Theoretical Study of the Ground and Excited Singlet States of Styrene

The singlet-state manifold of styrene is examined by use of extended PPP-CI with geometry optimization and CNDO/S-CI techniques. Excitation energies, natural orbitals, equilibrium geometries, and the ethylenic torsional potential of the lower singlet states are determined. The calculated ground-state geometry is in good agreement with experimental results. The effect of double-excitation Cl on the excited-state properties is found to be important for the S₃ state, whose excitation energy after geometry optimization is within 0.2 eV of the S₂ state identified in the one-photon absorption spectrum. The S₃ state, in spite of its large double excitation contribution, is calculated to have a significant one-photon oscillator strength. Planar minima are calculated for all four singlet states. In contrast to some earlier analyses, the barrier to rotation about the vinyl bond is found to be significant for the S₁ state. Implications of the excited-state potential surfaces for styrene photoisomerization dynamics are discussed.