Design of molecular switching and signaling based on proton transfer in 2-hydroxy Schiff bases: a computational study

Abstract

The present work aims to exploit the possibility of using the tautomerism in 2-hydroxy Schiff bases for molecular switching. The enol imine (E)⇔ enaminone (K) tautomerization in a series of 2-hydroxy Schiff bases have been investigated theoretically at the DFT/B3LYP/6-311G** level of theory. The intramolecular proton transfer processes have been explored, transition structures have been located and characterized. The kinetics and thermodynamics of the proton transfer process, and its time scale have been computed and discussed in the framework of the suitability as molecular switches. Substituent effects have been computed and its effect on the enthalpy changes (∆H*) and activation energies (∆G*) have been analyzed and discussed. Nonspecific solvent effects have also been taken into account by using the polarized continuum model (IPCM) of two different solvent. The tautomerization energies are decreased and hence the endothermic nature of the enol imine ⇔ enaminone tautomerization. The potential energy barriers, on the other hand, are increased due to the relative destabilization of the transition states. The NBO charge populations show that there is a high positive charge on the hydrogen atom during the process in all cases, which confirms that the proton transfer proceeds through a three-center interaction. The proton transfer processes, in all cases studied are kinetically allowed. The low potential energy barrier suggests that interconversion between the two tautomeric forms is spontaneous and the two forms may coexist.

Deuteration of Aromatic Rings under Very Mild Conditions through Keto-Enamine Tautomeric Amplification

“…This transformation disrupts aromaticity in the benzene ring and is strongly disfavored, hence the requirement for elevated temperatures. Schiff bases based on salicylaldehyde, on the other hand, are known to tautomerize to keto-enamine forms with diminished aromaticity. − This diminished aromatic character can presumably lower the energetic cost of the keto-mediated H/D exchange pathway described above. In this case, the keto-enamine tautomer A facilitates the formation of the all-keto form B , thereby enabling H/D exchange under milder conditions, Scheme b.…”

Toward Exploring Novel Organic Materials: MP4-DFT Properties of 4-Amino-3-Iminoindene

“…In order to increase the selectivity among asymmetrical tautomers (containing the NCCCN core) it is necessary to push the activation energy to a higher value. As can be seen in the previous studies [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ], aromaticity can be employed here to generate asymmetrical tautomers with different electronic structures; one is aromatic and the other is not aromatic. Additionally, introducing a skeletal constraint that guarantees pushing the two nitrogen atoms further apart from each other (compared to malonaldimine; N…N = 2.7088 Å) can participate in making the proton transfer more difficult and, thus, energetically more expensive.…”

“…Switching based on tautomerization reactions has been recognized in many molecular systems [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. The first recognized ideal tautomerism-based molecular switching process was identified for the porphyrins-category of molecules [ 11 ].…”