Relative hydrogen bonding affinities of imides and lactams

Acidity and basicity interplay in amide and imide self-association

Simple acid–base properties explain the differences in amide and imide dimerisation, and represent an alternative to the secondary interactions hypothesis.

Amides dimerise more strongly than imides despite their lower acidity. Such an unexpected result has been rationalised in terms of the Jorgensen Secondary Interactions Hypothesis (JSIH) that involves the spectator (C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O_S) and H-bonded (C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O_HB) carbonyl groups in imides. Notwithstanding the considerable body of experimental and theoretical evidence supporting the JSIH, there are some computational studies which suggest that there might be other relevant intermolecular interactions than those considered in this model. We conjectured that the spectator carbonyl moieties could disrupt the resonance-assisted hydrogen bonds in imide dimers, but our results showed that this was not the case. Intrigued by this phenomenon, we studied the self-association of a set of amides and imides via¹H-NMR, ¹H-DOSY experiments, DFT calculations, QTAIM topological analyses of the electron density and IQA partitions of the electronic energy. These analyses revealed that there are indeed repulsions of the type O_S···O_HB in accordance with the JSIH but our data also indicate that the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O_S group has an overall attraction with the interacting molecule. Instead, we found correlations between self-association strength and simple Brønsted–Lowry acid/base properties, namely, N–H acidities and C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O basicities. The results in CDCl₃ and CCl₄ indicate that imides dimerise less strongly than structurally related amides because of the lower basicity of their carbonyl fragments, a frequently overlooked aspect in the study of H-bonding. Overall, the model proposed herein could provide important insights in diverse areas of supramolecular chemistry such as the study of multiple hydrogen-bonded adducts which involve amide or imide functional groups.

Enhancement of efficiency in organic photovoltaic devices containing self-complementary hydrogen-bonding domains

Self-complementary hydrogen-bonding domains were incorporated as the electron deficient unit in “push–pull”, p-type small molecules for organic photovoltaic active layers. Such compounds were found to enhance the fill factor, compared with similar non-self-organized compounds reported in the literature, leading to higher device efficiencies. Evidence is presented that the ability of these molecules to form one-dimensional hydrogen-bonded chains and subsequently exhibit hierarchical self-assembly into nanostructured domains can be correlated with improved device efficiency.

More chemistry in small spaces

This Account is about coaxing molecules into spaces barely big enough to contain them: encapsulation complexes. In capsules, synthetic modules assemble to fold around their molecular targets, isolate them from the medium for relatively long times, place them in a hydrophobic environment, and present them with functional groups. These arrangements also exist in the interior spaces of biology, and the consequences include the familiar features of enzymes: rapid reactions, stabilization of reactive intermediates, and catalysis. But inside capsules there are phenomena unknown to biology or historical chemistry, including new structures, new stereochemical relationships, and new reaction pathways. In encapsulation complexes, as in architecture, the space that is created by a structure determines what goes on inside. There are constant interactions between the container and contained molecules: encounters are not left to chance; they are prearranged, prolonged, and intense. Unlike architecture, these reversibly formed containers emerge only when a suitable guest is present. The components exist, but they cannot assemble without anything inside. Modifications of the capsule components give rise to the results of the present Account. The focus will be on how seemingly small changes in the encapsulation complexes, exchanging a C═S for a C═O, reducing an angle here and there, or replacing a hydrogen with a methyl, can lead to unexpectedly large differences in behavior.

Clefts as receptor and enzyme analogues

Synthetic receptors for small biological targets have become a popular research topic in molecular recognition. This paper discusses the optimal functional group complements and the scaffolds that are ideal for such purposes. Specifically, remote steric barriers are used to control the conformation of (that is, to preorganize) hosts derived from acridine skeletons, triaryl benzenes and related systems. These structures separate entropic effects from enthalpic effects and show that entropy is an important contributor to high affinity. In a comparative study lactams are shown to be superior to imides in their capacity for self-association. Imides are shown to have higher affinity than lactams for adenine derivatives because of the presence of an unconventional hydrogen bond. Finally, preorganization in the context of chemical catalysis is demonstrated in two systems, one involving hemiacetal cleavage and a second involving a self-replicating system.

Spontaneously formed vesicles of sodium N-(11-acrylamidoundecanoyl)-glycinate and L-alaninate in water

Two N-acyl amino acid surfactants, sodium N-(11-acrylamidoundecanoyl)-glycinate (SAUG) and L-alaninate (SAUA), were synthesized and characterized in aqueous solution. A number of techniques, such as surface tension, fluorescence probe, light scattering, and transmission electron microscopy were employed for characterization of the amphiphiles in water. The surface and interfacial properties were measured. The amphiphiles have two critical aggregation concentrations. The results of surface tension and fluorescence probe studies suggested formation of bilayer self-assemblies in dilute aqueous solutions of the amphiphiles. The magnitudes of free energy change of aggregation have indicated that bilayer formation is more favorable in the case of SAUG. Steady-state fluorescence measurements of pyrene and 1,6-diphenyl-1,3,5-hexatriene (DPH) were used to study the microenvironment of the molecular self-assemblies. Temperature-dependent fluorescence anisotropy change of DPH probe revealed phase transition temperature of the bilayer self-assemblies. The effects of pH on the structure of the self-assemblies of SAUG and SAUA have been studied. The role of intermolecular hydrogen bonding between amide groups upon aggregation toward microstructure formation in solution has been discussed. Circular dichroism spectra suggested the presence of chiral aggregates in an aqueous solution of SAUA. The transmission electron micrographs revealed the presence of closed spherical vesicles in aqueous solutions of the amphiphiles. Dynamic light scattering measurements were performed to obtain average size of the aggregates.

Molecular recognition and the development of self-replicating systems

Weak intermolecular forces lie at the heart of biochemical recognition phenomenon and the last decade has seen much activity in the evaluation of these forces. A number of model systems have been developed including macrocyclic structures and molecular clefts. With these structures it has been possible to measure forces at the sub-kilocalorie level involving hydrogen bonding, aromatic stacking and van der Waals interactions. This manuscript deals with molecular clefts as synthetic receptors for nucleic acid components and their ultimate use in developing chemical reactions between components within a complex. This has led to an entirely synthetic, self-replicating system that shows the features of self-complementarity and autocatalysis. A general discussion of self-replicating systems and their implications for prebiotic chemistry is developed.