THE GROUND-STATE OF ANTIAROMATIC 1,3,5,7-TETRA-TERT-BUTYL-S-INDACENE

The electronic and vibrational properties of the antiaromatic molecule 1,3,5,7-tetra-tert-butyl-s-indacene (TTBI) in the ground state are investigated in this paper. MO ab initio calculations have been performed on TTBI and the parent molecule, s-indacene (1). The results show that the C-C bonds of s-indacene alternate in length and the corresponding C2h structure is more stable than the structure without bond alternation (D2h symmetry), regardless of the basis set used. Bond length alternation is decreased by alkyl substitution in the 1, 3, 5, and 7 positions. The optimized TTBI structure calculated with the 3-21G basis set is in good agreement with X-ray diffraction data on the molecule. The torsional motion of one butyl group with respect to the indacene ring gives rise to two stable conformers. Experimentally, ground-state properties have been studied by means of infrared and Raman spectroscopy. At low temperature both spectra show narrow bands. Using 568.2-nm excitation the Raman spectrum is enhanced by resonance effects due to the proximity of the strongly active S2 state and, as a consequence, a large number of Raman modes are observed. The vibrational assignment is discussed by means of semiempirical and ab initio calculations of frequencies and intensities of model molecules such as I, 1-tert-butyl-indacene (TBI) and 1,5-di-tert-butyl-indacene (DTBI). The infrared spectrum is in remarkably good agreement with predictions from semiempirical calculations. Comparison with calculated frequencies also allows the assignment of the most intense Raman lines.