The kinetics of the Diels−Alder reactions of di(2-pyridyl)-1,2,4,5-tetrazine (1) with substituted styrenes 2 was investigated in aqueous media and in organic solvents. The second-order rate constants of this reaction increase dramatically in water-rich media. A decrease in pH accelerates the aqueous Diels−Alder reaction even more. The Hammett ρ-values and also the electronic demand η-values of the reaction are solvent sensitive. In protic solvents the dipolar character of the activated complex is increased, but simultaneously hydrogen-bond interactions stabilize the activated complex. These effects are most pronounced in 2,2,2-trifluoroethanol, which shows that the aqueous accelerations cannot be solely attributed to solvent-induced changes of the reaction mechanism.

We have studied the microscopic solvent structure of dimethyl sulfoxide−water mixtures and its influence on the solvation structure of solute from a clustering point of view, by means of a specially designed mass spectrometric system. It was observed that the propensity to the cluster formation is nonlinearly dependent on the solvent composition, exhibiting the existence of a critical value of mixing ratios where drastic changes in the microscopic solvent structure occur. It was also demonstrated that in such a solvent mixture the solvation structure of solutes such as 2-butanol, cyclopentanol, cyclohexanol, and phenol is greatly related to the microscopic solvent structures, implying that solute species interact with already established solvent clusters, rather than with individual solvent molecules.

The microscopic structures of acetonitrile-water and DMSO-water binary mixed solvents and their influence on the solvation for solutes (some alcohols and phenol) have been studied on the basis of the cluster structures observed through a specially designed mass spectrometer. In acetonitrile-water mixtures, the water clusters were observed at water mole fractions: X w > 0.2; on the other hand, in DMSO-water mixtures, the water clusters were observed only at much higher water mole fractions: X w > 0.93. In the mixing processes, the water clusters were stabilized in the acetonitrile-water mixtures, whereas the DMSO clusters were stabilized in the DMSO-water mixtures. It is demonstrated that these microscopic structures directly affect the solvation for alcohols and phenol in these binary mixed solvents.