Vibronic mechanisms in the twohyphen;photon spectrum of benzene

摘要

Intensity measurements have been made on the single quantum bands of the active modes in the twohyphen;photon1B2ularr;1A1gspectra of C6H6and its isotopic homologs C6H5D,ohyphen;C6H4D2, C6HD5,phyphen;C6H4D2,phyphen;C6H2D4,shyphen;C6H3D3, and C6D6. ngr;14(b2u) and ngr;18(e1u) (D6hsymmetry) are found to be active in all the isotopic benzenes, and ngr;12(b1u) in all except C6H6,shyphen;C6H3D3, and C6D6where it is identity forbidden, andphyphen;C6H4D2where it is weak. The results show that the band strength of 1410is insensitive to deuterium substitution (confirming Duschinsky rotation of this mode), while 1210and 1810are strongly sensitive to the number and orientation of the deuterium atoms. INDO/S calculations of transition tensors at displaced nuclear coordinates, with and without doubly excited configurations, indicate that the ground state vibronic mechanisms usually neglected in onehyphen;photon spectra are important in the benzene twohyphen;photon spectra. The activity of ngr;14can be attributed to vibronic perturbation of the finalB2ustate by the ground state, and that of ngr;18by vibronic perturbation of the ground state by an excited state. The favored mechanism for the activity of ngr;12in the spectra of the deuterium benzenes involves intensity borrowing from ngr;18triggered by the kinematic perturbation of the deuterium atoms. The coupling states for ngr;12are thenE1uandE2g(D6hsymmetry). Evidence is presented for Duschinsky rotation of thee1umodes.