Synthesis and Spectroscopic Identification of Ethylidyne Adsorbed on

4952

J. Phys. Chem. B 1998, 102, 4952-4965

Synthesis and Spectroscopic Identification of Ethylidyne Adsorbed on Ni(111)⊥ T. Bu1 rgi,† T. R. Trautman,‡ K. L. Haug, A. L. Utz,§ and S. T. Ceyer* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed: February 20, 1998; In Final Form: April 15, 1998

The interaction of ethylene adsorbed on Ni(111) with gas-phase H atoms has been investigated. The major adsorbed reaction product is identified by high-resolution electron energy loss spectroscopy to be ethylidyne (C-CH3). This study is the first direct spectroscopic observation of a C-CH3 species adsorbed on Ni in an ultrahigh-vacuum environment. Spectra of four isotopomers, C-CH3, 13C-13CH3, C-CD3, and 13C-13CD3, are reported, and a complete and consistent vibrational assignment of their fundamental modes is presented. Based on this assignment, a force field is derived from the measured vibrational frequencies using a normalmodes analysis and is found to be in good agreement with that deduced from IR spectra of an ethylidyne species in an organometallic complex. Inspection of the eigenvectors of the normal-mode displacements reveals that substantial mixing of harmonic bond motions is the origin of the unusual upshift in frequency of the C-C stretching mode upon deuteration. A quantitative determination of the relative dynamic bond dipole moments demonstrates that the changes in intensity and dipole activity of the C-C stretching and symmetric CH3 deformation modes upon deuteration, phenomena common to all C-CD3 spectra, also arise from extensive mixing of bond motions. A detailed analysis of the spectra strongly suggests a C3V or C3 local environment for ethylidyne and a 3-fold hollow adsorption site.

I. Introduction Since the mid-1970s, ethylene has been known to adsorb molecularly on several different transition metals at low temperatures and to rearrange to the same adsorbed product when the surface was raised to room temperature or beyond. However, considerable confusion about the chemical identity of its product reigned, even despite use of vibrational spectroscopy1 as the probe. Presently, there is general agreement that the rearrangement of adsorbed ethylene leads to the formation of ethylidyne, C-CH3, which has since been identified by vibrational spectroscopy on a myriad of single-crystal surfaces in a UHV (ultrahigh vacuum) environment.2-19 Specifically, it is formed in a thermal process by rearrangement of ethylene on Pt(111),2,6,7 Pd(111),14,15 and Rh(111)11 at around 300 K, on Rh(100) between 200 and 300 K,13 on Ru(001) between 150 and 280 K,18 and on Ir(111) below 180 K.16 Ethylidyne is also formed from acetylene in the presence of coadsorbed hydrogen on Pt(111) at 350 K,4 on Pd(111) at 300 K,14,15 and on Rh(111) at 270 K.11 On Ru(001), acetylene rearranges at 230-250 K to a mixture of C-CH3 and C-CH.18 Finally, ethylidyne has been noted to form from methyl radicals adsorbed on Pt(111) at 230 K.19 Notably, on Ni, a spectroscopic identification under UHV conditions of adsorbed ethylidyne has not been reported. Unlike their adsorption on other transition metals, ethylene and acetylene do not rearrange to form C-CH3. Rather, C2H4 decomposes to C2H2 at 230 K,20,21 and C2H2 further decomposes † Present affiliation: Laboratory of Technical Chemistry, ETH Zentrum, Zu¨rich. ‡ Present affiliation: Paramins, Linden, NJ. § Present affiliation: Department of Chemistry, Tufts University. * To whom correspondence should be addressed. ⊥ Having missed inclusion in the J. L. Kinsey Festschrift issue (J. Phys. Chem. 1996, 100), S.T.C. dedicates this paper to Jim, in admiration of his brilliance and his elegant science and with deep appreciation for being my colleague, mentor (“Put methane in the beam!”), and friend.

to C-CH or forms benzene at high coverages22-24 above 400 K. Methyl radicals on Ni(111)22,23 do not form C-CH3, but instead dissociate to CH which then recombines to form C2H2. The purpose of this work is to present the vibrational spectroscopic identification of C-CH3 adsorbed on Ni(111) which is produced by the reaction between adsorbed ethylene, acetylene, or ethane and gas-phase hydrogen atoms.24,25 The use of gas-phase H atoms to form ethylidyne is not common, but it is not new either. Ethylidyne forms when an acetylenecovered Pt(111) surface held at 280 K is exposed to gas-phase H atoms,4 although the authors erroneously assigned the highresolution electron energy loss spectrum (HREELS) of the newly formed species to ethylidene (CH-CH3) instead of C-CH3 at that time. A similar effect was noted for C-CH3 formation from C2H2 on Rh(111).11 In contrast, the chemistry of the gasphase H atom has been extensively studied in the field of diamond film growth, once its critical role was recognized.26,27 H atoms not only adsorb on but also abstract hydrogen from diamond films, creating a dangling bond site for the hydrocarbon which acts as the growth species. An investigation of H atoms reacting with C:H films, using HREELS, showed that sp and sp2 carbon are hydrogenated to sp3 during exposure to H atoms28 and that H atoms induce chemical erosion of those C:H films.29 It was also shown that H atoms have the ability to abstract hydrogen from a hydrocarbon film.30,31 While vibrational spectroscopy has been used in a few studies to identify the adsorbed products of the reactions of adsorbed hydrocarbons with H atoms,32,33 most of the more recent studies34-40 have used thermal desorption measurements to probe the adsorbed reaction products. However, identification of C-CH3 by thermal desorption methods is tenuous because C-CH3 decomposes rather than desorbs upon raising the temperature. Another recent study reports the formation of C-CH3, as detected by temperature-programmed secondary ion emission, upon high exposures of Ni(111) to C2H4 and C2H2.41

S1089-5647(98)01223-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/30/1998

