BUTADIENE .3. CHARGE-DISTRIBUTION IN ELECTRONICALLY EXCITED-STATES

The vertical transition energies for butadiene have been calculated using the CIS/6-311(2+)G* theoretical model. The observed energies were satisfactorily reproduced. The charge distribution for each of the excited states was calculated so that the change from the ground-state distribution could be examined. The nature of the Rydberg states are discussed. Quantitative information on the degree of charge-transfer and bond-order changes was obtained for the excited states. The adiabatic geometries of the Valence and Rydberg states were examined, and vibrational frequencies for the excited states were calculated, providing agreement with experiment. The triplet states were also studied for both the vertical and adiabatic surfaces. Analysis of the charge density gave information on the charge reorganization that occurs in the excited states. It showed the remarkable similarity of the atomic orbital-like Rydberg states to the corresponding radical cation and, moreover, showed the unique nature of the 1(1)B(u) pi --> pi* valence state. The vertical 2(1)A(g) state was examined with the MCSCF method. For a comparison, the 1(1)B(u), 1(3)B(u), and 1(3)A(g) states were studied in the same fashion. The relationship between these states is discussed.