Inversion motion and S1 equilibrium geometry of 4-fluoroaniline:: Molecular beam high-resolution spectroscopy and ab initio calculations

The fluorescence excitation spectrum in the region of the 0(0)(0), 6a(0)(1), I-0(2) and 1(0)(1) S-1 <-- S-0 bands of 4-fluoroaniline expanded in a supersonic beam has been measured under high-resolution conditions with complete spectral resolution of the rotational components. The rotational constants of the four vibronic levels have been determined. The inversion motion in S-1 has been described by means of the flexible model approach, i.e., considering additional structural changes occurring in the molecule during the torsional oscillation. Structural relaxation is related to several internal coordinates, such as C1N bond stretching, the HNH bending and the angular displacements from planarity. The equilibrium geometry of S-1, taking into account the present results and reported data, is found effectively planar with a barrier to inversion lower than 10 cm(-1). Excited-state rotational parameters such as the shifts of the planar moments of inertia justify satisfactorily the use of this model. Ab initio calculations with 6-31G basis sets including polarization and diffuse orbitals are in good agreement with the flexible model results, thus confirming the nonlocal character of the amino inversion in the excited state. The potential well has been also modeled ab initio for the ground-state including correlation effects through the perturbative MP2 treatment.