Halogen bonding (XB) in complexes of diiodine with heteroaromatic N-oxides was examined via a combination of UV–vis spectral and X-ray structural measurements, as well as computational analysis. While all of these associates were formed by analogous I···O bonds, they showed considerable variations of formation constants (5–1500 M–1) and intermolecular I···O bond length (2.3–3.2 Å). In the solid state, both atoms of I2 molecules were involved in XB, and the I···O separations were determined by the electron-donor abilities of N-oxides and the strength of the bonding on the opposite side of the ditopic XB donor. The solution-phase formation constants of 1:1 complexes, K, as well as magnitudes of the calculated interaction energies, ΔE, increased with the shift of the values of the most negative potentials on the surfaces of N-oxides’ oxygen atoms, V min, toward more negative values. Yet, the interatomic contacts consistently deviated from the locations of V min. Instead, the structures of complexes were well suited for highest occupied molecular orbital/lowest unoccupied molecular orbital interactions of reactants. The values of K, ΔE, and the intermolecular distances d I···O in the calculated complexes were highly correlated with the charge-transfer interaction energies derived from the natural bond orbital analysis. This indicated that, besides electrostatic, molecular orbital interactions play a substantial role in XB between diiodine and N-oxides. This conclusion was supported by the analysis of the complexes using the quantum theory of atoms in molecules, noncovalent interaction index, and density overlap region indicator, which showed that the covalent character of I···O bonding increases with the rise of interaction energies in the complexes.

Halogen- and hydrogen-bonded complexes between trihalomethanes, CHX3, and (pseudo-)halide anions, A-, co-existing in acetonitrile solutions were identified and characterized via a combination of UV-vis and NMR spectral measurements with the results of X-ray structural and computational analyses. Halogen-bonded [CHX3, A-] complexes displayed strong absorption bands in the UV range (showing Mulliken correlations with the frontier orbital energies of the interacting species) and a decreased shift of the NMR signal of trihalomethanes' protons. Hydrogen bonding led to the opposite (increased) NMR signal shift and the UV-vis absorption bands of the hydrogen-bonded [CHX3, A-] complexes were similar in intensity to those of the separate CHX3 molecules. The simultaneous multivariable treatment of the results of UV-vis and NMR titrations of CHX3 with A- anions afforded formation constants of both halogen- and hydrogen-bonded complexes between these species, which existed side-by-side in the acetonitrile solutions. The relative values of the formation constants were consistent with the magnitudes of the positive potentials on the surfaces of the halogen or hydrogen atoms if the effects of the polarization of the trihalomethanes due to the presence of the anions were taken into account.