Magnetic Resonance in Chemistry

Iratçabal, P.; Grenier‐Loustalot, M. F.; Lichanot, A.; Metras, F.

doi: 10.1002/mrc.1260230103pmid: N/A

The conformational preferences of 1‐oxa‐ and 1‐thia‐spiro[2.5]octanes have been determined from lowtemperature 13C NMR spectra. The 13C chemical shifts are discussed in terms of the α, β and γ effects. The energy barrers to inversion of the cyclohexane ring correlate with the energy barriers to internal rotation of the methyl group in 1‐hetero‐2‐methylcyclopropanes.

Tretyakov, A. V.; Rudaya, L. I.; El'tsov, A. V.; Larin, M. F.; Lopyrev, V. A.

doi: 10.1002/mrc.1260230104pmid: N/A

The 13C NMR spectra of some 2‐arylimidazo‐[4,5‐c]‐ and ‐[4,5‐b]‐pyridines and their protonated forms have been studied. The nitrogen atom of the pyridine ring has been shown to be the most basic nitrogen in these compounds.

Liepinsh, E.; Sekacis, I.; Lukevics, E.

doi: 10.1002/mrc.1260230105pmid: N/A

Two‐dimensional NMR experiments on 29Si‐1H shift correlations in trimethylsilyl‐substituted sugars are described. This technique allows the unambiguous assignment of 29Si and 1H signals of all trimethylsilyl groups.

Wilson, Nancy K.

doi: 10.1002/mrc.1260230106pmid: N/A

Carbon‐13 nuclear magnetic resonance spin‐lattice relaxation times and nuclear Overhauser enhancements were measured as functions of temperature for the hydrogen‐bearing carbons in 9‐fluorenone, 4‐methyl‐9‐fluorenone and 4,5‐dimethyl‐9‐fluorenone. Reorientational diffusion constants were obtained from the dipolar T1 values. Isotropic overall molecular motion was assumed, and the methyl group rotation was separated from the overall molecular reorientation by the method of Woessner. Arrhenius plots of the dependence of the rotation rates on temperature provided values of the energy barriers for molecular reorientation and methyl group rotation. The energy barriers for molecular reorientation are essentially the same: 9‐fluorenone 11.8 ± 0.5 kJ mol−1, 4‐methyl‐9‐fluorenone 11.9 ± 0.5 kJ mol−1 and 4,5‐dimethyl‐9‐fluorenone 11.7 ± 0.2 kJ mol−1. The barriers to methyl group rotation are 4‐methyl‐9‐fluorenone 7.8 ± 0.9 kJ mol−1 and 4,5‐dimethyl‐9‐fluorenone 11.0 ± 0.5 kJ mol−1. Thus the bay methyl‐methyl steric interaction in 4,5‐dimethyl‐9‐fluorenone increases the methyl rotational barrier by 3.2 kJ mol−1.

Johns, S. R.; Willing, R. I.

doi: 10.1002/mrc.1260230107pmid: N/A

The DOUBTFUL pulse sequence is proposed as a method for the assignment of 13C chemical shifts and 13C, 13C coupling constants of quaternary resonances in compounds in which the chemical shift of an adjacent carbon is known.

Engelhardt, Michael; Hägele, Gerhard; Mikolajczyk, Marian; Balczewski, Piotr; Wendisch, Detlef

doi: 10.1002/mrc.1260230108pmid: N/A

2‐Dimethoxyphosphoryl‐1,3‐dithiane was obtained by the reaction of 2‐chloro‐1,3‐dithiane with trimethyl phosphite. Studies at 200, 360 and 400 MHz using 1H, 13C and 31P nuclei and 1D‐ and 2D NMR techniques are described. Long‐range coupling constants were extracted by iterative treatment of [AB]2CDMR6X spin systems. A rigid dithiane structure with axial phosphorus is consistent with the NMR results. Analogous dithianes and solid‐state structures are discussed.

Hasan, Misbah Ul; Asrof Ali, Sk.

doi: 10.1002/mrc.1260230109pmid: N/A

The steric and electronic interactions in several methanesulphonanilides and dimethane‐sulphonanilides have been examined on the basis of their 13C NMR chemical shifts. In aromatic amines there is evidence of nitrogen electron pair donation into the aromatic ring. However, the substitution of a methanesulphonyl group on nitrogen, as in N‐phenylmethanesulphonamide, causes a decrease in this resonance interaction. The chemical shifts of ortho‐ and para‐carbons therefore move downfield in comparison with the corresponding amines. Substitution of the aromatic ring at the para position by electron‐withdra wing and electron‐donating groups affects the resonance process. The substitution of methyl groups either at nitrogen or at the ortho positions causes severe steric inhibition of resonance, which is clearly reflected in the chemical shifts of these compounds. The chemical shifts in several dimiethane‐sulphonanilides have also been examined and compared with the shifts in the corresponding methanesulphonanilides. The observed differences are explained in terms of steric, mesomeric and inductive interactions in these molecules. The preferred conformations of these compounds in solution are also postulated on the basis of their chemical shifts.

Azzaro, Marcel; Geribaldi, Serge; Videau, Bruno

doi: 10.1002/mrc.1260230110pmid: N/A

Rates of rotation around the CN bond in a series of ten N,N‐disubstituted 3‐amino‐5,5‐dimethylcyclohex‐2‐enones were determined from total NMR line shape analysis. Linear correlations are shown to exist between Δ 298≠ and parameters describing the inductive and steric effects of N‐alkyl substituents. The substituent effects on the barriers seem to occur mainly through a change in the potential energy of non‐bonded interactions between the nitrogen substituents and the hydrogen atoms in the ortho position to the amino group.