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

Abstract

Charge density of the Zundel cation in the solid state reveals a covalent nature of its central O–H–O fragment.

Towards understanding π-stacking interactions between non-aromatic rings

“…The electron density between the rings in the dimer reaches almost 0.1 e Å À3 (Molčanov et al, 2019). This is not a high value (for comparison, the maximum electron density in medium-strong hydrogen bonds is about 0.2 e Å À3 ; Molčanov et al, 2015Molčanov et al, , 2017a but it extends over a large contact area. Therefore, multiple bonding (3,À1) critical points are found between the rings [ Fig.…”

“…Stacking interactions exhibit characteristics comparable with those of hydrogen bonding. They both cover a broad range from dispersion (the weakest hydrogen bonds, such as C-HÁ Á ÁS and C-HÁ Á ÁCl) to two-electron/three-centric covalent bonding (the strongest hydrogen bonds, such as in the Zundel cation) (Steiner, 2002;Molčanov et al, 2017a).…”

A quantum crystallographic approach to short hydrogen bonds

“…7 Specific features of these short, strong hydrogen bonds include a sharing of electron density distribution across the hydrogen bond, 8 in some cases this may equate to covalency in a 3-centre 2-electron system. 9,10 The interactions can have hydrogen potentials approaching that of a single well with a diminished barrier to proton transfer 11 with large values of hydrogen bond energy (>100 kJ mol −1 ). 12,13 Varying proton positions are often found, either bonded to the original donor (a neutral hydrogen bond), transferred to the acceptor (salt formation) or, in some cases, the donor and acceptor atoms compete for the hydrogen atom such that it may be centred 14 (salt-cocrystal continuum) or appear on the edge of proton transfer.…”

A Crystallographic Charge Density Study of the Partial Covalent Nature of Strong N⋅⋅⋅Br Halogen Bonds

“…In comparison, the in‐ vacuo‐optimised geometry of the bromonium ion from 3 is symmetric, with both N⋅⋅⋅Br contacts having lengths of 2.124 Å and ρ max of 0.655 e Å −3 . Therefore, the N−Br−N fragment of the bromonium cation is analogous to the covalent/hydrogen bonded Zundel cation (H 2 O⋅⋅⋅H + ⋅⋅⋅OH 2 ), which comprises two weak covalent bonds of order 0.5 . Moreover, a systematic trend of reduction of VSCC‐s around the Br atom is noted, going from an almost isolated N−Br bond in 1 (only weak interactions with its environment) to the bromonium cation (Figure c).…”