Ethylidyne Adsorbed on Ni(111) Once again, the present demonstration of the ease of C-CH3 formation by reaction of a radical such as a H atom with a hydrocarbon raises caution about utilization of an energetic environment as part of a technique for identification of C-CH3. In fact, this result41 remains unsubstantiated by a vibrational spectroscopic study of this system20-24 and underlines the importance of a direct spectroscopic method of detection. In the present study, high-resolution electron energy loss spectroscopy is used to identify C-CH3 as the nascent, adsorbed product of the reaction of H atoms with C2H4 adsorbed on Ni(111) at 120 K. Spectra of four different isotopomers of ethylidyne C-CH3, C-CD3, 13C-13CH3, and 13C-13CD3 are presented. The scarcity of 13C-labeled ethylidyne spectra has led to some confusion in the assignment of the C-C stretching mode42 because this mode shifts up in frequency upon deuteration, an observation common to all spectra of C-CD3.2,7,12,14,18 Another typical phenomenon observed in spectra of ethylidyne upon deuteration is the drastic change in the relative intensities of the C-C stretching and CH3 symmetric deformation modes which complicates mode assignments based solely on spectra of C-CH3 and C-CD3. The isotopic labeling of the carbon in combination with the high-resolution capabilities of the spectrometer used in this work permits the unambiguous assignment of the EEL spectra of all four ethylidyne isotopomers that are presented here. Based on this assignment, a normal-modes analysis and refinement of the force field for C-CH3 on Ni have been performed by fitting the calculated frequencies for the four isotopomers to the observed ones simultaneously, using a least-squares procedure. The resulting force field is similar to the force field for ethylidyne in (CO)9Co3(-C-CH3) deduced from infrared spectra.42 Inspection of the eigenvectors of the normal-mode displacements reveals substantial mixing of the C-C stretching and CH3 symmetric deformation motions. A bond dipole moment analysis of the intensities of the corresponding loss features yields the relative dynamic dipole moments of these bond motions in the four isotopomers and quantitatively confirms the mixing of the C-C stretching and CH3 symmetric deformation motions as the origin of the unexpected frequency shifts and intensity changes upon deuteration. Careful analysis of the dipole activity of the modes gives information about the local symmetry and binding site of C-CH3. The work is arranged as follows. Section II describes the apparatus and experimental procedures and is followed by the presentation of the spectra of the four isotopomers in section III. Section IV discusses first the vibrational mode assignments of the loss features and then presents the results from the force field refinement and the bond dipole moment analysis. Discussion of the mechanism for ethylidyne formation is deferred to a future publication.25 II. Experimental Section A. Apparatus and Reactants. The apparatus, which consists of a triply differentially pumped, molecular beam source precisely coupled to a UHV vacuum chamber, has been described in detail.23,43,44 The Ni(111) crystal, oriented to within 0.2°, is mounted on the end of a liquid nitrogen cryostat that is attached to a manipulator in the UHV chamber. The chamber is equipped with a cylindrical mirror analyzer for Auger spectroscopy, a quadrupole mass spectrometer, and a highresolution electron energy loss spectrometer. The elastically scattered, 6.5 eV electron beam in the EEL spectrometer typically has a full width at half-maximum (fwhm) of 37-45 cm-1 and an intensity of (1-4) × 105 counts/s. Unless noted

J. Phys. Chem. B, Vol. 102, No. 25, 1998 4953 otherwise in the figure captions, spectra are measured with a channel width of 16 cm-1, and the signal integration times per channel are given in the captions. Measurements of spectra in the off-specular direction are achieved by rotation of the crystal with respect to the incident electron beam and the analyzer. Gases and gas mixtures are handled in a bakeable manifold. The C2H4 (99.95% purity from MG Industries), C2D4 (98% purity from Cambridge Isotope Laboratories), and 13C2H4 (99% purity from Cambridge Isotope Laboratories) are introduced into the UHV chamber as a beam and are seeded in Ar. Ethylene is used as received without further purification. Hydrogen (99.9999% purity from MG Industries) and deuterium (99.8% purity from Cambridge Isotope Laboratories) are passed through a liquid N2 trap. B. Procedures. H atoms are produced by dissociating H2 on a hot tungsten filament placed 0.25 in. in front of the crystal. The filament is heated resistively to about 1800 K while the H2 pressure is held at 5 × 10-6 Torr during the H atom exposure. The H atom flux is estimated from the integral of the hydrogen and deuterium thermal desorption signal measured after a brief exposure of a partially deuterium-covered Ni(111) surface to the H atoms. Under the assumption that the incident H atoms either adsorb or abstract a D atom, the flux is estimated as 3.7 × 1014 H atoms/(cm2 s). Note that although the assumption of unit probability for adsorption plus abstraction is a physically reasonable one for the open-shell H atom, these probabilities have not been measured, and therefore this estimated flux is strictly a lower limit. While ethylidyne can be synthesized by exposure of adsorbed C2H4, C2H2, or C2H6 to H atoms,24,25 all spectra shown here are of the products of the reaction of C2H4 with H atoms. An ordered overlayer of C2H4 (0.25 ML) is formed by exposure of Ni(111) at 80 K to a beam of 2% C2H4 seeded in Ar. The reaction is carried out by exposure of 0.25 ML of C2H4 or 13C H to an equivalent of 24 ML of H atoms, 0.25 ML of C D 2 4 2 4 to 24 ML of D atoms, or 0.25 ML of 13C2H4 to 48 ML of D atoms. Upon exposure to H atoms, radiative heating from the filament immediately raises the surface temperature to 120 K. Upon completion of the exposure, the crystal cools quickly to 80 K and the HREEL spectra are recorded. However, all spectra shown here are measured after heating the crystal to 300 K at a rate of 2 K/s upon completion of the H atom exposure and immediately cooling it to 80 K. This anneal removes the small amount of hydrogen that is absorbed during the synthesis and therefore eliminates its loss feature from the vibrational spectrum of the adsorbed product. The anneal also increases the elastically scattered electron intensity by a factor of 2 and enhances the intensity of the loss features, presumably due to ordering of the adsorbates, but has no other effects on the spectra. During exposure to H atoms, the carbon coverage drops from 0.5 carbon atoms per Ni (0.25 ML of C2H4) to about 0.16 ML because some ethylene is displaced or is hydrogenated to ethane, which desorbs during the synthesis. The absolute carbon coverage is measured by comparison of the ratio of C (272 eV) to Ni (848 eV) Auger signals to that measured from a (2 × 2) ordered overlayer of C2H4 (0.5 ML of C) at 80 K,45 as observed by electron diffraction. The anneal does not affect the carbon coverage. III. Results A. C2H4 + H. High-resolution electron energy loss spectra of the products of the reaction of C2H4 adsorbed on Ni(111)

