Vibrational properties of C-20 isomers, a semi-empirical study

Self-consisent field Hartree-Fock calculations using MNDO, AM1 and PM3 hamiltonians were carried out for the vibrational spectra of the geometrical isomers of the smallest fullerene cluster, C-20 Comparisons with a recent density functional approach brought out similarities and differences among the methods and were in accord with earlier first principles stability ordering of the three ground state isomers: the planar cyclic form (the ring), the corannulene form (the bowl), and the closed cage fullerene form. Complete vibrational symmetry assignments are given for the fullerene and the bowl forms and an approximate grouping is presented for the ring isomer. The distribution of all vibrational levels is compared among the three forms and the density functional results for the PM3 method, and schematic infrared spectra are calculated for the three C-20 forms.