Protonation of Naphthalene-(Water)n Nanoclusters: Intracluster Proton Transfer to Hydration Shell Revealed by Infrared Photodissociation Spectroscopy

Solvation-dependent intracluster proton transfer (ICPT) within bare and Ar-tagged protonated naphthalene-(water)n clusters, H+(Np-Wn) with n≤3, is characterized by infrared photodissociation (IRPD) spectroscopy in a supersonic plasma expansion. IRPD spectra of size-selected clusters recorded in the CH and OH stretch range (2750-3800 cm-1) are analyzed with dispersion-corrected density functional theory (DFT) calculations (B3LYP-D3/aug-cc-pVTZ) to determine both the protonation site and the structure of the hydration network. Ar tagging of H+(Np-Wn) leads to colder spectra with higher spectral resolution. The position of the excess proton is controlled by a subtle balance between the difference in proton affinity (PA) of Np and Wn and the involved solvation energies. For n=1, the excess proton is localized on the Np ring, leading to a H+Np-W structure with a bifurcated CH…O ionic H-bond, because of the large difference in PA of Np and W. For n=2, ICPT occurs and the cluster has a structure, in which a symm...