Abstract
Intramolecular hydrogen bond (HB) formation was analyzed in the model compounds N-(2-benzoylphenyl)acetamide, N-(2-benzoylphenyl)oxalamate and N1,N2-bis(2-benzoylphenyl)oxalamide. The formation of three-center hydrogen bonds in oxalyl derivatives was demonstrated in the solid state by the X-ray diffraction analysis of the geometric parameters associated with the molecular structures. The solvent effect on the chemical shift of H6 [δH6(DMSO-d6)–δH6(CDCl3)] and Δδ(ΝΗ)/ΔT measurements, in DMSO-d6 as solvent, have been used to establish the energetics associated with intramolecular hydrogen bonding. Two center intramolecular HB is not allowed in N-(2-benzoylphenyl)acetamide either in the solid state or in DMSO-d6 solution because of the unfavorable steric effects of the o-benzoyl group. The estimated ΔHº and ΔSº values for the hydrogen bonding disruption by DMSO-d6 of 28.3(0.1) kJ·mol−1 and 69.1(0.4) J·mol−1·K−1 for oxalamide, are in agreement with intramolecular three-center hydrogen bonding in solution. In the solid, the benzoyl group contributes to develop 1-D and 2-D crystal networks, through C–H∙∙∙A (A = O, π) and dipolar C=O∙∙∙A (A = CO, π) interactions, in oxalyl derivatives. To the best of our knowledge, this is the first example where three-center hydrogen bond is claimed to overcome steric constraints.
Highlights
Non-covalent interactions have been considered as the driving forces involved in molecular recognition processes
The formation of two- and three-center hydrogen bonds in the solid state was demonstrated by the analysis of the geometric parameters associated with the molecular structure by X-ray diffraction
hydrogen bond (HB) is not allowed in N-(2-benzoylphenyl)acetamide neither in the solid state nor in DMSO-d6 solution, and this effect is attributed to the unfavorable steric effects of o-benzoyl group
Summary
Non-covalent interactions have been considered as the driving forces involved in molecular recognition processes. One acceptor and two donor groups (H···A···H) or one donor and two acceptor groups (A···H···A) can form two structurally different THBs (Figure 1) [9,10], known as three-center acceptor (bifurcated acceptor) and three-center donor (bifurcated donor) hydrogen bonds, respectively They can be composed of intermolecular, intramolecular or both components. A complete description of the effect of THB formation on the properties of several materials has been compiled elsewhere [23] In this contribution, N-(2-benzoylphenyl)acetamide (1) has been chosen as a model compound for a two centered or regular hydrogen bond (HB), ethyl N-(2-benzoylphenyl)oxalamate (2) for a. Three-centered hydrogen bond (THB) and N1,N2-bis(2-benzoylphenyl)oxalamide (3) as a model compound for cooperativity effects associated with THB formation (Figure 2).
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