Very Strong Parallel Interactions Between Two Saturated Acyclic Groups Closed with Intramolecular Hydrogen Bonds Forming Hydrogen-Bridged Rings

Abstract

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.

Stacking interactions between hydrogen-bridged and aromatic rings: study of crystal structures and quantum chemical calculations

“…Please do not adjust margins Please do not adjust margins common for stacking interactions, 3,[11][12][13][14]29,30 since most of them are between 3.0 Å and 4.0 Å. Normal distance dependence on offset values is given in Fig.…”

“…Although stacking (parallel) interactions have been considered typical for aromatic ring and studied extensively on model systems with aromatic rings, evidence of parallel interactions of other planar molecules and fragments has been reported. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] It has been discovered that the monomer aromaticity can even hinder stacking interactions in some cases. By analysing stacking energies calculated for a wide range of π-stacking systems, it has been concluded that stacking interactions involving non-aromatic polyenes are as favourable as, if not more favourable, than interactions among equivalent aromatic species.…”

“…20 Rings containing metals or hydrogen bonds can also form stacking interactions. 3,4,6,[11][12][13][14]21 Thus, planar metal-chelates with delocalized π-bonds can stack with aromatic species and with other metal-chelate rings, exhibiting large interaction energies. 3,4,6,11 Quasi-rings, formed by resonance-assisted hydrogen bonding, 12,21 can form π-stacking interactions.…”

“…Moreover, hydrogen-bridged rings with only single bonds in the ring form stacking interactions much stronger than stacking of two benzene molecules. 13,14 Namely, saturated hydrogen-bridged rings in crystal phase are mostly antiparallel with normal distances typical for stacking (3.0-4.0 Å). High level ab initio calculations of interaction energies of dimers showed that energies can be much higher than stacking in benzene dimer.…”

“…23 Guanidinium cation is among the most frequent hydrogen-bridged species found in the Cambridge Structural Database (CSD). 14 In this work we studied stacking between aromatic and hydrogen-bridged rings, by inspecting their mutual contacts in CSD and by calculating the interaction energies at high quantum mechanical level.…”