Volume 202, number 3,4 CHEMICAL PHYSICS LETTERS 22 January 1993 Vibronic effects in the MCD spectrum of buckminsterfullerene Giancarlo Marconi zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Islituto FRAE-CNR, Via de’ Castagnoli I, 40126 Bologna, Italy and Pier Remigio Salvi Laboratorio di Spettroscopia Molecolare, Dipartimanto di Chimica, Universitk di Firenze, Via G. Capponi 9, 50121 Florence, Italy Received 27 August 1992;in final form 10 November 1992 The manifestations of vibronic coupling responsible for the magnetic circular dichroism (MCD) A terms in the region of the lowest excited state of Css have been examined through quantum-mechanical semi-empirical calculations. The vibronic sequences present both in the MCD and in the fluorescence spectrum are attributed to interactions of a low-lying T,, state with higher permitted T,. states through t,. and h, modes. The main vibrational modes responsible for the borrowing are examined and a rationale for the sign shown by the MCD bands is provided, 1. Introduction Buckminsterfullerene, C6a,has one of the highest molecular symmetries [ 11. In the icosahedral point group, I,,, only T,, states are one-photon optically permitted, whereas T,, G and H states possess mag- netic moments, Calculations on the electronic states of Ceopredict of presence of several forbidden states below the first absorption system (‘T,,x380 nm) [2]. In particular, it is found that the lowest excited states of T,,, Tzg and G, symmetry are quasi-degen- erate, with calculated energies only 0.05 eV from the lowest to the highest. Therefore, in order to attribute the symmetry of S,, it is important to describe prop- erly the vibronic interactions responsible for the ap- pearance of bands in optical and magnetic circular dichroism (MCD) spectra in the region of the low- lying singlet states. In such a case of high symmetry the MCD spectrum is of particular relevance due to the prompt recognition of degenerate excited states through the sigmoidal shape of the A terms. More- over the bisignate pattern provided by the MCD spectrum provides information complementary with respect to the absorption spectrum. The MCD spec- trum of C6a detected by Gasyna et al. [ 3 ] in Ar ma- trix at 5 K offers a fine pattern of vibronically in- duced magneto-optical constants in the region of the lowest singlet. In this Letter, we present the results of calculations of the vibronic part of the MCD spectrum in the re- gion of ~600 nm and relate them to the vibronic transitions revealed by the fluorescence and absorp- tion spectra of CbO. Moreover, an overview of the al- lowed MCD spectrum of C6a is presented. It is found that vibrations of different symmetry give rise to A terms in MCD of different sign, providing support for the assignment of the vibrational bands seen in the one-photon spectra of the same wavenumbers. The different mechanisms governing the vibronic borrowing of the relevant modes are investigated. 2. Theory 2.1. The MCD constants The experimental basis for our analysis is pro- vided by the detailed absorption and MCD spectra 0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved. 335