4954 J. Phys. Chem. B, Vol. 102, No. 25, 1998

Bu¨rgi et al. TABLE 1: Compilation of Vibrational Frequencies in cm-1 at the Maximum of the Intensity of All Features Observed in Figures 1, 2, and 4-7 and Their Dipole Activity (DA)a C2H4 + H

13C

2H4 +

H

C2D4 + D

13C

2H 4

+D

frequency DA frequency DA frequency DA frequency DA 267 ( 6 362 ( 3 457 ( 5 860 950 1025 ( 5 1129 ( 4 1150 1220 1336 ( 3 1410 ( 8 1740 1880 2252 ( 10 2646 ( 11 2883 ( 3 2940 ( 8

Figure 1. EEL spectra predominantly of C-CH3 at 80 K. Frequencies in cm-1 are average values, as explained in Table 1. Frequency labels in italics mark features not associated with C-CH3. Left axis shows count rate for elastic feature while right axis shows count rate in counts/s for loss features. Spectrum in (a) recorded in specular direction with ∆Efwhm and signal integration time of 37 cm-1 and 34 s for 100-1500 cm-1; 43 cm-1 and 58 s for 1500-3200 cm-1; and 51 cm-1 and 110 s for 2700-3100 cm-1, respectively. (b) 10° from specular direction with ∆Efwhm and integration time of 45 cm-1 and 33 s for 100-1500 cm-1 and 43 cm-1 and 127 s for 2600-3200 cm-1, respectively; 20° off-specular and 52 cm-1 and 82 s for 1500-2600 cm-1; sum of 10° and 20° off-specular spectra presented on expanded scale with ∆Efwhm of 44 cm-1 and integration time of 280 s for 2600-3200 cm-1.

Figure 2. Expanded C-H stretching region, 2700-3100 cm-1, of Figure 1. Specular and off-specular spectra have ∆Efwhm of 51 and 54 cm-1, respectively, and integration times of 110 and 39 s, respectively.

with gas-phase H atoms are shown in Figure 1. Expanded spectra of the 2700-3100 cm-1 region are shown in Figure 2. These spectra are measured at 80 K after exposure of a (2 × 2) ordered overlayer (0.25 ML) of C2H4 to approximately 4.4 × 1016 H atoms/cm2 at a crystal temperature of 120 K and then heating the crystal to 300 K. Spectra are recorded both at the specular and at an off-specular angle for the purpose of identifying the dipole active loss features. Table 1 lists the frequencies of all the modes observed and notes their dipole activity.

yes 268 ( 11 yes 351 ( 5 431 ( 7 860 950 1013 ( 5 yes 1080 ( 4 1150

yes 272 ( 5 yes 357 ( 7 436 ( 5 640 700 836 ( 11 yes 1152 ( 7 850

yes 271 ( 8 yes 353 ( 15 420 ( 5 640 700 829 ( 3 yes 1116 ( 3 850

yes yes

yes

yes 1313 ( 9 yes 987 ( 9 971 ( 2 1406 ( 12 1740 1400 1400 1880 1570 1570 2162 ( 24 2285 ( 11 2199 ( 10 2624 ( 20 yes 2876 ( 9 yes 2079 ( 6 yes 2062 ( 16 yes 2153 ( 13 2113 ( 10 2920 ( 5 2201 ( 4 2199 ( 10

a Maximum intensity is determined from an approximate fit of the elastic line shape to each feature. Frequencies are averages of between 4 and 42 spectra. Uncertainties are 90% confidence limits of the standard deviation of the mean and are given only for features arising from ethylidyne and a Ni phonon mode.

Figure 3. EEL spectra of 1 ML (a) adsorbed H with ∆Efwhm of 49 cm-1 and integration time per 12 cm-1 channel of 135 s. (b) Adsorbed D with ∆Efwhm of 43 cm-1 and integration time per 12 cm-1 channel of 171 s. Spectra measured at 80 K and 8° from specular direction.

Not all of the loss features arise from the adsorbed hydrocarbon of interest, which will be identified as CCH3 in section IV. Given the synthetic procedure, some of the modes arise from coadsorbed hydrogen. To assist in the assignment of the hydrogen loss features, a spectrum of 1 ML of H at 80 K is shown in Figure 3a. The overlayer is formed by exposure of Ni(111) to 2400 langmuirs of H2 while the crystal cools from about 550 to 80 K. The spectrum exhibits two prominent, nondipole active fundamentals at 950 and 1150 cm-1 which have

Ethylidyne Adsorbed on Ni(111) been assigned to the antisymmetric and symmetric modes of surface bound H, respectively,46,47 and some overtones and combination bands between 1580 and 2300 cm-1, the most noticeable of which are observed at 1740, 1880, and 2240 cm-1. While the antisymmetric hydrogen mode at 950 cm-1 is easily identified as a non-dipole active feature in Figure 1, the symmetric mode at 1150 cm-1 appears as a shoulder on the high-frequency side of an intense hydrocarbon mode at 1129 cm-1 and is not labeled. The overtones and combination bands of surface H at 1740 and 1880 cm-1 are also apparent in Figure 1, while the broad feature at 2252 cm-1 represents an overtone of surface H unresolved from a hydrocarbon overtone. In addition, several weak features, the most intense of which appears at 860 cm-1, are not associated with the primary hydrocarbon of interest. These features are assigned to a small amount of acetylene that is formed as the result of a dynamic equilibrium between ethylidyne, the primary hydrocarbon of interest, and coadsorbed H and C2H2. The acetylene coverage is estimated to be about 10-20% of that of ethylidyne. Although this equilibrium is the subject of a dedicated investigation to be described in a future publication,24,25 some spectroscopic evidence is mentioned here as support for the presence of a small amount of acetylene. First, spectra of C2H2 adsorbed on Ni(111) measured with 6.5 eV electron energy have an intense, dipole active loss feature at 860 cm-1 which has been assigned by Ibach48 to the antisymmetric C-H bending mode. Comparison of the frequencies of the loss features in Ibach’s spectra and in our spectra23 to the ones in Figure 1 reveals that the feature at 860 cm-1 as well as the weak feature at 1220 cm-1 and a possible, very weak and unlabeled feature at 2970 cm-1 match those of adsorbed acetylene. Second, the intensities of the features at 860 and 1220 cm-1 are sensitive to minor changes in H coverage and together vary in opposition to those of the features associated with C-CH3, indicating that these two features are not associated with C-CH3. Finally, the feature at 267 cm-1 arises from the longitudinal S2 phonon mode that becomes observably dipole active when an adsorbate is present on a Ni(111) surface.49 With the vibrational features associated with hydrogen and acetylene now accounted for, the features remaining at 362, 457, 1025, 1129, 1336, 1410, 2252, 2646, 2883, and 2940 cm-1 are assigned to the adsorbate of interest. Two conclusions regarding the identity of the adsorbate can already be drawn. First, comparison of the spectrum in Figure 1a measured at the specular angle with the one in Figure 1b measured at an offspecular angle and comparison of the two spectra in Figure 2 reveals that the intensities of the three features at 362, 1129, and 1336 cm-1 decrease by about a factor of 4 and that of the 2883 cm-1 feature decreases by a factor of 2 as the detection angle is moved away from the specular direction while the intensities of the other features decrease by at most 25%. Therefore, these four loss features are dipole active and are assigned to modes of the totally symmetric representation of the symmetry point group to which the adsorbate belongs. The small number of dipole active modes relative to the total number of observed modes suggests a highly symmetric species. Second, only four modes, at 1025, 1129, 1336, and 1410 cm-1, are observed in the frequency region between 700 and 1600 cm-1, which is the typical frequency range for C-C stretching and C-H bending modes. This small number of modes is indicative of either a hydrocarbon containing only very few atoms or a hydrocarbon with many degenerate modes. B. 13C2H4 + H. The spectra shown in Figure 4 are obtained by exposure of 0.25 ML of 13C2H4 to H atoms under similar

