Structures and vibrations of ortho-, meta-, and para-fluoroanilines in the S-0 and S-1 states by ab initio calculations and resonant two-photon ionization spectroscopy

The structures and vibrations of ortho-, meta-, and para-fluoroanilines in the S-0 and S-1 states have been studied by ab initio quantum chemical methods and resonant two-photon ionization (R2PI) spectroscopy. The calculated results show that the structures of these three fluoroanilines are very similar to those of aniline in both states. Upon electronic excitation, the C-N bond length decreased considerably, whereas the C-F bond length became only slightly shorter. Thus, in the excited state the amino group has a stronger interaction with the ring than the fluorine substituent for these molecules. A comparison of vibrational modes in both the states is presented and discussed in terms of the changes in reduced masses, force constants, and mixing between the ring and the amino motions. We also report the R2PI spectra of these species near their S-1 <-- S-0 transition regions, in which some of the bands are observed for the first time. These spectral bands are successfully assigned with the help of computed results and comparison with those of other relevant substituted benzenes.