Infrared Reflection Absorption Spectroscopic Study of the Rotational

Infrared reflection absorption (IRA) spectra were measured for methyl ethyl ether ... (110) and Cu(110) at 80 K. The IRA spectra measured for MEE on A...
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J. Phys. Chem. B 2003, 107, 5008-5015

Infrared Reflection Absorption Spectroscopic Study of the Rotational Isomerism of Methyl Ethyl Ether on Cu(110) and Ag(110) Tairiku Kiyohara, Hironao Shinohara, Takahiro Kasahara, Riki Okubo, and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda UniVersity, Shinjuku-ku, Tokyo 169-8555, Japan ReceiVed: October 31, 2002; In Final Form: February 26, 2003

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-CH2 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/631++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 CH3(-CH2) 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-CH2 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.

Introduction A vast amount of studies have been performed by using vibrational spectroscopy to elucidate the rotational isomerism of organic molecules in gaseous, liquid, and solid states, providing valuable information regarding intramolecular, as well as intermolecular, interactions.1,2 The rotational isomerism of a series of ethers, including methyl ethyl, methyl propyl, and methyl butyl ethers has been studied by Perchard3,4 and Shimanouchi et al.5,6 The latter authors performed extensive normal frequency calculation analyses by using the GF method on the IR and Raman spectra of the ethers, determining the structures of isomers in each state and establishing the assignments of the spectra of the isomers. According to these studies, the ethers in the crystalline state always take on the all trans (abbreviated to T) conformation, and on conversion to glassy, liquid, and gaseous states, the ethers exist in a mixture of conformations comprised of T and gauche (abbreviated to G) forms. For example, methyl ethyl ether (CH3OCH2CH3, MEE) in the liquid and gaseous states takes on both T and G forms and methyl propyl ether (CH3OCH2CH2CH3) takes both TT and TG forms in the glassy state, and TT, TG, GT, and GG forms in the liquid and gaseous states.5 (The first and second symbols (T or G) in the notations expressing the rotational isomers indicate the conformations around the CH3O-CH2CH3 and OCH2-CH2CH3 bonds, respectively.) To extend the study on the rotational isomerism of molecules in the bulk states to that of adsorbates on metal surfaces, we

have been studying the isomerism of a series of methyl alkyl ethers adsorbed on Cu(110) and Ag(110) by using infrared reflection absorption (IRA) spectroscopy. In the present paper, we report the result of the measurement and the analysis of the spectra of methyl ethyl ether (this is abbreviated to MEE in the following) adsorbed on Cu(110) and Ag(110). The T form of MEE in the gaseous state is more stable than the G form with the enthalpy difference of 5.65 kJ/mol.3 Our IRA spectroscopic study7 has indicated that dimethyl ether (CH3OCH3, DME) adsorbs molecularly on Cu(110) and Ag(110) at 80 K at relatively lower surface coverages with the C2 axis almost perpendicular to the surfaces. The shifts to the lower frequency side observed for the νs(COC) band compared to that of DME in the gaseous state (920 cm-1), -25 cm-1 for DME on Cu(110) and -17 cm-1 for DME on Ag(110), suggest a weak coordination interaction of the oxygen atoms of DME to the substrate copper and silver atoms. This was substantiated by the result of density functional theory (DFT) calculations performed on cluster models of DME/Cu(110), where the oxygen atom takes a bridge site coordinating to the surface Cu atoms. Referring to these results, we can expect that MEE also adsorbs on the substrate through a coordination interaction of the oxygen atom to the Cu and Ag atoms. A steric repulsion between the CH3(-CH2) group of MEE and the Ag(110) and Cu(110) surfaces can be an additional factor to determine the conformations of MEE on the substrates. Thus, if lateral interactions between the adsorbates can be neglected, the

10.1021/jp022353l CCC: $25.00 © 2003 American Chemical Society Published on Web 05/03/2003

Rotational Isomerism of MEE on Cu(110) and Ag(110)

