The NMR chemical shieldings of selected tautomeric forms of the studied compounds were calculated using the GIAO approximation. The calculated absolute shieldings were transformed to chemical shifts using the reference compound tetramethylsilane, Si(CH 3 ) 4 , for hydrogen and carbon: δ = δ calc (ref) -δ calc . Both δ calc (ref) and δ calc were evaluated at the same computational level (M06-2X/def2TZVP) in the corresponding solvent environment.