Phosphine Oxides as Spectroscopic Halogen Bond Descriptors: IR and NMR Correlations with Interatomic Distances and Complexation Energy

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=PMe3 halogen bond energy Delta E, X...O distance r, halogen's sigma -hole size, QTAIM parameters at halogen bond critical point and changes of spectroscopic parameters of phosphine oxide upon complexation, such as P-31 NMR chemical shift, Delta delta P, and P=O stretching frequency, Delta nu. Some of the correlations are halogen-specific, i.e., different for F, Cl, Br, I and At, such as Delta E(r), while others are general, i.e., fulfilled for the whole set of complexes at once, such as Delta E(Delta delta 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.