Magnetic circular dichroism of azulene

The MCD spectrum of azulene has been measured. It is shown that the MCD in the first transition (O → A) and third transition (O → C) can be reproduced quite well by SCF-MO-CI calculations. From an analysis of the magnetic circular dichroism in the O → B band, which in contrast to that of the other bands has a very peculiar shape, it is demonstrated that the amount of Franck-Condon intensity in this transition is only 20 per cent of the total intensity ; 80 per cent of the absorption is induced by vibronic coupling via totally symmetric modes. Furthermore, the detailed distribution of allowed and forbidden intensity throughout the entire O → B absorption system is found from this analysis. The relevance of these data, which cannot be obtained that easily by means of other techniques, to some earlier explanations of the O → B transition will be discussed.