The solvation of Cl^-, Br^-, and I^- in acetonitrile clusters: Photoelectron spectroscopy and molecular dynamics simulations

We present the photoelectron spectra of Cl^-, Br^-, and I^- solvated in acetonitrile clusters (CH₃CN)_n with n = 1-33, 1-40, and 1-55, respectively, taken with 7.9 eV photon energy. Anion-solvent electrostatic stabilization energies are extracted from the measured vertical electron binding energies. The leveling of stabilization energies beyond n = 10-12 for the three halides signifies the completion of the first solvation layer. This is different from the behavior of anion-water clusters which probably do not fill the first solvation layer, but rather form surface solvation states. Classical molecular dynamics simulations of halide-acetonitrile clusters reproduce the measured stabilization energies and generate full solvation shells of 11-12, 12, and 12-13 solvent molecules for Cl^-, Br^-, and I^-, respectively. Ordered shell structures with high stability were found for the clusters of Cl^-, Br^-, and I^- with n = 9, 9, and 12. This special stability is reflected in the intensity distribution of the clusters in the mass spectra. Larger anion-acetonitrile clusters have the molecules beyond the first solvation layer packed in a small droplet which is attached to the first layer. It is suggested that in general, anions solvated in large clusters of polar solvents, might be located close to their surface.