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

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Abstract
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 S1 and S2 states, the molecules are planar; In S1, the main structural change is in the aromatic ring, that is somewhat expanded. In S2, the C=C vinyl double bond elongates, while the C1—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.