Infrared spectra and structures of (CH₃NH₂)_nH⁺ (n = 1-4). Binding features of an excess proton

Infrared photodissociation spectra of CH₃NH₃⁺ -Ar and )CH₃NH₂)_nH⁺ with n = 2-4 are measured in the 2600-3500 cm^-1 region and analyzed with the aid of ab initio calculations. The intensities of the CH-stretching transitions relative to the NH-stretching transitions increase with increasing n, suggesting the change of the binding features of an excess proton in the clusters. Two CH3_NH₂ molecules in (CH₃NH₂)₂H⁺ equally share the proton. On the other hand, the proton is localized on the central molecule in (CH₃NH₂)₄H⁺, forming the CH₃NH₃⁺ core solvated by three CH₃NH₂ molecules.