Orthogonal H-bonding synthons, actual and virtual structures in molecular crystals: a case study

Semicarbazides of p-substituted benzoic acids are a class of simple molecules endowed with H-bonding donor and acceptor groups capable of forming H-bonded rows along three linearly independent directions (orthogonal H-bonding synthons). When the acceptor group at the para position is strong (-NO2, -CN, -N=) the same crystal packing is observed, in which three linearly independent chains are formed, and the lattice parameters are easily predictable. In the case of halogen atoms at the para position (-F, -Cl) two different packings are observed in which only one or two H-bonded chains are present. Through ab initio periodic DFT-LCAO computations, we have calculated the lattice energy and density of each semicarbazide in each of the three packings, constructing the matrices U-ij and rho(ij) of the lattice energy and density. It is found that all of the different modes of packing observed experimentally for single members of the class correspond to minima of the lattice energy and to acceptable lattice densities for every member of the class.