J. Phys. Chem. B, Vol. 102, No. 25, 1998 4955

Figure 4. EEL spectra predominantly of 13C-13CH3 measured at 80 K. Spectrum in (a) recorded in specular direction with ∆Efwhm of 40 cm-1 and integration time per 8 cm-1 channel of 48 s. (b) 10° from specular direction with ∆Efwhm of 45 cm-1 and integration time per 8 cm-1 channel of 48 s.

conditions to those used prior to the measurement of the spectra in Figure 1. The frequency and the dipole activity of all features observed in Figure 4 are listed in Table 1. As in Figure 1, the features at 950, 1150, 1740, and 1880 cm-1 result from adsorbed H whereas the features at 860 and 1220 cm-1 are ascribed to a small amount of 13C2H2. The remaining features at 351, 431, 1013, 1080, 1313, 1406, 2162, 2624, 2876, and 2920 cm-1 are associated with the adsorbed hydrocarbon of interest. Correlating the features in the spectra shown in Figure 4 for 13C H + H with those in the spectra shown in Figure 1 for 2 4 C2H4 + H is straightforward, because the change in carbon isotope is a weak perturbation and hence results in a similar mode pattern with minor shifts in frequency. In Table 1, where corresponding features are aligned, it can be seen that the frequency of only four loss features is noticeably affected by the substitution of a heavier carbon isotope. The features at 1129 and 2252 cm-1 in the C2H4 + H spectra exhibit the larger percentage decreases in frequency to 1080 and 2162 cm-1 in the 13C2H4 + H spectra, respectively, while the features at 362 and 457 cm-1 in the C2H4 + H spectra decrease to 351 and 431 cm-1 in the 13C2H4 + H spectra, respectively. These loss features will be assigned in the following discussion to modes that mainly involve carbon motion. Note that the downward shift in the frequency of the feature at 1129 cm-1 to 1080 cm-1 allows the Ni-H antisymmetric stretch at 1150 cm-1 to be observed more clearly in Figure 4 than in Figure 1. As with their corresponding features at 1129 and 1336 cm-1 in the C2H4 + H spectra, the features at 1080 and 1313 cm-1 in Figure 4, respectively, are very dipole active, decreasing in intensity in the spectra shown by about a factor of 2 as the detection angle is moved away from the specular angle. The features at 351 and 2876 cm-1 in the 13C2H4 + H spectra are also dipole active, just as their corresponding features at 362 and 2883 cm-1 are in the C2H4 + H spectra.


Figure 5. EEL spectra predominantly of C-CD3 measured at 80 K. Spectrum in (a) recorded in specular direction with ∆Efwhm and integration time of 42 cm-1 and 26 s for 100-1400 cm-1 and 46 cm-1 and 153 s for 1400-2500 cm-1, respectively. (b) 10° off-specular with ∆Efwhm and integration time of 43 cm-1 and 43 s for 100-1400 cm-1 and 44 cm-1 and 62 s for 1900-2400 cm-1.

Figure 6. Expanded C-D stretching region, 1920-2380 cm-1, of Figures 5 and 7. ∆Efwhm and integration time are 44 cm-1 and 200 s for C-CD3; 47 cm-1 and 157 s for C-CD3 off-specular; 44 cm-1 and 175 s for 13C-13CD3; and 43 cm-1 and 163 s for 13C-13CD3 off-specular.

C. C2D4 + D. Figures 5 and 6 show spectra measured after exposure of 0.25 ML of C2D4 to D atoms under otherwise similar conditions to those used for the spectra in Figure 1. Table 1 lists the frequency and dipole activity of all features observed in these spectra. Again, not all of the features belong to the hydrocarbon adsorbate of interest. Modes associated with deuterium are identified by comparison to the vibrational spectrum of 1 ML of D adsorbed on Ni(111) shown in Figure 3b. The deuterium overlayer is prepared under conditions similar to those for the spectrum in Figure 3a. The features at

