13C CPMAS NMR study of solid arylazonaphthols. Evidence of 13C,14N self-decoupling induced by a solid-state proton transfer reaction

Abstract

Variable-field ¹³C CPMAS NMR spectra for several solid arylazonaphthols are discussed. In compounds existing in the pure hydrazone form, spectral changes observed in lowering the magnetic field can be satisfactorily explained as arising from incompletely averaged ¹³C,¹⁴N dipolar coupling interactions. Similar structures which are however known to exist in the solid state as rapidly interconverting mixtures of keto–hydrazone and enol–azo tautomers do not show significant effects from the nitrogen involved in the proton transfer. This result is ascribed to ¹³C,¹⁴N self-decoupling due to a fast quadrupole relaxation of this latter nitrogen, induced by jumps in its electric field gradient which accompany the solid-state proton exchange. This information helps to establish the existence of a double minimum potential for the proton motion in these intramolecularly hydrogen bonded compounds.