Vibrational Spectra and Quantum Calculations of Ethylbenzene

Normal vibrations of ethylbenzene in the first excited state have been studied using resonant two-photon ionization spectroscopy. The band origin of ethylbenzene of S₁←S₀ transition appeared at 37586 cm^-1. A vibrational spectrum of 2000 cm^-1 above the band origin in the first excited state has been obtained. Several chain torsions and normal vibrations are obtained in the spectrum. The energies of the first excited state are calculated by the time-dependent density function theory and configuration interaction singles (CIS) methods with various basis sets. The optimized structures and vibrational frequencies of the S₀ and S₁ states are calculated using Hartree-Fock and CIS methods with 6-311++G(2d,2p) basis set. The calculated geometric structures in the S₀ and S₁ states are gauche conformations that the symmetric plane of ethyl group is perpendicular to the ring plane. All the observed spectral bands have been successfully assigned with the help of our calculations.