Inversion symmetry and local vs. dispersive interactions in the nucleation of hydrogen bonded cyclic n-mer and tape of imidazolecarboxamidines

Substitutional changes to imidazolecarboxamidine that preserved intermolecular hydrogen bonding in the solid state were used to study the relationship between packing and the hydrogen bond motif. Various motifs competed, but the most common imidazole-carboxamidine crystalline phase was a C_i symmetric dimer that established inversion centers by associating enantiomeric tautomers. Counter to intuition, the calculated gas-phase energies per molecule of the solid state atomic coordinates of the C_i dimer motifs were higher than those of the C₁ dimer, trimer, tetramer and tape motifs, while the packing densities of C_i dimers were found to be higher. This result was interpreted as an enhanced ability of the C _i dimers to pack. If other motifs competed, the hydrogen bonds and conformations should be lower in energy than the C_i dimer. The results detail the effect of packing on the conformation in these molecules. The results are interpreted as a rough measure of the energetic compromise between packing and the energies related to the coordinates involving one dihedral angle and hydrogen bonding. The results establish a connection between solution and solid phase conformation.