Equilibrum and Nonequilibrium Chemical Reactions of Helicene Oligomers in the Noncovalent Bond Formation

Abstract

Chemical reactions involving noncovalent bond formation are discussed with regard to the equilibrium and nonequilibrium states: An equilibrium-to-equilibirum chemical reaction involves change of equilibrium states by changing the environment; a nonequilibrium-to-equilibrium reaction involves change of metastable state to equilibrium. Complex nature of the chemical reactions, especially in the latter, is shown in terms of the multiple-path nature in the microscopic molecular structure changes and macroscopic concentration changes. Irreversible and reversible nonequilibrium-to-equilibrium chemical reactions are also compared in terms of the multiple-path. Helicene oligomers, which reversibly form double-helix and random-coil by temperature changes, are discussed with regard to the reversible nonequilibrium-to-equilibrium chemical reaction with self-catalysis, where notable chemical phenomena appear under nonequilibrium conditions.

Synthesis of 1,128‐Octacosahectanediol and Its Sharp Thermoresponse in Solution with Concomitant Structural Change

“…The sharp peak in dissolution similar to melting is also notable. The large Δ H /Δ S and sharp peak in the dissolution are consistent with the sharp thermoresponse, which is due to a large structural change. The strong intermolecular van der Waals interactions between the methylene groups stabilize the extended structure in the solid state; a large entropic gain Δ S makes the globular structure thermodynamically favorable in solution.…”

“…The association and dissociation reversibly occurred upon cooling and heating, during which small temperature changes of 20–30 K induced large changes in the relative concentrations of dimeric aggregates and dissociated random coils. Accordingly, for example, the thermodynamic parameters obtained for a sulfonamide helicene ( P )‐tetramer were considerably large: Δ H =+2.7 × 10 2 kJ mol −1 and Δ S =+7.5 × 10 2 J mol −1 K −1 . Thus, a similar phenomenon appeared in the molecular aggregation/disaggregation of helicene oligomers to the case with the dissolution of extremely long polymethylene compound 1 .…”

“…Thus, 3, 5, and 6 g L −1 solutions remained unchanged for several hours; each solution was a metastable supercooled solutions with high thermal stability. It appears that the precipitation occurs only during a temperature decrease, and that precipitation itself promotes precipitation, analogous to self‐catalysis . This is an interesting observation of metastable supercooled solutions beyond equilibrium.…”

“…During our recent studies on helicene oligomers, it was found that two‐atom linker oligomers such as acetylene, sulfonamide, aminomethylene, and oxymethylene derivatives formed dimeric aggregates in solution . The association and dissociation reversibly occurred upon cooling and heating, during which small temperature changes of 20–30 K induced large changes in the relative concentrations of dimeric aggregates and dissociated random coils.…”

“…During the solid–liquid transition, a large change occurs in the shape of the extremely long polymethylene molecule from an extended structure in the solid state to a globular structure in solution. Substantial change of free energy, Δ G =Δ H − T Δ S , from competition of the large enthalpic loss Δ H and large entropic gain Δ S results in a sharp thermoresponse . The discussions of the solid‐liquid phase transition can be applied to the dissolution phenomenon in solution,, and a notable thermal property, namely, the sharp increase in solubility for a small temperature increase, can appear, which has not been previously examined.…”