12737
J. Phys. Chem. 1994, 98, 12737-12745
Carbon-Halogen Bond Scission and Rearrangement of p-Halohydrins on the
Rh(111) Surface Nicole F. Brown? and Mark A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received: July 20, 1994; In Final Form: September 28, 1994@
Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) studies were performed in order to investigate the decomposition behavior of B-halohydrins (XCH~CHZOH, X = F, C1, Br, I) on Rh(ll1). The goal of these experiments was to synthesize and to characterize oxametallacycles (-CH*CH20-) on the Rh(ll1) surface. The halohydrins did not follow the expected pathway, but a new reaction analogous to the pinacol rearrangement to acetaldehyde was discovered. 2-Iodoethanol and 2-bromoethanol decomposed via this path to release methane at 267 and 252 K, respectively, with about 25% selectivity. 2-Chloroethanol decomposed via a pathway in which methane was liberated at 260 K with a selectivity of 18%, while 2-fluoroethanol decomposition did not produce methane. H2 and CO were also observed as desorbing products during the TPD experiments, while carbon was also deposited on the Rh( 111) surface. Halogen atoms desorbed at high temperature in these experiments. The decomposition of ethylene glycol on Rh( 111) was also studied, since it has the same molecular structure as the ,8-halohydrins, but with an OH group replacing the halogen. Only CO and H2 were detected as desorbing products during ethylene glycol decomposition. No volatile methane was detected, nor was any carbon deposited on the metal surface. All of the carbon and oxygen atoms in the ethylene glycol desorbed in the form of molecular carbon monoxide; C - 0 bond scission did not occur. The reaction pathways and products observed during decomposition of the /3-halohydrins and ethylene glycol demonstrate the importance of the b-CX bond strength (where X = I, Br, C1, F, OH) in determining the reaction pathway and consequently the reaction products.
Introduction Previous studies of the chemistry of oxygenates on the Rh(1 11) surface have provided considerable circumstantial evidence for the formation of surface oxametallacycle intermediates in the decarbonylation of primary alcohols and This behavior is in sharp contrast with that of aldehydes, for which surface acyls appear to be the relevant intermediates and which, unlike alcohols and epoxides, give rise to volatile hydrocarbon products in temperature-programmeddesorption (TPD)experiments on Rh(lll).'s2 Aldehydes react on this surface to form CO, H2, and volatile hydrocarbons one unit shorter than the parent; in contrast, primary alcohols and epoxides release CO and H2 but deposit hydrocarbon fragments on the surface which dehydrogenate without producing volatile carbon-containing products. This difference in product distributions, along with the absence of any evidence from high-resolution electron energy loss spectroscopy (HREELS) for the formation of adsorbed aldehydes or their reaction products from alcohols or epoxides, suggests that alcohols and aldehydes decarbonylate on Rh( 111) via nonintersecting pathways. Previous spectroscopic studies have shown that alcohols form alkoxides by initial 0-H scission on the Rh( 111) s~rface~,~f',~ as on many other transition metal surfaces.*-13 However, Rh(111) appears to be unique with respect to the subsequent reactions preferred by these species; no a-CH scission occurs to form adsorbed aldehydes. We have proposed instead that primary alkoxides undergo C-H scission on Rh( 111) to form surface oxametallacycle intermediates (shown below). The same species would be formed from simple ring-opening reactions of epoxides, and we have previously demonstrated + Present address: Merck & Co., Inc., P.O.Box 2000, R801, Rahway, NJ 07065-0900. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, November 1, 1994. @
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Rh the congruent chemistry of primary alcohols and epoxides on Rh( 11l).4,5 Analogous intermediates have also been suggested by Xu and Friend14-17 to account for reactions such as olefin oxidation to ketones and tert-butyl alcohol dehydration on Rh(111). Although it is virtually impossible to account for the different reactivity patterns of simple oxygenates on Rh( 111) without invoking oxametallacycles, spectroscopic evidence for these intermediates on surfaces is, unfortunately, lacking. The situation is similar for these ligands in transition metal complex chemistry in solution; they are mechanistically indispensable but spectroscopically elusive.18 We have therefore undertaken a variety of studies on Rh(l1 l), attempting both enhancement and inhibition of oxametallacycle formation, in order to build the case for these species on both mechanistic and spectroscopic grounds. We report here an examination of the chemistry of P-halohydrins, undertaken to provide an additional probe of oxametallacycleformation. The original hypothesis behind this work was that, since the carbon-halogen bond in 2-iodoethanol is much weaker than the C-H bond in ethanol, an oxametallacycle might be formed from 2-iodoethanol at sufficiently low temperature to be isolated for spectroscopicexamination. Such an observation would demonstrate the existence of oxametallacycles on this surface and permit the chemistry to be compared with that of the corresponding alcohols and epoxides. However, as will be shown, 2-iodoethanol did not follow this postulated mechanism and actually decomposed through a pathway in which acetaldehyde was an intermediate. Further studies of
0022-365419412098-12737$04.50/0 0 1994 American Chemical Society
Brown and Barteau
12738 J. Phys. Chem., Vol. 98, No. 48, 1994 X-CHZCH~OH(where X = F, C1, Br, I, and OH) on Rh( 111) were carried out to test how these substituents at the /%position affect the reaction pathways leading to or circumventing oxametallacycle formation.
Experimental Section The experimental apparatus and procedures in the present study were the same as those described previously.'-6 Surface composition and structure were verified by AES and LEED. TPD data were acquired with a quadrupole mass spectrometer, multiplexed with an IBM XT. The HREEL spectrometer (McAllister Technical Services) was operated at a beam energy of 5 eV, which produced an elastic peak of ca. 2.0 x lo5 Hz with a fwhm of 70 cm-' when the beam was reflected from a clean Rh( 111) surface. The various organic reagents (2-iodoethanol, 99%; 2-chloroethanol, 99%; 2-bromoethanol, 95%; 2-fluoroethanol, 95%; and ethylene glycol, 99+%; all obtained from Aldrich) were stored in individual glass tubes attached to a stainless steel dosing line. Each was purified by repeated freeze-pump-thaw cycles. These reagents were dosed onto the Rh( 111) sample in ultrahigh vacuum through a 1.5 mm stainless steel needle. The principal methods for restoring the clean surface between TPD experiments were oxygen adsorption and TPD and hightemperature (1350 K) annealing. The latter removed halogen atoms deposited in the course of the various halohydrin decomposition reactions. Oxygen adsorption followed by TPD was used to bum off surface carbon, the amount of which was quantified by measuring the CO and COZsignals in these bumoff TPD experiments. All TPD spectra and coverages reported have been corrected for mass spectrometer sensitivity according to the procedure of KO et al.19 The exposure values reported in langmuirs have not been corrected for ionization gauge sensitivities. All surface coverages were calibrated with respect to the TPD signal for desorption of CO from an ordered (J3xJ3)R3O0 structure corresponding to 0.33 monolayer (ML).
