From hydrogen bonds to σ-hole interactions: decoding the interplay of hydrogen and halogen bonding in nitrile complexes with methane and halogenated methanes.

A systematic computational investigation of hydrogen and halogen bonding interactions in nitrile⋯CF3X (X = H, Cl, Br, and I) and nitrile⋯CH4 complexes has been carried out to elucidate the nature and governing factors of these noncovalent interactions. Nitrile nitrogen acts as an electron donor, forming both HB and XB interactions. The strength of halogen bonding increases with halogen polarizability, with CF3I donors exhibiting the strongest interactions due to a pronounced σ-hole, while hydrogen bonding varies with donor substitution, hence making CF3H a better donor in comparison with CH4. Interestingly, although CF3I complexes remain the strongest overall, the hydrogen bonded complexes of nitrile⋯CF3H display interaction strength that is comparable to or even greater than that of nitrile⋯CF3Br and nitrile⋯CF3Cl complexes. This observation highlights the ability of hydrogen bonding to compete with halogen bonding depending on the nature of the substituent attached to it. Stabilization energies, geometries and nitrile stretching frequency shifts were analysed to characterize these interactions. QTAIM and NBO analyses reveal closed-shell interactions with significant donor-acceptor charge transfers, especially in the case of nitrile⋯CF3I halogen bonded complexes, while NCE and EDA highlight the key roles of electrostatic contributions. These results provide clear insight into the factors governing HB and XB in nitrile systems.