Tautomerism of azo dyes in the solid state studied by 15N, 14N, 13C and 1H NMR spectroscopy, X-ray diffraction and quantum-chemical calculations

Abstract

The azo–hydrazo tautomerism of four model azo dyes in the solid state is investigated by multinuclear solid-state nuclear magnetic resonance (SS-NMR) spectroscopy, X-ray diffraction and quantum-chemical calculations. The measurement of 15N SS-NMR spectra is the most straightforward way to determine the tautomeric state of solid azo compounds. Unfortunately, the sensitivity of these nuclei for SS-NMR is very low and the acquisition of 15N spectra in a reasonable time may be challenging. It is demonstrated that a natural abundance of almost 100% of another nitrogen isotope, 14N, can be exploited for the investigation of the tautomerism. Indirect detection of 14N nuclei via protons is an elegant approach for the rapid and unequivocal determination of the tautomeric form. Furthermore, a comparison of experimental carbon SS-NMR chemical shifts with those obtained in solution or by quantum-chemical computations can be used for the same purpose.