Results A. Halohydrin TPD. In a previous report, it was shown that ethanol and acetaldehyde decomposed on Rh( 111) along dissimilar pathways.' Acetaldehyde decomposed to liberate volatile CH4, CO, and H2, while ethanol decomposed such that only CO and H2 were observed as desorbing products.' Temperature-programmed desorption (TPD) experiments with adsorbed X-CH~CHZOH(where X = F, C1, Br, and I) on Rh(1 11) were performed in order to probe these pathways further. Studies of 2-iodoethanol on Rh( 111) showed that the halohydrin decomposed via a pathway leading to methane as a volatile product. The observation of desorbing methane was in direct contrast to the behavior of ethanol on Rh( 11l), where essentially no methane was produced. It is a surprising result in that decreasing the number of hydrogens around the &carbon of the alcohol increased the yield of a hydrogen-rich hydrocarbon product, methane. Figure 1 shows the TPD spectra for reaction of 0.8 langmuir of 2-iodoethanol on the Rh(ll1) surface. Quantitative analysis of these data indicated that 0.044 ML of methane desorbed at 267 K, while 0.40 ML of HZdesorbed at 272 K and in a small peak at 386 K. Also, 0.18 ML of molecular CO desorbed at 440 K. By adding the number of H atoms contained in the methane and dihydrogen products, it was determined that the ratio of total H2 to CO was equal to 2.7, which reasonably approximates the stoichiometric value of 2.5. The yields resulting from decomposition of the 2-iodoethanol as well as other halohydrins are listed in Table 1. No
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Figure 1. TF'D after an exposure of 0.8 langmuir of 2-iodoethanol on the clean Rh(ll1) surface at 99 K. TABLE 1: Product Yields from the Various Cz Oxygenates co oxygenate exposure, Hz CH4 langmuirs (MU ( M U (ML) (Hz+ OSCI-Lt)/CO 2.1 (2.0)" 0.21 0.091 0.19 1.3, acetaldehyde 2.8 (2.5) 0.40 0.044 0.18 0.8,2-iodoethanol 2.7 (2.5) 2.3, 2-bromoethanol 0.27 0.033 0.13 2.7 (2.5) 1.8.2-chloroethanol 0.39 0.029 0.16 0.14 3.0 (3.0) 0.42 0.00 1.1, ethanol 2.3 (2.5) 0.16 1.7,2-fluoroethanol 0.37 0.00 0.41 1.4 (1.5) 0.58 0.00 0.6, ethylene glycol
Values in parentheses represent the expected stoichiometric ratios. halomethanes or other halohydrocarbons were produced from any of the halohydrins on the Rh( 111) surface. The temperatures at which CHq and HZdesorbed in all cases were very close to those observed for the reaction of acetaldehyde on this surface.' These similarities are indicative of desorption via a common intermediate which will be shown to be acetaldehyde. Molecular 2-iodoethanol was also observed in a peak at 188 K. The methane TPD spectra during decomposition of all of the halohydrins studied, as well as the methane spectra during ethanol and acetaldehyde decomposition, are shown in Figure 2. It can be seen from this figure that all of these molecules yielded volatile methane upon decomposition, except for 2-fluoroethanol and ethanol. Various exposures of these oxygenates on the Rh( 111) surface (between 0.8 langmuir for 2-iodoethanol and 2.3 langmuirs for 2-bromoethanol) were employed in order to produce similar extents of reaction (CO yields). As shown in Table 1, CO yields varied between 0.13 and 0.19 ML. The methane spectra in Figure 2 have been corrected in order to show the amounts of methane desorbed when equivalent amounts of the reactants decomposed; Le., each spectrum has been scaled to a total CO yield of 0.18 ML. As can be observed from Figure 2, a larger fraction of the acetaldehyde decomposed to liberate C b , as compared to that generated from the halohydrins. Approximately 50% of the acetaldehyde which decarbonylated did so via a pathway which produced CHq. From 2-iodoethanol and 2-bromoethanol, methane was produced with about 25% selectivity, as compared to the 18% selectivity to methane production from 2-chloroethanol decomposition. Since all of these reactants produce one molecule of CO for each molecule reacted, methane selectivities represent the ratio of methane to CO produced. Thus, for decomposition of 0.18 ML of the parent molecule, 0.090 ML of methane would be produced from the acetaldehyde as compared to 0.045 ML from the
Decomposition Behavior of P-Halohydrins
J. Phys. Chem., Vol. 98, No. 48, I994 12739
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Temperature (K) Figure 2. Comparison of the CHq TPD spectra for 2-fluoroethanol,
ethanol, 2-chloroethanol, 2-bromoethanol, 2-iodoethanol, and acetaldehyde adsorbed on the clean Rh(ll1) surface. acetaldehyde
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Figure 3. Comparison of the
H 2 TPD spectra for 2-fluoroethanol, ethanol, 2-chloroethanol, 2-bromoethanol, 2-iodoethanol, and acetaldehyde adsorbed on the clean Rh( 111) surface.
2-iodoethanol and 2-bromoethanol. This 50% selectivity toward methane production from acetaldehyde decomposition was consistent with the results of Houtman for acetaldehyde.' When methane was produced during decomposition of the halohydrins, it was observed in a single peak with a maximum rate of desorption at approximately 260 f 8 K. These temperatures are essentially equivalent to that observed during acetaldehyde decarbonylation, 257 K. Since methane does not adsorb on transition metal surfaces at temperatures above 150 K,20 the methane peaks were clearly reaction limited. Furthermore, the similarities in the shape and the position of the methane peaks strongly suggest that methane resulted from a common ratelimiting step, most likely decarbonylation of acetaldehyde. Figure 3 shows the H2 desorption spectra resulting from decomposition of these same halohydrins, ethanol, and acetaldehyde. Dihydrogen was liberated during the decomposition of all reactants. From Figure 3 and Table 1, it can also be seen that a larger amount of H2 desorbed from 2-fluoroethanol decomposition than from acetaldehyde decomposition. Again, this is because half of the hydrogen among the products of
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Figure 4. Comparison of the CO TF'D spectra for 2-fluoroethanol, ethanol, 2-chloroethanol, 2-bromoethanol, 2-iodoethanol, and acetaldehyde adsorbed on the clean Rh( 111) surface.
