A density functional study of vibrational frequencies of diphenylbutadiene in the ground and transient states

The Raman vibrational frequencies of all-trans-1,4-diphenyl-1,3-butadiene (DPB-d(0)) and alpha,alpha',beta,beta'-deuterated DPB (DPB-d(4)) in the ground (S-0) and the lowest triplet (T-1) states, and their cation and anion radicals have been calculated using the B3LYP exchange-correlation functional and the basis set of 6-31G*. Calculated B3LYP/6-31G* frequencies of both DPB isotopomers in the S-0 state are in good agreement with the experimental (Raman and IR) data except the polyenic C=C stretching mode with remarkably large isotopic shift. For DPB-d(0) and DPB-d(4) in the T-1 state and their cation and anion radicals, the correspondences between calculated and observed Raman frequencies are satisfied as a whole by the same calculation (B3LYP/6-31G*) level but for a few bands the observed frequencies have less agreement in a quantitative sense with the calculated values. Such deviations are discussed in relation to the delicate nature in mode couplings. The previously reported vibrational assignments for the species have been improved. (C) 1998 Elsevier Science B.V. All rights reserved.