Title: σ/π character of H-bonding in water clusters

Abstract: Hydrogen bonds are typically treated as sufficiently localized directional intermolecular bonds, in which dispersion and electrostatic contributions can be distinguished. However, being formed chiefly due to the overlapping of p orbitals of electronegative atoms, the corresponding electronic bonds are characterized by both {\sigma}- and {\pi}-kind binding, the former determining the directionality of bonds, while the latter, the coupling of molecules and the collective effects in H-bond networks. The latter contribution was never considered previously and is predetermined by overlapping pre-lone pair orbitals of oxygen atoms. This is manifested in the peculiarities of the electron density distribution, which are quantified based on the analysis of magnetic shielding tensors of oxygen and bridge hydrogen nuclei and illustrated by the shapes of cluster orbitals of water aggregates.