Structural symmetry study of some bis‐adduct of an acceptor with two donors

The protonated water dimer has a symmetric structure 1 in contrast to the asymmetric structure of protonated ammonia dimer 2 . To make a general understanding of structural symmetry difference for these D–A–D systems, we carried out a systematic theoretical study with the B3LYP/6‐31+G** method. We considered the electron donors (D) of the lone pair on either side of BF, CO, and N 2 molecules, as well as the electron acceptors (A) like Li + , CH 3 + , SiH 3 + , and PH 2 + beside the proton. We found the magnitude of the first bonding energy between D and A [BE(1)] is decisive for the symmetry of the D–A–D system. A larger BE(1) leads to a smaller BE(2), the bonding energy of D–A with the second donor. This gives rise to a stronger tendency toward asymmetry. The magnitude of BE(1) correlates well with the difference in ionization potential (ΔIP) of the orbitals involved in the D–A interaction. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002