Detection of OH stretching mode of CH3OH chemisorbed on Ni3+ 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 similar to 40 cm(-1) 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(+)(CH3OH)(m). The photodissociation was analyzed by assuming that Ni-3,4(+)(CH3OH)(m) dissociate unimolecularly after the photon energy absorbed by them is statistically distributed among the accessible modes of Ni-3,4(+)(CH3OH)(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(+)(CH3OH)] is related to the electron density distribution inside the methanol molecule.