Infrared spectra of HCl(H₂O)_n clusters from semiempirical Born-Oppenheimer molecular dynamics simulations

Infrared spectra of HCl(H₂O)_n clusters, with n = 4-10 and 21, are calculated at T = 50 K from semiempirical Born-Oppenheimer molecular dynamics simulations performed with the PM3-MAIS model. The specific focus of this study is on the relationship between spectroscopic features associated with the presence of the excess proton generated by the HCl dissociation as a function of n and the underlying water hydrogen-bonding topologies. Vibrational modes involving the motion of the excess proton are attributed to specific features appearing at ∼1175 cm^-1 for Zundel-type structures, in the 1670-1800 cm^-1 range for intermediate Zundel-Eigen-type structures, and at ∼2820 cm^-1 for Eigen-type structures. This broad range of vibrational frequencies correlates with the position of the excess proton within the clusters. Overall, the theoretical predictions are in good agreement with the available experimental data.