Kinetic investigations of the process of encapsulation of small hydrocarbons into a cavitand-porphyrin.

Exchange of guest molecules into capsule shaped host molecules is the most fundamental process in host-guest chemistry. Several examples of quantitative measurements of guest exchange rates have been reported. However, there have been no reports on the activation energies of these processes. A molecule known as cavitand-porphyrin (H 2 CP) has been reported to have a flexible host structure capable of facilitating moderate guest exchange rates suitable for kinetic measurements of the guest exchange process with 1 H NMR. In this article, various kinetic and thermodynamic parameters related to the process of encapsulation of small hydrocarbons into H 2 CP in CDCl 3 solution were determined by 2D exchange spectroscopy (EXSY): association and dissociation rate constants (k ass = 320 M -1 s -1 , k diss = 1.4 s -1 for methane at 25 °C), the corresponding activation energies (E a,ass = 27 kJ·mol -1 , E a,diss = 58 kJ·mol -1 ), and thermodynamic parameters for each process (ΔG ≠ ass = 59 kJ·mol -1 , ΔG ≠ diss = 72 kJ.mol -1 , ΔH ≠ ass = 25 kJ·mol -1 , ΔH ≠ dιss = 55 kJ·mol -1 , ΔS ≠ ass = -113 J·K -1 ·mol -1 , and ΔH ≠ diss = 58 J·K -1 ·mol -1 for methane). The thermodynamic parameters (AG° = -13 kJ·mol -1 , ΔH° = -31 kJ·mol -1 , ΔS° = -60 J·K -1 ·mol -1 for methane) for this encapsulation equilibrium determined by EXSY were comparable to those for methane determined by ID 1 H NMR titration (AG° = -11 kJ·mol -1 , ΔH° = -33 kJ·mol -1 , ΔS° = -75 J·K -1 ·mol -1 for methane). In addition, the structure of the methane encapsulation process was revealed by ab initio MO calculations. The activation energies for methane association/dissociation were estimated from MP2 calculations (E a,ass = 58.3 kJ·mol -1 , E a,diss = 89.1 kJ·mol -1 , and ΔH° = -30.8 kJ·mol -1 ). These values are in accord with the experimentally determined values. The observed guest exchange rates and energies are compared with the corresponding values of various reported capsule-shaped hosts.