6994
J. Phys. Chem. 1995, 99, 6994-7001
Infrared Reflection Absorption Spectroscopic Study on the Adsorption Structures of Acrolein on an Evaporated Silver Film Shuji Fujii, Naoki Osaka, Masato Akita, and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169, Japan Received: October 27, 1994; In Final Form: February 9, I995@
Infrared reflection absorption (IRA) spectra were measured for acrolein adsorbed on an evaporated silver film at 90 K under ultrahigh vacuum conditions. IRA spectra indicated that acrolein adsorbed on the silver substrate exists in a trans form and that, on increasing exposure levels, the adsorbate takes on type-A, -B, -C, and -D structures successively. The adsorption structures are characterized by the C-0 stretching (v(C=O)) and CH2 out-of-plane bending (u(CH2)) bands; they are observed at 1670 and 978 cm-' (type-A), 1693 and 985 cm-' (type-B), 1699 and 995 cm-' (type-C), and 1691 and 993 cm-' (type-D). Type-A and -B interact directly with the substrate, while the type-C and -D form molecular layers on top of the type-A and -B layer. The intensity ratios, w(CH~)/Y(C=O),observed for type-A, -B, -C, and -D are appreciably larger than the ratio observed for acrolein in a polycrystalline state, indicating that all the adsorbates have the molecular plane more or less parallel to the substrate surface. In the CH out-of-plane bending vibration region, type-C and -D give the CH out-of-plane bending bands of the vinyl group and formyl group near 1018 and 1005 cm-', respectively, in addition to the u(CH2) band, while type-A and -B give only the prominent band due to w(CH2). This trend was observed also for acrylic acid lying flat on a silver substrate (Fujii, S . ; et al. Sur$ Sci. 1994,306, 381). Thus, the selective enhancement of the u(CH2) band seems to be a general feature for a planar molecule containing a vinyl group and interacting directly with the silver surface. Ab initio molecular orbital method at the RHF/4-3 1G* level was applied to calculate the normal vibrational frequencies of a free acrolein as well as a silver ion-acrolein complex; the results reproduced frequency differences between IRA bands of type-A and the corresponding bands of acrolein in a gas state, proving that type-A forms a coordination bond between the oxygen atom of the C=O group and a positively charged site of the substrate.
Introduction Fourier-transform infrared reflection absorption (FT-IRA) spectroscopy has proven to be very powerful for clarifying orientations, structures, and adsorbate-substrate interactions of olefins, aromatic compounds, and organic acids adsorbed on metal Recently, we observed the IRA spectra of acrylic acid (CH2=CHCOOH) adsorbed on silver films under ultra-high vacuum (UHV) conditions, and reported that CH outof-plane bending bands observed in the 1000-900-~m-~region serve as a good marker to discriminate adsorption modes of acrylic acid.8 In the present paper we extended the IRA spectral study to acrolein (CHz=CHCHO, Figure 1) adsorbed on an evaporated silver film under UHV conditions in order to c o n f m the general applicability of the out-of-plane bending bands for determining the adsorption modes of planar molecules containing a vinyl group. In the previous papeI.8 it was also reported that, upon increasing exposure levels, acrylic acid takes on a series of adsorption structures successively; Le., at first it takes on a structure with the C-0 bond coordinated to the surface, then a hydrogen-bonded dimer lying flat on the surface, and finally an ordered structure similar to that of crystalline acrylic acid. In the present paper we observed the IRA spectra of acrolein at various exposure levels to elucidate successive adsorption structure changes and tried to elucidate adsorption characteristics of planar molecules such as acrylic acid and acrolein on the silver substrate. Acrolein has been known to exhibit conformational isomerism between s-trans and s-cis forms around the central C-C bond; near-ultraviolet spectroscopy indicated that 95-6% of acrolein in a gaseous state at @
Abstract published in Advnnce ACS Abstracts, April 15, 1995.