acetaldehyde decomposition was present in methane, while all of the H atoms liberated from 2-fluoroethanol and ethanol decomposition desorbed as H2. Thus, for 0.18 ML of desorbing CO from acetaldehyde, 0.09 ML of CHq and 0.18 ML of H2 would be evolved, as compared to 0.45 ML of H2 from decomposition of 0.18 ML of 2-fluoroethanol and 0.54ML of HZ from decomposition of the equivalent amount of ethanol. For the other halohydrins, which yielded methane with selectivites of 18-25%, less than 0.45 ML of H2, but more than 0.18 ML, was produced, as shown in Table 1. It is also apparent from Figure 3 that acetaldehyde, 2-iodoethanol, and 2-bromoethanol evolved H2 at approximately the same temperature, 272 K. 2-Chloroethanol released H2 in a peak at 282 K, while ethanol and 2-fluoroethanol gave rise to Hz peaks at approximately 290 K. Broad tails in the H2 spectra at higher temperatures were also observed during decomposition of these oxygenates and resulted from decomposition of surface hydrocarbon fragments. Similar tails in H2 desorption spectra have been observed after adsorption of a variety of hydrocarbons and oxygenates on the Rh( 111) surface. Figure 4 shows the CO desorption spectrum during decomposition of the haloethanols, ethanol, and acetaldehyde. Molecular CO desorbed at approximately 473 K in the TPD spectra of 2-chloroethanol, ethanol, and 2-fluoroethanol, while CO desorbed at lower temperatures for 2-iodoethanol and 2-bromoethanol, at 440 and 435 K,respectively. Acetaldehyde gave rise to CO desorption between 377 and 513 K, with maxima at 424 and 471 K. It has been reported previously by Thielz1that desorption of CO deviates from simple first-order kinetics at higher coverages of CO due to repulsive lateral interactions. For higher coverages of CO, a second desorption peak is generated at lower temperatures than the simple first-order desorption peak observed for small amounts of CO. Furthermore, studies of coadsorbed CO and ethylidynes have shown that ethylidynes produce similar deviations of CO desorption from first-order kinetics, causing CO to desorb at lower temperatures than in the absence of coadsorbates.22 The variations in peak temperature for CO in Figure 4 can also be explained in terms of repulsive interactions between adsorbed CO and the various hydrocarbon and halogen species present on the surface. These reflect the reaction selectivity as well; the methane-forming reagents exhibit preferential population of the CO state desorbing at lower temperatures, as shown in Figure
12740 J. Phys. Chem., Vol. 98, No. 48, 1994
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D (C-X) (kcal/mole) Figure 5. Ratio of the C W C O yield from decomposition of substituted ethanols versus the dissociation energy of the C-X bond.
4. The larger halogen atoms deposited by decomposition of bromo- and iodoethanol would be expected to obstruct a greater fraction of the surface and to cause greater perturbation of the CO desorption spectra, as observed. Sorface carbon was also detected after decomposition of each of these oxygenates. This is consistent with the observation that only a portion of the hydrocarbon fragments potentially available for hydrogenation to methane actually followed this path. The yield of methane from the haloethanols exhibited a clear dependence on C-X bond strength. This dependence is evident in Figure 5, where the ratio of CHq yield to CO yield for decomposition of each substituted ethanol is plotted against the dissociation energy of the C-X bond, where X = I, Br, C1, H, and F. The decomposition selectivity of the haloethanols resembled that of ethanol when the C-X bond strength was greater than approximately 92 kcaYmol (i.e., C-H). However, when the C-X bond strength was less than 68 kcal/mol (Le., C-Br and C-I), the decomposition behavior of the 2-haloethanols paralleled that of acetaldehyde on Rh( 111). 2-Chloroethanol, which has a C-C1 bond strength of 81 kcaYmo1, exhibited some characteristics of both ethanol and acetaldehyde, since the total yield of CH4 produced was noticeably less than that of either the 24odoethanol or the 2-bromoethanol but substantially more than the undetectable amounts from 2-fluoroethanol. The striking difference in methane yields suggests that 2-iodoethanol and 2-bromoethanol decarbonylate via a different mechanism than do ethanol and 2-fluoroethanol. The correlation of methane production with lower carbon-halogen bond strengths and the synthesis of a halogen-free hydrocarbon product (methane) in this case suggest that the route to methane formation involves scission of the C-X bond early in the reaction sequence for iodo- and bromoethanol. The intermediate level of the methane yield from chloroethanol suggests that, as the carbon-halogen bond strength increases, activation of this bond becomes less favorable, and the reaction sequence analogous to that for unsubstituted alcohols becomes kinetically competitive. From previous studies of ethanol and propanol on Rh( 11l), it has been proposed that scission of a P-CH bond of an alkoxide results in formation of an oxametallacycle.1s2 If this reaction sequence were applied to the halohydrins, it would be expected that if initial 0-H scission to form haloalkoxides preceded C-X bond scission, oxametallacycle formation would occur upon subsequent C-X scission. If, however, C-X bond scission initiated decomposition, then new reaction channels might be opened, since the hydroxyethyl (-CH*CHzOH) ligands which
would result from such a step were not implicated in any of the oxygenate chemistry previously observed on this surface. If methane production from the halohydrins occurs via an adsorbed acetaldehyde intermediate, as suggested above, then rearrangement of the halohydrins to acetaldehyde following initial C-X scission would be required. As discussed in detail below, one such rearrangement of P-halohydrins to acetaldehyde is a well-known variant of the pinacol rearrangement in organic chemistry, usually catalyzed by protons or by Lewis acids such as metal cations. This sequence, initial C-X scission followed by rearrangement to acetaldehyde, provides an explanation for the selectivity dependence in Figure 5 in terms of the competition between C-X and 0 - H bond scission. In order to form methane, initial C-X bond scission followed by rearrangement of the resulting intermediate must precede 0 - H scission, or else an oxametallacyclewould result. Figure 5 implies that there are two distinct regimes in which different bond scission steps are favored. For C-X bond strengths less than 70 kcaymol, initial C-X bond scission of the halohydrins occurs, ultimately producing methane as a desorbing product. When the C-X bond strengths are greater than approximately 90 kcal/mol, initial 0-H scission is favored, resulting in &oxide and subsequently oxametallacycle formation; methane is not formed. However, for molecules like 2-chloroethanol with C-X bond strengths between 70 and 90 kcaYmol, both pathways are operative. In the transition between these, the CHq yield is not constant but decreases as the C-X bond strength