J. Phys. Chem. B, Vol. 107, No. 21, 2003 5009

following three factors are considered to determine the rotational isomerism of MEE: (i) the enthalpy difference between the T and G forms, (ii) the coordination interaction between the oxygen atom of MEE and surface atoms, and (iii) the steric repulsion between the methyl group and the substrate surfaces. One of the important problems to be addressed by the present paper is to clarify how these factors interplay to determine the rotational isomerism of MEE on Ag(110) and Cu(110). The study on the rotational isomerism of molecules on metal surfaces has been done only on a few cases including 1-butene on Ag(110) performed by Pawela-Crew et al.8 Experimental Section Materials. MEE (99% purity) was purchased from Tokyo Kasei Kogyo Co., Ltd. and distilled prior to IR measurements. Substrates. Ag(110) and Cu(110) single crystals (99.999%, 15 mm (φ) × 1 mm) were purchased from Techno Chemics, Inc. The surfaces of the crystals were cleaned by Ar+ ion sputtering (0.7 µA cm-2, 700 eV, 15 min at 600 K) and annealing at the same temperature, as already described in detail.7,9 The formation of the reconstructed surfaces were confirmed by observing the anticipated LEED patterns. Measurement of IR Spectra. The IR spectral measurements were performed by the apparatus already explained.7,9 Briefly, the apparatus consists of a load-lock chamber and two ultrahigh vacuum (UHV) chambers; one of the UHV chambers containing a four-grid retarding field AES/LEED optics, a quadrupole mass spectrometer, and an Ar+ ion sputtering unit was used for preparing the clean and reconstructed surfaces and the other UHV chamber containing a Fourier-transform IR spectrometer (Brucker model 66v/S) was used for IRA spectral measurements. The IR spectra were recorded at an incident angle of 80° with a liquid nitrogen cooled MCT detector. The base pressure of the two chambers was below 1 × 10-10 Torr. The temperature of the substrates was cooled to 80 K by liquid nitrogen. MEE was dosed to the substrates precooled at 80 K through a 1/8 in. stainless tube by using a variable leak valve. Exact surface coverages could not be determined in the measurements, but because the formation of multilayers for MEE on Ag(110) was easily detected by observing IR bands ascribable to a bulk (or multilayered) crystalline state, it was assumed that a saturation coverage is formed just before the appearance of the bulk bands. As for MEE on Cu(110), the intensities of the IRA bands saturated at a certain level of exposure, and it was assumed that a saturation state was formed at this stage. All of the spectra were given by -log(R/R0) as a function of wavenumber in the 4000-750 cm-1 region, where R and R0 indicate measured reflection intensities with and without an adsorbate, respectively. Reflection intensities were recorded by adding 3000 scans at the resolution of 4 cm-1. Computation Procedure Normal Frequency Calculation. Ab initio quantum mechanical calculations were performed using Gaussian 9810 and a DFT method at the B3LYP level with the 6-311++G** basis set. The normal frequencies were calculated for the optimized structures of MEE with the T and G formes and corrected by multiplying the scale factor, which was obtained by the following equation proposed by Matsuura and Yoshida.11

νobs ) 1.0087 - 0.000 016 3νcalc νcalc

(1)

Figure 1. IRA spectra of MEE adsorbed on Ag(110) measured at 80 K with increasing surface coverage from the bottom to the top spectrum. See text.

Simulation of IRA Spectra of MEE on the Substrates. To determine the orientations of MEE on Cu(110) and Ag(110) from the observed IRA spectra, the simulation of the spectra were carried out for the adsorbate with various orientations on a metal surface on the basis of the following assumptions: (i) the transition dipole moment calculated for each normal mode of the free MEE molecules with the T and G forms can be used to estimate the intensity of the corresponding IRA band, (ii) the intensity of the IRA band is proportional to the square of the projection to the surface normal of the transition dipole moment, and (iii) each component band with a calculated frequency, νcalc,i, was assumed to have a Lorentzian shape with the half-width at half-maximum (σ) of 1 cm-1. Then, the spectra were calculated by using the following formula: N

S(ν) )

σ

Pi ∑ π i)1

1 (ν - νcalc,i)2 + σ2

(2)

where Pi was calculated by the formula

Pi ) (µ⊥)2

(3)

µ⊥ in eq 3 is the projection to the surface normal of the calculated transition moment vector for the normal vibration with the frequency νcalc,i of MEE with an assumed orientation on a metal surface. Results IRA Spectra of MEE on Ag(110). Figure 1 illustrates the IRA spectra in the 1600-700 cm-1 region observed for MEE on Ag(110) measured at 80 K with increasing amounts of exposure. The bottom three spectra, which are virtually identical with each other except for slight intensity changes, were measured at submonolayer and monolayer (or saturation) coverage, and the top two spectra were measured after the

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TABLE 1: Calculated and Observed Frequencies (cm-1) of Methyl Ethyl Ether calcd valuesa freq A′