(b) Figure 1. Structures of s-trans- (a) and s-cis-acrolein (b). The number of each atom is used for ab initio calculations. Ag(9) indicates the position of the silver ion in the optimized structure of an silver ionacrolein complex (see text).
room temperature assumes the s-trans form (see Figure 1) with a small amount of a second conformer with the s-cis The structure of the s-trans acrolein has been analyzed by electron d i f f r a c t i ~ n ' ~and J ~ microwave14 spectroscopies. Infrared spectra of the s-trans acrolein have been
QO22-3654l95/2Q99-6994$09.0QlQ 0 1995 American Chemical Society
Spectroscopic Study of Acrolein together with vibrational assignments, which were carried out on the basis of normal vibration frequencies calculated by molecular orbital methods.17J8 The IR spectra of the s-cis acrolein were also observed by using an inert gas matrix isolation method by Krantz et al.15 and Blom et al.;16 they produced the cis conformer by using a UV light irradiation or a high-temperature Knudsen cell technique. The IRA spectra reported in the present paper were analyzed referring to these IR studies on the s-trans and s-cis acrolein. In addition to the IRA studies, we carried out ab initio molecular orbital (MO) calculations in order to confirm the assignments of the IRA spectra of acrolein. Ab initio MO calculations of acrolein have already been performed by several De Mar6 et a l S 2 O and Loncharich et aL2' reported an optimized structure calculated by using the basis set of 6-31G*. Hamada et al.18 and Bock et calculated vibrational frequencies with the basis sets of 4-31G and 6-31G, respectively, and reproduced appreciably well the IR spectra of gaseous s-trans acrolein. In the present paper we performed the ab initio MO vibration analysis at the RHF/4-3 lG* level to give normal frequencies of a free acrolein as well as a silver ion-acrolein complex and tried to confirm the validity of models proposed to adsorption structures, in which acrolein interacts directly with the silver substrate.
Experimental Section Measurement of IRA and IR Spectra. The apparatus used for measuring IRA spectra has already been described in the previous paper.8 Briefly, it contains a single-level UHV chamber (base pressure below 1 x Torr) equipped with an infrared absorption spectrometer (JEOL Model 5500) and a thermal desorption mass spectrometer (ULVAC model MSQ400). IRA spectra, obtained in a single reflection mode at an angle of incidence of 80°, were recorded with a liquid nitrogen cooled MCT detector in the 4000-700-cm-* region by adding 1000 scans at the resolution of 4 cm-'. A silver substrate for IRA measurements was prepared by evaporating silver onto a mirror-finished copper plate (25 x 25 x 1 mm) with a thickness of more than 1000 8, by a resistive heating method; during the preparation the plate temperature was kept at 300 K. The substrate was connected to a liquid nitrogen reservior, which allows us to cool the substrate down to 80 K. The substrate temperature was monitored by a chromel-alumel thermocouple attached to the sample side of the substrate. Exposure of the sample to the silver substrate was performed through a variable leak valve positioned at a distance of 2 cm from the substrate at 90 K. The amount of the exposure was expressed by uncorrected Langmuir units (langmuir = Torrs). The measurement of an IR transmission spectrum was performed on acrolein exposed (900 langmuirs) to a Kl3r plate at 90 K by the same apparatus used for the IRA measurements. Materials. Acrolein was obtained from Tokyo Chemicals Co., Ltd., and purified by distillation under nitrogen atmosphere and by repeated freeze-pump-thaw cycles just prior to use. A deuterated analogue, acrolein-dl (CHFCDCHO), was prepared by the aldol condensation method from formaldehyde in D2O and a ~ e t a l d e h y d e . ~ The ~ , ~ reactions ~,~~ were carried out in the gas phase at 300 O C in a reactor filled with an alkaline catalyst. The catalyst was prepared as follows; silica gel 40 (70-230 mesh, Merck) was treated with a 10% sodium carbonate solution and dried; all the hydrogen atoms of hydroxyl groups in the catalyst were replaced with deuterium by treatment with D20 in the reactor at 300 "C. Crude product from the aldol condensation reaction was purified by distillation under
