GEOMETRY OF THE INTERMOLECULAR X-H...Y (X, Y=N, O) HYDROGEN-BOND AND THE CALIBRATION OF EMPIRICAL HYDROGEN-BOND POTENTIALS

The geometrical properties of intermolecular X-H...Y (X, Y = N, O) hydrogen bonds in organic crystals have been statistically analyzed using crystal data from the Cambridge Structural Database. To avoid complex interactions with other substituents, compounds have been selected in which only one hydrogen-bonding group (two for some acids or N-H...N systems) appears on a hydrocarbon framework; 91 crystal structures of monocarboxylic acids, 82 crystal structures of bicarboxylic acids, 79 crystal structures of amides, 43 crystal structures of alcohols, and 44 crystal structures of compounds with N-H...N hydrogen bonds (mainly N-heterocycles and purines) have been considered. No significant breakdown of the close packing principle appears in these H-bonded crystals, where all the O-H groups in carboxylic acids and all the N-H groups in amides (with only one exception) are engaged in single H-bonds; many alcohols do not form H-bonds in the solid, or form weaker or bifurcated bonds. Cases with more than one molecule per asymmetric unit are much more frequent for hydrogen-bonded substances than for general organic crystals. The ranges of all bond distances and angles within the H-bonded groups have been analyzed. Simple empirical ''6-exp'' potentials for the hydrogen bonds are proposed: they have been parametrized using the above geometrical results and 54 experimental heats of sublimation, which ate successfully reproduced by the calculated lattice energies within less than 10%. Thus, the force field can be considered to be satisfactory for the relatively weak H-bonds formed in the class of compounds investigated. Calculated lattice frequencies and equilibrium crystal structures are also satisfactory. Careful optimization can incorporate a substantial part of the electrostatic interactions, even in the absence of explicit electrostatic contributions in the functional form. A correction using charge parameters from empirical molecular orbital calculations improves the directional properties of the potentials.