The C-2-N bond of 2-N,iV-dimethylaminopyrylium cations has a partial a character due to the conjugation of the nitrogen lone-pair with the ring. The values of AG*, AH*, ASf parameters related to the corresponding hindered rotation have been determined by 13C NMR total bandshape analysis. This conjugation deereases the electrophilic character of carbon C-4 so that the displacement of the akoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N&nethylaminopyrylium cations and the corresponding AG* parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C-N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2-N bond decreases with increasing electron donating power of the substituent at position 4. AG' values decreases from 19.1 kcal mo1-l (79.9 kJ mol-l) to 12.6 kcal mol-' (52.7 U mol-') according to the following sequence for the R-4 substituentx -C&, -CH,, -OCH3, -N(CH,),. Dans les cations N,N-dimBthylamino-2 pyrylium, la conjugaison du doublet de l'azote avec le cycle confkre B la liaison C-2-N un caractbre partiel de double liaison. Les valeurs des paramktres AG', AH#, AS' relatifs B 1 I C J 1 ?R CCC-0030-4921/79/0012-0071$03.00 ORGANIC MAGNETIC RESONANCE, VOL. 12, NO. 2, 1979 71 ' J (C-5, H-5) = 175.7 3) =168*9 ' J (C-5.H-3) = 4.25 ' J (C-3, H-5) = 3 2 3J (C-5, CH&) = 4.25 CH, 1 ' J (G3, H-3) = 175.0 ' J (c-5. H-5) = 173.5 ' J (C-3,H-5) = 5.55 3J(C-5,H-3) = 6.5 CH, ' J (C-3, C H 3 4 = 5.55 ' J (C-5. CH3-6)= 4.35 N ' H 3 C )& 3 CH, I ' J (c-3, H-3) = 174.1 ' J (C-5, H-5) = 172.6 3J (C-5, CH3-6) = 4.0 =J (c-3.H-5) = 5.05 ' J (G5.H-3) = 6A5