Experimental evidence of a 3-centre, 2-electron covalent bond character of the central O-H-O fragment on the Zundel cation in crystals of Zundel nitranilate tetrahydrate

The charge density of the Zundel cation in the crystal of 2[H5O2](+)[C6O8N2](2-)center dot 4H(2)O is studied by X-ray diffraction and periodic DFT calculations. The covalent nature of the central O-H-O bonds in the Zundel cation is revealed: the electron density at the two O-H critical points is about 1.0 e angstrom(-3), corresponding to a three-centre, two-electron system. The bond orders derived from the topology of the electron density are 0.27-0.41, which are more similar to those of single O-H bonds (bond order 0.45-0.51) than those of H. O hydrogen bonds (bond orders = 0.05-0.10).