Bu¨rgi et al. 700 and 850 cm-1 in Figure 3b are assigned to the antisymmetric and symmetric vibrations of D on Ni(111), respectively, and correspond to the H modes at 950 and 1150 cm-1, respectively, in Figure 3a. In Figure 5, the D symmetric mode at 850 cm-1 is unresolved from a hydrocarbon mode at about the same frequency. The overtones of the deuterium fundamentals are observed at 1400 and 1570 cm-1. A weak feature at 640 cm-1, corresponding to the feature at 860 cm-1 in Figure 1, is associated with a small amount of C2D2. The remaining loss features at 357, 436, 836, 987, 1152, 2079, 2153, 2201, and 2285 cm-1 are associated with the main hydrocarbon adsorbate. The frequencies of the four features at 357, 436, 1152, and 2285 cm-1 shift little from the frequencies of their corresponding features at 362, 457, 1129, and 2252 cm-1 in the C2H4 + H spectra in Figure 1, and therefore these features will be assigned to modes that involve predominantly carbon motion. However, the small upward shift in frequency of the features at 1152 and 2285 cm-1 from 1129 and 2252 cm-1, respectively, upon deuteration is counterintuitive, but this peculiarity is resolved with the aid of the normal-modes analysis discussed below. The frequency of all other features decreases substantially upon deuteration, indicating that these modes involve considerable hydrogen motion. For example, the mode at 836 cm-1 in the C2D4 + D spectra has shifted from 1025 cm-1 in the C2H4 + H spectra while the mode at 987 cm-1 has shifted from 1336 cm-1, corresponding to frequency ratios of 1.23 and 1.35, respectively. In the C-D stretching region shown most clearly in Figure 6, four modes are observed at 2079, 2153, 2201, and 2285 cm-1. The features at 2153 and 2285 cm-1 are not C-D stretching modes, but rather are a combination mode and an overtone, respectively, as discussed below. The C-D stretching modes at 2079 and 2201 cm-1 in the C2D4 + D spectra correspond to the C-H stretching modes at 2883 and 2940 cm-1 in the C2H4 + H spectra with frequency ratios of 1.39 and 1.34, respectively. The C2D4 + D spectra exhibit no obvious counterpart to the weak feature in the C2H4 + H spectra at 1410 cm-1. The features at 357, 1152, and 2079 cm-1 have dipole activity similar to their corresponding features at 362, 1129, and 2883 cm-1 in the C2H4 + H spectra. However, while the feature at 987 cm-1 is not dipole active in the C2D4 + D spectra, its corresponding feature at 1336 cm-1 in the C2H4 + H spectra is dipole active. This behavior is clarified below. An experimental observation that will become important in the next section must be noted here. Similar spectra to those shown in Figures 5 and 6 can be obtained by exposure of 0.25 ML of C2H4 to D atoms for 4 min or by exposure of 0.25 ML of C2H4 to H atoms followed by exposure to D atoms for 4 min. These observations show that all hydrogen on the adsorbate as well as that bound to the surface efficiently exchanges in the presence of gas-phase D atoms. D. 13C2D4 + D. A fourth set of spectra for this system is shown in Figure 7, and higher quality spectra of the C-D stretching region are shown in Figure 6. The surface is prepared by exposure of 0.25 ML of 13C2H4 to D atoms for 4 min under otherwise similar conditions as those used prior to the measurement of the spectra in Figures 1, 4, and 5. As noted at the end of the previous section, the hydrogen bound to the hydrocarbon as well as bound to the surface completely exchanges with deuterium. The spectra in Figure 7 can therefore be assigned to a hydrocarbon containing exclusively 13C and deuterium, surface bound D, and a small amount of 13C2D2. This exchange is readily confirmed by a comparison of the spectra in Figure 7 with those in Figure 5 from the C2D4 + D system, which

Ethylidyne Adsorbed on Ni(111)

Figure 7. EEL spectra predominantly of 13C-13CD3 measured at 80 K. Spectrum in (a) recorded in specular direction with ∆Efwhm of 45 cm-1 and integration time of 40 s for 100-1200 cm-1; ∆Efwhm of 42 cm-1 and integration time per 8 cm-1 channel of 80 s for 1200-2400 cm-1. (b) 10° off-specular with ∆Efwhm of 45 cm-1 and integration time per 8 cm-1 channel of 48 s.

reveals that the frequency pattern is very similar. Only minor frequency shifts due to the 13C labeling are observed. Again, the frequency and dipole activity of all the observed features are listed in Table 1. The features at 700, 850 (unresolved from the 829 cm-1 feature), 1400, and 1570 cm-1 are associated with surface bound D while a small amount of 13C2D2 is the origin of the antisymmetric 13C-D bending mode at 640 cm-1. The remaining features at 353, 420, 829, 971, 1116, 2062, 2113, and 2199 cm-1 are associated with the hydrocarbon of interest. It is again straightforward to pair the features associated with the hydrocarbon of interest in the 13C2H4 + D spectra in Figure 7 with those of the C2D4 + D spectra in Figure 5. The largest frequency shift is associated with the feature at 1152 cm-1 in the C2D4 + D spectra which shifts to 1116 cm-1 in the 13C2H4 + D spectra. This feature will be assigned to a mode mainly involving carbon motion. Three loss features at 353, 1116, and 2062 cm-1 are dipole active as are their corresponding features at 357, 1152, and 2079 cm-1 in the C2D4 + D spectra. The 13C H + D spectra exhibit no obvious counterpart to the feature 2 4 in the C2D4 + D spectra at 2285 cm-1. IV. Discussion Examination of the spectra in Figures 1, 2, and 4-7 of the products of the reactions C2H4 + H, 13C2H4 + H, C2D4 + D, and 13C2H4 + D, leads to the conclusion that the major adsorbed product is ethylidyne, C-CH3, or one of its isotopomers, 13C13CH , C-CD , 13C-13CD , respectively. The spectra are 3 3 3 consistent with ethylidyne adsorbed in a 3-fold hollow site with a local C3 or C3V environment and with its C3 axis normal to the surface. The analysis leading to the identification, symmetry, and adsorption site of the product of the reaction of gasphase H atoms with adsorbed C2H4 is presented in this section.

