VIBRATIONAL AND ELECTRONIC-PROPERTIES OF ANTIAROMATIC SYSTEMS - A SPECTROSCOPIC STUDY OF 1,3,5-TRI-TERT-BUTYLPENTALENE

The vibrational and electronic spectra of the stable pentalene derivat ive 1,3,5-tri-tert-butylpentalene (TTBP) are reported, and their prope rties are discussed on the basis of QCFF-PI MO semiempirical calculati ons, which allow geometry optimization and normal mode analysis for th e ground and lowest excited states. The infrared and Raman spectra hav e intensities mostly arising from modes of pentalene origin. The obser ved frequencies are compared with those obtained from vibrational calc ulations on the parent molecule and on several tert-butyl derivatives. The pi pi electronic transitions of TTBP in the visible and UV regio ns are satisfactorily predicted by our calculations, including the int eraction between singly excited configurations. Optimization procedure s show that bond alternation, a distinctive feature of antiaromatics, is reduced in the excited states and that ground and excited state pot ential surfaces have minima displaced one with respect to the other. L ow-temperature absorption spectra in the S-2 and S-3 regions are inter preted in terms of Franck-Condon vibronic transitions Whose strength d epends directly on the structural change upon excitation. The theoreti cal results on S-0 --> S-2 and S-0 --> S-3 band profiles, with conside ration of the normal mode rotation in the excited state, that is, the Duschinsky effect, are in fair agreement with experiment.