The model behind hydrogen bonds and similar complexes is fundamentally flawed, Amsterdam researchers show in Chemistry – A European Journal.

Most everyone familiar with chemical concepts will recognise the hydrogen bond, a pretty strong intermolecular interaction between a hydrogen atom and for example oxygen, nitrogen or halogens (think of the base pairs in DNA). For most people, the idea behind this is that hydrogen is electropositive and thus has a relatively positive charge. The other molecule would contain electronegative atoms and thus have a relatively negative charge. This will cause an electrostatic attraction, similar to what occurs in magnets .

‘However, problems arose when people realised that two electronegative atoms could also attract one another’, says Lucas de Azevedo Santos, postdoc at the TheoCheM group at the Vrije Universiteit Amsterdam. You’ll find this phenomenon in so-called pnictogen, chalcogen and halogen bonds, which are Lewis acid-base interactions between group 15, 16 and 17 elements, respectively. ‘To explain this, the σ- (sigma) hole model was postulated. This model looks at the bond donating Lewis acid as having a positive region on the surface around the molecule, while the bond accepting Lewis base is treated like a concentrated negative point charge.’ But new calculations using molecular orbital (MO) theory shows that this model neglects two very important aspects: the physics behind both electrostatic interactions and the directionality of intermolecular interactions.