J. Phys. Chem. B, Vol. 102, No. 25, 1998 4957 A. Mode Assignments. 1. Preliminary Mode and Symmetry Assignments. Only one feature in a frequency region appropriate for a carbon stretching mode, at 1129 cm-1 (1080, 1152, and 1116 cm-1) in the C2H4 + H (13C2H4 + H, C2D4 + D, and 13C2H4 + D) spectra, respectively, shifts its frequency significantly upon carbon labeling. This feature must therefore result largely from carbon motion and is assigned to the C-C stretching mode, ν(C-C), of the adsorbate. The observation of a single C-C stretching mode coupled with the simplicity of the spectrum of the adsorbate (less than 10 fundamentals) leads to the conclusion that the adsorbate is a C2 hydrocarbon. This conclusion is consistent with the observed dipole activity of the C-C stretching mode, since this mode of any C2 hydrocarbon belongs to the totally symmetric representation, regardless of the symmetry point group of the adsorbate. The same is not necessarily true for hydrocarbons containing more than two carbon atoms. The small number of dipole active modes, four for the C2H4 + H system, suggests a highly symmetric adsorbate. In the following discussion of the symmetry of C-CH3 adsorbed on Ni(111), only the topmost layer of Ni atoms is explicitly considered, although inclusion of the second and third layers of Ni atoms does not significantly affect the conclusions. The discussion also assumes the C-CH3 species to be an isolated adsorbate, thus neglecting adsorbate-adsorbate interactions that could affect the symmetry. This approximation of an isolated adsorbate is justified by the low coverage of C-CH3 (0.08 ML) and by the qualitative invariance of the spectra for coverages between about 0.01 and 0.1 ML of C-CH3. There are four point groups to which C-CH3 adsorbed on Ni(111) can possibly belong. The highest symmetry group is C3V, which mandates that the C3 axis and the three mirror planes as symmetry elements be oriented perpendicularly to the surface. The symmetry can be lowered to C3 by removing the mirror plane symmetry, to Cs by destroying the C3 axis and two mirror planes, and to C1 by destroying all symmetry elements. In the case of the C3V point group, an adsorbed C-CH3 species transforms as 4A1 + A2 + 5E, meaning that, out of its 15 vibrational modes (3N - 6 internal modes, 3 hindered translations Tx, Ty, Tz, and 3 hindered rotations Rx, Ry, Rz), only four modes belong to the totally symmetric representation A1. If any of these modes has a significant dynamic dipole normal to the surface, then it will appear as a dipole active loss feature. Therefore, a maximum of four loss features could be dipole active: the symmetric C-Ni stretching mode, νs(Ni-C) or Tz; the C-C stretching mode, ν(C-C); the symmetric deformation (umbrella) mode, δs(CH3); and the symmetric C-H stretching mode, νs(C-H). An adsorbed C-CH3 radical transforms as 5A + 5E in the C3 point group, in which case one additional mode, the methyl torsion Rz, which is of A2 symmetry in the C3V point group, becomes totally symmetric, thereby allowing a maximum of five dipole active loss features. In the Cs point group, adsorbed C-CH3 transforms as 9A′ + 6A′′ and transforms as 15A in C1. Therefore, the nine modes of the totally symmetric representation A′ in Cs and all 15 modes of C-CH3 in C1 may be observed as dipole active loss features. Only four dipole active features are obvious in the spectra of C-CH3 (Figures 1 and 2) and 13C-13CH3 (Figure 4) while only three are obvious in the spectra of C-CD3 (Figures 5 and 6) and 13C-13CD3 (Figures 6 and 7). The low number of dipoleactive features is inconsistent with C-CH3 belonging to the low symmetry Cs or C1 point groups. In addition, the low total number of features observed, eight fundamentals, is inconsistent with an adsorbate of low symmetry. Both for Cs and C1

4958 J. Phys. Chem. B, Vol. 102, No. 25, 1998 symmetry, a total of 15 fundamental vibrational modes could possibly be observed, while only 10 could be observed for C3V or C3 point groups due to the degeneracies that are inherent in the higher symmetry species. Last, the strong dipole activity of the feature assigned to the ν(C-C) mode at 1129 cm-1 (1080, 1152, 1116 cm-1) for C-CH3 (13C-13CH3, C-CD3, 13C13CD ) and hence, the large dynamic dipole moment, are 3 consistent with the C-C axis oriented perpendicularly to the surface, as required by the C3V or C3 point groups. It is therefore concluded that C-CH3 on Ni(111) is of C3V or C3 symmetry. Further definition of the symmetry of this adsorbate requires assignment of the loss features to individual modes which is carried out in the next section. 2. Fundamentals. a. Totally Symmetric Modes. Ethylidyne adsorbed with a C3V symmetry has four totally symmetric modes, νs(Ni-C) or Tz, ν(C-C), δs(CH3), and νs(C-H). The dipole active loss feature at 1129 cm-1 (1080, 1152, 1116 cm-1) for C-CH3 (13C-13CH3, C-CD3, 13C-13CD3) has already been assigned to ν(C-C). The frequency ratio of ν(C-C) for the two carbon isotopes, 1.04, is close to that for the two carbon isotopes of deuterated ethylidyne, 1.03. The origin of the peculiar increase in the frequency of ν(C-C) upon deuteration is discussed in the next section. Similar frequencies have been observed for the C-C stretching mode of C-CH3 (1160 cm-1) in (CO)9Co3(-C-CH3)42 and for C-CH3 adsorbed on Pt(111) (1130 cm-1),2 Rh(111) (1120 cm-1),12 Ru(001) (1120 cm-1),17 Pd(111) (1080 cm-1),14 Ir(111) (1165 cm-1),16 and supported Ni particles under high ethylene pressure (1125 cm-1).50 The dipole active feature at 1336 cm-1 (1313 cm-1) in the C-CH3 (13C-13CH3) spectra in Figure 1 (Figure 4) is assigned to the totally symmetric CH3 deformation mode, δs(CH3). Its frequency is expected to decrease upon deuteration, and indeed, a feature at 987 cm-1 (971 cm-1) is present in the C-CD3 (13C13CD ) spectra in Figure 5 (Figure 7), corresponding to a 3 frequency shift upon deuteration of a factor of 1.35 (1.35). A feature at 1356 and at 1337 cm-1 in the IR spectrum of (CO)9Co3(-C-CH3)42 and of C-CH3 on supported Ni particles,50 respectively, has been similarly assigned to δs(CH3). For C-CH3 and C-CD3 adsorbed on Pt(111), δs(CH3) and δs(CD3) appear at 1350 and 990 cm-1 in the HREEL spectra,2 respectively, corresponding to a frequency shift upon deuteration of a factor of 1.36. Both the frequency and frequency shift of δs(CH3) are very similar for C-CH3 adsorbed on Pt(111) and Ni(111). It should be noted, however, that while δs(CH3) is dipole active in C-CH3 and 13C-13CH3, δs(CD3) is not dipole active in the C-CD3 and 13C-13CD3 species. A similar observation has been made for C-CH3 on Pt(111).2 Part of the loss of dipole activity upon deuteration is the result of a decrease in the vibrational displacement of δs(CH3) upon substitution of the heavier isotope and hence a decrease in the dynamic dipole moment. This effect is well established51 in IR spectroscopy of gas-phase molecules. However, it seems unlikely that this effect accounts for such a dramatic loss in dipole activity upon deuteration. Another mechanism that can account for intensity changes is the mixing of two or more bond motions of the same symmetry upon deuteration. Mixing of bond motions manifests itself both in an unusual intensity of the corresponding mode upon deuteration and in unusual frequency shifts. The spectra in Figures 1, 4, 5, and 7 show evidence of the mixing of the symmetric bending motion of the methyl group and the C-C stretching motion, both of which belong to the totally symmetric representation. For example, in addition to the loss of dipole activity of the δs(CH3) mode upon deuteration, the C-C stretching mode, ν(C-C), shifts up in frequency, although one

