Tautomerism in Hydroxynaphthaldehyde Anils and Azo Analogues:  a Combined Experimental and Computational Study

Abstract

The enol imine a enaminone tautomerization constants, K T , and thermodynamic parameters, ∆H T and ∆S T , of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the equilibrium toward the quinone form (for the unsubstituted derivative 1c, K T ) 0.20 (cyclohexane) and K T ) 1.49 (ethanol)). Both donor (MeO, NMe 2 ) and acceptor (CN, NO 2 ) substituents lead to a decreased K T independent of solvent polarity. In apolar solvents, for all derivatives 1a-1e, the enol imine a enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of ∆G T to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated ∆G T values. Solvent effects on 1c are also successfully reproduced by the calculations. Geometric (O‚‚‚N distance) and energetic criteria (conformer energy differences, homodesmotic reactions) establish the importance of intramolecular hydrogen bonding for the tautomerism of these compounds. The results obtained for 1a-1e are compared with tautomeric properties of the isomeric naphthaldehyde anils 2-4, the monocyclic analogues 5 and 6, the corresponding azo derivatives 7-9, and the N-alkyl derivative 10.

Solid state tautomerism in 2-((phenylimino)methyl)naphthalene-1-ol

“…The tautomeric constants in CHCl 3 and DMSO are 0.72 and 0.85 correspondingly, which is surprising taking into account the different proton acceptor/donor properties of these solvents. However, it has been proven by linear solvation energy relationship analysis [9] and by quantum chemical calculations [24] that due to the substantial strength of the intramolecular hydrogen bonding in the Schiff bases their interaction with proton acceptor solvents (like DMSO) is limited and the observed limited (in comparison with CHCl 3 ) changes result only from the net polarity of DMSO.…”

“…The salicylidene anilines (similar to compound 1 in the scheme below), being relatively simple in structure, are suitable model compounds for the study of the intramolecular proton transfer, and have therefore attracted considerable attention from both experimental [7][8][9] and theoretical [9,10] points of view. The proportioning between the tautomeric forms (enol E and keto K), which strongly depends on the conditions of the experimental/theoretical measurements, is the key moment in all investigations.…”

“…The measurements are extended from an isolated molecule in vacuum (quantum chemical calculations [9,10] and mass spectrometry [11,12]) through less (UV-Vis spectroscopy [9,12]) or more (NMR [8,12,13]) concentrated solution in various solvents to solid state X-ray structure determination [14]. However, the prediction of the relative stability of the tautomers by means of quantum chemical calculations depends strongly on the level of theory (methods and basis sets, solvation models, etc.)…”

Femtochemistry of orange II in solution and in chemical and biological nanocavities

“…It has been reported that it exists under azoenol (AZO) and ketohydrazone (HYZ) forms ( Fig. 1) (15)(16)(17)(18)(19)(20). In a water solution, for example, the H-atom within the O-H .…”

“…The inclusion complexes of OII and CDs have been studied by several groups showing the formation of a 1:1 entity (24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35). So far, the CD inclusion complex studies have considered that OII in water solutions exists under AZO tautomer, contrary to many studies in solutions where OII coexists under AZO and HYZ forms (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Recently, ultrafast transient lens technique with a response function of Ϸ300 fs has been used to study the dynamics of OII in CD (36).…”