The competitive strengths of hydrogen and halogen bonding to haloforms and their different spectroscopic markers

Haloforms (CX₃H) are paired with halide anions and with neutral N-bases, and the properties of the ensuing hydrogen (HB) and halogen bond (XB) are examined by DFT calculations. The strength of either sort of interaction diminishes in the order F^- > Cl^- > Br^- > I^- > NH₃ > NCH. The XB energy climbs rapidly as the haloform X atom grows larger, but the HB is much less sensitive to the identity of X. In most cases, the HB is energetically favored over the XB. Exceptions occur when CI₃H is paired with any of the halides, where the XB is more stable. In both cases, the X-H stretching frequency is shifted to the red, but the magnitude of this shift is far larger in the HB case. The NMR chemical shielding of the proton is substantially reduced by formation of a HB, but undergoes a small increase within the XB. The C nucleus of the haloform suffers a large shielding drop within the HB, but its shielding change is far smaller within the context of a XB, and can be of either sign.