The unusual T-shaped X-H...pi hydrogen bonds are found between the B=B double bond of the singlet state HB=BH and the acid hydrogen of HF, HCl, HCN and H2C2 using MP2 and B3LYP methods at 6-311++G(2df,2p) and aug-cc-pVTZ levels. The binding energies follow the order of HB=BH...HF>HB=BH...HCl>HB=BH...HCN>HB=BH...H2C2. The hydrogen-bonded interactions in HB=BH...HX are found to be stronger than those in H2C=CH2...HX and OCB identical withBCO...HX. The analyses of natural bond orbital (NBO) and the electron density shifts reveal that the nature of the T-shaped X-H...pi hydrogen-bonded interaction is that much of the lost density from the pi-orbital of B=B bond is shifted toward the hydrogen atom of the proton donor, leading to the electron density accumulation and the formation of the hydrogen bond. The atoms in molecules (AIM) theory have also been applied to characterize bond critical points and confirm that the B=B double bond can be a potential proton acceptor.
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