Bifurcated hydrogen bonds: Three-centered interactions

The nature of bifurcated or three-centered hydrogen bonds (HB) has been investigated. Different families of compounds were chosen: monomers with intramolecular three-centered HE, dimers with a HB donor (HBD) and a molecule with two HE acceptor (HBA) groups, and trimers with one HBD and two HBAs. All the systems were optimized at the B3LYP/6-31G* level, and, in the case of the complexes, the interaction energies were evaluated and corrected with the basis set superposition error (BSSE). The electronic nature of these three-centered HBs was analyzed by means of the atoms in molecules (AnM) approach. The present study indicates the existence of bifurcated bond paths in the AIM analysis with electron densities that can be classified as follows: (i) compounds with symmetric three-centered HBs presenting two symmetric bond critical points with equal values of electron density; (ii) compounds with asymmetric three-centered HBs presenting two bond critical points with different values of electron density; (iii) compounds with a regular HB and a van der Waals interaction showing two bond critical points with different electron density values one of which is very small; (iv) van der Waals complexes with two bond critical points having very small electron densities. Therefore, looking at the geometry, electron density, and energy results, the nature-as HB of these three-centered interactions has been confirmed.