Unravelling the factors affecting the stability and reactivity of dehydro-pyrazole, isothiazole and isoxazole radical isomers: a computational study

Abstract

Reactive open-shell ring systems comprising C, N, O, and S atoms are crucial for the radiation-induced molecular evolution and relevant to life and interstellar regions. Herein, we targeted the dehydro radical isomers derived from five-membered heterocycles pyrazole 1, isothiazole 2, and isoxazole 3. Using electronic structure and unimolecular decomposition-based mechanistic calculations, we attempted to envisage the variables influencing the thermodynamic and kinetic stability of radical isomers 1a–1d, 2b–2d and 3b–3d. Evidence suggests that the factors including (hetero)aromaticity, the radical electron's role in delocalization (resonance), and the ring strain play a significant role. The N-centered pyrazole radical 1a was found to be entirely delocalised, which impacts its higher thermodynamic and kinetic stability. Conversely, the C-centered radicals (1b–1d, 2b–2d, and 3b–3d) show important contributions from the interaction between the radicals and lone pair electrons, evident from the NBO analysis. The mechanistic studies through unimolecular decomposition channels show the importance of ring-opening channels as the decisive factor in their kinetic stability. Preferential cleavage of X–N or C–X bonds and formation of fragmented products like acetylene, HCN, HNC, CO, CS, NHCCH radicals, SCCH radicals, OCCH radicals, HNNC radicals, SNC radicals, and ONC radicals were also characterized.