The electronic absorption spectra of the alkyl-substituted derivatives of bis(benzene)chromium(0) in the vapour phase

Abstract

The UV and visible absorption spectra of (arene) 2chromium(0) (arene = benzene (I), toluene (II), ethylbenzene (III), cumene (IV), tert-butylbenzene (V), mesitylene (VI) in the vapour phase have been investigated. Four band systems A,B,C and D are revealed in the spectra. The bands of the system with the shortest wavelengths, D, represent the Rydberg series. The first ionisation potentials IP a 1 g , 5.18 and 5.01 eV respectively. The Rydberg bands correspond to the allowed electrodipole transitions from the highest occupied molecular orbital (MO) a 1 g to the vacant MO of either the a 2 u or e 1 u type. System C corresponds to the intense band of the solution spectra. The electronic transition e 2 g → e ★ 2 g obviously makes a great contribution to this system. System B is assigned to the transition from a 1 g to vacant a 2 u or e 1 u MO, which can be Rydberg orbitals. System A can be assigned to the a 1 g → e ★ 2 u transition or to the Rydberg transition, which is forbidden in the D 6 h point group but becomes allowed upon reduction of symmetry.