Morphology of protonated methanol clusters: An infrared spectroscopic study of hydrogen bond networks of H⁺(CH₃OH)_n (n = 4-15)

Infrared spectroscopy of large-sized protonated methanol clusters, H ⁺(MeOH)_n (n = 4-15), was carried out in the OH stretch region to characterize the development of the hydrogen bond network with the cluster size, n. The band intensity of the free OH stretching mode decreased with n, and the band finally disappeared at n = 7. On the other hand, the broad absorption band due to hydrogen-bonded OH stretches exhibited a remarkable shift with the cluster size, and it finally converged on 3300 cm^-1 for n ≥ ∼ 10. The size dependence of the infrared spectra was morphologically interpreted in terms of the formation of the bicyclic hydrogen-bonded structure of the clusters.