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, 2B2u 2B3g, is severely affected by nonadiabatic interaction of the two states, to which the aforementioned features are attributed. In particular, three b1u fundamental peaks for 2B3g PBQ^· + gain large intensities from the electronic transition through the vibronic coupling. On the other hand, transition to another b1u 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 b1u level is significantly scrambled with nearby vibronic states of b2u symmetry, which accounts for the weak broad band experimentally observed in the 1560--1600 cm-1 region.