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 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...Y distances and related E(HB) energies of the 81 X-H...:Y bonds are found to correlate with Delta(pKa) = pKa(X-H)-pKa(Y-H+), validating the pKa equalization principle for which strong H-bonds occur only when Delta(pKa) tends to zero. Moreover, by redefining all X/C-H...:Y bonds as X/C-H<--:Y 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.
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
BERTOLASI, Valerio;GILLI, Paola;GILLI, Gastone
2011
Abstract
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 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...Y distances and related E(HB) energies of the 81 X-H...:Y bonds are found to correlate with Delta(pKa) = pKa(X-H)-pKa(Y-H+), validating the pKa equalization principle for which strong H-bonds occur only when Delta(pKa) tends to zero. Moreover, by redefining all X/C-H...:Y bonds as X/C-H<--:Y 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.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
