The geometric structures of the precursors C 18 -(CO) n (n = 2, 4, and 6) without imaginary frequency are shown in Fig. . For corresponding Cartesian coordinates, please refer to the supplementary material of Ref. . All precursors are observed to be strictly planar, and molecules with n = 2, 4, and 6 belong to C 2v , C 2v , and D 3h point groups, respectively.