Resonance enhanced multiphoton ionization spectroscopy and theoretical calculations of cis- and trans-m-aminostyrene rotamers

• The REMPI spectra of rotamers were obtained.
• The S1 ← S0 excitation energies and ionization energies were determined.
• The relative stability of two rotamers in each of the S0, S1 and D0 states was evaluated by experimental method.
• La/Lb mixing transition was predicted by theoretical calculation.
• The calculated results compared well with experimental observations.

The ab initio and DFT calculations predict that two rotamers, cis and trans m-aminostyrene, are stable for each of the S0, S1 and D0 states. In the one-color resonant two-photon ionization (1C-R2PI) spectra, the band origins of the S1 ← S0 electronic transitions (00 band) of cis and trans m-aminostyrene appear respectively at 30,937 ± 3 and 31,141 ± 3 cm−1. The electronic transition energies (E1’s) of both rotamers of m-aminostyrene are lower than that of p-aminostyrene, which is contrary to the prediction according to the previous studies. The redshift of E1’s may be related to the mixing character of “1Lb” and “1La” in the S1 states of the two rotamers. The 2C-R2PI spectra give the adiabatic ionization energies of 61,278 ± 15 and 61,495 ± 15 cm−1 for cis and trans rotamers. The observed active modes of rotamers in the S1 states involve mainly the in-plane ring deformation and substituent-sensitive bending vibration. It is derived from the 1C- and 2C-R2PI spectroscopic data that the cis rotamer of m-aminostyrene is more stable than the trans rotamer by 30 ± 30, 234 ± 30 and 247 ± 30 cm−1 for the S0, S1 and D0 states, respectively. This is different from the m-aminophenol and m-aminoanisole that have a more stable trans rotamer.

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