J. Phys. Chem., Vol. 99,No. 18, 1995 6995 nitrogen atmosphere. The deuteration ratio of the product was proved to be more than 95% by measuring 'H-NMR and mass spectra. Ab Initio MO Calculation. Ab initio MO calculations were carried out with the GAUSSIAN 9025using a super computer (Fujitsu vp2200110) of Centre for Informatics at Waseda University. Optimized geometries and force constants of acrolein and a silver ion-acrolein complex were calculated at the RHF/SCF level with 4-3 lG* basis set except for silver atom, for which Huzinaga's contracted (333331333133) basis set26was used. Ab initio MO vibrational analyses in the SCF level systematically overestimates the force constants and adjustments with scale factors are required to bring the calculated frequencies into agreement with the observed values. Then, the calculation of in-plane normal frequencies was performed through the following steps; first, we made ab inito MO vibrational analyses for 1,3-butadieneand trans-glyoxal, for which ample vibrational data were a c c u m ~ l a t e d , ~ ' and - ~ ~ determined the scale factors of each molecule by a least-squares fitting program made by Shimojima et al.;30 second, we corrected the calculated force constants of both the s-trans and s-cis acrolein by assuming that the scale factors of 1,3-butadiene and trans-glyoxal could be transferred to the ethylenic and aldehyde moieties of acrolein and calculated normal vibration frequencies. As an adsorption structure model we postulated a silver ion-acrolein complex and performed ab initio MO vibrational analyses by assuming that the sets of the scale factors of acrolein could be applied also the complex. As for out-of-plane vibrations, however, scale factors for 1,3-butadiene and trans-glyoxal could not be determined, because only a small number of out-of-plane vibration frequencies have been reported for these molecules. As a consequence, scale factors for the out-of-plane vibrations of acrolein were determined so that good fits could be obtained between calculated frequencies and observed ones, for which reliable assignments were obtained based on the IR spectra of various deuterium-substituted analogues of a~rolein.~-llThese factors were also used to correct the force constants of out-ofplane vibrations of the silver ion-acrolein complex. The scale factors of force constants associated with intemal coordinates containing the silver ion were tentatively assumed as follows; 0.8 for Ag-0 stretching (v(Ag-0)) and Ag-C-0 bending (d(Ag-C=O)) force constants and 0.75 for a C=O torsional (z(C=O)) force constant.
Results and Discussion
IRA Spectra of Acrolein Adsorbed on a Silver Film. Figure 2 summarizes the IRA spectra of acrolein exposed to a silver substrate with increasing amounts at 90 K. The spectral changes indicated that, on increasing the exposure, the adsorbate takes on different structures successively. At an exposure of 0.05 langmuir (Figure 2a), the adsorbate gave rise to IRA bands at 1670 and 978 cm-'; hereafter we call the adsorbate associated with these bands as type-A. On increasing the exposure to 1.0 langmuir (Figure 2b), there appeared bands at 1693 and 985 cm-' in addition to those ascribable to type-A. The adsorbate giving the 1693- and 985-cm-' bands is called as type B. The weak feature at 920 cm-' is also ascribed to type B. At the exposure of 4.0 langmuirs (Figure 2c), a new set of bands appeared at 1699, 1018, and 995 cm-', which are assigned to type-C. At the exposure of 8.0 langmuirs (Figure 2d), the intensities of these bands increased further, and at the same time the bands at 1691, 1007, 939, 933, and 926 cm-l newly appeared. The spectrum at the exposure level of 10.0 langmuirs (Figure 2e) showed similar features as those of the spectrum at 8.0 langmuirs except for a minor difference; e.g., the intensity
Fuji et al.
