Internal Rotation of the CF3 Group in the (Trifluoromethyl)anilines: A Zero-Kinetic-Energy Pulsed-Field-Ionization Study

Abstract

Internal rotation in the ionic ground states of 1-, 2-, and 3-(trifluoromethyl)aniline (ABTF) have been studied by high-resolution photoelectron spectroscopy (ZEKE-PFI) using the S1 state as the resonant intermediate. Coefficients of the torsional potential of the CF3 group obtained from a one-dimensional rigid rotor model were as follows: 2-ABTF, V3=720 ± 10 cm-1, V6 = −3 ± 3 cm-1; 3-ABTF, V3 = 222 ± 10 cm-1, V6 = −4 ± 2 cm-1; and 4-ABTF, V6 = 12 ± 5 cm-1, with little change in the rotational constant of the rotor. These values display trends consistent with those found for the CH3 rotor in the same environment. In particular, the equilibrium conformation of the rotor changes on ionization from the S1 state by 60° in the 3-isomer, but is unchanged in the 2- and 4-isomers. Symmetry forbidden transitions, leading to marked non-Franck−Condon behavior of intensities in the ZEKE spectra of the meta isomer are observed to be far more extensive than in the corresponding CH3 case. A further factor directly affecting the ZEKE spectrum in the ortho isomer is the coupling between the adjacent CF3 and NH2 rotors, which is far more pronounced than with CH3 and NH2.