Excited-state structure and vibrations of p-diaminobenzene studied by ab initio calculations

The structures and vibrations of p-diaminobenzene (PDAB) in the S₀ and S₁ states have been studied by ab initio quantum-chemical calculations. Results from geometry optimization show that the two stable cis and trans conformers of PDAB are non-planar in the S₀ state. Upon electronic excitation to the S₁ state, enhanced interaction between the ring and the amino substituent causes the molecule to become planar and contract along the long in-plane axis. A detailed analysis of the normal vibrations of PDAB in both states has been done on the basis of the motions of individual atoms as well as reduced masses, force constants and frequencies. The computed frequencies are in reasonably good agreement with the available experimental data.