A study of the tautomers of N-salicylidene-p-X-aniline compounds in methanol

Abstract

We have studied the enol-imine→keto-amine tautomeric equilibrium of N-salicylidene-p-X-aniline compounds with X = Me, OMe, NMe 2 as electron donor substituents and X = COMe, CN and NO 2 as electron acceptor substituents. The equilibrium constants (KЊ tau ) and standard thermodynamic properties ∆GЊ tau , ∆HЊ tau and ∆SЊ tau were measured and calculated in methanol solution at various temperatures, by means of excitation fluorescence spectroscopy. We have analyzed the p-phenylaniline substitution effect on KЊ tau and the thermodynamic properties through the Hammett parameters σ. We have performed molecular orbital calculations at the semiempirical AM1 and ab initio HF/3-21G levels to interpret the experimental results, explicitly including the solute-solvent interaction through the formation of an intermolecular hydrogen bond between the salicylidene and methanol molecules. These computational results show a good correlation with the experimental values. An interpretation of the experimental values of T∆SЊ, based on changes in the molecular structure produced in the enol-imine→keto-amine tautomeric reaction, is proposed.

Insights into the Origin of Solid‐State Photochromism and Thermochromism of N‐Salicylideneanils: The Intriguing Case of Aminopyridines

“…[49] Thus, 57 Fe Mçssbauer spectroscopy confirms the presence of one crystallographic site as observed in the crystal structures of 1-5. The decrease of temperature leads to small parameter changes as expected (see Table S1 in the Supporting Information).…”

Enol-keto tautomerism of aromatic photochromic Schiff base N,N′-bis(salicylidene)-p-phenylenediamine: Ground state equilibrium and excited state deactivation studied by solvatochromic measurements on ultrafast time scale

“…8,12,13,17,18 In nonpolar solvents most or even all molecules are present in its enol form in the ground state. 8,12,13,17,18 In nonpolar solvents most or even all molecules are present in its enol form in the ground state.…”

Photochromic cycle of 2′-hydroxyacetophenone azine studied by absorption and emission spectroscopy in different solvents

“…25,[45][46][47] It is also consisted with our theoretical calculations which predict the first longwavelengths strong transitions at 378 nm (oscillator strength 0.69) and 448 nm (oscillator strength 0.54) for the enol (conformer II) and cis-keto (conformer X) structures in HEX, respectively (Table I).…”

“…It is known that the cis-keto tautomer of photochromic Schiff bases is more stabilized than the enol one in protic and polar solvents, 25,45,46 and the enol and cis-keto tautomers can exist in equilibrium if the energy for both tautomers in nearly the same. Interestingly, such equilibrium was not observed for SAA/HFIP.…”