Spectroscopic Studies of Keto-Enol Equilibria. VII. Nitrogen-15 Substituted Schiff Bases

NMR study of proton transfer equilibrium in Schiff bases derived from 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-acetonaphthone. Deuterium isotope effects on13C and15N chemical shifts

“…Owing to both of these factors, 6 exists in solution at 220 K as the pure NH form. The observed solvent effects on tautomeric equilibria are similar to those obtained by Dudek and Pitcher-Dudek 18 and Abbas et al 19 The Lewis acidity of the solvents 27 seems to be a more important factor than their polarity or basicity. For 1 at 270 K the mole fraction of the NH form was 0.84 and 0.83 in CDCl 3 and CD 3 CN, respectively, and decreased to 0.71 in acetone and 0.40 in toluene.…”

Synthesis of Some New Anils: Part 1. Reaction of 2-Hydroxy-benzaldehyde and 2-Hydroxynaphthaldehyde with 2-Aminopyridene and 2-Aminopyrazine

“…2-Hydroxy Schiff base ligands are of interest mainly due to the existence of (O-H···N and N-H···O) type hydrogen bonds and tautomerism between the enol-imine and keto-enamine forms (Scheme 1). Tautomerism in 2-hydroxy Schiff bases both in solution and in the solid state was investigated using different spectroscopic techniques [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. In the spectra of solutions of these compounds different Schiff bases have been studied in both polar and non-polar solvents [5,6,8,9,11,13,16,20].…”

“…The results indicate that the absorption band at greater than 400 nm belongs to the keto-enamine form of the Schiff base; the enol form has no appreciable absorbance in this region. Different explanations for the source of this band have been proposed [5,8,15,17,20]. It was suggested that this new band is due to the keto form in salicylidene anilines (Scheme 1).…”

Aromatic Versus Heteroradialene Character in Extended Thiophloroglucinol Ligands and their Trinuclear Nickel(II) Complexes

“…[97][98][99][100][101][102][103][104][105][106][107] Whereas a 15 N chemical shift of about À49 ppm is characteristic of a pure imino group with no contributions of hydrogen bonds, this resonance shifts to about À80 ppm on hydrogenbond formation. [106,107] The 15 N NMR spectrum of H 3 O 3 shows four doublets at À249.1, À249.4, À251.8, and À252.9 ppm, each with a coupling constant of 1 J NH = 90 Hz. The coupling constants are 1 J NH = 0 Hz for the OH tautomers and up to 1 J NH = 94 Hz for NH tautomers or secondary amines in general.…”