Crystal structure and tautomerism of Pigment Yellow 138 determined by X-ray powder diffraction and solid-state NMR

The crystal structure of C.I. Pigment Yellow 138 was determined from X-ray powder diffraction data using real-space methods with subsequent Rietveld refinements. The tautomeric state was investigated by solid-state 1D and 2D multinuclear NMR experiments. In the crystals, the compound exhibits the NH-tautomer with a hydrogen atom situated at the nitrogen of the quinoline moiety. Direct evidence of the presence of the NH-tautomer is provided by 1H-14N HMQC solid-state NMR at very fast MAS. Solid-state dispersion-corrected density functional theory calculations with BLYP-D3 confirm the correctness of the crystal structure and support the N-H tautomer. The NH hydrogen atom forms an intramolecular resonance-assisted N-H⋯O hydrogen bond to the neighbouring indandione moiety. The indandione moiety is almost coplanar with the quinoline fragment, whereas the phthalimide group forms an angle of 57° with it. Quantum-mechanical calculations show that for an isolated molecule this tautomeric form is by approximately 60 kJ/mol more stable than the OH-form with a hydrogen atom placed at an oxygen atom of the indandione moiety. The out-of-plane rotation of the phthalimide group is not a packing effect, but caused by intramolecular steric hindrance.