Structure and IR Spectra of 3(5)-Aminopyrazoles and UV-Induced Tautomerization in Argon Matrix.

Alina Secrieru,Rui Fausto,Susy Lopes,Maria L S Cristiano

doi:10.3390/molecules26144299

Alina Secrieru, Rui Fausto + Show 2 more

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https://doi.org/10.3390/molecules26144299

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Abstract

The prototropic tautomerism in 3(5)-aminopyrazoles was investigated by matrix isolation infrared (IR) spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. In consonance with the experimental data, the calculations predict tautomer 3-aminopyrazole (3AP) to be more stable than the 5-aminopyrazole (5AP) tautomer (calculated energy difference: 10.7 kJ mol−1; Gibbs free energy difference: 9.8 kJ mol−1). The obtained matrix isolation IR spectra (in both argon and xenon matrices) were interpreted, and the observed bands were assigned to the tautomeric forms with help of vibrational calculations carried out at both harmonic and anharmonic levels. The matrix-isolated compound (in argon matrix) was then subjected to in situ broadband UV irradiation (λ > 235 nm), and the UV-induced transformations were followed by IR spectroscopy. Phototautomerization of the 3AP tautomer into the 5AP form was observed as the strongly prevalent reaction.

Highlights

IntroductionMolecules 2021, 26, 4299. https://Like other nitrogen-containing heterocyclic compounds, pyrazoles (five-membered heterocycles with two adjacent nitrogen atoms) have been attracting a growing interest due to their unique features and extensive range of applications [1,2,3,4]
Molecules 2021, 26, 4299. https://Like other nitrogen-containing heterocyclic compounds, pyrazoles have been attracting a growing interest due to their unique features and extensive range of applications [1,2,3,4]
[17], in with the bined with calculations performed density functional theory level present work, we studied thethe tautomerism in unsubstituted

Summary

Introduction

Molecules 2021, 26, 4299. https://Like other nitrogen-containing heterocyclic compounds, pyrazoles (five-membered heterocycles with two adjacent nitrogen atoms) have been attracting a growing interest due to their unique features and extensive range of applications [1,2,3,4]. Pyrazoles are prone to prototopic tautomerism, which appears as a key factor in determining their chemical reactivity In these compounds, 1,2-H shifts between the two vicinal nitrogen atoms are in general easy, energetically accessible processes that lead to tautomerism. Their use spans from the synthesis of different types of compounds with applications in pharmaceutics and agrochemical industries to the development of functional materials [5,6,7,8] In spite of their widespread practical interest, these compounds have only been scarcely studied from the structural point of view at the molecular level, and the available information about their vibrational properties and photochemistry is lacking.

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