increases. B. HREELS Experiments. High-resolution electron energy loss spectroscopy (HREELS) experiments were performed in order to probe the intermediates produced during the decomposition of these halohydrins. Figure 6 shows the spectrum of 2-iodoethanol adsorbed on the Rh( 111) surface at 99 K. The v(0H) mode at 3360 cm-I in the liquid infrared spectrum23was not readily observed; however, this mode was not always detected during studies of the hydrogenated alcohols Furthermore, it is assumed that the alcohols must adsorb molecularly, since if the alkoxide were formed and then C-I bond scission followed, oxametallacyclesbut no methane would be generated. The assignments of the modes observed in the 99 K spectrum of Figure 6 are given in Table 2. Between the spectra at 152 and 200 K, the 910 and 1080 cm-' modes diminished while the modes at 1230 and 1325 cm-' increased in intensity. A peak at 655 cm-' also began to grow and was assigned to the d(CC0) mode of the aldehyde, since spectroscopic studies of y2-acetaldehydealso resolved a prominent peak at 610 cm-' assigned to this mode.' The peak at 655 cm-' shifted to about 620 cm-I (closer to the 610 cm-I value observed for Vbldehydes) at higher temperatures. Even though the exact identity of the adsorbates characterized by the 200 K spectrum is not known, the spectrum is also consistent with assignment to a hydroxyalkyl since the OH bond is still intact, as evidenced by the 1230 cm-' peak assigned to the in-plane y(0H) mode. This observation of the y(0H) mode at 1230 cm-I is consistent with the y(0H) mode at 1241 cm-I for solid ethanol.24 This mode was shifted to 815 cm-' for adsorbed ethanol on Rh( 111) since ethanol was bonded to the surface via the lone pair of electrons on the oxygen.' Furthermore, since the most intense peak in spectra of adsorbed alkoxides is typically the v(C-0) mode at approximately 1100 cm-', it is unlikely that the species at 200 K, with a relatively weak feature at this frequency, is an alkoxide. The low intensity and small frequency shift of this peak relative to its value for the intact molecule are both consistent with its assignment to the v(C0) mode of an intact C-OH function not closely aligned with the surface normal. The hydroxyalkyl formed by scission of the
Decomposition Behavior of P-Halohydrins
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Figure 6. HREELS after an exposure of 1.2langmuir of 2-iodoethanol on the Rh( 111) surface at 99 K and HREELS after subsequent heating of the crystal to higher temperatures.
C-I bond of 2-iodoethanol would be bound to the surface via the P-carbon. Studies of adsorbed alkyl iodides on Pt( 111)25 showed that C-I bond scission occurred between 170 and 240 K, while C-I bond scission occuned by 100 K on Ag( 111).26 Thus, cleavage of the C-I bond of 24odoethanol by 200 K on Rh( 111) is consistent with the typical barriers for brealung such bonds on metal surfaces. The spectrum at 224 K is characterized by the onset of decarbonylation, illustrated by the v(C0) mode of linear carbon monoxide at 1970 cm-l and the v(C0) mode of bridging CO at 1710 cm-'. No mode was detected at 1700 cm-' during the decomposition of ethanol, but a peak consistent with bridging CO was observed in this region during decomposition of acetaldehyde. The strong mode at 1285 cm-' representative of the intact OH group of the hydroxyalkyl was still observed at 224 K. However, the peaks in the 249 K spectrum at 620, 990, 1120, 1315, 1415, and 2900 cm-' were consistent in both position and relative intensity with those observed following adsorption of acetaldehyde on this surface upon heating to this temperature.' Thus, the spectrum at 249 K can be explained by the presence of only adsorbed aldehyde species and adsorbed carbon monoxide. By 279 K, the aldehydes were nearly completely decarbonylated, which is consistent with the TPD results which showed that methane
J. Phys. Chem., Vol. 98, No. 48, I994 12741 TABLE 2: Mode Assignments and Frequencies (cm-l) for Adsorbed Halohydrins X-CH~CHIOHon Rh(111) at ca. 99 K, X = liquid phase2* modesZ8 chloroethanol F C1 Br I v(OH), t 3500 3365 WW. g 3255 v(CW 3000 2975 2950 2855 2940 2957 2932 2876 (CHz) scissor 1454 1430 1430 1425 1430 1430 1430 1390 1325 (CHd wag Y(OH), g 1384 (CH2) twist, g 1297 1250 1280 1250 1135 1237 1250 1280 1145 1135 CHd twist. t (OH) twist, t V(C0) 1073 1080 1070 1045 1080 1055 WC) 1029 1080 1070 1045 1080 e(CH2) 938 885 830 975 910 869 815 846 748 v(C-X), t 661 v(C--X), g Y(CC0) 474 525 470 410
desorbed in a peak at 267 K. By 331 K, the intensity of the CO peak increased. The loss at 465 cm-' is characteristic of the v(M-CO) mode of adsorbed CO. The remaining lowintensity modes in this spectrum were assigned to hydrocarbon fragments on the surface. A high-temperature hydrogen peak was detected at 386 K during TPD experiments with 2-iodoethanol. A similar peak was detected during acetaldehyde decomposition and assigned to the dehydrogenation of surface hydrocarbon fragments observed in the HREELS. The spectrum at 399 K in Figure 6 shows only the modes associated with adsorbed CO. No Rh-I modes (which should have frequencies of about 175 cm-' 27) could be resolved from the elastically scattered peak in HREELS, although atomic I was observed to desorb at approximately lo00 K in TPD experiments. Adsorbed CO desorbed in a peak with a maximum at 440 K; no CO modes remain in the 527 K spectrum of Figure 6. As shown by the TPD experiments, 2-iodoethanol and 2-bromoethanol decomposed to yield the same products at essentially the same temperature and with comparable selectivities. These results imply that these two halohydrins decompose via a common pathway with common intermediates. These similarities were evident from HREELS. As was the case for 2-iodoethanol, 2-bromoethanol adsorbed intact at 99 K. A comparison of the vibrational modes observed with those of the other halohydrins is given in Table 2. The v(0H) mode was not readily observed, but this bond was assumed to be intact for the same reasons discussed for 2-iodoethanol. After heating to 205 and 228 K, the losses for the bromoethanol-derived adlayer were slightly more intense, especially for the y(0H) mode at ca. 1280 cm-l, but these spectra were essentially otherwise indistinguishable from those for the iodoethanolderived layer at comparable temperatures. As noted above for 2-iodoethanol, the spectra at 224-249 K are attributable primarily to adsorbed acetaldehyde. By 256 K, the adsorbed acetaldehyde from bromoethanol had almost completely decarbonylated, as determined from the loss of intensity of vibrations between 1000 and 1500 cm-' and the appearance of a strong v(C0) mode for molecular CO at 2040 cm-'. At 324 K some CH modes still persisted, consistent with the TPD results which showed a high-temperature H2 peak at 378 K. The HREELS results for 24odoethanol and 2-bromoethanol were thus quite similar. Both halohydrins adsorbed molecularly
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12742 J. Phys. Chem., Vol. 98, No. 48, 1994 x 9001 x9
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. . . .
