摘要:

AbstractThe results ofab initioGIAO‐CHF13C nuclear shielding calculations for some solid amino acids, performed with the 6—311G** and a locally dense basis set are reported. Comparison of the present calculation with the observed shieldings shows an RMS error of 6.6 ppm. Shieldings of the carbon nuclei which are multiply bonded are strongly influenced by electron correlation effects. Experimental molecular geometries used in these calculations were taken from crystal structures of amino ac

ISSN:0749-1581    

DOI:10.1002/mrc.1260330902

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA modified BIRD pulse, called the DODO pulse, was developed and incorporated into one version of the FLOCK sequence. Addition of the two‐dimensional spectra obtained with this modified sequence and with the unmodified FLOCK sequence produces a normal indirectly bonded13C1H shift correlation spectrum. Subtraction of one spectrum from the other generates a difference spectrum which allows a clear distinction between two‐bond and three‐bond13C1H connectivities among protonate

ISSN:0749-1581    

DOI:10.1002/mrc.1260330903

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractWideline1H NMR and selective high‐resolution13C cross‐polarization magic angle spinning (CP/MAS) NMR spectroscopy was used to investigate the polymer structure in methacrylate thermoset resins modified by the incorporation of a polyether soft‐block. Spin diffusion experiments indicate phase separation between a hard‐ and soft‐block phase. Similar soft‐block domain sizes were found regardless of the soft‐block content, reflecting the discriminatory nature of the delay used to create the magnetisation gradient. The main effect of increasing the polyether content is to ensure that more of the sample adopts a two‐phase hard/soft morphology. The selective13C CP/MAS NMR spectra show that partial mixing of the methacrylate resin and polyether must occur and adding a Goldman‐Shen preparation period establishes that the mixing occurs through the bulk and is not s

ISSN:0749-1581    

DOI:10.1002/mrc.1260330904

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA one‐ and two‐dimensional NMR study was performed on three commercial resol phenol—formaldehyde (PF) prepolymer resins.1H,13C, CSCM and DQF COSY NMR spectral data, in acetone‐d6, were obtained on each resin and on PF model compounds: phenol, five methylolphenols, four diphenylmethanes, two formals, two dibenzyl ethers and two dibenzylamines. Gated‐decoupled13C experiments, using 2,4,6‐trimethylphenol as internal standard, were used to quantitate the major components present in each of the three resins. The major chemical differences of the three resins were noted. A DQF COSY method was developed that allowed the qualitative identification of most of the major phenolic components present in each of th

ISSN:0749-1581    

DOI:10.1002/mrc.1260330905

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractBroad13C resonances, as the result of partially relaxed scalar13C11B coupling, were observed for the C‐1 and to a lesser extent C‐2 carbons of 1‐alkynyldioxaborolanes at 25°C. Measurement of the13C spectra at 102°C resulted in a slower11B quadrupolar relaxation rate and allowed the determination of1J(13C,11B) scalar coupling from the resulting quadruplets for the C

ISSN:0749-1581    

DOI:10.1002/mrc.1260330906

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractDi‐ and tricarbonylnickel phosphine complexes (22) were prepared and their liquid‐state61Ni NMR spectra were recorded with the use of special solenoid glass sample tubes and a solenoid probehead, which are described. The acquired chemical shifts δ(61Ni), linewidths v1/2(Ni), relaxation timesT1and coupling constants1J(61Ni,31P) are discussed with reference to the electronegativity of the substituents at the phosphine ligands and the Tolman angle θ of the l

ISSN:0749-1581    

DOI:10.1002/mrc.1260330907

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractSolid‐state31P magic angle spinning (MAS) NMR spectra of Ag[P(m‐tolyl)3]2NO3were investigated as a function of the magic angle spinning frequency. Examination of the31P MAS NMR spectra obtained at 4.70 and 9.40 T indicates that the two phosphorus nuclei have identical isotropic chemical shifts, that is, they are crystallo‐graphically equivalent, δ = 11.2 ppm. However, since the orientation of their respective chemical shift tensors is not coincident, the two phosphorus nuclei are magnetically non‐equivalent and exhibit spinning‐frequency dependent31P NMR lineshapes. Analysis of the spinning‐frequency dependent31P MAS NMR spectra at 4.70 and 9.40 T indicates that2J(31P,31P) = 140 Hz. This value was confirmed by 2DJ‐resolved spectroscopy. The determination of an indirect spin‐spin coupling constant between two nuclei which constitute an ‘isolated’ spin pair with identical isotropic chemical shifts is not possible in conventional solution‐state NMR studies unless a third spin is introduced. The31P MAS NMR spectra of Ag[P(m‐tolyl)3]2NO3also exhibit resolvable splittings due to109Ag and107Ag;1J(109Ag,31P) = 517±5 Hz and1J(107Ag,31P) = 453 ± 5 Hz. In solution NMR studies these couplings are not always observed because of r

ISSN:0749-1581    

DOI:10.1002/mrc.1260330908

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA procedure for the estimation of the mixing time between the last two 90° pulses in the classic three‐pulse sequence NOESY/EXSY is proposed and tested and some considerations for the treatment of the two‐dimensional (2D)1H NMR exchange spectra are given. The rate constants are thus obtained with reasonable precision. This procedure was followed to obtain the 2D spectra of the model compound α‐[bis(dimethylamino)methylene]‐4‐nitrophenylacetonitrile, which represents a four‐site exchange system. The barriers to restricted rotations found in this compound were also determined from one‐dimensional (1D)1H NMR spectra, which were processed with the iterative complete lineshape analysis (CLSA) method. The double‐fit approach was incorporated in the CLSA method. It is shown that the results from the 2D dynamic NMR spectral studies corroborate those obtained by the CLSA

ISSN:0749-1581    

DOI:10.1002/mrc.1260330909

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe solvent and concentration dependence of the isomeric equilibria of 1‐methylamino‐2‐niroethene were studied by1H and13C NMR spectroscopy. The results are used for clarification of the controversy regarding the content of theEform in CDCI3solution, correcting an earlier suggestion concerning the stereochemical assignments around the C‐1N bond in theEform and verification of the values of the olefinic one bond proton—carbon spin—spin coupling constants,1J(C,H), reported recently. An unambiguous means of assigning the stereochemistry around the C‐1N bond in theEisomer is suggested, based on using the complementary information from3J(C‐2,NH) and3J(H‐1,NH), in addition to the3J(Me,H‐1) vi

ISSN:0749-1581    

DOI:10.1002/mrc.1260330910

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA newent‐kaurane diterpenoid glycoside has been shown to be the main sweetening constituent of the traditional medicinesAster tongolensisandA. ageratoidesnative to China. This study involved the total1H and13C spectral assignment of the compound and its degraded product by means of two‐dimensional NMR techniques (COSY, NOESY, HETCOR and TOCSY) and resulted in unambiguous spectral assignments, particularly in the heavily convoluted regions of the spectra. Concerning the absolute stereochemistry, the sweetener was determined to be (4R,5S,8S,9S,10,13R)‐16,17‐dihydroxykauran‐19‐oic acid β‐D‐glucopyranosyl ester through a combination of circular dichroism and 2

ISSN:0749-1581    

DOI:10.1002/mrc.1260330911

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY