Theoretical Study On Some Typical Chemical Processes Taking Place Inside The Molecular Container

Molecules or molecular assemblies with special cavities can be defined as 'molecular container'.The inner space of such micro-containers provides peculiar environment which is different from common solvents or bulk surroundings.Thus they have shown bright application future in many important areas such as accelerating reactions, stabilizing active intermediates and selectively separating ions.More and more attentions have been paid to the synthesis of molecular container compounds and their complexation properties in recent years both experimentally and theoretically.In the present thesis,the study of some typical chemical processes taking place inside two kinds of molecular containers has been carried out using quantum chemistry methods.Our investigation focuses on the detailed proceeding mechanism in the encapsulated state for each target system.The main contents can be summarized into the following sections:1.The rearrangement reaction of phenylcarbene proceeding inside the molecular container:Cram's hemicarcerand has been investigated with AM1 semi-empirical optimization and B3LYP/6-31G^* single point energy correction methods.The results show that the encapsulation effect from the molecular container has not changed the rearrangement mechanism substantially compared with the free-state process.However, the energy barrier for the rate-determining step has been lowered by 5.2 kcal/mol when the rearrangement reaction is confined in the molecular container,which is about 20%of that in the free state,indicating that the participation of the molecular container can facilitate the rearrangement reaction.The experimentally undetected intermediate bicyclo[4.1.0] -heptatriene seems still hard to be observed under the protection of the molecular container as the rearrangement reaction mechanism remains the same to the free state,which also means that the stabilization effect from the molecular container should mainly come from its shielding effect.2.The investigation of the rearrangement reaction mechanism of p-OCH₃ substituted phenylcarbene inside Cram's hemicarcerand was carried out following the same procedure as the phenylcarbene system.It is found that the introduction of the p-OCH₃ group could not significantly influence the rearrangement mechanism,and the rearrangement mechanism of p-OCH₃ substituted phenylcarbene is basically similar to the phenylcarbene system.Compared with the free state,the energy barrier for the rate-determining step is also lowered in the encapsulated state,with a value of 2.5 kcal/mol,which accounts for 8.5%of the cooresponding free-state barrier,suggesting that the molecular container can play certain role in facilitating the rearrangement reaction,while it increases a little to the second step.3.The potential energy surface for the CN₂H rotation of 1-bicyclo[2.2.1]heptyldiazirine encapsulated by Cram's hemicarcerand has been explored by ONIOM(B3LYP/6-31G^*:AM1)and ONIOM(B971/6-31G^*:AM1)optimization methods,as well as B3LYP/6-31G^**and B971/6-31G^**single point energy correction methods.The results obtained show that the conversion process with the largest rotation barrier has changed in the encapsulated state,and the potential rotation barrier is increased by more than 2 kcal/mol,which exceeds the highest rotation barrier in the free state.The conformations of the relatively stable isomers are not in consistency under the two different states:a conformation that is stable in the free state might become no longer stable or a free-state unstable conformation might become stable when it is confined inside the molecular container.This demonstrates that the molecular container can present some conformational preference towards different guest isomers to achieve optimum matching between the host and guest molecules.4.The tautomerization reaction of formamide to formamidic acid encapsulated in a Rebek's molecular container has been investigated systematically by ONIOM(B3LYP/6-31G^*:AM1)optimization and B3LYP/6-31G^**single point energy correction methods.The findings display that the molecular container can only encapsulate a formamide molecule or a formamide molecule plus one H₂O molecule,which means that merely bare and single-H₂O catalyzing tautomerization mechanisms are possible inside the container.The self-catalyzing and mutiple-H₂O catalyzing mechanisms cannot take place due to the confinement effect of the molecular container.The tautomerization energy barriers are both increased either for the bare or single-H₂O catalyzing mechanism in the encapsulated state.The bare tautomerization barrier in the encapsulated state increases by 5.7 kcal/mol,accounting for 12%of the corresponding total energy barrier in the free state,and the increased values for the single-H₂O catalyzing process are 3.0 kcal/mol and 16%,respectively, suggesting that the encapsulation could make the tautomerization process a little difficult.Thus it is concluded that the molecular container could mildly modulate the inner-phase tautomerization reaction through steric and electronic effects.However these cannot change the tautomerization mechanism substantially,and just make the process slightly hard to occur because of the weakness in such effects.