The strong OHO hydrogen bond. How much covalency?

A quantum crystallographic approach to short hydrogen bonds

“…5 ) whilst only a negative value of the Laplacian ∇ 2 ( r BCP ) for the H⋯O interaction is found in 2 (−1.71(11) e Å −5 ) where full proton transfer has occurred across the hydrogen bond. In agreement with previous findings, 9 as the electron density reflects proton position in 1–3 , it therefore appears sensitive to the extent of static transfer of the H-atom across the hydrogen bond. The large values of the electron density in the H⋯O region of the short hydrogen bonds given by the topological parameter ρ ( r ) BCP (0.79 to 0.9 e Å −3 ) combined with the ring shaped deep blue/covalent bond of the respective calculated NCI isosurface ( Fig.…”

“…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.…”

Inequivalence of substitution pairs in hydroxynaphthaldehyde: A theoretical measurement by intramolecular hydrogen bond strength, aromaticity, and excited‐state intramolecular proton transfer reaction

“…1) and visually appear to be superimposing to one another whereby supporting the conclusion that electron density at hydrogen bond critical point (HBCP) is primarily a local property, and to a large extent is independent of the nature of the hydrogen bond itself, that is, all of the hydrogen bonds exhibit essentially similar topological properties. 36,37 The present findings appear to be consistent with an earlier report by Alkorta et al, 38 who showed the electron density at HBCP for a large number of complexes to be an additive property. Our findings also project that q c and hydrogen bond distance are directly dependent.…”

“…HN21, and HN23 is not as high as to allow the development of a partial covalent character (as in the aforementioned case of ref. 36), but still are quite appreciably strong. However, to be specific, this comparison is not quantitative as because of the differential basis sets used in the respective reports.…”

“…36 Majrez in a recent report 36 has calculated the ! 2 q(r) value at the BCP in the OHO hydrogen bond of 1:1 phosphoric acid-urea complex to be varying within the 0.0149-0.091 a.u.…”