摘要:

AbstractOn 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 protonorthoto 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 andcisto 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, the13C 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 liestransto the other substituent, as would be expected electrostatically. Large twists out of the ring plane are not consistent w

ISSN:0030-4921    

DOI:10.1002/mrc.1270220802

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractThis paper reports the77Se 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. The77Se chemical shift values of the ring selenium and the CSe groups are compared with the13C chemical shift values of neighbouring carbon atoms. The relationships between the77Se 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 screeni

ISSN:0030-4921    

DOI:10.1002/mrc.1270220803

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractThe33S 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. The33S 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,

ISSN:0030-4921    

DOI:10.1002/mrc.1270220804

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractZ,Eisomers 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 fo

ISSN:0030-4921    

DOI:10.1002/mrc.1270220805

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

ISSN:0030-4921    

DOI:10.1002/mrc.1270220806

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractObservable coupling over five formal bonds between the methoxy group protons and theorthoring 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 planarcisconformer, in which conjugation is favoured and repulsions between the methyl group and theorthohydrogen 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 thecisthan in thetransconformer. Molecular orbital calculations at the STO‐3G level, with complete geometry optimization, support the conduskus drawn from experiment

ISSN:0030-4921    

DOI:10.1002/mrc.1270220807

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractPyropheophytin a, which is an unsymmetric porphyrin, has been titrated with trifluoroacetic acid (TFA) in tetrahydrofuran, the protonation reaction being followed by13C NMR spectroscopy. TFA was added in small increments to a 0.28Msolution 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. The13C 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 discu

ISSN:0030-4921    

DOI:10.1002/mrc.1270220808

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractThe13C NMR spectra of some hydroxylatedent‐kaurane diterpenoids were measured in CDCl3‐Py‐d5(1:1) solution and also after addition of boric acid. Complexation ofent‐3β,18‐,ent‐7α,15β‐,ent‐7α,15α‐,ent‐15β,16β‐ andent‐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 th

ISSN:0030-4921    

DOI:10.1002/mrc.1270220809

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractIn 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

ISSN:0030-4921    

DOI:10.1002/mrc.1270220810

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractThe configurations and conformations ofcis‐ andtrans‐N‐methyl‐ and ‐N‐benzyl‐4,5‐ and ‐5,6‐tetramethylenetetrahydro‐1,3‐oxazines were determined by1H and13C NMR spectroscopy. Thecisisomers are conformationally homogeneous, having the hetero atom attached to the cyclohexyl ring, in the axial and equatorial positions, respectively, in the 5,6‐ and 4,5‐tetramethylene compounds, similar to the case of the 2‐p‐nitrophenyl‐substituted

ISSN:0030-4921    

DOI:10.1002/mrc.1270220811

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY