bonding (ΔδHB), ranging from ‐14 to ‐47 ppm, depends on the donor properties of the amino groups and the type of substituents. The 17O chemical shifts of these enaminones correlate well with each other and with the pKa values of the corresponding alkylamines. Fair correlations of the 17O data of carbonyl groups with the 1H‐2 chemical shifts are observed, suggesting that the change of electron density on the carbonyl O atom parallels that on C‐2. © 1997 by John Wiley & Sons, Ltd.

Parella, Teodor; Sánchez‐Ferrando, Francisco; Virgili, Albert

doi: 10.1002/(SICI)1097-458X(199701)35:1<30::AID-OMR21>3.0.CO;2-0pmid: N/A

Vicinal 1H, 13C coupling constants were measured for rigid unsaturated 1H—C—C=13C molecular frameworks at natural abundance. As a general trend, an empirical range of 4.5–6.5 Hz is observed independent of the value of the dihedral angle between the CH bond and the plane of the double bond. Furthermore, through‐bond and through‐space substituent effects also influence the magnitude of the coupling constants in a similar way to that known for saturated systems. In summary, two different additive transmission mechanisms take part in these coupling paths depending on the orientation of the π‐system with respect to the allylic bond. The experimental evidence presented demonstrates the breakdown of the Karplus rule in this particular structural fragment. © 1997 by John Wiley & Sons, Ltd.

CLARAMUNT, ROSA MARÍA; SANZ, DIONISIA; LÓPEZ, CONCEPCIÓN; JIMÉNEZ, JOSÉ ANTONIO; JIMENO*, MARÍA LUISA; ELGUERO, JOSÉ; FRUCHIER, ALAIN

doi: 10.1002/(SICI)1097-458X(199701)35:1<35::AID-OMR25>3.0.CO;2-Kpmid: N/A

The 15N chemical shifts and a large collection of coupling constants pertaining to azoles have been gathered from the literature. To complete this collection and to check some anomalies, the spectra of 14 compounds in several solvents were recorded again and 31 compounds were studied for the first time; in all, data for 420 compounds (pyrroles, imidazoles, pyrazoles, triazoles, tetrazoles, indoles, benzimidazoles, indazoles, benzotriazoles and carbazoles) are reported. Additive models are used to discuss the substituent chemical shifts. © 1997 by John Wiley & Sons, Ltd.