摘要:

Abstract183W NMR spectra for several tungsten carbonyl complexes were recorded using heteronuclear multiple‐quantum spectroscopy. The measurements involved the use of both one‐bond and long‐range coupling constants, with the latter technique demonstrating a general approach to obtaining183W spectra in Tp′ [Tp′ = tris(3,5‐dimethylpyrazolyl)borohydride] and Cp tungsten compounds. Differences in the183W chemical shifts between the Tp′ and Cp complexes are explained in terms of σ‐donor and π‐acceptor capabilities of the ligands. It is suggested that the very small1J(183W,1H) value of 7 Hz in WH(Tp′)(CO)3arises from an interaction of an H+with a [W(Tp′)(CO)3]−orbital with a relatively large d‐component. For WH(Tp′)(CO)(PHCCMe) (3), the very lowfield hydride chemical shift, δ = 10.8, is suggested to arise, in part, from anisotropic effects.1H NOESY measurements on 3 show that the hydride lies in a deshielding region with respect to both the acetylene and two pyrazolylborate rings, the acetylene is bent such that the CH3and Ph groups move toward one another and there is a second isomer of 3, presumably related to the geometry of the coordinated acetylen

ISSN:0749-1581    

DOI:10.1002/mrc.1260310112

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe reaction between 6‐ and 7‐ketocyclopenta[d] [1,3]dioxane and methyl‐phenylmagnesium bromide gave in each case only one isomer of the 6‐hydroxy‐6‐methyl‐phenyl‐ and 7‐hydroxy‐7‐methyl‐phenylcyclopenta [d] [1,3]dioxanes. The configurations of these derivatives were determined from a detailed analysis of the1H NMR spectra. All the compounds were found to adopt theO‐insi

ISSN:0749-1581    

DOI:10.1002/mrc.1260310113

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractA new method for lanthanide‐induced shift (LIS) calculations of highly flexible structures was developed using molecular geometries obtained from the random variation of dihedral angles (up to ten) which are rotating freely and independently of each other. The result of the new method (program JLLISMUL) is a data set containing the specified random dihedral angles and the corresponding agreement factorR(as a criterion for the fitting of the calculated and experimental LIS values). For comparison, the same random procedure was also used for parallel force field calculations using the molecular mechanics program PIMM. The results obtained by the new method are in very good agreement with known or reported preferred conformations of five highly flexible molecules which were used as examples for examining the new strategies of obtaining the preferred conformers. The minima of both the ‘LISRfactor hyper surface’ and the ‘energy hyper surface’ are rapidly a

ISSN:0749-1581    

DOI:10.1002/mrc.1260310114

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractLong‐range deuterium isotope effects on13C chemical shifts,nΔC(OD), were studied in the intramolecularly hydrogen‐bonded purpurogallins (benzotropolones). A very large long‐range isotope effect from the hydrogenbonded 4‐OH(D) is observed over six bonds at C‐7. Further, long‐range isotope effects are transmitted over as many as eight bonds. The transmission pathway including the carbonyl carbon is favoured, and some of the long‐range effects are transmitted via the carbonyl group. Large long‐range isotope effects are thus observed in hydrogen‐bonded systems with effective conjugation to the carbonyl group via alternating double bonds. This study indirectly supports recent theoretical predictions. The13C and1H NMR spectra of purpurogallin and its 2,3‐dimethyl, 2,3,6‐trimethyl and 2,3,4,6‐tetramethyl derivatives are assigned, and the hydrogen bond patterns are established based on the OHchemical shifts an

ISSN:0749-1581    

DOI:10.1002/mrc.1260310115

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThesyn/antidependences of the alkyl13C chemical shifts provide a useful probe of stereochemistry around the amide bonds in peptides and proteins. To investigate these dependences theab initioIGLO method was used to obtain the13C chemical shielding for the following series of amide molecules as models for the peptide bond:syn‐ andanti‐N‐methylformamide (NMF),cis‐ andtrans‐N‐methylacetamide (NMA),N,N‐dimethylformamide (DMF) andN,N‐dimethylacetamide (DMA). Molecular geometries for shielding calculations are optimized ones at the HF/6–31G* level. Good correlations were observed between the calculated13C chemical shift results and the experimental data. Thesyn/antidependence of13C chemical shifts around amide bonds is perdicted either with double‐ζ or triple‐ζ quality basis sets. Based on an analysis of the paramagnetic bond contributions (PBC) to the chemical shieldings, it is concluded that thesyn/antidependence of the13C chemical shifts in amides arises almost entirely from the PBC of thesynandantiNC bonds, and is completely analogous to the electronic origin for thecis/tran