6996 J. Phys. Chem., Vol. 99, No. 18, 1995
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Figure 2. IRA spectra measured at 90 K after exposing acrolein on a silver substrate at the levels of (a) 0.05 langmuir, (b) 1.O langmuir, (c) 4.0 langmuirs, (d) 8.0 langmuirs, and (e) 10.0 langmuirs, and (f) the spectrum measured after keeping the substrate at 90 K for an hour since the measurement of the spectrum (e). The asterisks indicate supurious bands due to an artifact. The vertical line and the accompanying factor in each spectrum indicate absorbance scale.
ratio of the 1691-cm-' band to the 1699-cm-' one in the former spectrum is larger than the ratio in the latter. When the sample of Figure 2e was kept at 90 K for a prolonged time, the band intensities at 1699,939, and 926 cm-I gradually decreased, and finally (after about 1 h) the sample gave the spectrum in Figure 2f. The adsorbate giving the final feature is called as type-D. Comparison of Figure 2c with Figure 2f indicates that type-C and -D give common features at 1018 and 995 cm-', while the
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Figure 3. IRA spectra measured at 90 K after exposing acrolein at the level of (a) 0.05 langmuir, (b) 0.6 langmuir, (c) 1.75 langmuirs, and (d) 78 langmuirs, (e) the spectrum measured after annealing the sample of the spectrum (d) to 110 K and cooling down again to 90 K, and (f) an IR transmission spectrum of a large amount of acrolein exposed (ca. 900 langmuirs) on a KBr plate at 90 K. The measurements of the spectra (a)-(d) were performed under similar conditions as those of Figure 2 except for prolonged exposure time; i.e., it took about 1 h to expose acrolein at each exposure level. The vertical line and the accompanying factor in each spectrum indicate absorbance scale. bands at 939 and 926 cm-' are observed only for type C. Table 1 summarizes the frequencies of the IRA bands observed for the type-A, -B, -C, and -D adsorbates. Figure 3, parts a-e, illustrates the IRA spectra observed for acrolein with increasing amount of exposure; the measurement
J. Phys. Chem., Vol. 99, No. 18, 1995 6991
Spectroscopic Study of Acrolein
TABLE 1: Frequencies and Assignments of the IRA Bands Observed at 90 K for the Type-A, -B, -C, and -D Adsorbates of Acrolein and Acrolein41 and the Infrared Absorption Bands of Acrolein in a Gaseous State and in a Polycrystalline State at 90 K gaso
type-Ab
type-A-dc
type-Bb
type-B-dc
type-Cb
type-C-dc
type-Db
polycrystal"
1724 1625 1420 1360 1275
1670
1664
1693
1693
1363
1360
1699 1618 1425 1365 1284
1697 1603 1431 1360
1691 1616 1425 1365 1284
1684 1618 1425 1365 1284
1158 993 972 959
1164
1161 1018 1007 995
1159 1016 1009 993
1219
1210
912
1164 84 1
978
985
987
assignmentse
1169 1018 l00lf 995
999 99 1
920
Data taken from ref 18. Data taken from Figure 2. Data observed for acrolein-dl (Figure 4). Data taken from Figure 3f. e Y, 6, y , e, and w denote stretching, in-plane deformation, out-of-plane bending, in-plane rocking, and out-of-plane wagging vibrations, respectively. f and v indicate the formyl and vinyl moieties of acrolein, respectively. f Taken from Figure 3c. 8 Assignment based on the ab initio MO calculation (see Table 7). * Doublets observed for type-C and -Dand the polycrystalline acrolein may be ascribed to crystal field splitting. was performed under similar conditions as those of Figure 2 except for prolonged exposure time; i.e., we took more than 1 h to expose acrolein at each level. Figure 3f exhibits an IR transmission spectrum of a large amount of acrolein exposed (ca. 900 langmuirs) on a KBr plate at 90 K; the spectrum can be ascribed to a polycrystalline state. Although the spectrum of Figure 3a was observed at the same exposure level (0.05 langmuir) as that of Figure 2a, it gave different features from those of Figure 2a, showing IRA bands at 1693 and 987 cm-' ascribable to type-B. The IRA spectra of both type-A and -B (Figures 2a and 3a) are observed at the very low exposure (0.05 langmuir); this result suggests that both adsorbates interact directly with the substrate surface; presumably, type-B observed for the prolonged exposure experiment takes on a more stable structure than type-A. As can be seen from Figure 3, parts b and c, upon increasing the exposure level, the bands ascribed to type-C appear at 1699, 1020, 995, 939, and 926 cm-l. (The 1001-cm-' band in Figure 3c may be ascribed also to type-C, although