Internal rotation of the CF3 group in the (trifluoromethly)anilines: A zero-kinetic-energy pulsed-field-ionization study

Internal rotation in the ionic ground states of 1-, 2-, and 3-(trifluoromet hyl)aniline (ABTF) have been studied by high-resolution photoelectron spect roscopy (ZEKE-PFI) using the S-1 state as the resonant intermediate. Coeffi cients of the torsional potential of the CF3 group obtained from a one-dime nsional rigid rotor model were as follows: 2-ABTF, V-3=720 +/- 10 cm(-1), V -6:= -3 +/- 3 cm(-1); 3-ABTF, V-3 = 222 +/- 10 cm(-1), V-6 = -4 +/- 2 cm(-1 ); and 4-ABTF, V-6 12 +/- 5 cm(-1) with Little change in the rotational con stant of the rotor. These values display trends consistent with those found for the CH? rotor in the same environment. In particular, the equilibrium conformation of the rotor changes on ionization from the S-1 state by 60 de grees in:the 3-isomer, but is unchanged in the 2- and 4-isomers. Symmetry f orbidden transitions, leading to marked non-Franck-Condon behavior of inten sities in the,ZEKE spectra of the meta isomer are observed to be far more e xtensive than in the corresponding CH3 case; A further factor directly affe cting the ZEKE spectrum in the ortho isomer is the coupling between the adj acent CF3 and NH2 rotors, which is far more pronounced than with CH3 and NH 2