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

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.

Hydrogen Migration in the Phenylethen-2-yl Radical

“…To investigate whether this is at all reasonable, we examine the geometry of styrene, the nonradical counterpart obtained from the addition of a hydrogen to the radical site. Hartree−Fock calculations − give a very low torsional barrier, 0.21 kcal/mol, and a nonplanar optimized geometry with a 15−20° torsion angle. An MP2/6-31G(d) optimization of the styrene molecule produced a dihedral angle of 27.6°, indicating that the MP2 procedure predicts larger dihedral angles than the Hartree−Fock calculations.…”

Two-State Model of Antiaromaticity: The Low Lying Singlet States

“…This is the case of molecules, such as biphenyl ( IV ) and styrene ( V ), that formally are even parity systems, having six and four π electron pairs, respectively. Therefore, both molecules behave as a substituted benzene, with their ground state being essentially aromatic, and the lowest excited singlet is a twin state having the characteristic exalted frequency of the Kekulé mode . Note that such separation is impossible for the bicyclic systems pentalene ( VI ) and heptalene ( VII ).…”

Benchmarking the Molecular Mechanics−Valence Bond Method: Photophysics of Styrene and Indene

“…In the case of styrene ,− we have recently demonstrated 8i that changes in bonding within the benzene ring following excitation cannot be neglected, as the two mechanisms for S 1 decay set out in Scheme (where 1a , 1b , 1g , 1h , and 1m refer to optimized geometries of Figure that we shall discuss subsequently) show. The lowest energy pathway to efficient internal conversion (IC) is via an S 1 /S 0 conical intersection that results from a distortion of the benzene ring alone 8h (Scheme , left).…”