Theoretical Studies On The Host-Guest Complexes: Cyclohexane And Its Ramification Encapsulated Within A Cylindrical Capsular Host

This project focuses on the theoretical study of the host-guest complexes: cyclohexane and its ramifications encapsulated within the cavity of the cylindrical capsular host. The guest molecule's conformational conversion mechanism inside the capsule and the good "fit" between the concave inner surface of the host capsule and the convex outer surface of the guest were investigated theoretically through quantum chemistry methods.1 . The conformational conversion mechanisms of cyclohexanes encapsulated within two molecular capsules were investigated by using AM1 semiempirical method and density functional methods. The influences of the inner phase of the capsules on the cyclohexane guest conformational conversion were discussed by analyzing the computed results. It was found that the two molecular capsules can accommodate cyclohexane guest inside their cavities but with distinct orientations. The encapsulated cyclohexanes will undergo the same conversion processes as those of free ones. The conversion frequency between the encapsulated chair and the twist-boat forms can be modulated by the host-guest interactions and hydrogen bonds at the middle of the capsules.2. The mechanism for the conformational conversion of 1, 3-dioxane guest encapsulated inside a capsular host was theoretically investigated using semiempirical PM3 method and DFT methods. The influences of the inner phase of the capsule on the conformational conversion of guest molecule were discussed via analyzing the comparative results. It was found that the capsular host could accommodate 1, 3-dioxane within its cavity by the weak attractive interactions between host and guest, and it responds to the conformational conversion of guest by the deformation of hydrogen-bonding seam at the middle of the capsule. When entrapped in the capsule, the guest molecule undergoes the conformational conversion from chair form to twist-boat form slower than that under the free condition. The deformation of the capsule is favorable to maximize the attractive interactions between host and guest.3. The weak interaction complexes of the cylindrical capsular host encapsulating H-substituted (methyl and hydroxyl) cyclohexane guest molecules within its cavity were investigated theoretically through two different DFT methods: B3LYP and PW91. This study addresses the question how the host and guest molecules match each other to form the stable weak interaction systems. Computational analyses revealed that PW91 functional provides better performance for the studied weak interaction systems than the popular B3LYP method. Although there are weak repulsive interaction energies between the host and guest molecules, the weak repulsions cannot disrupt the host-guest complex. An important factor is that numerous, stable hydrogen bonds (16 hydrogen bonds) at the center of the capsule largely decrease the total energy of the host-guest complex. Different from the methyl cyclohexane guest (at the middle of the molecular capsule), the hydroxyl cyclohexane guest may be nearer to the lower half of the molecular capsule in order to achieve hydrogen bonding with the cylindrical capsular host.4. The weak interaction complexes of the cylindrical capsular host coencapsulating two different guest molecules within its cavity were investigated theoretically through DFT (PW91) method. Different from the single guest (at the middle of the molecular capsule), two different coguests may be nearer to the upper and lower halves of the molecular capsule, respectively, in order to achieve hydrogen bonding with the cylindrical capsular host.