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 pKa Equalization 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 center dot center dot center dot :Y (X,Y = N,O) and 108 weaker C-H center dot center dot center dot :O H-bonds and as 49 C/N <-:O pi*(<-)n or pi*(k)(<-)n and four C <-:C pi*<-pi electron donor acceptor (EDA) interactions, and carefully scrutinized to single out the general rules (if any) the 242 contacts are conforming to. X center dot center dot center dot Y distances and related E-HB energies of the 81 X-H center dot center dot center dot :Y bonds are found to correlate with Delta pK(a) = pK(a)(X-H)-pK(a)(Y-H+), validating the Delta pK(a) equalization principle for which strong H-bonds occur only when Delta pK(a) tends to zero. Moreover, by redefining all X/C-H center dot center dot center dot:Y bonds as X/C=H <-(:)sigma*<- 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 pi-bonding pairs of multiple bonds) become engaged in EDA interactions with the electron acceptors (X-H, C-H, pi*(k), or pi*) 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.