ISSN:0749-1581    

DOI:10.1002/mrc.1260310116

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe possibility of assigning the configuration of a substituent in six‐membered ring compounds from the13C chemical shifts of a single epimer was examined. The chemical shifts of the ring carbons in saturated six‐membered ring compounds were calculated using standard substituent parameters obtained fromtert‐butyl systems, and these calculated values were compared with the observed values. It was found that in isomeric 1,3‐ and 1,4‐disubstituted cyclohexanes the configuration could be assigned correctly for each isomer by comparing the observed chemical shifts with the calculated values, an exception beingtrans‐1,3‐dihydroxycyclohexane. However, in the 1,2‐disubstituted cyclohexanes the configuration could be assigned correctly only by comparing the observed and calculated chemical shifts for the γ‐ or β‐methylene carbons. The configuration cannot be assigned correctly in heterocyclic systems by using the individual chemical shifts as probes. However, when the sum of the chemical shifts of the ring carbons could be calculated for equatorial and axial orientations of the substituent using standard substituent parameters to a reasonable degree of accuracy, the configuration for any epimer could be assigned correctly by comparing the observe

ISSN:0749-1581    

DOI:10.1002/mrc.1260310117

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractTwo‐dimensional long‐range13C1HJ‐resolution spectroscopy (LRCJR) was used to measure three‐bond13C1H coupling constants [3J(C,H)] fortrans‐ andcis‐chrysanthemic acid, methyltrans‐pyrethrate and some microsomal metabolites of thetrans‐chrysanthemate biophenothrin. The carbon of the methylcis‐disposed to an attached proton shows a larger3J(C,H) value than does thetrans‐carbon for the dimethyl‐substituted cyclopropane and epoxide rings. The reverse situation applies for the analogous dimethyl vinyl grouping. The3J(C,H) values are not altered on conversion of one of the olefinic geminal methyl groups to a hydroxymethyl or methoxycarbonyl functionality, but increase on transformation to an aldehyde. These3J(C,H) values are in agreement previous results from long‐range CH COSY experiments, and provide a useful method for determining the stereochemistry of c

ISSN:0749-1581    

DOI:10.1002/mrc.1260310118

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

Abstract1H NMR spectra of eight naturally occurring brassinosteroids, plant hormones with a steroid skeleton, and five synthetic analogues were analysed by COSY and long‐range COSY measurements. Based on the1H resonances, the13C signals were assigned by a combination of CH COSY and long‐range CH COSY spectra detecting13C nuclei, or by HMQC and HMBC spectra detecting1H nuclei. Although the steroids showed complex1H signals around 1–2 ppm, the resonances for the two angular methyl groups and the other methyl groups in the side‐chain could be assigned unambiguously and used as a reference point for the other assignments. Strong correlation peaks between the methyl protons and carbons connected with the protons through two or three bonds were observed in the long‐range CH COSY or the HMBC spectra. These spectra provide valuable information for assigning the13C NMR resonances of this cla

ISSN:0749-1581    

DOI:10.1002/mrc.1260310119

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe13C NMR chemical shifts and coupling constants for diphenyl ether and 28 environmentally interesting polychlorinated diphenyl ethers were measured and analysed.

ISSN:0749-1581    

DOI:10.1002/mrc.1260310120

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe13C NMR chemical shifts of ten arylmethylatedtrans‐β‐methyl‐β‐nitrostyrenes and the parent compound were measured and the spectra interpreted. Chemical shift assignments for the aryl‐methylated compounds have not, to our knowledge, been reported previously. The data are based on 1 D and 2D experiments perform

ISSN:0749-1581    

DOI:10.1002/mrc.1260310121

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY