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

The ab initio and DFT calculations predict that two rotamers, cis and trans m-aminostyrene, are stable for each of the S-0, S-1 and D-0 states. In the one-color resonant two-photon ionization (1C-R2PI) spectra, the band origins of the S-1 <- S-0 electronic transitions (0(0) band) of cis and trans m-aminostyrene appear respectively at 30,937 +/- 3 and 31,141 +/- 3 cm(-1). The electronic transition energies (E-1'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 E-1's may be related to the mixing character of "L-1(b)," and "L-1(a)" in the S-1 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 Si 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. (C) 2013 Elsevier B.V. All rights reserved