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Energy Loss (cm-l) Figure 8. HREELS after an exposure of 1.3 langmuir of 2-chloroethanol on the Rh(ll1) surface at 93 K and HREELS after subsequent heating of the crystal to higher temperatures.
heating of the crystal to higher temperatures. at 99 K via the lone pair of electrons on the oxygen; however, rearrangement occurred by about 200 K such that a hydroxyalkyl bonded via the P-carbon and not through the oxygen was formed. This hydroxyalkyl was converted to y2-acetaldehyde species by approximately 225 K, and the chemistry thereafter followed that of adsorbed acetaldehyde. HREELS studies of 2-fluoroethanol adsorbed on the Rh( 111) surface at 95 K were also performed, and these results are illustrated in Figure 7. As was the case with the other halohydrins, 2-fluoroethanol was adsorbed molecularly. The vibrational frequencies and their assignments are contained in Table 2. These assignments were made by comparing the vibrational frequencies for the adsorbed alcohol with the alcohol in the liquid phase.23 The spectrum at 208 K did not show any significant changes in most vibrational frequencies, although the y(CC0) mode increased in frequency to 580 cm-' and the falloff of the v(C-H) peak at high frequency (due to removal of weak contributions from 0-H stretch modes above 3000 cm-') was sharper. These changes parallel those observed for dissociation of ethanol to form ethoxides, and the 208 K spectrum for fluoroethoxides in Figure 7 is remarkably similar to that for the nonhalogenated variety reported previously.' This similarity is not surprising, since the only contribution of CF modes to the fluoroethanol spectrum directly overlaps the prominent C - 0 stretching modes at 1080 cm-'. The spectrum at 224 K, however, showed that the e(CH2) mode at 885 cm-' was attenuated, while the CH2 wag mode at 1400 cm-I was still visible. The coupled v(C-0), v(C-F), and v(C-C) modes at 1095 cm-I were still observed. The spectrum at 247 K was quite similar to that observed at 224 K, and the changes in this spectrum in this temperature region are also analogous to those for ethanol, with much stronger attenuation of the 880 and 1400 cm-I modes than of the 1100 cm-' modes. By 278 K, new modes at 1185 and 2025 cm-' developed. The loss at 2025 cm-' is the v(C0) mode of adsorbed carbon monoxide and is indicative of decarbonylation. The peak at 1185 cm-' is consistent with the peak observed at 1200 cm-I in the 258 K spectrum for 2-chloroethanol and at 1210 cm-' in the 272 K spectrum for ethylene oxide5 and is assigned to CH2 fragments on the surface. The intensity of the v(C0) loss was surprisingly weak, as was the case for this same mode during studies of 2-chloroethanol, These decreases in the intensity are most likely
a result of the electronegative fluorine and chlorine atoms altering the surface dipole. Similar effects can be observed in the 314 and 365 K spectra. The persistence of v(C0) and Y(CH) modes in the 365 K spectrum is consistent with the TPD results which showed CO desorbing in a peak with a maximum at 473 K and H2 desorbing at temperatures up to 450 K. The HREELS and TPD results for 2-fluoroethanol on Rh(1 11) showed that decomposition occurred via a different pathway from that for 2-iodoethanol and 2-bromoethanol decomposition on Rh(111); the former produced distinguishable fluoroalkoxide species, and the latter two reacted to form adsorbed acetaldehyde. The methane yields in Figure 5 showed that the decomposition of 2-chloroethanol on Rh( 111) fell between these limiting cases, and the HREELS studies for 2-chloroethanol on Rh( 111) confirm this conclusion. The HREEL spectra for adsorbed 2-chloroethanol on Rh( 111) are depicted in Figure 8. The 93 K spectrum exhibits vibrations associated with the molecular alcohol, and these modes and their assignments are given in Table 2. In this spectrum, the Y(OH) mode at 3255 cm-' is clearly visible, and this spectrum is well resolved and shows a strong peak at 1070 cm-' which is associated with the v ( C 0 ) mode of the alcohol. The spectrum for molecular 2-chloroethanol most closely resembled that of 2-bromoethanol, as expected, but with greater intensity and resolution of nearly all modes. No noticeable changes in the spectrum were observed at 150 K, but by 204 K there was a significant decrease in the intensity of the peaks, as well as a degradation in resolution. The 204 K spectrum showed weaker modes at 1070 and 1430 cm-' than previously observed. A peak at 610 cm-' began to grow and was assigned to the 6(CCO) mode of acetaldehyde, as previously discussed for 2-iodoethanol. More changes in the spectrum were observed by 258 K. In fact, the 258 K spectrum of 2-chloroethanol resembled the spectra at approximately 225 K for 2-iodoethanol and 2-bromoethanol, although the y(0H) mode of the hydroxyethyl species was less intense in the former case, consistent with the lower yield of methane formed via this pathway. Also at 258 K, decarbonylation of the 2-chloroethanol had begun, as is evident from the peak at 2060 cm-'. A small peak at ca. 1200 cm-l appeared in the spectrum above 250 K, as for 2-bromoethanol and 2-fluoroethanol. A similar peak was observed at 272 K during spectroscopic studies of ethylene oxide on Rh(111) and was assigned to CH2 fragment^.^ The spectrum of
Decomposition Behavior of P-Halohydrins
1
J. Phys. Chem., Vol. 98, No. 48, 1994 12743
I
214K
x
20 1
x 10
236 K
2920
1075
i
2910
100
200
300
400
500
MM
700
800
Temperature (K)
Figure 9. TPD after an exposure of 0.6 langmuir of ethylene glycol on the clean Rh( 111) surface at 99 K.