A′′

int Trans Form 14.7 17.5 5.1 162.4 32.4 3.2 26.7 0.0 7.9 3.9 2.8 0.3 0.0 9.3 1.6 6.1 8.8 Gauche Form 804.1 1.8 834.5 15.0 971.0 26.9

857.4 1012.9 1101.9 1132.8 1216.9 1375.7 1399.7 1450.2 1474.0 1484.2 1499.8 823.9 1148.8 1178.5 1282.1 1456.0 1458.7

obsd freqsb (liquid) 853 1015 1094 1120 1208 1365 1392 1456 1462 1472 1485 815 1150 1169 1269 1445 1456 800 843 979

1063.7

31.2

1068

1121.8 1154.4 1176.5 1219.4 1318.5 1368.5 1386.5

44.8 17.2 85.7 7.9 4.7 3.9 21.4

1120 1150 1164 1208 1304 1365 1383

1445.7 1452.9 1456.0 1470.6 1484.9 1493.5

0.5 5.2 6.7 4.1 0.4 10.6

1445 1456 1456 1462 1472 1485

obsd freqs (IRAS)c low cov 843 1009 1090 1205

1176 1439

829(G) 971(G)

1094(T)

1212(G) 1381(G) 1441(G) 1446(G)

high cov

assignmentsd

On Ag(110) skeletal str.-1 skeletal str.-2 CH3(-C) rock. 1121 skeletal str.-3 1211 CH3(-O) rock. CH2 wag. + CH3(-C) sym. def. 1395 CH3(-C) sym. def. + CH2 wag. CH3(-O) sym. def. 1439 CH3(-C) asym. def. 1462 CH3(-O) asym. def. 1485 CH2 sciss. 819 CH2 rock. CH3(-O) rock. + CH2 rock. CH2 rock. + CH2 twist. 1275 CH2 twist. CH3(-O) asym. def. CH3(-C) asym. def. On Cu(110) CH3(-C) rock. + CH2 rock. 831(G) skeletal str.-1 971(G) skeletal str.-2 855(T) 1016(T) skeletal str.-3 1054 CH3(-C) rock. + C-C str. 1094(T) 1124(G, T) CH3(-C) rock. + CH3(-O) rock. 1152(G) CH3(-O) rock. 1174(G, T) skeletal str.-3 1212(G,T) CH3(-O) rock. + CH2 rock. CH2 twist. CH2 wag. + CH3(-C) sym. def. 1381(G) CH3(-C) sym. def. + CH2 wag. 1394(T) CH3(-O) sym. def. 1441 CH3(-O) asym. def. + CH2 sciss 1446 CH2 sciss. + CH3(-C) asym. def. CH3(-O) sym. def. CH3(-C) asym. def. 1490 CH3(-O) asym. def.

855 1020

a Frequencies and intensities calculated for the optimized structure of MEE in the T and G forms by using a B3LYP/6-311++G** level calculation (see text). The calculated frequencies were corrected by using a formula proposed by Matsuura and Yoshida.11 See text. b Data taken from ref 6. c Frequencies observed for MEE at low and high surface coverages on Ag(100) (trans form) and Cu(110) (gauche form). Frequencies with T and G are assigned to the T and G forms, respectively, of MEE. See text. d Skeletal str.-1 and -2 indicate steletal stretching modes associated with C-O and C-C stretching modes. The former is mainly due to a C-O-C symmetric stretching and the latter due to a C-O-C asymmetric stretching. Rock., def., sciss., and str. indicate rocking, deformation, scissoring, and stretching modes, respectively.

formation of multilayers. The observed frequencies are summarized in Table 1 and compared with the frequencies of the IR bands of MEE in the crystalline state, which have been assigned to the T form by Shimanouchi et al.5,6 From the table, it is clear that all of the IRA bands measured at 843, 1009, 1090, 1176, and 1205 cm-1 for the adsorbate forming a monolayer correspond to the 853, 1015, 1094, 1169, and 1208 cm-1 of MEE in the T form. Thus, the adsorbate in the monolayer state takes on the T form. Upon formation of the multilayer, the IRA spectra exhibit drastic changes giving the 819, 1121, 1275, 1395, 1462, and 1485 cm-1 bands, which are absent in the spectra of the monolayer. The spectral features of the multilayer adsorbate are almost identical with the IR spectrum of MEE in the bulk crystalline state reported by Perchard (Figure 1B of ref 3). The result indicates that the adsorbate exists in an isotropic orientation state keeping the T form. The much simpler IRA spectral feature of the monolayer spectra compared to the multilayer spectra and especially the absence of the 1121 cm-1 band, which is the strongest for the multilayer spectra, in the monolayer spectra indicate that a

