A computational study on the characteristics of open-shell H-bonding interaction between carbamic acid (NH₂COOH) and HO₂, HOS or HSO radicals

Quantum chemical computations were applied to investigate the characteristics of open-shell hydrogen-bonding interactions in the complexes of carbamic acid (NH₂COOH, CA) with HO₂, HOS and HSO radicals. All the resulting complexes were studied using the MP2, B3PW91 and B3LYP computational levels and 6311++G** basis set. Geometry optimizations show that the O–H⋯O contact is stronger than N–H⋯O and S–H⋯O. The interaction energies revealed that all the radicals form stronger hydrogen bonded complexes at site-1, as confirmed by electron-density (ρ) and corresponding Laplacian (∇²ρ) values obtained by atoms in molecule (AIM) analysis. Non-covalent interaction and reduced density gradient analysis support the AIM results. Natural bond orbital analysis was employed to obtain the stabilization energies (E⁽²⁾) due to charge delocalization between the interacting units. Energy decomposition analysis suggests that, for the title complexes, the exchange energy makes a larger contribution to the total interaction energy compared to other energy terms. [Figure not available: see fulltext.].