Hydrogen-Bond-Assisted Symmetry Breaking in a Network of Chiral Metal-Organic Assemblies.

Herein we elucidate the interplay of chiral, chelate, solvent, and hydrogen-bonding information in the self-assembly of a series of new three-dimensional metal-organic architectures. Enantiopure ligands, each containing H-bond donors and acceptors, form different structures, depending on the ratio in which they are combined: enantiopure components form M4L4 assemblies, whereas racemic mixtures form M3L3 stacks. Chiral amplification within M3L3 enantiomers was observed when a 2:1 ratio of R and S subcomponent enantiomers was employed. Simply switching the solvent (from MeCN to MeOH) or chelating unit (from bidentate to tridentate) increased the diversity of structures that can be generated from these building blocks, leading to the selective formation of novel M2L2 and M3L2 assemblies. The addition of achiral ligand building blocks resulted in the formation of further structures: When an achiral subcomponent was combined with its R and S chiral congeners, a three-layer heteroleptic architecture was generated, with the achiral unit sitting at the top of the stack. When combined with the S enantiomer only, however, the achiral unit assembled in the center of the structure, thus demonstrating the selective placement of achiral units within chiral systems. Further sorting experiments revealed that combining R and S stereocenters within a single ligand led to diastereoselective product generation. These results show how geometric complementarity between different ligands impacts upon the degree of hydrogen-bonding within the assembly, stabilizing specific low-symmetry architectures from among many possible structural outcomes

Solvatochromic probes for detecting hydrogen-bond-donating solvents

Hydrogen bonding heavily influences conformations, rate of reactions, and chemical equilibria. The development of a method to monitor hydrogen bonding interactions independent of polarity is challenging as both are linked. We have developed two solvatochromic dyes that detect hydrogen-bond-donating solvents. The unique solvatochromism of the triazine architecture has allowed the development of probes that monitor hydrogen-bond-donating species including water

A comparative molecular dynamics study of sulfuric and methanesulfonic acids

The molecular dynamics computer simulation method has been used to study sulfuric and methanesulfonic acids. Calculations have been carried out between 200 K and 400 K using reliable force fields. Thermodynamic properties, such as the density, the heat of vaporization and the melting temperature, have been computed. Moreover, structural and dynamical quantities, such as the radial distribution functions, the shear viscosity and the diffusion coefficients, have also been calculated. The results display a noticeable good agreement with the available experimental data. A hydrogen bond analysis has also been performed, which shows, on one hand, that sulfuric acid has a hydrogen bond network which resembles the one of water; and, on the other hand, that methanesulfonic acid has a hydrogen bond structure which, in some details, recalls the one of methanol, but with a more important presence of single bonds and, to a lesser extent, of branching. Finally, the dynamics of the formation and rupture of hydrogen bonds has also been analyzed. To this end, the interrupted or slow hydrogen bonding lifetimes have been calculated using two different procedures. Our findings suggest that the sulfuric acid hydrogen bond network is more labile than the methanesulfonic acid one.Postprint (author's final draft

The Hydrogen Bond of QCD

Using the Born-Oppenheimer approximation, we show that exotic resonances, X and Z, may emerge as QCD molecular objects made of colored two-quark lumps, states with heavy-light diquarks spatially separated from antidiquarks. With the same method we confirm that doubly heavy tetraquarks are stable against strong decays. Tetraquarks described here provide a new picture of exotic hadrons, as formed by the QCD analog of the hydrogen bond of molecular physics.Comment: 5 pages, 3 figures, comments and references added. Table 1 extende