specific orientation is taken by MEE on Ag(110) at the monolayer coverage state. IRA Spectra of MEE on Cu(110). Figure 2 illustrates the IRA spectra in the 1500-700 cm-1 region observed for MEE on Cu(110) measured at 80 K with increasing amount of exposures. The dashed line exhibits the background spectrum, showing the noise level of the spectra. The bottom three spectra (solid lines) were measured before the saturation was completed, and the top two spectra were measured after the saturation was almost completed. In contrast to the case of MEE/Ag(110), further increase in the exposure did not cause any spectral change from that of the top spectra in Figure 2, indicating that the multilayer formation does not take place on Cu(110) at 80 K. The IRA spectra measured at submonolayer coverages are appreciably different from those of MEE on Ag(110). Because the spectra exhibited low S/N ratios, we repeated the measurements, confirming the existence of the IRA bands at 829, 971, 1094, 1212, 1381, and 1445 (doublet) cm-1, as shown in Figure 2. The frequencies are listed in Table 1 together with the

Rotational Isomerism of MEE on Cu(110) and Ag(110)

Figure 2. IRA spectra of MEE adsorbed on Cu(110) measured at 80 K with increasing surface coverage from the bottom to the top spectrum. The bottom dashed line indicates a background spectrum. See text.

frequencies of IR bands assigned to MEE in the G form.5,6 From the table, it is clear that the IRA bands except for the 1094 cm-1 band have the counterparts in the IR bands of MEE in the G form. (The 1094 cm-1 band can be ascribed to the T form, which exists as a minor component.) Thus, MEE forming the submonolayer coverages on Cu(110) takes on mainly the G form, contrasting the case of MEE on Ag(110), for which the adsorbate exists in the T form. Upon formation of the monolayer or saturation coverage, the IRA spectra gives the IRA bands at 855, 1016, 1054, 1124, 1152, 1174, and 1394 cm-1, in addition to those observed for the submonolayers. Comparison of the frequencies of the IRA bands with those of the IR bands due to the T and G forms listed in Table 1 indicates that (i) the 855, 1016, 1094, and 1394 cm-1 bands are assigned to the T form, (ii) the 1124, 1174, and 1212 cm-1 bands are ascribable to both the T and G forms, and (iii) the 1054 and 1152 cm-1 bands are due to the G form. Thus, MEE forming the monolayer on Cu(110) exists in both the T and G forms. The absence of the 1054, 1124, 1152, 1174, and 1212 cm-1 bands ascribable to the G form indicates that MEE in the G form at the submonolayer coverages takes on a specific orientation on Cu(110). Comparing the bottom spectrum with the top one in Figure 2, we notice that, upon formation of the monolayer, the intensity ratio, I(971 cm-1)/I(829 cm-1), appreciably decreases with concomitant decrease in the total intensity of both bands. These results suggest that orientation changes of the G form and a partial conversion from the G to T form take place during the formation of the monolayer. DFT Calculations on MEE in the T and G Forms and the Orientation of MEE on Ag(110) and Cu(110). As mentioned above, the IRA spectra in the 1500-800 cm-1 region indicate that the adsorbate on Ag(110) and that on Cu(110) exist in a preferred orientation, although the conformations taken by

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Figure 3. Schematic representations of the transition moments projected on the molecular plane of normal modes of MEE in the T form corresponding to the IRA bands that appear (A) at a saturation coverage and (B) at multilayer coverages on Ag(110). The figure under each representation indicates the observed frequency of the IRA band, and the figure in the parentheses indicates the calculated frequency. See text.

the adsorbates are different from each other. According to the surface selection rule developed by Greenler,11 the intensity of an IRA band should be proportional to the square of the projection to the surface normal of the transition moment of the band. In the case of a highly symmetric molecule, the rule provides valuable information about its orientation on a metal surface. To determine the orientations of MEE in the T and G forms on the substrates by using the surface selection rule, however, we need to know the value and orientation of the moments of normal modes corresponding to the observed IRA bands. (Although MEE in the T form has the Cs symmetry, we need to know the orientation of the transition moments within the Cs symmetry plane for discussing its detailed orientation on a metal surface based on its IRA spectra.) We applied the DFT method to determine the transition moments of MEE in the T and G forms. The calculated frequencies and intensities in the 800-1500 cm-1 region for MEE both in the T and in the G forms are listed in Table 1 together with the observed frequencies. From the table, it is clear that the calculated frequencies reproduce the observed ones within +15 to -7 cm-1. The order of the calculated intensities of the IR bands of MEE in the T form reproduces that of the observed intensities of MEE in the crystalline form, indicating that the DFT calculation can give the reliable relative values of the transition moments at least for MEE in the T form. Figure 3A is the schematic representation of the calculated transition moment vectors of the normal modes, which correspond to the IRA bands measured for MEE on Ag(110) at

