°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.

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excited states. Chloroform, however, assumed to be a scavenger of electrons (7, 8), reduces both monomer and excimer emission by equal amounts whilst only slightly decreasing their respective lifetimes. These results suggest that both the monomer and excimer arise from common precursors. It seems most likely that the latter generate either monomer or excimer species followed by rapid equilibration.The intense Cerenkov emission monitored during this work suggests that, since such emission has maximum intensity in the direction of the irradiating particle beam, the benzene luminescence yield is appreciably less than that for Cerenkov emission over the spectral region 260-360 mp. It is likely, therefore, that some benzene emission will arise from absorption of Cerenkov photons; the chloroform scavenging experiment shows that this is not a major contribution, however. The signs of the intermethyl proton spin-spin coupling constants in ortho, meta, and para xylene derivatives (interbenzylic coupling) have been determined by double resonance techniques. JCH3,CH30, JCH ,CH3n(, and J C H 3 , C H B P are positive, negative, and positive, respectively, in agreement with the predict~ons of theor~es based on o-n interactions. The magnitudes of the coupling constants are discussed in terms of the values predicted by these theories. Signs of methyl-methyl proton spin-spin coupling constants in xylene derivatives (interbenzylic coupling)Canadian Journal of Chemistry, 48, 1002Chemistry, 48, (1970 IntroductionThe signs of the long range a-side-chain proton -ring proton (benzylic) coupling constants in toluene and substituted toluenes,

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Computer-assisted structure elucidation (CASE) using a combination of 1D and 2D NMR data has been available for a number of years. These algorithms can be considered as "logic machines" capable of deriving all plausible structures from a set of structural constraints or "axioms", defined by the spectroscopic data and associated chemical information or prior knowledge. CASE programs allow the spectroscopist not only to determine structures from spectroscopic data but also to study the dependence of the proposed structure on changes to the set of axioms. In this article, we describe the application of the ACD/Structure Elucidator expert system to help resolve the conflict between two different hypothetical hexacyclinol structures derived by different researchers from the NMR spectra of this complex natural product. It has been shown that the combination of algorithms for both structure elucidation and structure validation delivered by the expert system enables the identification of the most probable structure as well as the associated chemical shift assignments.