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The ^(13)C nmr spectra of several aryl nitrile oxides have been taken. The magnitude of ^1J^_(C(13)-^(15)N) for 2,4,6-trimethylbenzonitrile-^(15)N has been found to be 77.5 Hz, by far the largest one-bond carbon-nitrogen spin-spin coupling constant yet reported. The ^(13)C resonance of the carbon atom bonded to the nitrogen atom in this nitrile oxide is considerably shifted upfield by comparison with the analogous ^(13)C resonances of nitriles; however, little change is seen in the paramagnetic shielding term for ^(15)N upon conversion of a nitrile to a nitrile oxide. The ^(13)C and ^(15)N spectra are reported for a precursor and some reaction products of 2,4,6-trimethylbenzonitrile-^(15)N oxide. The low-field position of the ^(15)N resonances of isoxazole systems is discussed in relation to nitrogen chemical shifts for other five-membered heterocycles.
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Christl et al. (1973) studied this question.