Volume 208, number 5,6 CHEMICAL PHYSICS LETTERS 18 June 1993 The electronic structure and vibrational frequencies of the stable zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA CT6 isomer of D2 symmetry Giorgio Orlandi, Francesco Zerbetto zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF Dipartimento di Chimica “ G. Ciamician” , Universitbdi Bologna. ViaF. Selmi 2, 40126 Bologna, Italy Patrick W. Fowler Department of Chemistry, University ojExeter, Exeter EX4 4QD, UK and David E. Manolopoulos Department of Chemistry University of Nottingham, University Park, Nottingham NG7 2RD, UK Received 5 March 1993; in final form 3 May 1993 We report QCFF/PI quantum chemical calculations of the molecular structure, the vibrational frequencies, and the energy ordering of the states at the onset of the electronic spectrum of the Clb isomer of Dz symmetry. Comparison is made with the limited amount of experimental data available. 1. Introduction After the initial preparation of macroscopic quan- tities of fullerenes [ 11, some progress has been achieved in the isolation of other allotropic forms of carbon. Recently, Ettl et al. [ 21 reported the partial spectroscopic characterization of an isomer of CT6 of D2 symmetry. This molecule is of great interest be- cause of its chirality which sets it apart from the other more abundant fullerenes, namely CsO and GO. Al- though, to date, the two enantiomers have not been separated, partial data on the visible and the infrared spectra of CT6 have been published. To the best of our knowledge, there has been no previous theoret- ical study of these data. It is the purpose of this note to provide some anal- ysis of the spectra as a contribution to the increasing amount of theoretical and experimental spectro- scopic information that is being gathered for the mo- lecular allotropes of carbon. To do this, we use the quantum chemical force field for K electrons (QCFF/PI) [ 31. This model em- ploys an empirical potential for the a-electron framework and uses a quantum chemical scheme for the x; electrons. The o core is simulated by an ex- pansion of the potential energy functions in terms of harmonic and Morse oscillators. Three- and four- body interactions describe bending deformations to- gether with pyramidalization and torsional contri- butions. In passing, we note that it is the simulation of the o core by classical potentials that, on the one hand, allows good accuracy in the calculation of the vibrational frequencies and, on the other hand, does not allow the calculation of reliable infrared inten- sities. The x-electron system is simulated by a self consistent field procedure. Both the one-electron (t or hopping) integrals and the two-electron (U or Coulomb) integrals are functions of the interatomic distances. The electron-electron repulsion is calcu- lated for any pair of electrons, i.e. atoms, while the one-electron integrals are limited to one center and nearest-neighbour interactions. With this quantum chemical procedure, we successfully predicted the vi- brational frequencies of C& several years before it 0009-2614/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved. 441