Ionization spectroscopies and theoretical calculations of cis and trans 3-fluoro-N-methylaniline-Ar(n=1,2) van der Waals clusters: Structures and binding energies.

The ab initio and dispersion-corrected density functional theory (DFT) calculations of the van der Waals (vdW) clusters of cis and trans 3-fluoro-N-methylaniline-Ar (3FNMA-Ar) (n=1,2) predict that, for cis and trans 3FNMA-Ar clusters, the π-bound geometry with the Ar atom sitting over the ring is the global minimum in both neutral and cationic states, while for cis and trans 3FNMA-Ar clusters, the [π|π]-bound sandwich structure with two Ar ligands occupying nearly equivalent π-bound positions above and below the ring plane is the global minimum. The vibronic spectra of cis and trans 3FNMA-Ar clusters in the S state were recorded by using one-color and two-color resonant two-photon ionization (R2PI) techniques, the comparison of which yields an estimate of the binding energy of cluster in the S state. It is found that the linear correlation between the redshift of the S←S electronic transition energy (E) of cluster and the E of the monomer also holds for the Ar clusters of hetero-di-substituted aromatics. By recording the photoionization efficiency (PIE) curves and mass-analyzed threshold ionization (MATI) dissociation spectra of cis and trans 3FNMA-Ar clusters, the ionization energies (IE) and binding energies of clusters in the D state are obtained. The MATI spectra of the cis and trans 3FNMA-Ar cations exhibit significant progressions of the vdW bending mode (b), indicating the structural changes of the clusters in the bending coordinate upon ionization. The combination of the three kinds of spectroscopic methods provides the binding energies of cis and trans 3FNMA-Ar clusters in the S, S and D states with good accuracy, and the values estimated by dispersion-corrected DFT calculations compare well with the experimental results. From our studies, the ωB97X-D and TD-ωB97X-D methods using high quality basis set are recommended for studying the intermolecular interaction of such vdW clusters in the ground and excited states.