Magnetic Resonance in Chemistry

Pehk, T.; Kooskora, H.; Lippmaa, E.

doi: 10.1002/mrc.1270080103pmid: N/A

Carbon‐13 chemical shifts of all twenty‐two dimethylcyclohexanols, formed by the hydrogenation of isomeric xylenols, have been measured and assigned. Conformational peculiarities of dimethylcyclohexanols are discussed on the basis of their carbon‐13 chemical shifts.

Schwarzenbach, R.; Meili, J.; Könitzer, H.; Clerc, J. T.

doi: 10.1002/mrc.1270080104pmid: N/A

A computerised library search system for 13C NMR data is described. Given the spectral data of an unknown compound, the system will retrieve from the library those reference compounds exhibiting similar spectra. For comparison the spectral data are converted into a binary code, designed to reflect the underlying structure rather than exact values for chemical shifts. Thus, the ability of the system to retrieve compounds similar to the unknown (as opposed to identical) is greatly enhanced. A sophisticated search strategy adapting itself automatically to the problem at hand makes the system highly efficient.

Diehl, P.; Bösiger, H.; Tracey, A. S.; Ader, R.

doi: 10.1002/mrc.1270080105pmid: N/A

The structure of pyrazine partially oriented in a liquid crystal has been determined from its proton magnetic resonance spectrum utilising the 13C satellites (at natural abundance). Carbon–carbon, carbon‐hydrogen and hydrogen–hydrogen internuclear distance ratios have been determined.

Rueppel, M. L.; Marvel, J. T.

doi: 10.1002/mrc.1270080106pmid: N/A

Proton and phosphorus magnetic resonance spectra of substituted methylphosphonic acids have been determined as a function of pH. A method has been developed for measuring the 31P shift indirectly by optimal heteronuclear decoupling of the 1H spectra of samples and standards. Control experiments have demonstrated the broad applicability of this technique to the characterization of low milligram samples of N‐phosphonomethylglycine and potential metabolites.

Takeuchi, Yoshito; Dennis, Nicholas

doi: 10.1002/mrc.1270080107pmid: N/A

13C NMR spectra of 1‐methyl‐, 1‐phenyl‐ and 1, 6‐dimethyl‐pyridinium‐3‐oxide, 3‐methylphthalazinium 1‐oxide and anhydro‐3‐methanesulphonamido‐1‐methylpyridinium hydroxide, together with the corresponding amines and quaternary salts, were determined with and without proton noise‐decoupling. The assignment of resonances was based on the analysis of the fine splitting pattern caused by long range couplings whenever appropriate. A procedure for the estimation of chemical shifts of the salts and betaines is proposed.

Thiault, B.; Mersseman, M.

doi: 10.1002/mrc.1270080108pmid: N/A

Quantitative analyses have been carried out by 13C NMR spectroscopy using Fourier transformation. The Overhauser effect, due to the broad band decoupling of protons, has been suppressed using the two existing methods (gated decoupling and paramagnetic species), and results from each are compared. Both require a calibration curve, and the second method, when applied to aromatic substances, needs the ratio between the sample to be analysed and the paramagnetic species to be less than a certain value. Experiments have been made with pure products and blends of pure products containing both saturated and aromatic carbons. The precision of the results is given.

Mitchell, Terence N.

doi: 10.1002/mrc.1270080109pmid: N/A

The 13C and 119Sn NMR spectra of 33 organotin compounds of the type RSnMenCl3 − n and related types are discussed. The substituent effects of the groups SnMe3, SnMe2Cl, SnMeCl2 and SnCl3 (and of some related groups) on the carbon chemical shifts in the alkyl group R have been determined; the SnMe3 group causes a small upfield shift of the carbon attached to it, while the other groups cause downfield shifts. The shifts show a monotonic change on replacing methyl groups in Me3Sn by chlorine atoms. The effects on carbons further removed from the tin atom are discussed. Variation in R causes little change in nJ(SnC) or δ(119Sn).

Shapiro, B. L.; Proulx, T. W.

doi: 10.1002/mrc.1270080110pmid: N/A

Proton and 13C NMR data are presented for six different compounds containing the fragment C6H5CCH2SiMe3. In a number of instances it was observed that, in the 1H NMR spectrum, the SiMe3 groups had a chemical shift significantly upfield from internal tetramethylsilane (δ = −0·14 to −0·36). These unexpected upfield chemical shifts of the SiMe3 groups are suggested to result from the predominance, on a time averaged basis, of conformations which place the methyl groups attached to silicon in the face of an aromatic ring. The preference for such conformations is, in turn, the result of rotational preferences exhibited by the ‘flat’ aromatic ring. These results suggest that conformational analysis of systems containing a phenyl ring should take more explicit account of the fact that the preferred orientation of this phenyl ring can have a profound influence on the conformation adopted by the remainder of the molecule. In addition, the preferred conformation of the phenyl ring can have a significant effect upon the observed 1H NMR chemical shifts, while the 13C chemical shifts are relatively insensitive to conformational factors and can be explained by well‐known substituent effects previously delineated for all‐carbon systems.