Vibronic mixing and vibronic coupling of nearby electronic states. II: CNDO calculations of vibronic mixing in p-benzoquinone

The electronic structure of the lowest excited states of p-benzoquinone is given by calculations in the CNDO approximation. It is shown that for the description of the two nearly degenerate nπ* B1g and Au states the molecule can approximately be considered as a dimer. [A single p-benzoquinone molecule is considered as a dimer, the monomer unit being one-half of the molecule, i.e., a (CH)2C=O fragment.] The vibronic mixing of these two states is evaluated from calculations at displaced nuclear positions, which specify the relative importance of the different coordinates of b1u symmetry. The computed parameters are compared with parameters obtained, within the dimer model, from experimental data. The results of the calculations support the interpretation of Stark measurements of this compound which suggested a pseudo Jahn–Teller distortion of the lowest excited states.