Bu¨rgi et al. would normally expect this mode to decrease slightly in frequency upon deuteration. A normal-modes analysis, presented below, confirms the extensive mixing between the symmetric bending motion of the CD3 group and the C-C stretching motion. The totally symmetric Ni-C stretching mode, νs(Ni-C), is observed as a dipole active feature at 362 cm-1 (351, 357, 353 cm-1) in the C-CH3 (13C-13CH3, C-CD3, 13C-13CD3) spectra. The analogous mode was reported at 401 cm-1 for (CO)9Co3(-C-CH3)42 and at 430 cm-1 for C-CH3 on Pt(111).2 While the feature observed at 457 cm-1 (431, 436, 420 cm-1) in the C-CH3 (13C-13CH3, C-CD3, 13C-13CD3) spectra could alternatively be assigned to νs(Ni-C) because the behavior of its intensity upon deuteration and carbon labeling is consistent with this assignment, this feature is not dipole active. The νs(NiC) mode is expected to be dipole active because of the perpendicular orientation of its vibrational vector with respect to the surface. Indeed, the feature at 430 cm-1 assigned to the νs(Ni-C) mode of C-CH3 on Pt(111)2 is dipole active. Therefore, the assignment of the loss feature at 362 cm-1 to νs(Ni-C) of C-CH3 on Ni(111) is retained. The assignment of the loss feature at 457 cm-1 is discussed below. The final totally symmetric mode to be assigned in the case of C3V symmetry is the symmetric C-H stretching mode, νs(C-H). This task is deferred to a section dedicated to the discussion of the C-H stretching region. The possibility of a fifth totally symmetric mode, the methyl torsion mode, Rz, which reflects C3 symmetry, is discussed in detail below. b. Degenerate Modes. Five doubly degenerate modes of E symmetry are expected for C-CH3 belonging to either the C3V or C3 point group. None of them can appear as dipole active loss features. The frequency of the non-dipole active feature at 457 cm-1 (431, 436, 420 cm-1) in the C-CH3 (13C-13CH3, C-CD3, 13C13CD ) spectra decreases by a factor of about 1.05 both upon 3 substitution of the 13C isotope and upon substitution of deuterium. The magnitude of these shifts is consistent with that expected from the ratio of reduced masses of a Ni3-adsorbate oscillator within a harmonic approximation. The appropriate magnitude of the frequency shifts coupled with the non-dipole activity of this feature leads to its assignment as the degenerate antisymmetric Ni-C stretching mode, νa(Ni-C). The same mode has been observed at 600 cm-1 for C-CH3 adsorbed on Pt(111).2 A second degenerate mode is the hindered rotation, δ(NiC-C) (Rx and Ry). Typically very low in frequency, this mode has been assigned to a feature in the HREEL spectra of C-CH3 on Pt(111)2 at 300 cm-1 and to a feature in the inelastic neutronscattering (INS)52 and IR42 spectra of (CO)9Co3(-C-CH3) at 181 and 220 cm-1, respectively. In the spectra of C-CH3 in Figure 1, the feature at 267 cm-1 has a frequency that is approximately that of these previous assignments, but because of its dipole activity and the constancy of its frequency upon isotopic substitution (268, 272, 271 cm-1 for 13C-13CH3, C-CD3, 13C-13CD3), it has been assigned to the longitudinal S2 phonon mode that becomes observably dipole active when an adsorbate is present on a Ni(111) surface.49 It is therefore concluded that the δ(Ni-C-C) mode is both so low in frequency and so low in intensity that it is masked by the elastic peak. The third degenerate mode is the methyl rocking mode, F(CH3), which typically has a frequency around 1000 cm-1. In the IR spectrum of (CO)9Co3(-C-CH3) ((CO)9Co3(-C-CD3)), a feature at 1004 cm-1 (828 cm-1) was assigned to it,42 whereas



TABLE 2: Mode Assignments of the Ethylidyne Loss Features Given in Table 1 (All Values in cm-1) mode

C-CH3

13C-13CH

3

C-CD3

νs(Ni-C) 362 ( 3 351 ( 5 357 ( 7 νa(Ni-C) 457 ( 5 431 ( 7 436 ( 5 F(CH3) or F(CD3) 1025 ( 5 1013 ( 5 836 ( 11 ν(C-C) 1129 ( 4 1080 ( 4 1152 ( 7 δs(CH3) or δs(CD3) 1336 ( 3 1313 ( 9 987 ( 9 δa(CH3) or δa(CD3) 1410 ( 8 1406 ( 12 obscured 2ν(C-C) 2252 ( 10 2162 ( 24 2285 ( 11 2δs(CH3) 2646 ( 11 2624 ( 20 ν(C-C)+δs(CD3) 2153 ( 13 νs(C-H) or νs(C-D) 2883 ( 3 2876 ( 9 2079 ( 6 νa(C-H) or νa(C-D) 2940 ( 8 2920 ( 5 2201 ( 4

