The dependence of the preferential protonated site in (CH₃)₃N-H⁺-(CH₃OH)_n on the cluster size was investigated using theoretical calculations and infrared spectroscopy measurements. While simple estimation from the magnitude of proton affinity suggested that the excess proton prefers the methanol site in n > 4, density functional theory calculations of the stabilization energy indicated the clear preference as protonation of the trimethylamine site, even for n = 9. Infrared spectra of the clusters were observed for n = 3 - 7. Spectral simulations were also performed using the quantum harmonic superposition approximation. The observed (CH₃)₃N-H⁺-(CH₃OH)_n spectra were well interpreted by simulations of the isomers with the protonated trimethylamine ion core. It was shown that both the proton affinity and the mutual solvation energy govern the preferential location of the excess proton in binary component clusters.

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