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RESEARCH PRODUCT

Rotational State Change of Acetonitrile Vapor on MCM-41 upon Capillary Condensation with the aid of Time-Correlation Function Analysis of IR Spectroscopy

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The infrared spectra of adsorbed acetonitrile on MCM-41 (porewidth = 3.2 nm) were measured at 303 K. In the CN stretching ν2 region, two bands were observed at 2265 cm− 1 and 2254 cm− 1, assigned to hydrogen-bonded molecules on surface hydroxyls of MCM-41, and physisorbed molecules in mesopores, respectively. We designate here the 2265 cm− 1 band as the ν2α band and the 2254 cm− 1 band as the ν2β band. The bandwidth of the fundamental transition ν2fβ, was obtained by removing the overlap with hot band transitions of the same mode, ν2α band, and other modes by least-squares fitting. Before capillary condensation, the relaxation time τ obtained from the bandwidth of the ν2fβ band was smaller than that of the bulk liquid, indicating presence of weakly hindered rotation. After capillary condensation, τ was slightly longer than that of the bulk liquid, suggesting that the motion of acetonitrile molecules condensed in mesopores is prohibited in comparison with that of liquid acetonitrile at the same temperature.