Structures and dissociation channels of protonated mixed clusters around a small magic number: Infrared spectroscopy of ((CH ₃) ₃N) _n-H ⁺-H ₂O (n = 1-3)

The magic number behavior of ((CH ₃) ₃N) _n-H ⁺-H ₂O clusters at n = 3 is investigated by applying infrared spectroscopy to the clusters of n = 1-3. Structures of these clusters are determined in conjunction with density functional theory calculations. Dissociation channels upon infrared excitation are also measured, and their correlation with the cluster structures is examined. It is demonstrated that the magic number cluster has a closed-shell structure, in which the water moiety is surrounded by three (CH ₃) ₃N molecules. The ion core (protonated site) of the clusters is found to be (CH ₃) ₃NH ⁺ for n = 1-3, but coexistence of an isomer of the H ₃O ⁺ ion core cannot be ruled out for n = 3. Large rearrangement of the cluster structures of n = 2 and 3 before dissociation, which has been suggested in the mass spectrometric studies, is confirmed on the basis of the structure determination by infrared spectroscopy.