Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1270220801pmid: N/A

Schaefer, T.; Peeling, James; Wildman, Timothy A.

doi: 10.1002/mrc.1270220802pmid: N/A

On the basis of the presence or absence of long‐range spin–spin coupling constants between side–chain and ring nuclei in 2‐methoxyacetophenone, some literature ambiguities about the conformational preferences of the side‐chains in this compound can be resolved. The long‐range coupling between the methoxy protons and the ring proton ortho to the methoxy group, 5J(H, CH3)o, is (−)0.28 ± 0.02 Hz, as expected for a conformation in which the methoxy group lies in the benzene plane and cis to H‐3. The methyl protons of the acetyl group do not couple to H‐6, implying that this methyl group does not approach H‐6 closely. However, the 13C nucleus of this methyl group couples by +0.4 Hz to H‐5 and not to H‐3. This stereospecific five‐bond coupling implies that the acetyl group predominantly prefers an arrangement in which the carbonyl group lies trans to the other substituent, as would be expected electrostatically. Large twists out of the ring plane are not consistent with the observed couplings.

Poleschner, Helmut; Radeglia, Reiner; Meyer, Hinrich

doi: 10.1002/mrc.1270220803pmid: N/A

This paper reports the 77Se NMR chemical shifts of 1,3‐dithiole‐, 1,3‐thiaselenole‐ and 1,3‐diselenole‐2‐ones, ‐thiones and ‐selones, of the corresponding saturated compounds 1,3‐diselenolane‐2‐one, ‐thione and ‐selone, and the 1,3‐thiaselenolium tetrafluoroborates, either unsubstituted or substituted by morpholino, ethylthio or ethylseleno groups in the 2‐position. The 77Se chemical shift values of the ring selenium and the CSe groups are compared with the 13C chemical shift values of neighbouring carbon atoms. The relationships between the 77Se chemical shifts of the CSe groups and the wavelengths of their n→* absorption in the UV‐visible spectrum are discussed with respect to the significance of the δE term in the contribution of the paramagnetic screening and the electron density distribution.

Cassidei, L.; Fiandanese, V.; Marchese, G.; Sciacovelli, O.

doi: 10.1002/mrc.1270220804pmid: N/A

The 33S NMR spectra of some selected sulphones demonstrate additive substituent‐induced chemical shift (SCS) effects. In dimethyl sulphone (1), replacement of a methyl group by a vinyl or a phenyl group causes an SCS effect of −7 to −8.5 ppm or −4 to −5 ppm, respectively. The 33S chemical shift in 1 is also sensitive to substitution of methyl protons. The ß‐substituent effect for methyl and phenyl groups is in the range +7 to +8 ppm and +5.5 to +6 ppm, respectively.

Durand, R.; Geneste, P.

doi: 10.1002/mrc.1270220805pmid: N/A

Z,E isomers of rigid enol‐ethers were studied by proton nuclear magnetic resonance. As with oximes, it was found that the chemical shift difference (Δδ=δz−δE) for the protons α to the function in question depends on the dihedral angle between the CαH and CC bonds. This phenomenon can be explained by an electric field effect and not by a magnetic anisotropy effect. The present study has allowed the derivation of values of the volume magnetic susceptibility and the product of b×the dipole moment for the CO bond.

Jovanovic, Misa V.; Biehl, Edward R.

doi: 10.1002/mrc.1270220806pmid: N/A

Blonski, Wayne J. P.; Hruska, Frank E.; Wildman, Timothy A.

doi: 10.1002/mrc.1270220807pmid: N/A

Observable coupling over five formal bonds between the methoxy group protons and the ortho ring proton in 2‐methoxypyridine, coupliugs between the methoxy group carbon and ring protons, and methoxy carbon spin‐lattice relaxation times are all consistent with a preference for the planar cis conformer, in which conjugation is favoured and repulsions between the methyl group and the ortho hydrogen are reduced. Small‐amplitude torsioas about the C‐2–O bond may carry the methoxy group away from this orientation, but more distant conformations can probably be excluded. Methyl group rotation is less hindered in the cis than in the trans conformer. Molecular orbital calculations at the STO‐3G level, with complete geometry optimization, support the conduskus drawn from experimental evidence.

Lötjönen, Simo; Hynninen, Paavo H.

doi: 10.1002/mrc.1270220808pmid: N/A

Pyropheophytin a, which is an unsymmetric porphyrin, has been titrated with trifluoroacetic acid (TFA) in tetrahydrofuran, the protonation reaction being followed by 13C NMR spectroscopy. TFA was added in small increments to a 0.28 M solution of pyropheophytin a in tetrahydrofuran, and the chemical shift changes of the macrocyclic carbons were determined as a function of the TFA increments. On the addition of TFA the signals of the α‐carbons of ring II experienced a large upfield change, whereas the signals of all other macrocyclic carbons moved only slightly downfield or remained constant. These observations were interpreted as indicating the formation of a monocation in which the proton is attached to the nitrogen of ring II. The 13C protonation shifts of pyropheophytin a were compared with those previously reported for symmetric porphyrins. On the basis of this comparison, the basicity of the macrocyclic nitrogen atoms, the N–H tautomerism and the electron delocalization in structurally different porphyrin macrocycles are discussed.

Fernández‐Gadea, Francisco; Jimeno, María L.; Rodríguez, Benjamín

doi: 10.1002/mrc.1270220809pmid: N/A

The 13C NMR spectra of some hydroxylated ent‐kaurane diterpenoids were measured in CDCl3‐Py‐d5 (1:1) solution and also after addition of boric acid. Complexation of ent‐3β,18‐, ent‐7α,15β‐, ent‐7α,15α‐, ent‐15β,16β‐ and ent‐16β,17‐dihydroxy derivatives with boric acid produced considerable chemical shift changes and marked broadening of the signals of the hydroxy‐bearing and neighbouring carbon atoms. This behaviour provides a useful and reliable method for assigning the 13C NMR spectra of these compounds.

Sevenster, Arjen J. L.; Tabner, Brian J.

doi: 10.1002/mrc.1270220810pmid: N/A

In dimethylformamide the ESR spectra of the radical anions of dibenzo[f,h]quinoxaline and dibenzo[a,c]phenazine both exhibit line broadening due to slow molecular tumbling over a wide temperature range. Similar effects are also observed in the spectra of the radical anions of quinoxaline and phenazine. The ESR spectra of the radical anions of dibenzo[f,h]quinoxaline and dibenzo[a,c]phenazine have been interpreted by computer reconstruction and the hyperfine splitting constants assigned with the aid of Hückel unpaired electron density calculations.