High levelab initiocalculations predicted a possibility for energetically low-cost (±1 kcal mol⁻¹) change of the bond angle and bond lengths in wide range,from 96.4° to 112.8° and from 0.930 Å to 0.989 Å, respectively.

Although CH…O hydrogen bonds are generally very weak, here investigated CH…O interactions of coordinated 1,10-phenanthroline (phen) are very frequent and quite strong. In the crystal structures from the Cambridge Structural Database, 8344 CH…O interactions between coordinated phen and water molecule in the second coordination sphere were found. We calculated all possible types of CH…O interaction energies at DLPNO-CCSDT/CBS level for non-coordinated and coordinated phen with a water molecule. The data for non-coordinated phen exhibited the weakest interactions, from −2.09 to −2.94 kcal/mol. Upon coordination of phen, interactions become stronger. In octahedral cobalt(II) complexes, interaction energies are from −3.37 to −4.35 kcal/mol. With the decrease in the complex coordination number, interaction energies become stronger, the strongest are for square planar palladium(II) complexes from −3.91 to −4.94 kcal/mol. There is a linear correlation between interaction energies and electrostatic potential values at the interacting hydrogen atom, with a correlation coefficient of 0.97. For all studied systems, the weakest is always a linear interaction, and the strongest is a bifurcated interaction. The strongest calculated CH…O interactions of coordinated phen with water in the second coordination sphere (−4.94 kcal/mol) are as strong as the hydrogen bond between two water molecules (−5.0 kcal/mol).

Copper–semicarbazone ligands have been extensively investigated for several medicinal applications. In this contribution, a novel copper(II) complex with a pyridoxal–semicarbazone ligand, [Cu(PLSC)Cl(H₂O)](NO₃)(H₂O), was synthesized and characterized by X-ray crystallography, elemental analysis, UV-VIS, and FTIR spectroscopies. The stabilization interactions within the structure were assessed using the Hirshfeld surface analysis. The crystallographic structure was optimized at the B3LYP/6-311++G(d,p)(H,C,N,O)/LanL2DZ(Cu) level of theory. A comparison between the experimental and theoretical bond lengths and angles was undertaken to verify the applicability of the selected level of theory. The obtained high correlation coefficients and low mean absolute errors confirmed that the optimized structure is suitable for further investigating the interactions between donor atoms and copper, along with the interactions between species in a neutral complex, using the Quantum Theory of Atoms in Molecules approach. The electrostatic potential surface map was used to reveal distinct charge distributions. The experimental and calculated FTIR spectra were compared, and the most prominent bands were assigned. The interactions with human serum albumin (HSA) were assessed by spectrofluorometric titration. The spontaneity of the process was proven, and the thermodynamic parameters of binding were calculated. Molecular docking analysis identified the most probable binding site, providing additional insight into the nature of the interactions.