this band could not be clearly identified in Figures 2d and 2e.) At the exposure level of 78 langmuirs (Figure 3d), the adsorbate exhibits the spectrum similar to that of Figure 2e, giving the IRA bands ascribable to both type-C and -D. When the sample of Figure 3d was annealed to 110 K and cooled down again to 90 K, the adsorbate gave the IRA spectrum in Figure 3e, which are almost identical with the spectrum of type-D (Figure 20. Thus, the annealing process (or keeping at 90 K for a prolonged time in the case of the rapid exposure experiment) converted type-C to type-D. Further, the spectral features observed for type-D (Figures 2f and 3e) are similar to those for the polycrystalline sample (Figure 30; especially, the IRA bands at 1020, 1009,995, 933, and 920 cm-l (Figure 3e) characteristic of type-D are observed also for the polycrystalline sample at 1016, 1009,993,933, and 918 cm-l, respectively. These results suggest that type-D exists in a more stable state or in a more ordered state than type-C. Assignments of In-Plane Vibration Bands. According to Hamada et a1.,18the trans-acrolein in a gaseous state gives rise to IR bands at 1724, 1625, 1420, 1360, 1275, 1158, and 912 cm-l, which are assigned to CEO stretching (v(C=O)), C=C stretching (v(C=C)), CH2 scissoring (B(CH2)), CH in-plane bending vibrations in the formyl and vinyl groups (6(CH)f and d(CH),), C-C stretching (v(C-C)), and CH2 in-plane rocking (e(CH2)) vibrations, respectively. Bair et al.,9 Alves et al.,1° and Osbome et al." reported that the cis-acrolein gives IR bands at 1722 (v(C=O)), 1624 (v(C=C)), 1403 (d(CH2)). 1285 (6(CH)f), and 919 cm-' (v(C-C)). Thus, the d(CH2) and v(C-
C) bands give characteristic features for the trans and cis forms; Le., 1420 and 1158 cm-' for the trans and 1403 and 919 cm-' for the cis form. As summarized in Table 1, both the type-C and -D adsorbates show IR bands corresponding to all the IR bands observed for the s-trans-acrolein and do not give any feature ascribable to the cis form; the result indicates that type-C and -D take on the trans form. From Table 1 it is also clear that type-A and -B give IR bands at 1164 cm-' due to the v(C-C) band of the trans form; the adsorbates do not give any band due to the cis form. These results indicate that type-A and -B also assume the trans form. (The 1363-cm-' band due to type-B is not clearly observed in Figure 2a; the small population of type-B in the adsorbate may cause the corresponding band to be buried in a rather noisy background of the spectrum in the figure.) The frequencies of the v(C=O) band observed for the typeA, -B, -C, and -D adsorbates are appreciably lower than that of the gaseous acrolein (1724 cm-'). Further, the frequency of type-A is lower by about 20 cm-l than those of the other adsorbates. This frequency lowering may be ascribed to the formation of a coordination bond of the C=O group with the silver substrate. This is confirmed by the results ab initio MO calculation (vide infra). Assignments of Out-of-Plane Vibration Bands. The prominent features in the 1050-950-cm-' region in Figures 2 and 3 have been ascribed to C-H out-of-plane bending As already explained, these features clearly change with the types of adsorption of acrolein. In order to make explicit assignments to the out-of-plane vibrations we measured the IRA spectra of acrolein-dl (CH2sCDCHO) under the same condition as those of Figure 3. Figure 4 illustrates the results. At the exposure of 0.6 langmuir, there appeared a peak at 1693 cm-' ascribable to v(C=O) of type-B. When the exposure level was raised to 2.0 langmuirs, the v(C=O) band was observed at 1697 cm-', which could be assigned to type-C. This band increased intensity upon further exposure to 5.0 langmuirs. Comparing the spectra of the type-B and -C adsorbates of acrolein (Figure 3, parts a and c) to those of the type-B and -C of the deuterated sample (Figure 4,parts a and c), we recognize the following facts. (i) Upon deuteration of the hydrogen atom of the vinyl group (the H(6) position in Figure la) the 1020-cm-l band of type-C (Figure 3c) disappeared, indicating that it is assigned to a CH out-of-plane bending vibration of the vinyl group (y(CH),); weak features at 1016 and 1012 cm-' in Figure 4c may arise from undeuterated acrolein, which exists as a minor component. (ii) The deuteration did not cause any frequency
6998 J. Phys. Chem., Vol. 99, No. 18, 1995
Fujii et al.
