SEMIEMPIRICAL QUANTUM-CHEMICAL ASSIGNMENT OF THE CIRCULAR-DICHROISM SPECTRA OF SMALL CHIRAL FULLERENES

The semiempirical quantum-chemical simulation of the circular dichrois m, CD, spectra of the lowest chiral fullerenes allows matching of an e nantiomer and its spectrum. Comparison of calculations and experimenta l spectra for C-78 and C-84 makes it possible to establish that the CN DO/S method (Complete Neglect of Differential Overlap/Spectroscopic pa rameterization) is well suited to model the electronic CD response of fullerenes up to 400-500 nm. Two isomers of C-82, for which no experim ental spectra are as yet available, are added. The linewidth of the CD spectra of C-78 and C-84 is simulated through two quantities: the fir st is the density of vibrational states of the electronic ground state and partly accounts for the non-radiative lifetime of the excited sta tes that decreases with the increase of the excitation energy; the sec ond is the Stokes shift of the individual electronic states and accoun ts for the vibronic broadening. (C) 1997 Elsevier Science B.V.