Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260320401pmid: N/A

Esteban, Angel L.; Galache, Maria P.; Diez, Ernesto; Biekofsky, Rodolfo R.; Contreras, Rubén H.

doi: 10.1002/mrc.1260320402pmid: N/A

The proton‐coupled 13C NMR spectra of 1‐trans‐methoxy‐ (2) and 1‐cis‐methoxy‐1,3‐trans‐butadiene (3) are consistent with a methoxy heavy atom planar conformation with s‐syn and s‐anti orientations, respectively. Ab initio 6–31G* calculations confirmed such conformations. They were taken as model compounds to determine the influence of electrostatic interactions on the methoxy 13C NMR parameters. A shielding increase of 4 ppm is observed in 2, with respect to 3, for the OMe 13C chemical shift in an s‐syn conformation and is ascribed to the attraction between the methyl and vinyl moieties as proposed by Li and Chesnut. The methyl 1J(C,H) coupling is not affected by this interaction, showing that the carbon 2p electrons are more polarizable than those in the 2s orbital.

Basso, Ernani A.; Kaiser, Carlos; Rittner, Roberto; Lambert, Joseph B.

doi: 10.1002/mrc.1260320403pmid: N/A

The carbon‐13 chemical shifts were assigned for all ring positions in cyclohexanone and 4‐tert‐butylcyclohexanone with H, F, Cl, Br, I, MeO, MeS, MeSe, Me2N, Me or tert‐butyl in the 2‐position. The substituent‐induced shifts were calculated by difference from unsubstituted cyclohexanone or 4‐tert‐butylcyclohexanone. Both stereoisomers (cis and trans) were available for the 4‐tert‐butylcyclohexanones in all but one case. Comparison of the substituent‐induced shifts for the cis (equatorial 2‐substituent) and trans (axial 2‐substituent) isomers provides stereochemical insight into the interactions between the 2‐substituent and the carbonyl group that bring about non‐additivity of the substituent effects. In the 2‐equatorial isomer, the dipole–dipole interaction between the functional groups causes non‐additivities for the C‐2 carbon that depend largely on the electronegativity of the 2‐substituent. In the 2‐axial isomer, hyperconjugation or other orbital interactions between the groups cause non‐additivities for the C‐2 carbon that depend largely on the polarizability in addition to the electronegativity of the 2‐substituent.

Camps, Pelayo; Vázquez, Santiago; Jaime, Carlos; Virgili, Albert

doi: 10.1002/mrc.1260320404pmid: N/A

A high preference for the boat conformation of tricyclo(4.3.1.12,5)undec‐3‐en‐10‐one was established on the basis of 1H NMR spectroscopy with the aid of molecular mechanics calculations (MM3 program).

Pant, Geeta; Purohit, Mahesh C.; Morris, Gareth A.; Halstead, Adam G. W.; Thompson, Robert I. G.

doi: 10.1002/mrc.1260320405pmid: N/A

A glycoside extracted from the rhizomes of Agave americana was identified as 3‐O‐({{β‐D‐xylosyl(1 → 3)}{β‐D‐xylosyl(1 → 3) ‐ β ‐ D ‐ glucopyranosyl(1 → 2)} ‐ β ‐ D ‐ glucopyranosyl(1 → 4)} ‐ β ‐ D ‐ galactopyranosyl) ‐ (25R) ‐ 5α ‐ spirostan‐3β‐ol by concerted use of proton and carbon‐13 2D NMR methods. Complete assignments for the sugar resonances were obtained with standard methods, but the presence of chemical shift degeneracies prevented an unequivocal determination of two of the glycosidic linkage points by the HMBC experiment. Further two‐dimensional experiments failed to resolve the ambiguity, but a clear identification of the linkages was obtained using a selective reverse INEPT experiment with homonuclear double resonance.