n
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Figure 4. IRA spectra measured at 90 K after exposing acrolein-dl at the level of (a) 0.6 langmuir, (b) 2.0 langmuirs, and (c) 5.0 langmuirs. The vertical line and the accompanying factor in each spectrum indicate
absorbance scale. shift for the 987-cm-' band of type-B (Figures 3a and 4a). (iii) Upon deuteration the IRA bands at 1001 and 995 cm-' in Figure 3c slightly shift to 999 and 991 cm-' (Figure 4c), respectively. As explained in the next section, the ab initio MO calculation indicates that the deuteration does not cause any frequency shift for a band mainly associated with a CH2 out-of-plane wagging vibration (w(CH2)). Then, the result (ii) suggests that the 987cm-' band is ascribable to w(CH2). The IRA band strongly observed at 978 cm-' for the type-A adsorbate (Figure 2a) corresponds the 987-cm-' band of type-B and it is ascribed to w(CH2). On the basis of these results and the assignments of the out-of-plane vibrations of the gaseous acrolein16J8the IRA bands near 1018, 1007, and 995 cm-' observed for type-C and -D(Figures 2, parts d-f, and Figures 3, parts c-e) are assigned to y(CH),, a CH out-of-plane bending vibration of the formyl group (y(CH)f) and w(CHz), respectively. These assignments, which conform to the results of the MO calculation in the next section, are summarized in Table 1. As Figure 2, parts a and b, and Figure 3a show, the type-A and -B adsorbates give rise to only the strong and broad band due to w(CH2) near 980 cm-'. This is contrasted to the case of the type-C and -D adsorbates, which give the sharp features due to y(CH),, y(CH)f, and o(CH2) (Figure 2d,e, Figure 3ce). Similar phenomenon has been observed also for acrylic acid adsorbed on a silver substrate;8Le., the acid with the C=O group coordinated directly to the silver substrate exhibits only a prominent band at 978 cm-' due to w(CH2) (Figure 2a of ref 8), while the hydrogen-bonded dimer lying flat on the substrate gives well-defined bands at 999 and 978 cm-' (Figure 2d of ref 81, which are ascribable mainly to y(CH), and w(CH2), respectively. The selective enhancement of the w(CH2) band observed for the adsorbates interacting directly with the substrate surface cannot be explained only by the surface selection rule.31 A charge-transfer effect between the adsorbate and the substrate
may cause the enhancement through a vibronic coupling. In order to confirm this point we should perform detailed MO calculations taking account of interactions between the n-electrons of acrolein and silver atoms of the substrate. The w(CH2) frequencies of the adsorbates are much larger than the frequency (959 cm-') observed for acrolein in the gas state (see Table 1). The frequency increase of 19 cm-l for type-A is ascribable to the coordination of the C-0 group to the silver substrate. The increase of 26 cm-' for type-B may be explained by considering that the adsorbate interacts with the substrate through the C=C bond in addition to the coordination of the C=O group. The surface-enhanced Raman scattering (SERS) spectrum of 1,3-butadiene adsorbed on a coldly evaporated silver film indicated that an interaction through the C=C bond with a positively charged site of the film causes a high-frequency shift of an w(CH2) band.32 The existence of the interaction through both the C=O and C-C bonds in the type-B adsorbate is suggested also by the results of the ab initio MO calculation, as explained in the next section. The w(CH2) frequency of