°13C shift referred to TMS.respectively.33 The effect of self-association is probably negligible, since the concentration of the solute is low (1 mol %) in polar solvents. Then it is found that the 14N shifts of imidazole in acetone and DMSO (proton acceptors) are shifted downfield with respect to methanol, water, and trifluoroacetic acid (proton donors). The difference of 14N shifts is 6-9 ppm over the range of the experimental error.Similarly upfield 14N shifts of 6 ± 3 ppm and 7.4 ± 3-9.8 ± 5 ppm are observed in pyrazole and 1,2,4-triazole, respectively. These values are recognized as about half of the corresponding values for A-methyl derivatives, taking into account the argument described above.Sometimes difficulty arises in observing 14N shifts in protonated species, because a longer correlation time due to protonation tends to broaden nmr lines and make the two-and three-bond N-H couplings unobservable. Protonation shifts on imidazole and thiazole are obtained by the decoupling of one-bond N-H couplings. The upfield 14N shifts are 41 ± 3 ppm and 123.1 ± 3 (34) H.

SynopsisA description is given of an analytical temperature-rising elution fractionation (TREF) system for the purpose of determining short-chain branching (SCB) or copolymer distributions in polyethylenes and ethylene copolymers. The system achieves fractionation on the basis of crystallhbility and is shown to be very little influenced by molecular weight in the normal high polymer range. Sample preparation by slow cooling from relatively dilute solution followed by continuous elution with a simultaneous and fairly rapid rate of temperature rise proves to be an efficient fractionation process. An on-line detector and data system allows application of a calibration curve to give realistic SCB distribution data in a convenient manner. The potential value of the TREF technique for providing structural information is illustrated by examples which include low-density high-pressure resins made by both tubular and autoclave reactors, high-and low-density resins made by lowpressure processes, and copolymers of ethylene with vinyl acetate and ethyl acrylate.

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Correlations of vinyl 'H and 136= chemical shifts and charge densities with field, F, and resonance, R, parameters probide a self-consistent picture of electronic effects in these compounds. 'H chemica! shifts for some derivatives are affected by magnetic effects but this does not obscure the overall pattern of electronic effects. This pattern of electronic effects can be completely accounted for by a nlodel which assumes that substituent effects can be transmitted through space (field effects), via conjugative interactions (resonance effects) or by polarization of the styrene n electron system by the polar C-X bond (n polarization effects). The latter effect appears to be more important than pr-evious!y realized, IH correlations with F and K are used to estimate self-consistent and apparently reasonable values for C=N and C=C-H groups and F a n d R values for carbonyl substituents and (CH3),M sub- Les correlations entre les deplacements chimiques du 'H vinyle et I3C, dcs densites de charge avec le champ F et les parametres de resonance R fournissent une image coherente des effets Clectroniques dans ces con~poses. Dans certains derives les deplacements cliimiques du 'H sont affectis par les effets magnetiques mais cela ne change pas I'allure generale des efTets electroniques qui peut Ctre totalement expliquee par un modele qui suppose que les effets de substituants peuvent etre transmis a travers I'espace (effets de champ), via des interactions de conjugaison (effets de resonance) ou par polarisation du systeme Clectronique du styrene par la liaison C-X (effets de polarisation n). Ce dernier effet est plus important qu'on ne le pensait precCdemmant. Introduction compounds can be accounted for in terms of ziircher (1) and *llinger and coworkers (21, the dipolar electric field effect and the anisohave shown that the substituent dependence of tropic magnetic susceptibility of the substituent.chemical shifts for many aliphatic The substituent dependence of proton chemical shifts (S.C.S.) for aromatic co~npounds is less