the 2-chloroethanol adlayer at 303 K showed adsorbed CO and surface hydrocarbons, consistent with the TPD results which showed both CO and some HZdesorbing above this temperature. Even with some of the ambiguity in detailed interpretation of the HREEL spectra of the halohydrins, it is clear that 2-fluoroethanol reacted via a pathway unlike that for the other halohydrins. In fact, 2-fluoroethanol on Rh( 111) decomposed via a pathway similar to that for ethanol on Rh( 11l), forming the corresponding fluoroalkoxide and, presumably, the transient (fluorinated) oxametallacycle. The HREEL spectra for 2-iodoethanol, 2-bromoethanol, and 2-chloroethanol did not differ significantly from each other once the parent molecules had desorbed. This observation is consistent with the TPD results which showed that these three halohydrins liberated volatile methane, unlike 2-fluoroethanol. C. Ethylene Glycol. The decomposition behavior of ethylene glycol on Rh( 111) was also investigated. TPD experiments showed that C-X bond scission of X-CHzCHzOH when X = OH did not occur at any point in the decomposition sequence, unlike the previously described cases when X = I, Br, C1, H, or F. No volatile methane was observed during decomposition of the ethylene glycol, nor would it be expected based on the correlation of CHq selectivity with C-X bond strength shown in Figure 5. The bond strength of the C-OH bond of the ethylene glycol (91 k~al/mol)*~ is approximately the same as that of the C-H bond of ethanol (95 k c a l / m ~ l ) , ~ ~ and the results of Figure 5 suggest that initial 0-H bond scission is preferred for such species. The epoxide^^,^ and tertbutyl a l c o h 0 1 ~ are~ ~ the~ ~only reactants on Rh( 111) observed to date that decompose via C - 0 bond cleavage. Figure 9 shows the TPD spectrum for ethylene glycol decomposition on Rh(1 11). The only volatile decomposition products were CO and Hz. Essentially no surface carbon was detected after the TPD experiments. This observation supports the notion that C-C scission but not C-0 scission occurred, liberating two CO and three H2 molecules per molecule of ethylene glycol decomposed. The product yields found in Table 1 show that 0.41 ML of CO and 0.58 ML of HZdesorbed during ethylene glycol decomposition. These yields correspond to a H2KO ratio of 1.4, essentially the value expected for the above reaction scheme. The desorption of molecular ethylene glycol included a second layer with a coverage of 0.20 ML. This intense peak was observed at 214 K, while a small shoulder for adsorbed
95K I
0 200
I
1
600
I
1000
I
I
1400
ISW
l
I
2200 2600
1
3ow
3, 0
Energy Loss (cm-I)
Figure 10. HREELS after an exposure of 0.87 langmuir of ethylene glycol on the Rh(ll1) surface at 95 K and HREELS after subsequent heating of the crystal to higher temperatures.
TABLE 3: Mode Assignments and Frequencies (cm-') for Ethylene Glycol mode liquid3' ~ g (10132 i Rh(ll1) 3275 3290 3220 2935 2900 2930 2875 2900 2930 1459 1450 1445 1405 1445 1410 1332 1212 1205 1090 1080 1087 1038 887 870 875 864 870 875 700 710 745 478 540 360
molecules was observed at 230 K. This shoulder is consistent with desorption of the parent molecule from the monolayer state. HREELS studies of ethylene glycol on Rh( 111) were also performed in order to aid in elucidating the decomposition sequence of the diol on Rh( 111). The results of these experiments are illustrated in Figure 10. Ethylene glycol adsorbed molecularly at 95 K on Rh(ll1). Comparison of the 95 K spectrum with the spectra for ethylene glycol in the liquid phase31 and adsorbed on Ag(l10)32 is shown in Table 3. Ethylene glycol was stable on the Rh( 111) surface up to about 211 K, by which temperature the v(0H) mode at 3295 cm-l clearly diminished. There are also other subtle changes in the 211 K spectrum, especially the broad shoulder on the lowfrequency side of the 1430 cm-' peak, which are most likely indicative of adsorbed 1,2-dioxyethyleneintermediates. By 236 K, all modes began to attenuate, and by 262 K, all were poorly resolved. The v ( C 0 )mode at 2010 cm-' was identifiable above
Brown and Barteau
12744 J. Phys. Chem., Vol. 98, No. 48, 1994 ca. 260 K, indicating that decarbonylation had begun. By 309 K, very few modes were observed; the mode at 2025 cm-’ for molecular carbon monoxide was the only intense mode detected. Spectra obtained after heating to higher temperatures (353 and 381 K, not shown) further demonstrated that hydrocarbon fragments were not formed during ethylene glycol decomposition; there was no evidence for perturbation of the CO HREEL or TPD spectra by such species, nor were there high-temperature hydrogen desorption peaks characteristic of the dehydrogenation of such fragments.
Discussion On the basis of the TPD results for halohydrins on Rh(l1 l), it is clear that the strength of the carbon-halogen bond is the dominant factor in determining the decarbonylation pathway of these alcohols on Rh( 111). If the carbon-halogen bond is sufficiently strong, the haloethanol decomposes via the same pathway as that for ethanol, Le., via sequential formation of ethoxide and oxametallacycle intermediates. However, when the C-X bond is relatively weak, carbon-halogen scission appears to precede 0-H scission, ultimately leading to methane evolution during decarbonylation. This path leads through an aldehyde intermediate, and spectroscopic evidence for adsorbed acetaldehyde in the course of 2-iodoethanol and 2-bromoethanol decomposition was obtained by HREELS. There are numerous examples in the organic and organometallic literature of similar rearrangements to acetaldehyde. Examples include the Wacker p r o c e ~ s , where ~ ~ . ~ethylene ~ is oxidized by a palladium(I1)copper(I1) chloride solution to form acetaldehyde. The pinacol through which a diol or halohydrin is converted into either an aldehyde or a ketone can be catalyzed by Bransted or Lewis acids. Even though the actual mechanism of acetaldehyde formation from the halohydrins in the present work is not known and is not obvious from spectroscopic results, a mechanism based on the precedents above can be proposed which explains the experimental results while maintaining consistency with the details of oxygenate chemistry previously demonstrated on Rh(111). The dehalogenation, dehydrogenation, and rearrangement of the halohydrins to acetaldehyde are most easily described by a sequence of steps similar to those in the pinacol or semipinacol rearrangement. This reaction is illustrated by the following Bransted acid-catalyzed examples: 40,41
P
CH3 CH,
I I
1 I
CH,-C-C-CH3
L
H p 4
OH OH Pinacol (diol)
+ H20
CH,-C-C-CH,
I
CH3 Pinacolone (ketone)
H
H
H
O
I
1
I
11
I
I
H-C-C-H
OH OH Ethylene glycol (diol)
+ H p 4
H-C-C-H
I
+H20
H Acetaldehyde (aldehyde)
The term “semipinacol rearrangement” is used to describe rearrangement through removal of the secondary hydroxyl group from a diol which contains both secondary and tertiary hydroxyl g r o ~ p s . ~When ~ , ~the ~ secondary hydroxyl is replaced by a better leaving group, such as a halogen, then the semipinacol reaction can be forced to take place at the expense of, as well as to the exclusion of, removal of the tertiary hydroxyl group. Numerous examples exist of carbon-halogen bond scission of halohydrins in solution to form ketones or aldehydes via semipinacol rearrangements. For example, Alexander and Dittmer35reported
HZC=CHOH
I H
I
\ (5)
CH3-CHOH
H
’
‘
cg-Lo
i
/ \ I
H
4
cHq(g)
Figure 11. Possible paths for the decarbonylation of 2-iodoethanol on the Rh( 111) surface.