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Figure 4. Schematic representations of the transition moments projected on the plane formed by the two CO bonds of normal modes of MEE in the G form corresponding to the IRA bands that appear (A) at submonolayer coverage and (B) at a saturation coverage on Cu(110). The figure under each representation indicates the observed frequency of the IRA band, and the figure in the parentheses indicates the calculated frequency. The figure inserted into each projection indicates the angle between the plane formed by the two CO bonds and the transition moment. See text.

the submonolayer and saturation coverages (see Figure 1). Figure 3B is the representation of the transition moments for the normal modes corresponding to the IRA bands, which appear after the multilayer formation on Ag(110). The abscissa in the figure is taken along the line connecting the carbon atom of the CH3(CH2) group and the oxygen. All of the IR bands belong to the A′ symmetry species, and the vectors are shown as the projections on the molecular plane. From Figure 3A,B, it is clear that each of the IRA bands observed at the submonolayer and saturation coverages has a transition dipole moment that has an appreciable component perpendicular to the abscissa, whereas the IRA bands appearing after the multilayer formation have the transition moments more or less parallel to the abscissa. Then, MEE at the submonolayer and saturation coverages takes an orientation with the line connecting the CH3 group and the oxygen atom more or less parallel to the surface. The coexistence of the IRA bands listed in Figure 3A,B, in the spectra of the multilayers (the top two spectra in Figure 1) corroborates the conclusion that the adsorbate forming the multilayers exists in an isotropic orientation state, as already presumed by the resemblance between the IRA spectra of the multilayer (Figure 1) and the IR spectrum of MEE in the crystalline state. The appearance of the 1176 cm-1 band at the submonolayer and saturation coverages (see Figure 1), which can be ascribed to the A′′ species (see Table 1), suggests that the molecular plane of MEE in the T form tilts away from the surface normal. A quantitative discussion of the tilt angle will be presented in the next section. Figure 4A is the schematic representation of the calculated transition moments of the normal modes of MEE in the G form

Kiyohara et al. corresponding to the IRA bands that appear at submonolayer coverages on Cu(110) in Figure 2, and Figure 4B is the representations of the transition moments for the normal modes of MEE in the G form, which correspond to the IRA bands appearing after the monolayer formation. The moments are shown by projecting them on the plane formed by the two C-O bonds of MEE in the G form with the abscissa parallel to the O-CH2 bond. The directions of the projections in Figure 4A are similar to each other and almost perpendicular to the directions of the projections in Figure 4B, which also have more or less similar orientations. These results suggest that MEE in the G form has a preferential orientation at the submonolayer coverage state on Cu(110) and that upon forming the saturation coverage the adsorbate takes on a more or less isotropic or random orientation. Simulation of the IRA Spectra of MEE on Ag(110). To estimate the orientation of MEE on Ag(110) at the saturation state, the IRA spectra in the 1500-800 cm-1 region were simulated for MEE in the T form with various orientations on a metal surface. The results are illustrated in Figure 5. As shown by the insert of the figure, the z-axis is taken to be perpendicular to the molecular plane and the x-axis parallel to the line connecting the oxygen atom and the CH3(-CH2) group and the y-axis perpendicular to the xz-plane. In the initial orientation, the xz plane is parallel to the substrate surface, and an orientation change from the initial orientation is expressed by two rotation angles, that is, SD defined by a rotation around the axis parallel to the z axis in the initial orientation and PD defined by a rotation around an axis parallel to the x axis in the initial orientation. The spectral simulation was performed only for the cases in which either SD ) 0° and PD * 0° or SD * 0° and PD ) 0°, for the sake of simplicity. As already explained, one of the specific features for the IRA spectra of MEE on Ag(110) at the submonolayer and saturation coverages is the absence of the 1121 cm-1 band, which is the strongest band of MEE in the T form in the multilayer state. Figure 5 indicates that only the calculated spectra with SD nearly equal to zero give negligible intensities at the 1133 cm-1 component corresponding to the 1121 cm-1 band. Another specific feature of the spectra is the presence of the 1176 cm-1 band belonging to the A′′ species, which means that the molecular plane (the xy-plane) is tilted from the surface normal (i.e., PD * 0°). Comparison of the IRA spectra (the bottom three curves in Figure 1) and the calculated spectra with SD ) 0° and PD ) 22.5-67.5° (the calculated spectrum with SD ) 0° and PD ) 90° is ruled out because it gives only the components belonging to the A′′ species) indicates that the relative intensities of the IRA bands in the observed spectra are in a good agreement with those in the calculated spectrum with SD ) 0° and PD ) 45°. The calculated spectrum gives a broad band near 1458 cm-1, which is an overlapped band of the bands calculated at 1456 and 1459 cm-1, both of which belong to the A′′ species. This result suggests that the broad IRA band near 1439 cm-1 observed for MEE on Ag(110) at the submonolayer and satulation coverages is assigned to an overlapped band of the CH3(-C) and CH3(-O) asymmetric deformation modes belonging to the A′′ species. Simulation of the IRA Spectra of MEE on Cu(110). Figures 6 and 7 exhibit the results of the spectral simulations for MEE in the G form with various orientations on a metal surface. As shown in the insert of each figure, the molecular xy-plane is defined by the two C-O bonds with the x-axis parallel to the O-CH2 bond, and the z-axis is taken to be perpendicular to the xy-plane. The initial orientation is taken

