Ab initio study of styrene and β-methyl styrene in the ground and in the two lowest excited singlet states

The structure and vibrational frequencies of styrene and trans-β-methyl styrene in the lowest three singlet states (S₀, S₁, and S₂) have been calculated using ab initio quantum chemical methods. The frequencies are compared with experimental data obtained in the bulk and in a supersonic jet. The calculation shows that in the ground state the molecules have a broad shallow potential as a function of the torsional angle, are essentially planar, but may be slightly bent. In the S ₁ and S₂ states, the molecules are planar; In S ₁, the main structural change is in the aromatic ring, that is somewhat expanded. In S₂, the C=C vinyl double bond elongates, while the Cl - Cα single bond becomes shorter, bringing these two bonds to almost equal length. Correlation diagrams connecting ground state vibrational modes with ones belonging to electronically excited states are given; they show that for many out-of-plane modes the vibrational frequencies decrease upon electronic excitation. This is accounted for in terms of the changes in the π electron distribution taking place upon optical excitation that result in decreasing the force constants characterizing these vibrations. The frequencies of most in-plane modes change very little, but mixing between S₀ modes is indicated in some cases, and a few vibrations, among them a Kekulé-type mode, undergo considerable change. The relation to the spectroscopy of the corresponding transitions in benzene is briefly discussed.