Infrared Reflection Absorption Spectroscopic Study on the Rotational Isomerism of Methyl Propyl Ether on Cu(110) and Ag(110)

Abstract

Infrared reflection absorption (IRA) spectra were measured for methyl ethyl ether (MEE) adsorbed on Ag(110) and Cu(110) at 80 K. The IRA spectra measured for MEE on Ag( 110) at monolayer (or saturation) and multilayer coverages are characterized by the trans (T) form around the O-CH 2 bond of the adsorbate. Upon increase of exposures of MEE on Cu(110), the intensities of the IRA spectra saturate at a certain level. MEE on Cu(110) at submonolayer coverages gives IRA bands mainly ascribable to the gauche (G) form, whereas the adsorbate at the saturation state gives IRA bands ascribable to both the G and T forms. The spectrum of MEE on Ag(110) at the saturation state and that of MEE on Cu(110) at the submonolayer state indicate the existence of a specific orientation state for each substrate. To determine the orientations, the simulation of the IRA spectra of MEE with the T and G forms with varying orientations on a metal surface was carried out by using the transition moments of the normal modes of MEE in both forms calculated by the B3LYP/6-31++G** method. Comaprison between the results of the simulation and the observed spectra indicates that MEE in the T form adsorbs on Ag(110) with the molecular plane tilted about 45° from the surface normal with the line connecting the carbon atom of the CH 3 (-CH 2 ) group and the oxygen atom more or less parallel to the surface and that MEE in the G form adsorbs on Cu( 110) with the plane formed by the two CO bonds almost perpendicular to the substrate surface and with the O-CH 2 bond tilted away from the surface by about 22.5°. The orientation of MEE in the G form on Cu(110) is favorable for the adsorbate to take a bridging site through the coordination of the oxygen atom to the surface Cu atoms. The intrinsically stronger coordination interaction between the oxygen atom and the surface atoms on Cu(110) compared to that on Ag(110) was considered to cause MEE to take the G form on Cu(110), even if the enthalpy of the G form is higher than that of the T form by 5.65 kJ/mol.