AB-INITIO STUDY OF GEOMETRIES AND FORCE-FIELDS OF DIPHENYLACETYLENE IN THE GROUND-STATE, RADICAL-CATION, RADICAL-ANION, AND LOWEST EXCITED TRIPLET-STATE

Ab initio calculations of geometries and force fields using the 4- 3 1 G basis set were carried out at the RHF and ROHF versions of the SCF level on the ground state S0, radical cation R.+, radical anion R.-, and lowest excited triplet state T1 of diphenylacetylene. It was shown that the molecular geometry approaches a quinoidal structure in the following sequence: S0, R.+, R.-, and T1. Force constants were obtained in terms of local symmetry coordinates and were adjusted with the scale factors transferred from the ones determined in benzene and methylacethylene. Good agreement between observed and calculated normal frequencies was obtained for S0, R.+, and R.- but not for T1. For the T1 state, the scale factors for the C-Ph stretch and phenyl skeletal stretches were found to be not transferable. In addition, off-diagonal force constant matrix elements representing the interactions between these coordinates of the different phenyl groups were overestimated, which may be correlated with the instability of the restricted Hartree-Fock solutions.1