Planar hydrogen-bridged rings form parallel interactions in crystal structures. The interactions can be as strong as −4.89 kcal mol ⁻¹ .

Geometric analysis of data from Cambridge Structural Database (CSD) reveals that contacts between planar hydrogen-bridged rings and C₆-aromatic rings are mostly parallel stacked geometries. High-level quantum chemical calculations show that their interaction energies are comparable with interactions between two hydrogen-bridged rings. Namely, the interaction energy, at the CCSD(T)/CBS level, of the most stable geometry is −4.38 kcal mol⁻¹, which is comparable with that of interaction between two hydrogen-bridged rings (−4.89 kcal mol⁻¹) and significantly stronger than that of stacking between two benzene rings (−2.73 kcal mol⁻¹).

Saturated acyclic four-atom groups closed with a classic intramolecular hydrogen bond, generating planar five-membered rings (hydrogen-bridged quasi-rings), in which at least one of the ring atoms is bonded to other non-ring atoms that are not in the ring plane and, thus, capable to form intermolecular interactions, were studied in this work, in order to find the preferred mutual positions of these species in crystals and evaluate strength of intermolecular interactions. We studied parallel interactions of these rings by analysing crystal structures in the Cambridge Structural Database (CSD) and by quantum chemical calculations. The rings can have one hydrogen atom out of the ring plane that can form hydrogen bonds between two parallel rings. Hence, in these systems with parallel rings, two types of hydrogen bonds can be present, one in the ring, and the other one between two parallel rings. The CSD search showed that 27% of the rings in the crystal structures form parallel interactions. The calculations at very accurate CCSD(T)/CBS level revealed strong interactions, in model systems of thiosemicarbazide, semicarbazide and glycolamide dimers the energies are −9.68, −7.12 and −4.25 kcal/mol. The hydrogen bonds between rings, as well as dispersion interactions contribute to the strong interaction energies.

A SAPT (Symmetry Adapted Perturbation Theory) decomposition analysis was performed for various types of noncovalent interactions (stacking, hydrogen bonds, antiparallel) for various distances of interacting fragments. Although total interaction energy does not significantly change within the small range around the optimal distance, the energy contributions can significantly change in some systems. This is particularly pronounced for stacking interactions, while for hydrogen bonds and antiparallel interaction the energy contributions do not significantly change around the optimal distance. Thus, it is very important to precisely determine the optimal distance between the interacting fragments when performing SAPT analysis of noncovalent interactions, especially for stacking interactions, since the general conclusions about the nature of interactions can differ in the small range of distances between the interacting fragments.