摘要:

AbstractThe13C NMR spectra of 48 polychlorobromoalkanes have been studied. Unlike the13C signals of chlorine‐containing groups (38–105 ppm), those of bromine‐containing fragments, with the exception of CBr2(60–70 ppm), appear in a rather narrow range (25–50 ppm) and are shifted to higher field in relation to similar chlorine‐containing groups. The spin–spin coupling constants in similar bromine‐ and chlorine‐containing groups practically coinciEN. Calculation of the chemical shifts for the polyhaloalkanes under study according to the additivity scheme, as previously observed for polychloroalkanes, renders values which are in considerable discord with experimental values (up to –32 ppm for CBr3). These discrepancies may be compensated for by corrections for the binary interaction of halogen atoms by grouping the halogen‐containing fragments according to the geminal, vicinal, 1,3‐, 1,3,5‐ and 1,2,3‐arrangement of halogen atoms, and by introducing an increment for the position of the halogen at the secondary atom. It is established that as compared to 1‐monohaloalkanes: (a) in the case of the geminal arrangement of halogen atoms the α‐ and γ‐effects diminish (Δ α from –3.2 to –8 ppm; Δγ = 2.6 ppm), while the β‐effect increases slightly (from 0 to 1.2 ppm); (b) in the case of a vicinal arrangement both the α‐ and β‐effects diminish (by about –3.5 ppm) and the γ‐effect remains constant, as if the vicinal system of the halogens was topologically insulated; (c) for the 1,3‐ and 1,3,5‐arrangement of halogens their mutual influence is weak (about –0.5 ppm for each halogen atom in the α‐ and γ‐positions); (d) the 1,2,3 system (serial arrangement of halogen atoms) is the sum of two vicinal fragments and hardly deviates from the additivity scheme; (e) the arrangement of a halogen at the secondary C atom enhances the α‐effect (Δα = 2.8 and 1.0 for methyl and methylene, respectively, in the case of Cl, and 3.5 and 3.7 ppm in the case of Br); the variation of the β‐effect has a different sign in relation to CH3and CH2groups (+1.2 and –1.7 for Cl, and +2.5 and –1.0 for Br). More distant effects of halogens (δ and ϵ) were not considered. The determined increments (Δα, Δβ and Δγ) for the α‐, β‐ and γ‐effects of chlorine and bromine atoms allow the prediction of the 13Cchemical shifts in polyhaloalkanes with an accuracy up to ±1.5 ppm. Some deviations of up to ±5 ppm may be connected with the influence of a three particle interaction of ha

ISSN:0030-4921    

DOI:10.1002/mrc.1270160402

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe preferred conformation of 8‐methyl‐cis‐thiahydrindane has been both estimated by13C NMR chemical shifts and determined by low temperature13C NMR spectroscopy to be the conformer with the methyl group equatorial with respect to the cyclohexane ring. This result is in disagreement with the interpretation of the temperature dependence of the CD spectra of (+) and (−) 8‐methyl‐cis‐2‐thiahydrindane, whereby the conformation with the methyl group axial with respect to the cyclohexane ring was claimed to be the preferred conformation. The preferred conformation of the related oxygen heterocycle, 8‐methyl‐cis‐2‐oxahydrindane, has been estimated by13C NMR chemical shifts to be the conformer with the methyl group axial with respect to the cyclohexane ring. Possible reasons for these obs

ISSN:0030-4921    

DOI:10.1002/mrc.1270160403

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe results of a13C NMR spectral investigation involving 5,6‐dihydro‐1,4‐oxathiins, 1,4‐tetrahydro[9,10]benzoxathiin,trans‐tetrahydro‐1,4‐benzoxathiin, and the corresponding sulfoxides and sulfones are reported. An interpretation involving a dipolar structure with (2p→2p)π conjugation as opposed to (2p→3d)π interactions with the vinyloxy sulfides seems consistent with trends in the13C NMR shifts. For the sulfoxides and sulfones, the substitutent‐induced chemical shift (SCS) effects at the β vinylic carbons (βSO and βSO2effects) are considerably less than those atsp3carbons. The γSO and γSO2values at thesp2γ carbons indicatedeshielding, in contrast to the

ISSN:0030-4921    

DOI:10.1002/mrc.1270160404

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractLithium and magnesium enolates of an α‐sulphinyl ester 83%13C enriched at C‐1 and C‐2 are studied by13C NMR at different temperatures. It is shown that two metaliated species are present in the case of lithium which exchange at −60°C, ΔG≠ −60° = 37.8 KJ mol−1(9.05 Kcal mol−1) and that there is no rapid exchange between the non‐metallated ester and the metallated species (on the NMR time scale). In the case of magnesium, two or three metallated species are formed, according to the temperature which do no

