15min:

KRZYSZTOF PIECH, THOMAS BALLY, Department of Chemistry, University of Fribourg, CH-1700 Fribourg, Switzerland; TAKATOSHI ICHINO AND JOHN F. STANTON, Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, TX 78712.

X-irradiation of an Ar matrix doped with p -benzoquinone (PBQ) at 10 K leads to formation of the PBQ radical cation (PBQ^· +) and radical anion (PBQ^· -). The IR spectrum of PBQ^· + exhibits broad and dense absorption bands in the 2000 cm^-1 and higher energy region. Another characteristic of the spectrum is the presence of three intense peaks in the lower energy region. Equation-of-motion coupled-cluster calculations have been performed to analyze the spectrum with the quasi-diabatic model Hamiltonian technique. A spectral simulation based on the model Hamiltonian reproduces the observed IR spectrum very well, revealing that the electronic transition to the low-lying excited state, ²B_2u ²B_3g, is severely affected by nonadiabatic interaction of the two states, to which the aforementioned features are attributed. In particular, three b_1u fundamental peaks for ²B_3g PBQ^· + gain large intensities from the electronic transition through the vibronic coupling. On the other hand, transition to another b_1u fundamental level (anti-symmetric CO stretch) in the state has a diminished intensity due to cancellation of the electronic contribution and the usual dipole derivative contribution. Furthermore, this b_1u level is significantly scrambled with nearby vibronic states of b_2u symmetry, which accounts for the weak broad band experimentally observed in the 1560--1600 cm^-1 region.