Detection of OH stretching mode of CH₃OH chemisorbed on Ni ₃⁺ and Ni₄⁺ by infrared photodissociation spectroscopy

Structures of nickel cluster ions adsorbed with methanol, Ni ₃⁺(CH₃OH)_m (m = 1-3) and Ni ₄⁺(CH₃OH)_m (m = 1-4) were investigated by using infrared photodissociation (IR-PD) spectroscopy based on a tandem-type mass spectrometer, where they were produced by passing Ni _3,4⁺ through methanol vapor under a multiple collision condition. The IR-PD spectra were measured in the wavenumber region between 3100 and 3900 cm^-1. In each IR-PD spectrum, a single peak was observed at a wavenumber lower by ∼40 cm¹ than that of the OH stretching vibration of a free methanol molecule and was assigned to the OH stretching vibrations of the methanol molecules in Ni_3,4⁺(CH ₃OH)_m. The photodissociation was analyzed by assuming that Ni_3,4 ⁺(CH₃-OH)_m dissociate unimolecularly after the photon energy absorbed by them is statistically distributed among the accessible modes of Ni_3,4⁺(CH ₃OH)_m. In comparison with the calculations performed by the density functional theory, it is concluded that (1) the oxygen atom of each methanol molecule is bound to one of the nickel atoms in Ni_3,4 ⁺ (defined as molecular chemisorption), (2) the methanol molecules in Ni_3,4⁺(CH₃OH)_m do not form any hydrogen bonds, and (3) the cross section for demethanation [CH₄ detachment from Ni_n⁺(CH₃-OH)] is related to the electron density distribution inside the methanol molecule.