Computational analysis of substituent effects on proton affinity and gas-phase basicity of TEMPO derivatives, and their hydrogen bonding interactions with water molecules

Abstract

Abstract The molecular structure of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives (X: CH3, NH2, CHO, and NO2) in the gas phase, as well as their hydration, is investigated using the B3LYP method and the 6-311 + + G(d,p) basis set. Intermolecular interactions are analyzed using the natural bond orbital (NBO) and atoms in molecules (AIM) techniques. NBO analysis reveals the stability and formation of compounds, while AIM calculations show all hydrogen bonding interactions in the hydrated forms of TEMPO derivatives. The chemical reactivity parameters show that NO2-substituted TEMPO, with a small HOMO-LUMO energy gap, is more reactive, less stable, and exhibits softer qualities. Furthermore, the NBO results show that the stability of the investigated TEMPO derivatives is mainly influenced by LP(e)→σ∗ electronic delocalization effects, with the highest stabilization observed on the oxygen atom of the nitroxide moiety. The study considers electron density, atomic charges, and energetic and thermodynamic properties of the studied nitroxide radicals and their relative stability. The study computed the proton affinity (PA) and gas-phase basicity (GB) of TEMPO derivatives at T = 298 K for the O-protonation and N-protonation.The computed PA values in case of O-protonation of TEMPO derivatives (X: H, CH3, NH2, CHO, NO2) are 896.04, 894.17, 892.57, 870.36, and 851.29 kJ/mol, respectively, while the corresponding values for N-protonation are 806.50, 806.92, 804.08, 787.45, and 763.11 kJ/mol, respectively. The AIM analysis reveals that the N−O…H interaction in H2O complexes has the most favorable H-bond energy. The electronic structure characterization of N−O…H bonds in stable conformers of studied TEMPO derivatives revealed H-bond energies of 6.90, 6.90, 6.87, 6.62, and 6.46 kcal/mol for H, CH3, NH2, CHO, and NO2 substitutions, computed at bond critical points (3,−1).