Vibronic and cation spectroscopy of structural isomers p- and m-diaminobenzene and the amino substitution effect

Abstract

► We report the R2PI and MATI spectra of p -diaminobenzene and m -diaminobenzene. ► The first electronic transition and ionization energies are precisely measured. ► The data help us to learn the amino substitution effects on molecular properties. We applied the resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of p -diaminobenzene (PDAB) and m -diaminobenzene (MDAB). The band origins of the S 1 ← S 0 electronic transition of PDAB and MDAB were found to be 29 805 ± 2 and 33 395 ± 2 cm −1 , whereas the adiabatic ionization energies were determined to be 54 624 ± 5 and 58 321 ± 5 cm −1 , respectively. Most of the observed active vibrations of these two structural isomers in the electronically excited S 1 and cationic ground D 0 states result from in-plane ring deformation and bending vibrations. Comparing the present data with those of aniline suggests that the interaction of the amino substituent and aromatic ring in the upper (S 1 or D 0 ) state is stronger than that in the S 0 state. In addition, the frequency of observed active vibrations may depend on the nature, relative location, vibrational pattern, and the extent of the amino group participated in the normal vibration.