No abstract

Contrary to the simple expectations from Coulomb's law, Weinhold proposed that anions can stabilize each other as metastable dimers, yet experimental evidence for these species and their mutual stabilization is missing. We show that two bisulfate anions can form such dimers, which stabilize each other with self-complementary hydrogen bonds, by encapsulation inside a pair of cyanostar macrocycles. The resulting 2:2 complex of the bisulfate homodimer persists across all states of matter, including in solution. The bisulfate dimer's OH⋅⋅⋅O hydrogen bonding is seen in a H NMR peak at 13.75 ppm, which is consistent with borderline-strong hydrogen bonds.

Monovalent ions play significant roles in various biological and material systems. Recently, four new water models (OPC3, OPC, TIP3P-FB, and TIP4P-FB), with significantly improved descriptions of condensed phase water, have been developed. The pairwise interaction between the metal ion and water necessitates the development of ion parameters specifically for these water models. Herein, we parameterized the 12-6 and the 12-6-4 nonbonded models for 12 monovalent ions with the respective four new water models. These monovalent ions contain eight cations including alkali metal ions (Li + , Na + , K + , Rb + , Cs + ), transition-metal ions (Cu + and Ag + ), and Tl + from the boron family, along with four halide anions (F − , Cl − , Br − , I − ). Our parameters were designed to reproduce the target hydration free energies (the 12-6 hydration free energy (HFE) set), the ion-oxygen distances (the 12-6 ion-oxygen distance (IOD) set), or both of them (the 12-6-4 set). The 12-6-4 parameter set provides highly accurate structural features overcoming the limitations of the routinely used 12-6 nonbonded model for ions. Specifically, we note that the 12-6-4 parameter set is able to reproduce experimental hydration free energies within 1 kcal/mol and experimental ion-oxygen distances within 0.01 Å simultaneously. We further reproduced the experimentally determined activity derivatives for salt solutions, validating the ion parameters for simulations of ion pairs. The improved performance of the present water models over our previous parameter sets for the TIP3P, TIP4P, and SPC/E water models (Li, P. et al J. Chem. Theory Comput. 2015 11 1645 1657) highlights the importance of the choice of water model in conjunction with the metal ion parameter set.

Contrary to the simple expectations from Coulombs law,Weinhold proposed that anions can stabilize each other as metastable dimers,y et experimental evidence for these species and their mutual stabilization is missing. We showt hat two bisulfate anions can form such dimers,w hichs tabilizee ach other with self-complementary hydrogen bonds,b ye ncapsulation inside apair of cyanostar macrocycles.The resulting 2:2 complex of the bisulfate homodimer persists across all states of matter,i ncluding in solution. The bisulfate dimersO H···O hydrogen bonding is seen in a 1 HNMR peak at 13.75 ppm, which is consistent with borderline-strong hydrogen bonds.The fundamental role of Coulombsl aw [1] is so ingrained in all areas of chemistry,b iology,a nd physics that it is the starting point for formulating hypotheses on the behavior of matter.Itgoverns the bonding in ionic solids,the salt-bridges that guide protein structure and function, and the movement of ions in electric fields.W er ely daily upon the notion that like charges repel (Figure 1a)a nd opposite charges attract ( Figure 1b). Fort his reason, it came as some surprise when Weinholdsr ecent theoretical work [2] and its commentary [3] suggested that anions can attract, instead of repel, each other to form dimers. [4] Propositions that anions can stabilize each other are rare and the only unequivocal evidence for this phenomenon is in the solid state. [5] We found over 80 examples of HSO 4 À dimers (see the Supporting Information, Table S1), [6] as well as chains of HSO 4 À , [7] water-bridged sulfates, [8] and oligomers [9] and cyclic forms of hydrogen-bonded phosphates. [10] However, many forces other than anti-electrostatic hydrogen bonding, [2] for example,ionic bonding,can stabilize such anion dimers in crystals.Insolution, where solvation overtakes ionic bonding, dimer salts do not appear to survive.S upramolecular encapsulation offers aw ay to stabilize unstable species in solution. [11] Theonly reports of complexes of anion dimers are with H 2 PO 4 À and are based on the observation of a2:1 ratio of phosphate and receptor.N evertheless,t here are no direct observations of anion dimers in solution. With good reason, the authors of those works,R einhoudt, [5a, 12] Kubo, [13] Fabbrizzi, [5b, 14] Sessler, [15] and Tomisič, [16] took care to circumscribe these claims with caveats such as "tentativelySupportinginformation (experimentald etails, determination of binding constants, X-ray data collection and DFT calculation details) and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.

Shape-persistent macrocycles are attractive functional targets for synthesis, molecular recognition, and hierarchical self-assembly. Such macrocycles are non-collapsible and geometrically well-defined, and they are traditionally characterized by having repeat units and low conformational flexibility. Here, we find it necessary to refine these ideas in the face of a highly flexible yet shape-persistent macrocycles. A molecule is shape-persistent if it has a small change in shape when perturbed by external stimuli (e.g., heat, light, and redox chemistry). In support of this idea we provide the first examination of the relationships between a macrocycle’s shape persistence, its conformational space, and the resulting functions. We do this with a star-shaped macrocycle called cyanostar that is flexible as well as being shape-persistent. We employed molecular dynamics (MD), density functional theory (DFT), and NMR experiments. Considering a thermal bath as a stimulus we found a single macrocycle has 332 accessible conformers with olefins undergoing rapid interconversion by up-down and in-out motions on short time scales (0.2 ns). These many interconverting conformations classify single cyanostars as flexible. To determine and confirm that cyanostars are shape-persistent, we show that it has a high 87% shape similarity across these conformations. To further test the idea, we use the binding of diglyme to the single macrocycle as guest-induced stimulation. This guest has almost no effect on the conformational space. However, formation of a 2:1 sandwich complex involving two macrocycles enhances rigidity and dramatically shifts the conformer distribution towards perfect bowls. Overall, the present study expands the scope of shape-persistent macrocycles to include flexible macrocycles if and only if their conformers have similar shapes.