type-C and -D (995 cm-l) is similar to that of the polycrystalline acrolein (993 cm-'). Then, the frequency increase of 36 cm-' relative to the w(CH2) frequency of the gaseous acrolein is due to a stacking interaction between the molecular planes. The intensity ratios, w(CHz)Iv(C=O), observed for type-A and -B (Figure 2, parts a and b, and Figure 3, parts a and b) are much larger than the ratio for acrolein in the polycrystalline state (Figure 30. According to the surface selection rule,31this result indicates that the adsorbates lie flat on the substrate. The intensity ratio of the 1020-cm-' band to the 1699-cm-' band (Le., y(CH)f/v(C=O) for type-C) in Figure 3c is larger than the corresponding intensity ratio of the 1018-cm-' band to the 1699-cm-' band in Figure 2c; the former ratio is also appreciably larger than the intensity ratio of the 1016-cm-' band to the 1684-cm-' band observed for the polycrystalline sample (Figure 30. These results indicate that type-C formed by the prolonged exposure experiment takes more flat orientation than type-C formed by the rapid exposure. Normal Vibration Analyses of Acrolein and a SilverAcrolein Complex. In order to make assignments on IRA spectra and elucidate interaction modes of the adsorbates with the silver substrate, we performed normal vibration analyses of acrolein and a silver ion-acrolein complex by ab initio MO calculations with the 4-31G* basis set. Table 2 summarizes the optimized geometrical parameters of s-trans- and s-cis-acrolein together with the experimental re~u1ts.l~ The calculated geometries are similar to those already and in good agreement with the experimental ones except for the ClXC2-H6 angle, which is appreciably larger than the observed one. As a model for the type-A adsorbate we postulated a complex in which a silver monocation is coordinated to the oxygen atom of the C=O group of acrolein. The complex was assumed to take on a planar structure. The optimized geometrical parameters for the complex are listed in Table 2. The total charge of the complex was assumed to be 1, and the silver atom in the optimized structure has a formal charge of +0.982. The distance between silver ion and oxygen atom was found to be 2.26 A, which is reasonable for oxygensilver complexes,33and the Ag-O=C angle to be 172.2' (see also Figure 1). From Table 2, it is also clear that the coordination causes an increase in the length of the C=O (+0.018 A) and C=C (+0.006 A) bonds and a decrease in the length of the C-C bond (-0.023 A); these bond length changes are mainly due to increase in the n-electron delocalization caused by the complex formation.
+
Spectroscopic Study of Acrolein
TABLE 2: Observed and Calculated Structural Parameters of Acrolein and Silver Ion-Acrolein Complex Ag-trunstrans cis acrolein obs" calc obs" calc calc bond length, 8, 01=Cz 1.340 1.317 1.340 1.318 1.323 CZ-c3 1.468 1.477 1.478 1.484 1.454 c3=04 1.214 1.187 1.215 1.188 1.205 C3-H5 1.113 1.095 1.106 1.093 1.089 CZ-H6 1.084 1.075 1.088 1.076 1.074 cI-H7 1.090 1.076 1.098 1.074 1.075 C I-HX 1.080 1.074 1.081 1.074 1.073 Ag9-04 2.260 bond angle, deg 119.9 Cl=Cz-C3 120.4 121.3 121.5 121.3 124.3 O~=C~-CZ 124.0 123.9 124.2 124.3 119.5 04=Cs-Hs 121.3 120.9 120.1 120.3 122.6 CI==C~-H~ 117.2 122.4 117.4 121.6 121.8 Cz=Cl-H7 119.7 121.5 118.5 120.6 121.8 Cz-Cl-Hx 122.2 122.1 121.5 121.9 Ag9-04-G 172.7 ~
Data taken from ref 14.
TABLE 3: Local Symmetry Coordinates of Acrolein and Silver Ion- Acrolein Complex description" local symmetry coordinateb A' v(C
