NMR Studies of Tautomerism

In porphycene, the rates, barriers and the overall tautomerization mechanism seem to be completely different. Comparison of the reaction for 1 and 2 in the crystalline state was performed using i N CPMAS NMR (Fig. 8.5) [40, 68]. At elevated temperatures both molecules reveal only one peak, characteristic of a rapid inner hydrogen exchange. For porphyrin, lowering of the temperature from 356 to 192 K leads to line broadening and, finally, to the separation of =N- and NH 

Three alkyl derivatives of porphycene, lb, Ic, and le have also been studied using CPMAS NMR [71]. As in the case of parent 1, the tautomerization was found to be so rapid that the rate constants could not be determined from the line shape analysis. It was thus not possible to establish a correlation between the cavity parameters and tautomerization dynamics, lb and Ic revealed narrow doublets, whereas le showed one line that did not broaden, even at 173 K. le has the largest N- N distance among porphycenes and should therefore exhibit the weak- 

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Very recently, a low-temperature NMR study of tautomerism in purine derivatives has been conducted. The temperature dependences of the H NMR spectra were studied in DMF- /j solution. Two sets of signals were observed in the NMR spectra of the two purine derivatives at 213 K, whereas at laboratory temperature, there was only a single set of signals, reflecting the time-averaged contribution of the two components. Based on the characteristic values of the and N chemical shifts and the three-bond scalar coupling constants, the two components were determined to be the N -H and N -H tautomers. The NMR parameters obtained for the two tautomers of 6-methoxy purine are shown in Fig. 12. 

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