Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260230501pmid: N/A

Tashma, Zeev

doi: 10.1002/mrc.1260230502pmid: N/A

Replacement of chloroform by benzene as an NMR solvent for a series of (thio)phosphonothioamides results in an exceptionally wide range of shifts for the NCH proton signals, from an upfield shift of approximately 1 ppm to a downfield shift of about 0.1 ppm, depending on the phosphonate esterifying groups, the type of the N‐alkyl moiety and whether it is a PO or a PS compound. The last factor is believed to operate indirectly by affecting the aggregation state of the compounds in benzene.

Hasan, Misbah Ul; Arab, Mohammad; Pandia Rajan, K.; Sekar, R.; Marko, Dale

doi: 10.1002/mrc.1260230503pmid: N/A

The 13C NMR spectra of several methyl substituted 2,6‐diaryl‐4‐piperidones (aryl = phenyl, o‐chlorophenyl) have been measured. Substituent parameters for methyl and chloro groups have also been derived and are discussed in terms of their steric and electronic interactions. Both the methyl substitution at C‐3 and chlorine substitution in the ortho position of the phenyl ring cause significant changes in the 13C chemical shifts, which can be satisfactorily explained on the basis of steric interactions taking place in the preferred conformations of these molecules. In compounds with o‐chloro groups, the aryl ring is oriented such that the chlorine is syn to the axial hydrogen on the aryl‐bearing carbon. The shielding effect of the carbonyl group is found to be more pronounced on an adjacent equatorial methyl than on an adjacent axial methyl group.

Contreras, R. H.; Ferraro, M. B.; de Kowalewski, D. G.; Díaz, L.

doi: 10.1002/mrc.1260230504pmid: N/A

A quantitative estimation of the proton deshielding effect produced by a sterically crowded environment was obtained experimentally, based on departures from the chemical shift additivity rule of protons ortho to OCH3 and NO2 groups in benzene derivatives, where a conformational change from these side‐chain groups is expected. The influences of these conformational changes were also analysed from a theoretical point of view. Calculations of the magnetic shielding tensor were carried out at the INDO approximation with gauge invariant atomic orbitals, together with the uncoupled Hartree‐Fock approach. Theoretical results for these proton deshielding effects are in very good agreement with those determined experimentally.

Lopyrev, V. A.; Larina, L. I.; Vakul'skaya, T. I.; Larin, M. F.; Shibanova, E. F.; Titova, I. A.; Voronkov, M. G.; Liepin'sh, E. E.

doi: 10.1002/mrc.1260230505pmid: N/A

The influence of substituents on the 1H, 13C and 15N NMR chemical shifts of 2‐substituted 5(6)‐nitrobenzimidazoles and their analogues labelled with 15N in the nitro group was investigated. It was found that the substituent effects are transmitted to the carbon and hydrogen nuclei in positions 4–7 mainly by the resonance mechanism, but to the nitro‐nitrogen atom with approximately equal contributions from the resonance and inductive components. Comparison of the transmission of substituent effects in nitro‐benzimidazoles with para and meta derivatives of nitrobenzene showed that the nitro group seems to be more tightly conjugated with the benzimidazole fragment than with the benzene ring.

Lopyrev, V. A.; Larina, L. I.; Vakul'skaya, T. I.; Shibanova, E. F.; Titova, I. A.; Voronkov, M. G.

doi: 10.1002/mrc.1260230506pmid: N/A

The substituent effects in dianion radicals of fourteen 2‐substituted 5(6)‐nitrobenzimidazoles, electrochemically generated in acetonitrile, were studied by ESR. It was found that the substituent influence on the nitro group is transmitted with approximately equal contributions from the inductive and resonance components, independently of the pairs of the σIσR, σIσRo,σIσR+ and ℱℛ substituent constants used. The radical stabilization constant σ contributes much to the correlation of the hyperfine splitting constants of H‐6 and the heterocyclic nitrogen atoms. The transmission through the benzimidazole ring in dianion radicals of nitrobenzimidazoles is half that of the benzene ring in anion radicals of para‐substituted nitrobenzenes.

Häkkinen, A.‐M.; Ruostesuo, P.; Kurkisuo, S.

doi: 10.1002/mrc.1260230507pmid: N/A

The 17O, 15N and 13C NMR chemical shifts have been determined for N,N‐dimethylmethanesulphinamide (1), both as the neat liquid and in dimethyl sulphoxide, acetone, chloroform and various alcohols. The 17O and 15N nuclei of 1 resonate at a lower frequency than those of the corresponding sulphonamide, and depend on the solvent and concentration of the components in binary solution mixtures. Hydrogen bond‐forming solvents shift the signals to lower frequency than those of the neat amide. On changing the solvent, the change in the 17O NMR chemical shift of the sulphinamide is smaller than that of carboxamides, but the difference in the 15N NMR chemical shifts between these two systems with a solvent change is much lower and opposite in direction, reflecting the differences in the resonance structures of carboxamides and sulphinamides.

Woyciesjes, Peter M.; Janes, Nathan; Ganapathy, S.; Hiyama, Yukio; Brown, Theodore L.; Oldfield, Eric

doi: 10.1002/mrc.1260230508pmid: N/A

14N nuclear quadrupole resonance (NQR) spectra of the NO functional group in a series of 4‐substituted pyridine 1‐oxides (R = H, CH3, C6H5, OCH3, OCH2C6H5, Cl, CN, NO2) and the natural abundance 17O NQR and nuclear magnetic resonance (NMR) solution spectra of a number of selected systems were obtained. The orientations of the principal axes of the 14N electric field gradient tensor (efg) in the NO group were determined using a Townes‐Dailey treatment of the NQR results, combined with a 13C cross‐polarization magic‐angle sample‐spinning NMR line shape analysis of carbons bonded directly to nitrogen. The major principal axis (Z) is coincident with the NO bond. The orientation of the principal (Z) axis of the 17O efg tensor also lies along the N‐O bond, as determined by Townes‐Dailey analysis. The NO π‐bond orders show considerable sensitivity to the nature of the para‐substituent, ranging from 0.17 to 0.31. 17O and 15N NMR chemical shifts parallel these changes in π‐bond order.

Aversa, M. C.; Giannetto, P.; Saija, A.

doi: 10.1002/mrc.1260230509pmid: N/A

By observing the shifts of the 1H NMR signals of 1,4‐benzoquinones induced by Eu(fod)3, and simulating them with the aid of a novel computer program TWOCEN, it has been shown that the two carbonyl groups of 1,4‐toluquinone and 2,6‐dimethyl‐1,4‐benzoquinone compete in the Eu coordination. The metal binds at two sites on both the carbonyl groups, with 4‐CO having a greater complexing ability with respect to 1‐CO, in line with steric and electronic considerations.

Meese, Claus O.; Walter, Wolfgang

doi: 10.1002/mrc.1260230510pmid: N/A

The 13C NMR. chemical shifts of several selenoimidates (3) are reported in comparison with imidates (1) and thioimidates (2). Distinct resonance signals of 3 show additional satellites arising from 77Se couplings. Extremely large three‐bond couplings [3J(13C77Se) = 28–39 Hz] are observed in the Z configuration of the N‐methylselenoimidate double bond, where the coupling nuclei (77SeCN13C) are cis‐orientated.