Vibronic interaction in a copper oxide cluster Original Research Article

The orbital vibronic interaction in a D4h copper oxide cluster Cu4O12H8, which is one of the smallest cluster models for the CuO2 sheet of copper oxide compounds, is investigated to increase our knowledge of high-Tc superconductors. Only the B1g and B2g vibrational modes can couple to the degenerate eu HOMO of Cu4O12H8. The orbital vibronic coupling constants of the B1g and B2g modes to the eu HOMO are calculated and analyzed with the hybrid UB3LYP method that can reasonably describe the antiferromagnetic interactions between neighboring copper ions. The B1g mode of 191 cm−1 and the B2g modes of 51 and 505 cm−1 give very large coupling constants. The lowest frequency B2g mode of 51 cm−1 most strongly couples to the eu HOMO. The HOMO is localized diagonally in the broken-symmetry antiferromagnetic state, and therefore it is reasonable that this diamond-shaped distortion mode significantly couples to the HOMO.