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Figure 5. Simulated IRA spectra of MEE in the T form with varying orientations, as defined by the angles SD and PD in the insert, on a metal surface. See text.

Figure 6. Simulated IRA spectra of MEE in the G form with varying orientations, as defined by the angles SD in the insert, on a metal surface. See text.

for the xz-plane to be parallel to the substrate surface, and the orientation is expressed by rotation angles, SD and PD, from the initial orientation, where SD is a rotation angle around an axis parallel to the z-axis in the initial orientation (see the insert of Figure 6) and PD a rotation angle around an axis parallel to the x-axis in the initial orientation (see the insert of Figure 7). The simulation was performed for either the case of PD ) 0° and SD * 0° (Figure 6) or the case of PD * 0° and SD ) 0° (Figure 7). Specific spectral features of the IRA spectra of MEE on Cu(110) are the absence of the 1054, 1124, and 1174 cm-1 bands, which are expected to be strongly observed for the G form in an isotropic orientation, and the presence of the relatively strong bands at 829 and 971 cm-1 accompanying weak bands at 1212 and 1381 cm-1. From Figures 6 and 7, it is clear that only the calculated spectra for PD ) 0° and SD ) 22.5° and 45° give negligibly small intensities for the 1054 (1064), 1124 (1122), and 1174 (1177) cm-1 bands (the numbers in the

parentheses indicate the corresponding calculated frequencies). The relative intensities of the 829 (835), 971 (971), 1212 (1219), and 1381 (1387) cm-1 bands in the calculated spectrum for the case of PD ) 0° and SD ) 22.5° are in better agreement with the observed relative intensities for MEE on Cu(110) at the submonolayer coverages (Figure 2) than those calculated for the case of PD ) 0° and SD ) 45°. Thus, the simulation suggests that MEE in the G form takes on an orientation in which the plane formed by the two C-O bonds is almost perpendicular to the surface of Cu(110) and the line connecting the carbon atom of the CH2 group and the oxygen atom is tilted away from the substrate surface by 22.5°. Discussion The Rotational Isomerism and DFT Calculation Analyses of the IRA Spectra of MEE Adsorbed on Ag(110) and Cu(110). The IRA spectra clearly proved that MEE takes the T

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Figure 7. Simulated IRA spectra of MEE in the G form with varying orientations, as defined by the angles PD in the insert, on a metal surface. See text.