that 3-chloro-2-butanol in the presence of AgN03 reacted to form methyl ethyl ketone. This reaction occurred via carbonhalogen bond scission followed by a shift of hydrogen from the 2-position to the carbenium center on the third carbon. Lane and W a l t e r ~studied ~~ the action of mercuric ion upon an aqueous dioxane solution of 2-bromo- 1,l ,2-triphenylethanol to yield benzhydryl phenyl ketone. Collins and B ~ n n e studied r~~ this same reaction using 14Cto track the reaction sequence. The results using the isotopic labeling showed that the phenyl on the 1-carbon shifted to the 2-carbon (to which the bromine was originally attached) of the carbenium ion initially produced by halogen removal by a Lewis acid. A mechanism which can account for halohydrin decarbonylation on Rh(l11) is illustrated in Figure 11. It is proposed that the halohydrin (1) bonds initially to the Rh(ll1) surface via the lone pair of electrons on the oxygen atom. When the energetics favor carbon-halogen bond scission, the resulting hydroxyalkyl species (2) is formed. This species is bonded to the surface through the P-carbon, not through the oxygen. Activation of halohydrins by concerted elimination of HX is discounted for two reasons. First, no HX desorption from any of the 2-haloethanols was observed, although the formation and desorption of HF from 2,2,2-trifluoroethanol demonstrated that such products could be detected if formed.30 Second, the observation of relatively intense y(0H) modes in the HREEL spectra of these molecules above 200 K also provided support for the formation of stable hydroxyethyl intermediates. This ligand is converted to bound acetaldehyde as in the Wacker reaction. The mechanism of this conversion is not clear, although two possibilities, both of which include P-hydride scission, may be considered. The first involves P-hydride scission of the hydroxyethyl intermediate (2) to form species (3), adsorbed vinyl alcohol, and the second involves a P-hydride transfer step to form intermediate (4). Vinyl alcohol, which is of course unstable even in the absence of a reactive surface, rapidly undergoes tautomerization to acetaldehyde. Intermediate (4) would also produce acetaldehyde via 0 - H bond scission. Decarbonylation of an y*-aldehyde intermediate (5) produced by either of these routes occurs via a-CH bond scission to form an acetyl, which then undergoes C-C bond scission to release a methyl group to the surface; this ligand is hydrogenated to methane. The intramolecular hydrogen shift which would be involved in the direct conversion of (2) to (4) is directly analogous to the intramolecular shift in the semipinacol rearrangement of halohydrins to form aldehydes. If this pathway indeed describes the sequence of halohydrin reactions on Rh-
Decomposition Behavior of P-Halohydrins (1 1l), it would represent the first example of this reaction on a zerovalent metal. It should be noted, however, that extended surfaces may provide opportunities for hydrogen shift and hydrogen transfer reactions not available in homogeneous media. For example, concerted intermolecular hydride transfer is the crucial step in the base-catalyzed Cannizzaro reaction in solution,"2but on basic metal oxides it has been suggested that this transfer occurs via sites on the ~urface.4~ Thus, the net semipinacol rearrangement of halohydrins on metal surfaces may well involve hydrogen transfer to and from the surface, rather than the intramolecular 1,2-hydride shift as in solution. Unfortunately, because the ultimate volatile product of this reaction on Rh( 111) is methane (which must acquire hydrogen from the surface) rather than acetaldehyde, isotopic labeling experiments provide little opportunity to resolve the mechanism to this level of detail. Ethylene glycol, a 1,2-diol, clearly did not react via pinacol rearrangement on Rh(ll1). No volatile methane and no adsorbed acetaldehyde were observed. The ratio of HdCO was approximately 1.5, which implies that this molecule dehydrogenateddecarbonylated cleanly, releasing both C-0 functions as molecular CO. In TPD experiments after exposing the clean Ag( 110) surface to various doses of ethylene glycol, Capote and Madix observed that the ethylene glycol desorbed molecularly without reacting.32 Monolayer desorption was observed at 250 K while desorption from a multilayer state was observed at 225 K. When 20 langmuirs of ethylene glycol was dosed to a Ag(ll0) surface covered with 0.25 ML of oxygen atoms, ethylene glycol reacted with the atomic oxygen to form adsorbed ethylenedioxy (-OCH2CH20-) intermediates and H ~ 0 . 3The ~ ethylene dioxy intermediates were stable up to 350 K, decomposing to produce ethylene glycol at 365 K, glyoxal [(CH0)2] at 380 K, and H2 at both 350 and 380 K. However, when only 1.5 langmuirs of (CH2160H)2 was exposed to an Ag( 110) surface covered with 0.25 ML of I80(ad), i.e., with oxygen in excess, the surface oxygen atoms reacted with the ethylenedioxy at 300 K to form gaseous water and formaldehyde, along with surface formate which decomposed to COz and H2 at 415 K.44 Based on the results for ethylene glycol adsorption on Ag(110) and the HREELS results of this study, adsorption of the ethylene glycol on Rh( 111) most likely also occurs via both oxygen atoms. Next, 0-H bond scission occurs to form a dioxy intermediate. This intermediate could decompose via either C-C or C-H bond scission, but the formyl and formaldehyde intermediates which would be produced if C-C scission occurred before complete dehydrogenation are both unstable. Adsorbed formaldehyde decomposes completely, presumably via formyls, by 150 K on Rh(l.1l);45only CO and H2 desorb, just as for ethylene glycol. Thus, the sequence of C-C and C-H steps cannot be resolved. Finally, the results for ethylene glycol decomposition are not surprising, since C-0 bond scission is not a prevalent step for any of the monoalcohols examined on Rh( 111); 0 - H scission is the typical first step in alcohol activation on group VIII metals. Indeed, it is the ability of the @-halohydrinsto provide an alternative point of attack that has few precedents in the surface science literature.
