Classic Isomeric 1,2- and 2,1-nitrosonaphthols are Oximes in Solid State and Solutions

A detailed characterization by UV-visible, IR and NMR (1H, 13C, 15N) spectroscopies of two isomeric 1,2- and 2,1-nitrosonaphthols (1 and 2) has been carried out. The crystal structures of these iconic compounds, which were widely used in analytical chemistry in the past, were determined and evidenced their quinone-oxime nature in solid state. Compound 1 exists in the crystal as a syn- diastereomer in the closed form due to a strong intramolecular H-bond, while compound 2 is an anti- diastereomer forming a dimer with two intermolecular H-bonds. The oxime character of 1 and 2 in solutions was confirmed after careful studies of the NMR spectra of both compounds in solvents of different polarity and donor properties: CD2Cl2, acetone-d6 and DMSO-d6. An equilibrium state between oxime syn- and anti- isomers exists and can be modulated in a controlled way by changing media's polarity. Both compounds were also prepared labeled with 15N (50%) for recording their NMR nitrogen-15 spectra, and for the identification/assignment of vibrations with the participation of the >C=N-OH fragment. During synthesis of compound 1 a persistent, very polar, red-colored impurity 3 was detected, isolated and crystallographically characterized. It was found to be a rather unusual π-complex between the final compound 1 and its precursor 2-naphthol. The red color originates from the CT-band in the visible region of the UV/Vis-spectrum where the naphthol acts as a donor with 1 being an acceptor. Both are separated at 3.03 A.