form on Ag(110) and the G form on Cu(110) at the submonolayer coverages. Table 1 indicates that most of the frequencies of the IRA bands ascribable to the T and G forms of MEE coincide with those of the corresponding IR bands within (10 cm-1, indicating that the molecular force fields determining the vibrational frequencies of MEE in the T and G forms are not strongly perturbed by the adsorption process. In this case, the DFT calculation analyses based on the B3LYP/6++31G** level performed on MEE in the T and G forms are expected to give reliable information with regard to the frequencies, as well as the assignments, of the IRA bands. Actually the results of DFT calculations reproduce the observed frequencies of the IRA spectra of MEE on Cu(110) and Ag(110). To discuss the specific orientations of MEE on the substrates by applying the surface selection rule to the IRA spectra of the adsorbates, it was assumed that the calculated transition moments of the normal modes of MEE in the T and G forms can be applied to the moments of the adsorbates. The calculated transition moments should be taken as qualitative ones in the present level of the DFT calculations, but insofar as the absence of the 1121 and 1395 cm-1 due to the T form in the IRA spectra of MEE on Ag(110) at the saturation coverage and the absence of the 1054, 1124, 1152, and 1174 cm-1 bands due to the G form in the IRA spectra of MEE on Cu(110) at submonolayer coverages can be reasonably explained in terms of the calculated transition moments of the IRA bands in question and the assumed orientations, the discussion using the surface selection rule based on the calculated moments is very efficient to analyze the IRA spectra of MEE on Ag(110) and Cu(110). This is further corroborated by the simulation of the IRA spectra based on the calculated transition moments. The Adsorption Structures of MEE on Ag(110) and Cu(110). As can be seen from Figure 5, the spectrum calculated for MEE in the T form in the orientation with SD ) 0° and PD ) 45° corresponds quite well to the observed IRA spectra for MEE on Ag(110) at the submonolayer and saturation coverages in Figure 1, indicating that MEE adsorbs with the molecular plane tilted about 45° from the surface normal and the line connecting the CH3(-CH2) group and the oxygen atom nearly parallel to the substrate surface. As for the simulation of the IRA spectra of MEE on Cu(110) at submonolayer coverages,

Figure 8. Schematic representations of the adsorption modes of MEE on Ag(110) (A) and MEE on Cu(110). See text.

the calculated spectrum for the orientation with the molecular plane formed by the two CO bonds almost perpendicular to the surface and the tilt angle of the O-CH2 bond of about 22.5° (i.e., SD ) 22.5°, see Figure 6) reproduces the observed spectra in Figure 2. MEE in the T form adsorbed on Ag(110) at the saturation coverage gives the IRA band at 843 cm-1 associated mainly with νs(COC), which is observed at 853 cm-1 for MEE in the crystalline state (see Table 1). On the other hand, MEE in the G form on Cu(110) gives the νs(COC) band at 829 cm-1, which is the counterpart of the IR band at 843 cm-1 observed for MEE in the liquid state (Table 1). The frequency lowering of the modes due to adsorption (∆ν ) -10 cm-1 for MEE on Ag(110) and -14 cm-1 for MEE on Cu(110)) indicates the existence of the coordination interaction between the adsorbates and the surface metals. The orientations of the adsorbates on Ag(110) and Cu(110) determined by the spectral simulation are schematically shown in Figure 8. As can be seen from Figure 8B, the orientation of MEE in the G form on Cu(110) is favorable for the oxygen atom to coordinate to a bridge site of the surface metal atoms through the two lone pair electrons as in the case of DME on Cu(110).7 On the other hand, MEE in the T form cannot take on the bridge coordination site on Ag(110) because of the steric repulsion between the CH3(-CH2) group and the surface.

