Dynamic NMR study of the electronic state of heteroaromatics. III—Pyrylium salts or intramolecularly bridged pentamethine cyanines?

Abstract The 13 C NMR spectra and the temperature‐dependent 1 H NMR spectra of a series of NR 2 ‐substituted pyridines and pyrones were studied, and the 13 C chemical shifts and the rotational barriers about the exocyclic partial C,N double bonds were compared with the corresponding values for suitable pyridinium, pyrylium and thiopyrylium salts. The NMR spectroscopic results are discussed in the light of the electronic state and mesomerism in the species studied; the salts in question should be named, from the electronic point of view, more realistically as bridged pentamethine cyanines rather than pyridinium, pyrylium and thiopyrylium salts.