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 CH3NH2 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.