The ^(15)N NMR spectra of meso-tetraphenylporphyrin, its dication, and zinc tetraphenylporphyrin have been investigated both at the natural-abundance level and with isotopic enrichment. The resonances of tetraphenylporphyrin at ambient temperatures are broadened by chemical exchange of the central hydrogen atoms. At lower temperatures, the ^(15)N spectra provide structural information concerning the bonding of these hydrogens. The variable-temperature ^(15)N spectra of mixtures of tetraphenylporphyrin and its dication also reveal chemical-exchange phenomena. Spectra for tetraphenylporphyrin dication and zinc tetraphenylporphyrin at the natural-abundance level are reported. Spin-lattice relaxation times and nuclear Overhauser effect data have been determined for ^(15)N enriched samples of these two compounds.
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Gust et al. (1977) studied this question.