The geometries of 1:1 and 1:2 host-guest complex, C 18 @OPP and 2C 18 @OPP, are observed to be C 2 and D 2 point group symmetric, respectively (Table and Figure ). The orientation of guest C 18 in the complexes almost coincides with the plane of the loop which it resides, and the calculated included angle is only slightly greater than 2.0º (Table ). The angle between two loops of the host OPP in two complexes still remains about 70.0º as that in its free state, making the dihedral angle between two guests C 18 in 2C 18 @OPP as high as about 65.0º. It is also found that the OPP is separated from the encapsulated C 18 by a distance of 3.8 Å in C 18 @OPP and 2C 18 @OPP, which just between the corresponding values of C 70 @[10]CPP (3.4-3.8 Å) and C 70 @[11]CPP (3.8-4.2 Å) and is close to the van der Waals (vdW) distance between graphite sheets (3.4 Å). Therefore, there should be a considerable degree of vdW attraction between C 18 and OPP in the complexes.