The First Experimental Charge Density Studies and Comparative Periodic DFT Calculations for Crystalline Hydrogen Peroxide Adducts: Peculiarities of the HO–OH Covalent Bond

Charge density distributions in crystalline peroxosolvates of parabanic acid C₃H₂N₂O₃·0.5H₂O₂ and ammonium salt of 3,4,5-trinitropyrazole NH₄⁺·C₃N₅O₆^-·0.5H₂O₂ derived from the high-resolution single-crystal X-ray experiments are reported with comparative periodic DFT calculations. Both multipole refinement and theoretical studies showed that the O-O bonds exhibit a (3, -1) critical point with electron density (ρ_b) of approximately 0.24 a.u. Experimental deformation density maps show not an accumulation but a depletion of charge density around O-O bond centers. The Laplacian values of electron density (∇²ρ_b) obtained from X-ray data are definitely positive and lie within 0.70-0.79 a.u. Theoretical values of ∇²ρ_b noticeably depend on the basis sets. They exhibit small, both positive and negative values for the modest basis sets and are around 0.3 a.u. for the pob-TZVP basis set. Thus, the O-O linkage can be considered as a specific ≪charge-shift≫ bond. The F-F bond in crystalline α-F₂ exhibits similar features.