Bifurcated hydrogen bonds: Three-centered interactions

The nature of bifurcated or three-centered hydrogen bonds (HB) has been inv estigated. Different families of compounds were chosen: monomers with intra molecular three-centered HE, dimers with a HB donor (HBD) and a molecule wi th 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 t he basis set superposition error (BSSE). The electronic nature of these thr ee-centered HBs was analyzed by means of the atoms in molecules (AnM) appro ach. 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 b ond critical points with equal values of electron density; (ii) compounds w ith 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 differen t electron density values one of which is very small; (iv) van der Waals co mplexes 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.