Study On The Endohedral NMR And [2+3] Reaction Mecahnisms Of Fullerenes

Fullerenes and their derivatives have been expected to have applications in material science,biological science and medical science.Theoretical studies on fullerenes should be important to the experimental investigation of fullerenes.Our aims are assigning the structure of fullerenes and their derivatives,and exploring the fullerene reaction mechanisms by theoretical studies.The endohedral ³He and ¹H NMR chemical shifts were studied theoretically.The stereoselectivity for the reaction of epoxides with C₆₀,and the mechanism of N₂ extrusion after the 1,3-diploar cycloaddition of azides with C₆₀ were also studied.The main works are the followings:The endohedral ³He NMR chemical shifts for ³He@C_n(n=82,84 and 86), which were difficult to be separated individually,were predicted by the GIAO-B3LYP/3-21G//AM1 method.In comparison of the predicated values with the experimental data,we assigned their structures successfully.The accurate prediction for endohedral ³He NMR chemical shifts of fullerenes and their derivatives encouraged us to attempt the possible linear relation between the experimental and theoretical endoheral ¹H NMR chemical shifts of fullerenes and their derivatives.Among the four methods of GIAO-HF/3-21G//AM1,GIAO-HF/3-21G//PM3,GIAO-B3LYP/3-21G//AM1 and GIAO-B3LYP/3-21G//PM3,the theoretical value at the level of GIAO-B3LYP/3-21G//PM3 had excellent linear relation with the experimental ¹H NMR chemical shifts of fullerenes and their derivatives,and its cost was much less in comparison with other higher-level NMR calculations.We studied the stereoselectivity for the reaction of C₆₀ with epoxides at the level of B3LYP/6-31G^*//AM1.After the discussions on the theoretical results,the possibility of non-linear Eyring-plots for the reaction temperature effect was concluded.Further experiments confirmed our conclusion.A new concerted mechanism for the N₂ extrusion from the cycloaddition product of azides to C₆₀ was proposed.With the same calculation at the level of B3LYP/6-31G^**//AM1,the new concerted mechanism was located.More sophisticated studies done at the levels of B3LYP/STO-3G,B3LYP/3-21G^* and ONIOM(B3LYP/6-31G^*:AM1) confirmed the preferenceof the concerted reaction mechanism,which could also explain the experimental phenomena.