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

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 N-15 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 N-15 spectra in a reasonable time may be challenging. It is demonstrated that a natural abundance of almost 100% of another nitrogen isotope, N-14, can be exploited for the investigation of the tautomerism. Indirect detection of N-14 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.