ISSN:0030-4921    

DOI:10.1002/mrc.1270160405

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractComplete assignments are made for 23 4‐quinolones and related compounds. Literature data are used to calculate chemical shifts for 17 of these compounds: calculated and experimental shifts are in excellent agreement—all within 5 ppm for nine of the compounds and a maximum discrepancy of 7.6

ISSN:0030-4921    

DOI:10.1002/mrc.1270160406

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe use of additive parameters for the prediction of NMR chemical shifts is widely practised. However, no correlations are available for highly deshielded methine protons. In this work, methine chemical shifts have been studied using both multiple linear regression analysis and Simplex function minimization in an effort to determine under what limiting circumstances the additivity of shift parameters can be expected to apply. As expected, it was not possible to explain satisfactorily all methine shifts with a single set of substituent parameters. However, if only the deshielded cases were considered (H—CXYZ, where at least two of the three groups X, Y, and Z are electron‐withdrawing), a much better set of parameters could be determined. Using a data set of 440 of these deshielded methine shifts, involving 31 different substituents, a standard error of estimate of 0.20 ppm is found for protons spread between 3.0 and 7.5

ISSN:0030-4921    

DOI:10.1002/mrc.1270160407

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

Abstract13C NMR spectra ofms‐tetraphenylchlorins, new aminoalkyl‐ and hydroxy‐pyrroline substitutedms‐tetra‐phenylchlorins, andms‐tetraphenylisobacteriochlorins are presented and discussed. Significant changes in the chemical shifts of the α‐pyrrole, α‐pyrroline andmesoskeletal carbons are found in chlorins and isobacteriochlorins in comparison to porphyrins. In contrast, the chemical shifts of the β‐pyrrole carbons are almost unaffected by the structural modifications in chlorin and isobacteriochlorin. The chemical shifts of the α‐pyrrole carbons in the various chlorins and isobacteriochlorins are strongly affected by the substitutents of the pyrroline ring, or by the introduction of an additional pyrroline ring in isobacteriochlorins. The results show that most of the electron density is concentrated in the unreduced part of the molecule, i.e. in the pyrrole rings and, especially, on the α‐pyrrole carbons and is transferred or removed through the aromatic pathway by substituents on the pyrroline ring. These observations are supporting evidence that the α‐pyrrole, α‐pyrroline andmeso‐carbons are in the aromatic pathway and favour the proposal of a 16 atom dianion as the preferred delocalization pathway i

ISSN:0030-4921    

DOI:10.1002/mrc.1270160408

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractAn approach for the automatic assignment of13C spectra, based on the chemical shift/charge density relationship, is suggested. All permutations of spectral signals are computer‐generated, and for every permutation a least squares adjustment is carried out. The permutation presenting the highest correlation coefficient, or the lowest Hamilton's agreement factor, is considered to be correct. The application is exemplified by the13C chemical shifts of a series of aromatic compounds. It is shown that more reliable assignment is achieved if the considered permutations are restricted by taking intoaccount the signal multiplicit

ISSN:0030-4921    

DOI:10.1002/mrc.1270160409

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe13C NMR spectrum (at natural abundance) of monomeric chlorophyll α in acetone‐d6has been recorded to re‐examine the assignments of the low field (aromatic‐olefinic) region of the spectrum. The assignments, made by the examination of the fully coupled spectrum and by the use of long‐range selective1H decoupling (LSPD) with low‐power irradiation, were compared with those of the previous reports. The results of the present work clarify the ambiguities previously encountered in the assignment of the 10a‐ester, 7c‐propionyl, P‐2‐phytyl, 2b‐vinyl, γ‐ and β‐methine carbon atoms, as well as the β‐pyrrolic carbon‐6 and α‐pyrrolic carbons −16 and −17 of chlorophyll α. Reassignment of the three last carbons was found necessary. Knowledge of the chemical shifts of these carbon atoms was considered to be particularly valuable, as it yields relevant information on the delocalized π electron system which is crucial for the f

ISSN:0030-4921    

DOI:10.1002/mrc.1270160410

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

Abstract15N Nitrogen screening data are reported for some polycyclic azines and compared with previous results where available. INDO/S parameterized screening calculations for cinnoline and quinazoline help to decide between alternative nitrogen assignments. Nitrogen solvent shifts of several ppm are reported for all of the molecules studied.

ISSN:0030-4921    

DOI:10.1002/mrc.1270160411

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY