Theoretical study of hydrogen bonding interactions in substituted nitroxide radicals

Abstract

Intermolecular NO⋯H hydrogen bonding of nitroxide radicals with various electron acceptors accounts for their interesting properties and key applications. In this work we have performed a comprehensive and quantitative theoretical analysis of the intermolecular NO⋯H hydrogen bonds formed by nitroxide radicals with HF, H₂O, and CH₄ as electron acceptors. The electronic effects of nitroxide radicals are assessed in terms of the absolute minimum of molecular electrostatic potential (V_min) around nitroxide oxygen atom and compared with those of iminoxyl radicals. The observed V_min values clearly reflect the electron donating and withdrawing features of the substituents present in the structural framework of nitroxide radicals. Accordingly a linear relationship has been established between V_min and interaction energy (E_int) values of hydrogen bonded complexes of both nitroxide and iminoxyl radicals. Quantum theory of atoms in molecules (QTAIM) analysis demonstrated that the nature of hydrogen bonding in systems with electron donating substituents is a mix of closed shell and shared interactions, while it is mostly closed shell in systems with electron withdrawing substituents. Symmetry-adapted perturbation theory (SAPT) calculations show that the hydrogen bonds are largely dependent on electrostatic components of the interaction energy in NO⋯HF and NO⋯H₂O complexes, whereas the dominant contributor is dispersion in NO⋯HCH₃ complexes. The results herein suggest that modifying the substituents in the structural motif of nitroxide radicals helps to fine-tune the strength and nature of hydrogen bonding interactions in their complexes with electron acceptors. Moreover, quantification of the strength and nature of the hydrogen bonding interactions in nitroxide radicals can be effectively done with topological features of molecular electrostatic potential and QTAIM parameters.