Rotational Isomerism of MEE on Cu(110) and Ag(110) Presumably, one of the lone pair electrons of the oxygen atom participates in a coordination interaction, as shown in Figure 8A. A preliminary temperature desorption spectroscopic (TDS) measurement indicated that MEE on Ag(110) exhibits a broad desorption peak (Td) centered at about 150 K, whereas MEE on Cu(110) gives a relatively sharp Td peak at about 170 K.12 Thus, the adsorbate-substrate interaction for MEE on Cu(110) is stronger than that for MEE on Ag(110). This is consistent with the adsorption models depicted in Figure 8, that is, the two-coordination site (or bridge-coodination site) for MEE on Cu(110) and the one-coordination (or on-top coordination site) for MEE on Ag(110). Factors Determining the Adsorption Structures of MEE on Ag(110) and Cu(110). The frequencies of the IRA bands observed for MEE adsorbed on Ag(110) and Cu(110) do not show any appreciable dependence on surface coverages in the submonolayer region, indicating that a lateral interaction between the adsorbates is negligible. As already explained in the Introduction section, the rotational isomerism of MEE on the metal substrates at submonolayer coverages are mainly determined by the following factors: (i) the enthalpy difference between the T and G forms; (ii) the coordination interaction between the oxygen atom of MEE and the surface metals; (iii) the steric repulsion between atoms or groups within the adsorbate and metal surfaces. As for the factor i, it has been known that the T form of MEE in the gaseous state is more stable than the G form with the enthalpy difference of 5.65 kJ/ mol.3 The results of the IRA study on dimethyl ether (DME) adsorbed on Cu(110) and Ag(110)7 suggests that the intrinsic coordination interaction between the oxygen atom of MEE and the surface metals on Cu(110) is stronger than that on Ag(110). Presumably, this is one of the main reasons for the fact that MEE takes the G form on Cu(110), even if it is energetically unfavorable compared to the T form. On the other hand, in the case of MEE on Ag(110), where the coordination interaction is relatively weak, the factor i plays a dominant role to determine the structure of the adsorbate, resulting in the formation of the T form. Upon increase of exposures to the substrates, MEE on Ag(110) makes a multilayer, whereas MEE on Cu(110) does not form such a multilayer, suggesting that the temperature of multilayer formation on Cu(110) is below 80 K. Presumably, intermolecular interactions between the adsorbates in the ordered T form stabilize the multilayered structure on Ag(110). On the other hand, there does not exist such an intermolecular interaction between the adsorbates in the G form. This may be one of the reasons for the fact that the temperature of multilayer formation on Cu(110) is lower than that on Ag(110). Conclusion The IRA spectral measurement proved that MEE adsorbs molecularly on Ag(110) and Cu(110) at submonolayer coverages, forming the T form on Ag(110) and mainly the G form on Cu(110). The IRA spectra clearly indicated that each adsorbate exists in a specific adsorption state (orientation and

J. Phys. Chem. B, Vol. 107, No. 21, 2003 5015 adsorption site). To get more quantitative information with regard to the states, the simulation of the IRA spectra of MEE in the T and G forms on a metal surface with various orientations was carried out by using the transition moments of the normal modes calculated by the B3LYP/6-311++G** method. The comparison between the results of the simulation and the observed IRA spectra indicates that (1) 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 CH3(CH2) group and the oxygen atom more or less parallel to the surface and (2) 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-CH2 bond tilted away from the surface by about 22.5°. Thus, the simulation of IRA spectra by using the calculated transition moments by the DFT methods was found to be one of the efficient methods to determine adsorption states of molecules such as MEE on Ag(110) and Cu(110). As shown in Figure 8, it was suggested that MEE in the T form adsorbs on an atop bonding site, coordinating the oxygen atom to one of the Ag atoms on Ag(110), and that MEE in the G form occupies a bridge site with the oxygen atom coordinated to two of the Cu atoms on Cu(110). This is corroborated by the experimental fact that the desorption temperature observed for MEE adsorbed on Ag(110) (ca. 150 K) is appreciably lower than that for MEE on Cu(110) (ca. 170 K). The intrinsic coordination interaction between the oxygen atom of MEE and the Cu atoms is stronger than that between the oxygen atom and the Ag atoms. It was presumed that this is one of the main reasons for the fact that MEE takes the G form on Cu(110), even if it is energetically unfavorable compared to the T form. References and Notes (1) Mizushima, S. Structure of Molecules and Internal Rotations; Academic Press: New York, 1954. (2) Morino, Y.; Hirota, E. Annu. ReV. Phys. Chem. 1969, 20, 139. (3) Perchard, J. P. Spectrochim. Acta 1970, 26A, 707. (4) Perchard, J. P. J. Mol. Struct. 1970, 6, 457. (5) Shimanouchi, T.; Ogawa, Y.; Ohta, M.; Matsuura, H.; Harada, I. Bull. Chem. Soc. Jpn. 1976, 49, 2999. (6) Shimanouchi, T.; Matsuura, H.; Ogawa, Y.; Harada, I. J. Phys. Chem. Ref. Data 1978, 7, 1323. (7) Kiyohara, T.; Akita, M.; Ohe, C.; Itoh, K. J. Phys. Chem. B 2002, 106, 3469. (8) Pawela-Crew, J.; Madix, R. J.; Vasquez, N. Surf. Sci. 1995, 340, 119. (9) Akita, M.; Hiramoto, S.; Osaka, N.; Itoh, K. J. Phys. Chem. B 1999, 103, 10189. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998. (11) Matsuura, H.; Yoshida, H. Handb. Vib. Spectrosc. 2001, 3, S4203. (12) Kiyohara, T.; Itoh, K. Unpublished work.