Vibrational and electronic properties of antiaromatic systems: a spectroscopic study of 1,3,5-tri-t-butylpentalene.

The vibrational and electronic spectra of the stable pentalene derivative 1,3,5-tri-tert-butylpentalen(eT TBP) are reported, and their properties are discussed on the basis of QCW-PI MO semiempirical calculations, which allow geometry optimization and normal mode analysis for the ground and lowest excited states. The infrared and Raman spectra have intensities mostly arising from modes of pentalene origin. The observed frequencies are compared with those obtained from vibrational calculations on the parent molecule and on several tert-butyl derivatives.