Cation vibrational spectroscopy of trans and gauche n-propylbenzene rotational isomers. Two-color threshold photoelectron study and ab initio calculations

In this paper, a full account of an earlier report [Takahashi, Okuyama, and Kimura, J. Mol. Struct. 249, 47 (1991)] on the cation vibrational spectra of trans and gauche n-propylbenzene, which were obtained by means of a two-color (1 + 1′) multiphoton ionization threshold photoelectron technique is presented. The trans and gauche cations were separately produced by (1 + 1′) multiphoton ionization resonant through the S₁ vibronic levels of n-propylbenzene in supersonic free jets in which both the trans and gauche isomers exist. From the observed threshold photoelectron spectra, adiabatic ionization energies are determined as I_a(trans) =70 278 cm^-1 (8.7134 eV) and I_a(gauche) =70 420 cm^-1 (8.7311 eV) with an accuracy of ± 8 cm^-1 (±1 meV). Furthermore, four benzene ring modes (6b⁺, 1+, 12⁺, and 18a⁺) as well as several low-frequency torsional and bending modes have been identified which are sensitive to the relative conformations of the n-propyl group with respect to the benzene ring [(trans)⁺: 82, 212, and 300 cm^-1; (gauche)⁺: 46, 73, 207, and 252 cm ^-1]. The present vibrational assignments of the trans and gauche cations are based on a normal mode analysis performed for the trans cation by ab initio calculations. In the present work, this technique is demonstrated to be quite powerful for distinguishing the vibrational spectra of different cation isomers.