Dynamic NMR study of the electronic state of heteroaromatics. V-Influence of the counter anion, ortho substituents and the ring heteroatom on the exocyclic C,N rotational barrier of pyrylium and thiopyrylium salts

The 13C NMR spectra and variable‐temperature 1H NMR spectra of some NR2‐substituted pyrylium and thiopyrylium salts were studied, and the rotational barriers thus obtained about the exocyclic partial CN double bonds are discussed according to their dependence on the counter anion and other structural variations. The counter anion has a negligible influence on the barrier, but substituents on the ring can either strengthen electronically or weaken sterically the mesomerism of the NR2‐heteroaromatic system. The competition between these two effects is readily estimated from the rotational barriers obtained. The pyrylium salts display higher rotational barriers than the identically substituted thiopyrylium salts.