Conclusions @-Halohydrinsdecompose on Rh( 111) via different decomposition pathways, depending on the strength of the carbonhalogen bond. When the C-X bond strength is greater than approximately 90 kcaUmo1, decomposition occurs via initial 0-H scission of the alcohol to form the alkoxide, which undergoes P-CH scission to form an oxametallacycle. However,
J. Phys. Chem., Vol. 98, No. 48, 1994 12745 when the C-X bond strength is less than 68 kcallmol, initial carbon-halogen scission initiates decomposition via a pathway in which an aldehyde is formed and decomposes to liberate volatile methane. Between these two regimes, these two pathways compete. The strength of the C-0 bonds in ethylene glycol did not allow decomposition via this pinacol rearrangement mechanism. The use of halohydrins did not provide a low-temperature route to stable oxametallacycles on Rh( 11l), as the dissociation of C-I and C-Br bonds preceded scission of the OH bond, opening a new reaction pathway not accessible from the oxygenate reagents previously examined.
Acknowledgment. We gratefully acknowledge the support of this research by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences (Grant FG0284ER13290). References and Notes Houtman, C. J.; Barteau, M. A. J. Catal. 1991, 130, 528. Brown, N. F.; Barteau, M. A. Langmuir 1992, 8, 862. Brown, N. F.; Barteau, M. A. J . Am. Chem. SOC.1992,114,4258. Brown, N. F.; Barteau, M. A. In Selectivity in Catalysis; Davis, M. E.,Suib, S. L., Eds.; ACS Symp. Ser. 1993, 517, 345. Brown, N. F.; Barteau,-M:A. Surf. Sci. 1993, 298, 6. Houtman, C.; Barteau, M. A. Langmuir 1990, 6, 1558. Davis, J. L.; Barteau, M. A. S u ~ Sci. . 1990, 235, 235. Gates, J. A.; Kesmodel, L. L. J. Catal. 1983, 83, 437. Hrbek, J.; DePaola, R.; Hoffman, F. M. Surf. Sei. 1986, 166, 361. Christ", K.; Demuth, J. E. J. Chem. Phys. 1982, 76, 6308. Bare, S. R.; Stroscio, J. A.; Ho, W. Surf. Sei. 1985, 150, 399. Gates, S. M.; Russell, J. N.; Yates, Jr.. J. T. Sur$ Sei. 1985, 150, (1) (2) (3) (4)
Barteau, M. A. Catal. Lett. 1991, 8, 175. Xu, X.; Friend, C. M. J . Am. Chem. SOC. 1991, 113, 6779. Xu, X.; Friend, C. M. Langmuir 1992, 8, 1103. Xu, X.; Friend, C. M. Sulf. Sci. 1992, 260, 14. Xu, X.; Friend, C. M. J. Phys. Chem. 1991, 95, 10753. Jprrgensen, K. A.; Schiott, B. Chem. Rev. 1990, 90, 1483. KO,E. I.; Benziger, J. B.; Madix, R. J. J. Catal. 1980, 62, 264. Ceyer, S. T. Annu. Rev. Phys. Chem. 1988, 39, 479. Thiel, P. A.; Williams, E. D.; Yates, Jr., J. T.; Weinberg, W. H. Surf. Sei. 1979, 84, 54. (22) Blackman, G. S.; Kao, C. T.; Bent, B. E.; Mate, C. M.; Van Hove, M. A,; Somorjai, G. A. Surf. Sci. 1988, 207, 66. (23) Wyn-Jones, E.; Orville-Thomas, J. J . Mol. Struct. 1967-1968, 1, 79. (24) Barnes, A. J.; Hallam, H. E. Trans. Faraday SOC.1984, 66, 1960. (25) Henderson, M. A,; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 184, L325. (26) Zhou, X.-L.; White, J. M. J. Phys. Chem. 1991, 95, 5575. . (27) Adams, D. M. Metal-Ligand and Related Vibrations; St. Martin's Press: New York, 1967; p 69. (28) Buckley, P.; Giguere, P. A.; Schneider, M. Can. J. Chem. 1969, 47, 901. (29) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL;1989; p F213. (30) Brown, N. F. Ph.D. Dissertation, University of Delaware, 1993. (31) Sawodny, W.; Niedenzu, K.; Dawson, J. fV. Spectrochim. Acta 1967, 23A, 799. (32) Capote, A. J.; Madix, R. J. J. Am. Chem. SOC. 1989, I l l , 3570. (33). Smidt, J.; Hafner, W.; Jira, R.; Sedlmeier, I.; Sieber, R.; Riittinger, R.; Kojer, H. Angew. Chem. 1959, 71, 176. (34) Smidt, J.; Hafner, W.; Jira, R.; Sieber, R.; Sedlmeier, J.; Sabel, A. Agnew. Chem., hi.Ed. Engl. 1962, 1, 80. (35) Alexander, E. R.; Dittmer, D. C. J. Am. Chem. SOC.1951,73, 1665. (36) Bennett. G. M.: Chaoman. A. W. Ann. ReD. Chem. Soc. ( h u f o n ) 1930, 27, 114. (37) Collins, C. J.: Bonner, W. A. J . Am. Chem. SOC. 1953, 75, 5379. (38) Lane, J. F.; Walters, D. R. J. Am. Chem. SOC.1951, 73, 4234. (39) Tiffeneau, M.; Levy, J. Bull. SOC.Chim. Fr. 1923, 33, 758. (40) Fittig, R. Ann. Chem. 1859, 110, 17. (41) Collins, C. J.; Eastham, J. F. In The Chemistry of the Carbonyl Group; Patai, S., Ed.; Wiley: New York, 1966; p 761. (42) March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure; Wiley: New York, 1985. (43) Peng, X. D.; Barteau, M. A. Langmuir 1989, 5, 1051. (44) Capote, A. J.; Madix, R. J. Surf. Sei. 1989, 214, 276. (45) Houtman, C. J.; Barteau, M. A. Surf. Sci. 1991, 248, 57.
