Observation of thermal tautomerism of thermochromic salicylideneaniline derivatives in the solid state by 15N CPMAS NMR down to cryogenic temperatures

The temperature dependent solid state 15N CPMAS NMR spectra of 15N enriched salicylideneaniline derivatives, N‐salicylideneaniline (SA), N,Nî‐bissalicylidene‐p‐phenylenediamine (BSP) and N,Nî‐di(2‐hydroxy‐1‐naphthylidene)‐p‐phenylenediamine (DNP) were measured in order to study possible proton transfer processes in the NHO hydrogen bonds. For DNP a remarkable change of the 15N chemical shifts of more than 50 ppm was observed between 26 and 388 K by using the CPMAS NMR technique down to cryogenic temperatures. The result clearly indicates very small enthalpy differences among the four tautormers associated with the combination of two possible forms, OH and NH form, of the two NHO hydrogen bonds in a DNP molecule.