Structures of hydrogen bond networks formed by a few tens of methanol molecules in the gas phase: Size-selective infrared spectroscopy of neutral and protonated methanol clusters

In this work, we report infrared spectra of large neutral and protonated methanol clusters, (MeOH)_n and H⁺(MeOH)_n, in the CH and OH stretching vibrational region in the size range of n = 10-50. The infrared-ultraviolet double resonance scheme combined with mass spectrometry was employed to achieve moderate size selection of the neutral clusters with the addition of a phenol molecule as a chromophore. Infrared dissociation spectroscopy was performed on the protonated methanol clusters by using a tandem quadrupole mass spectrometer to enable the precise size selection of the clusters. While the neutral clusters showed essentially the same spectra in all the observed size range, the protonated clusters showed remarkable narrowing of the H-bonded OH stretch band with increasing n. In n ≥ ∼30, the spectra of the neutral and protonated clusters become almost identical. These spectral features demonstrate that hydrogen bond networks of methanol prefer simple cyclic structures (or "bicyclic" structures in protonated methanol) and branching of the hydrogen bond networks (side-chain formation) is almost negligible. Implications of the spectra of the clusters are also discussed by comparison with spectra of bulk phases.