Hydrogen Bonding and Electron Donor−Acceptor (EDA) Interactions Controlling the Crystal Packing of Picric Acid and Its Adducts with Nitrogen Bases. Their Rationalization in Terms of the pKaEqualization and Electron-Pair Saturation Concepts

The structures of picric acid and 14 of its adducts with N -bases were determined by X-ray diffraction. All intermolecular contacts shorter than the sum of the van der Waals radii were retrieved, classified as 81 conventional X−H···:Y (X,Y = N,O) and 108 weaker C−H···:O H-bonds and as 49 C/N←:O π*←n or π* ( k ) ←n and four C←:C π*←π electron donor−acceptor (EDA) interactions, and carefully scrutinized to single out the general rules (if any) the 242 contacts are conforming to. X···Y distances and related E HB energies of the 81 X−H···:Y bonds are found to correlate with Δp K a = p K a (X−H)−p K a (Y−H + ), validating the p K a equalization principle for which strong H-bonds occur only when Δp K a tends to zero. Moreover, by redefining all X/C−H···:Y bonds as X/C−H←:Y σ* ← n EDA interactions, all contacts become EDA interactions, leading to formulate the electron-pair saturation rule for which “all electron donors of a closed-shell molecule (nonbonding pairs of lone pairs or π-bonding pairs of multiple bonds) become engaged in EDA interactions with the electron acceptors (X−H, C−H, π* ( k ), or π* ) present, as far as they are available; when the acceptors are insufficient, they are saturated in order of decreasing EDA interaction strength”. It is shown that this novel rule provides a particularly easy way to look at crystal packing.