Non-additivity between substitution and cooperative effects in enhancing hydrogen bonds

Ternary systems XO2F:NCH:NCY and XO2F:CNH:CNY (X = P and As; Y = H and Li) as well as the corresponding binary ones were studied at the MP2/aug-cc-pVTZ level. Interestingly, the π-hole pnicogen bond in the PO2F complex is stronger than that in the AsO2F counterpart. The substituent Li in the Lewis base strengthens the hydrogen bond and pnicogen bond, but the more prominent enhancing effect is found for the pnicogen bond. The substitution effect is governed mainly through electrostatic interaction for the hydrogen bond but a combination of electrostatic and polarization interactions for the pnicogen bond. In the ternary systems, the π-hole pnicogen bond exhibits a positive cooperative effect with the hydrogen bond. Energy decomposition analysis indicates that the cooperativity is mainly attributed to the polarization energy. There is positive non-additivity between the substitution and cooperative effects, which is an effective measure for strengthening the hydrogen bond. The largest interaction energies occur in AsO2F:CNH:CNLi, amounting to -130.24 kJ/mol for the pnicogen bond and -119.90 kJ/mol for the hydrogen bond, and the former is a covalent interaction and the latter is an ion-pair hydrogen bond.