Resonant two-photon ionization spectra of van der Waals complexes p, m, o-C6H4F2⋯NH3(ND3)

Abstract

We have studied the resonant two-photon ionization (R2PI) spectra of three van der Waals (vdW) complexes p, m, o-C6H4F2⋯NH3(ND3) through the S1←S0 transition with mass selectivity. The stretching frequencies of the three vdW complexes were found to be quite close (86 cm−1). From the photodissociation mechanism and the relationship between anharmonicity of the stretching vibration and the dissociation energy, we estimated the dissociation energies of all the three vdW complexes in the S0 and S1 states. A quantum ab initio calculation on p-C6H4F2⋯NH3 at the MP2/6-31G** level gave the following geometry: The N atom of NH3 is located on the symmetry axis (Z-axis) and 3.53 Å above the benzene ring; the C3 axis of NH3 is at an angle of 52.5° with the Z-axis of p-C6H4F2 with one of the hydrogen atoms pointing towards the benzene ring; the rotation of NH3 around the Z-axis is nearly free. The calculated bond dissociation energies and the expectation of internal rotation are consistent with our experimental results.