Abstract
The energy involved in the structural switching of acyl and hydroxyl substituents in the title compounds was evaluated combining experimental and computational studies. Combustion calorimetry and Knudsen effusion techniques were used to determine the enthalpies of formation, in the crystalline state, and of sublimation, respectively. The gas-phase enthalpy of formation of both isomers was derived combining these two experimental data. Concerning the computational study, the G3(MP2)//B3LYP composite method was used to optimize and determine the energy of the isomers in the gaseous state. From a set of hypothetical reactions it has been possible to estimate the gas-phase enthalpy of formation of the title compounds. The good agreement between the experimental and computational gas-phase enthalpies of formation of the 1-acetyl-2-naphthol and 2-acetyl-1-naphthol isomers, provided the confidence for extending the computational study to the 2-acetyl-3-naphthol isomer. The structural rearrangement of the substituents in position 1 and 2 in the naphthalene ring and the energy of the intramolecular hydrogen bond are the factors responsible for the energetic differences exhibited by the isomers. The gas phase tautomeric keto ↔ enol equilibria of the o-acetylnaphthol isomers were analyzed using the Boltzmann’s distribution.
Highlights
The o-acetylnaphthol isomers present an orientation and close proximity of the oxygen of the acetyl group towards the hydrogen of the hydroxyl group, enabling the formation of an intramolecular hydrogen bond (IMHB) [1,2,3,4,5,6,7,8]
Static-bomb combustion calorimetry of at theseveral isoperibol type was by theKnudsen technique used totechnique, perform pressures of the two isomers were measured, temperatures, effusion the combustion experiments in oxygen for the two o-acetylnaphthol isomers
1-acetyl-2-naphthol caused by the distortion of the acetyl group
Summary
The o-acetylnaphthol isomers (structural formulae presented in Figure 1) present an orientation and close proximity of the oxygen of the acetyl group towards the hydrogen of the hydroxyl group, enabling the formation of an intramolecular hydrogen bond (IMHB) [1,2,3,4,5,6,7,8]. This process is known as excited state intramolecular proton transfer (ESIPT). The ESIPT process in 2-acetyl-1-naphthol has been in controversy for long, primarily because of its quantitative ESIPT conversion in most solution conditions and due to small Stokes shift of the ESIPT products (keto type prototautomer) compared to those products in other typical ESIPT molecules [1,14].
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