Can B-H···N/O σ-Interaction Be Classified as a Nonclassical Hydrogen Bond?

This study examines the potential B-H···N/O σ-interaction between difluoroborane (F2BH), dichloroborane (Cl2BH), dicyanoborane ((CN)2BH), and dinitroborane ((NO2)2BH) (acting as proton donors), with NH3 and H2O serving as proton acceptors. The analysis was conducted using DFT (M06-2X), MP2, and CCSD(T) methods, employing a triple-ζ basis set with diffuse and polarization functions (6-311++G(3df,2pd)). To compare this interaction with classical and nonclassical hydrogen bonds, the water dimer and the F3CH···NH3/OH2 systems, respectively, involving O-H···O and C-H···N/O hydrogen bonds were considered. Most characteristics of the O-H···O and C-H···N/O interaction types, including structural features, electron redistribution, bond critical points, NBO second-order perturbation energy (E2), vibrational frequency shifts, and NMR, are also observed in the B-H···N/O interaction, except that the bond strength is relatively weak. It is a maximum at the MEP at the site of the H atom that is located along the B-H bond of F2BH, Cl2BH, and (CN)2BH that is responsible for these attractive interactions, in which not only dispersion-driven forces but also electrostatic contributions play a significant role. Results from the interaction between diborane (B2H6) and BH4_ with NH3/OH2 indicate that the maximum at the MEP at the site of the H atom appears necessary to form B-Hδ-···N/O bond, which may be termed a hydridic hydrogen bond.