Kinetic Investigations of the Process of Encapsulation of Small Hydrocarbons into a Cavitand—Porphyrin

摘要

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_2CP) 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 ~1H NMR. In this article, various kinetic and thermodynamic parameters related to the process of encapsulation of small hydrocarbons into H_2CP in CDCl_3 solution were determined by 2D exchange spectroscopy (EXSY): association and dissociation rate constants (k_(a,ass)=320 M~(-1) s~(-1), k_(diss)= 1.4 s~(-1) for methane at 25 ℃), the corresponding activation energies (E_(a,ass)= 27 kJ·mol~(-1), E_(a,dss)= 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_(diss)= 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 (ΔG° = -13 kJ·mol~(-1), ΔH° = -31 kJ·mol~(-1), ΔS° = -60 J·K~(-1)·mor~(-1) for methane) for this encapsulation equilibrium determined by EXSY were comparable to those for methane determined by 1D ~1H NMR titration (ΔG° = -11 kJ·mol~(-1), ΔH° = -33 kJ·mor~(-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.