| Summary: | An extensive series of 128 halogen-bonded complexes formed by trimethylphosphine oxide and various F-, Cl-, Br-, I- and At-containing molecules, ranging in energy from 0 to 124 kJ/mol, is studied by DFT calculations in vacuum. The results reveal correlations between R−X⋅⋅⋅O=PMe<sub>3</sub> halogen bond energy Δ<i>E</i>, X⋅⋅⋅O distance <i>r</i>, halogen’s σ-hole size, QTAIM parameters at halogen bond critical point and changes of spectroscopic parameters of phosphine oxide upon complexation, such as <sup>31</sup>P NMR chemical shift, Δ<i>δ</i>P, and P=O stretching frequency, Δν. Some of the correlations are halogen-specific, i.e., different for F, Cl, Br, I and At, such as Δ<i>E</i>(<i>r</i>), while others are general, i.e., fulfilled for the whole set of complexes at once, such as Δ<i>E</i>(Δ<i>δ</i>P). The proposed correlations could be used to estimate the halogen bond properties in disordered media (liquids, solutions, polymers, glasses) from the corresponding NMR and IR spectra.
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