Can BB double bond be as potential proton acceptor: A UB3LYP and UMP2 theoretical study on unusual intermolecular T-shaped XH…π interactions between the triplet state HBBH (3Σg-) and HF, HCl, HCN or H2C2

The unusual weak T-shaped XH…π hydrogen bonds are found between the BB double bond of the triplet state HBBH (3Σg-) and the acid hydrogen of HF, HCl, HCN and H2C2 using UMP2 and UB3LYP methods at 6-311++G(2df,2p) and aug-cc-pVTZ levels. The binding energies follow the order of HBBH…HF > HBBH…HCl > HBBH…HCN > HBBH…H2C2, and the hydrogen-bonded interactions in the triplet state complexes HBBH…HX (3B1) are found to be weaker than those in HCCH…HX and OCBBCO…HX. The analyses of natural bond orbital (NBO) and the electron density shifts reveal that the nature of the T-shaped XH…π hydrogen-bonded interaction is that the lost density from the π-orbital of BB bond is shifted toward the hydrogen atom of HX, leading to the electron density accumulation and the formation of the hydrogen bond. The atoms in molecules (AIM) theory has been also applied to characterize bond critical points and confirm that it is difficult for the ground electronic state of HBBH to be as the hydrogen-bond proton acceptor, perhaps due to the nature of electron-deficient BB double bond.