π-Hole Interactions Involving Nitro Compounds: Directionality of Nitrate Esters

The MEPs of a variety of nitro compounds (R-NO2) suggest the existence of a pi-hole with a potential of up to +54 kcal/mol in 10 (R = CF3). Several of these nitro compounds were considered as partners for anions (F-, Cl-, NC-) and the electron rich molecules acetonitrile and dimethyl ether. In most cases a pi-hole complex was obtained with calculated binding energies of up to 20 kcal/mol with anions and 5 kcal/mol with the neural molecules. A thorough analysis of the CSD revealed that nitrate esters are highly directional pi-holes in the solid state, for at least sp(2) O atoms. This was further illustrated by highlighting several crystal structures where more than 0.2 angstrom van der Waals overlap was observed between the N atom of the nitrate ester and an electron rich atom like oxygen.