Hoebeke, Johan; Busatto‐Samsoen, Catherine; Davoust, Daniel; Lebrun, Evelyne

doi: 10.1002/mrc.1260320406pmid: N/A

Antipain ((1‐carboxy‐2‐phenylethyl)carbamoyl‐L‐arginyl‐L‐valyl‐DL‐arginal) is a potent inhibitor of papain and related proteases. The assignment of 1H NMR signals of the interconverting forms in equilibrium in aqueous solution at pH 5.8 were investigated using two‐dimensional DQF‐COSY, DOUBLE‐RELAYED‐COSY, HOHAHA and ROESY nuclear magnetic resonance techniques. The assignments of protons of several forms in equilibrium of the D‐ and L‐arginal antipain isomers were determined in aqueous solution at 300 K, and their corresponding stereochemistry is tentatively proposed. Addition of equimolar amounts of papain resulted in spectral changes and chemical shifts which are compatible with enzyme‐L‐carbinolamine interactions.

Adams, Bruce; Lerner, Laura E.

doi: 10.1002/mrc.1260320407pmid: N/A

Chemical shifts and coupling constants of hydroxyl protons for a series of simple carbohydrates are reported and several trends extracted: (1) chemical shifts are relatively independent of the experimental conditions if referenced to water; (2) chemical shifts are sensitive to the orientation of the HO group and its neighbors; and (3) coupling constants fall within a relatively narrow range, which is indicative of rotational averaging. These observations suggest that NMR parameters of hydroxyl protons can provide important diagnostic information about the stereochemistry of simple carbohydrates and may prove useful for analysis of more complex compounds.

Farrant, R. D.; Lindon, J. C.

doi: 10.1002/mrc.1260320408pmid: N/A

The measurement of the 1H NMR spectrum of a sealed sample of benzene dissolved in a nematic liquid crystal provides a convenient and sensitive test of both NMR probe temperature stability and of the temperature gradient across the sample.

Macchioni, Alceo; Pregosin, Paul S.; Rüegger, Heinz; van Koten, Gerard; van der Schaaf, Paul A.; Abbenhuis, Rob A. T. M.

doi: 10.1002/mrc.1260320409pmid: N/A

183W NMR spectra for a series of octahedral W(VI) imidophenyl, ‘WNPh’ and W(VI) imidoethyl, ‘WNEt,’ complexes were obtained using inverse detection based on non‐specific long‐range interactions. The metal couples to both the ortho and para protons of the NPh moiety (four and six bonds, respectively). The 183W shift is shown to be sensitive to the nature of the ligand and the coordination number. The effect of geometric isomerism on δ183W is shown to be several hundred ppm. Some δ14N and δ15N and 1J(183W,15N) data are reported. A number of new geometric isomers, involving the position of the NR ligand with respect to the remaining ligands, have been identified.

Jokisaari, Jukka P.; Ingman, L. Petri; Schrobilgen, Gary J.; Sanders, Jeremy C. P.

doi: 10.1002/mrc.1260320410pmid: N/A

The effects of the various xenon and krypton isotopes on the nuclear shielding of 19F in xenon and krypton difluoride in solution were studied by 19F NMR spectroscopy. For both molecules, linear relationships were observed between the one‐bond secondary isotope shift of the 19F nucleus, 1Δ; 19F(m′/128Xe) and 1Δ 19F(m′/78Kr), and the relative mass factor, (m′ − m)/m′, and also between the shifts and the quadratic mean bond displacement, 〈(Δr)2〉. On the basis of the latter correlation and the linear relationship between 〈Δr〉 and 〈(Δr)2〉 for the linear combination of the first and second derivatives of the 19F shielding, the values −1742 and −5804 ppm Å−2, respectively, were determined. Further, the temperature dependence of the 129Xe and 19F chemical shifts and the 129Xe, 19F indirect spin‐spin coupling of XeF2 were determined. The 129Xe chemical shift is extremely sensitive to temperature with Δδ/ΔT = −0.4718 ppm K−1, whereas for 19F Δδ/ΔT = +0.0042 ppm K−1. In contrast, the 129Xe, 19F indirect spin–spin coupling is independent of temperature over the range 243–303 K, the mean value being 5644.2 ± 0.6 Hz.