# Substituent Effects in the Thermal Decomposition of 1,2,4-Triazol-3(2H)-Ones and Their Thione Analogues: A DFT Study with Functional Performance

## TL;DR

This study uses computational methods to explore how substituents affect the thermal decomposition of triazole compounds and identifies optimal DFT functionals for accurate predictions.

## Contribution

The paper introduces a dual-methodology framework using specific DFT functionals for accurate activation energy predictions in sulfur- and oxygen-containing triazole systems.

## Key findings

- Hybrid functional APFD best predicts activation energies for sulfur-containing systems due to higher polarizability.
- Electron-donating substituents decrease activation energy, while electron-withdrawing groups increase it.
- Sulfur analogues decompose faster than oxygen counterparts, with ~40 kJ/mol lower activation energies.

## Abstract

This computational study investigates the thermal decomposition of 1,2,4-triazol-3(2H)-ones and their thione analogues using Density Functional Theory (DFT). The reaction proceeds via a concerted, six-membered cyclic transition state, primarily driven by the breaking of the N–N bond. A key finding is that the accuracy of the calculated activation energies (Ea) strongly depends on the choice of DFT functional. For sulfur-containing systems (thiones), the hybrid functional APFD (with 25% Hartree–Fock exchange) provides the most reliable results, effectively describing their higher polarizability. In contrast, for oxygen-containing systems (triazolones), the dispersion-corrected functional B97D-GD3BJ (with 0% Hartree–Fock exchange) delivers superior accuracy by better modeling electrostatic and dispersion interactions. The -CH2CH2CN group at the N-2 position acts not only as a protecting group but also stabilizes the transition state through non-covalent interactions. Electron-withdrawing substituents slightly increase the Ea, while electron-donating groups decrease it. Sulfur analogues consistently show significantly lower activation energies (by ~40 kJ/mol) than their oxygen counterparts, explaining their experimentally observed faster decomposition. This work establishes a dual-methodology computational framework for accurately predicting the kinetics of these reactions, providing valuable insights for the regioselective synthesis of biologically relevant triazole derivatives via controlled pyrolysis.