13C-13CD

3

353 ( 15 420 ( 5 829 ( 3 1116 ( 3 971 ( 2 obscured 2199 ( 10 2113 ( 10 2062 ( 16 2199 ( 10

in the HREEL spectra of C-CH3 (C-CD3) adsorbed on Pt(111) a weak feature at 980 cm-1 (790 cm-1) was assigned to F(CH3).2 By analogy to these previous assignments, the weak feature at 1025 cm-1 in the spectra of Figure 1 is assigned to the F(CH3) mode of C-CH3 on Ni(111). The corresponding mode at 1013 cm-1 in 13C-13CH3 (Figure 3) is partially obscured by the intense ν(C-C) mode at 1081 cm-1. In the spectra of C-CD3 and 13C-13CD3 in Figures 5 and 7, features at 836 and 829 cm-1 are assigned to F(CD3) and F(13CD3), respectively. The ratio of frequencies of this mode in the normal and deuterated species is 1.23, which compares well to those, 1.24 and 1.21, for (CO)9Co3(-C-CH3)42 and C-CH3 on Pt(111),2 respectively. The antisymmetric CH3 deformation mode, δa(CH3), is the fourth degenerate mode and is expected at slightly higher energy than its symmetric counterpart, δs(CH3). In (CO)9Co3(-CCH3) ((CO)9Co3(-C-CD3)), the δa(CH3) (δa(CD3)) mode was observed at 1420 cm-1 (1031 cm-1), 64 cm-1 higher than its symmetric counterpart,42 while for C-CH3 (C-CD3) adsorbed on Pt(111) the corresponding mode was observed at 1420 cm-1 (1030 cm-1), 70 cm-1 (40 cm-1) higher than its symmetric counterpart,2 with a frequency ratio for normal and deuterated C-CH3 of 1.38. In the spectra of C-CH3 in Figure 1, a feature at 1410 cm-1 is present, 74 cm-1 higher in frequency than δs(CH3) and is hence assigned to δa(CH3). The corresponding feature is observed in the spectra of 13C-13CH3 in Figure 5 at 1406 cm-1, 93 cm-1 higher in frequency than δs(13CH3). Assuming the same frequency ratio of 1.38 for normal and deuterated C-CH3 (13C-13CH3) on Ni(111) as for C-CH3 on Pt(111), the corresponding mode for the deuterated analogue is expected at approximately 1010 cm-1 (1000 cm-1). A weak, high-frequency shoulder present on the feature assigned to δs(CD3) at 987 cm-1 in the C-CD3 spectra in Figure 5 and at 971 cm-1 in the 13C-13CD3 spectra in Figure 7 likely represents this mode in the deuterated cases. The degenerate mode that remains to be assigned is the antisymmetric stretching mode, νa(C-H). The assignment of this mode is deferred to a section dedicated to the discussion of the C-H stretching region. The mode assignments, as presented above, of the spectra of C-CH3, 13C-13CH3, C-CD3, and 13C-13CD3 adsorbed on Ni(111) are compiled in Table 2. 3. Adsorption Site and C3V Versus C3 Symmetry. Given the excellent correlation between the number and kinds of modes observed to be dipole active and those predicted to be dipole active for a species of C3V or C3 symmetry, it is now appropriate to consider both an adsorption site consistent with the observed symmetry and, possibly, a further clarification of the adsorbate symmetry. Only two sites on a Ni(111) surface, the 3-fold hollow and the on-top site, fulfill the requirements for an overall C3V or C3 symmetry of the adsorbate-surface complex. The 3-fold hollow

site is preferred for the following reasons. First, the degenerate νa(Ni-C) modes or, equivalently, the frustrated translational modes Tx and Ty occur at a relatively high frequency, 457 cm-1. This high frequency strongly suggests adsorption at a 3-fold hollow site where the constraints on the parallel translational motion of C-CH3 are larger. In contrast, the parallel motion of C-CH3 adsorbed on a on-top site is relatively unhindered to the extent that, most often, the frequencies of these frustrated translational modes are unresolved from the elastic feature.1 Second, binding of the carbon atom of C-CH3 to the three Ni atoms of a 3-fold hollow site, instead of to a single Ni at an on-top site, achieves a quasi-tetrahedral coordination of the carbon atom. Finally, the 3-fold hollow site has been proposed for the adsorption site of C-CH3 on other close-packed surfaces such as Pt(111)53 and Rh(111)54 based on LEED studies. The question remains as to the precise symmetry of C-CH3, C3V or C3. In principle, the symmetry of an adsorbate can be determined from the number of dipole active features observed in the HREEL spectra. In practice, the observation of a dipole active loss feature depends critically not only on the absolute magnitude of the dynamic dipole moment of a vibrational mode and its orientation with respect to the surface but also on the experimental capability to resolve the critical feature that allows the symmetry to be distinguished. It is the failure to meet the latter criterion that precludes determination of the exact ethylidyne symmetry. The only mode that allows C3V to be distinguished from C3 symmetry is the methyl torsion mode, Rz. In C3 symmetry, it belongs to the totally symmetric representation A and is therefore expected to be dipole active, but in C3V symmetry, it belongs to the nontotally symmetric representation A2 and therefore cannot be dipole active. In either point group, this mode is expected to be low in frequency and to shift significantly upon deuteration. A careful examination of the low-frequency region of the spectra in Figures 1, 4, 5, and 7 leads to the conclusion that no feature can be ascribed to Rz. In fact, this mode has never been reported for a C-CH3 species adsorbed on a metal surface although it has been reported at 383 cm-1 in an INS spectrum of the (CO)9Co3(-C-CH3) cluster.52 The likely reason for its absence in HREEL spectra of adsorbed C-CH3 is that its frequency is too low, in the present case below 260 cm-1, to be resolved from the elastic feature. Such a low frequency for this mode is justifiable because its torsional motion should be largely unhindered. In contrast, the methyl torsion mode of CH3 (CD3) adsorbed on Ni(111) appears at 485 cm-1 (375 cm-1)23 because the H atoms are much closer to and interact more strongly with the surface than in the case of C-CH3. The stronger interactions and steric hindrances in the cases of both CH3 on Ni(111) and the (CO)9Co3(-C-CH3) cluster result in a higher potential barrier for methyl rotation and hence a higher frequency for this mode. Between a free rotor and a strongly hindered case, there is a wide range of potential barrier heights where the H atoms can exchange. For example, the H atoms on C-CH3 adsorbed on small Pt particles were found to interchange at 77 K on the time scale of a NMR experiment (≈2 × 10-5 s)55 while they do not interchange on the time scale of an IR experiment (≈2 × 10-12 s),6,7 even at 300 K. These findings are consistent with a frequency for Rz below 260 cm-1 for C-CH3 adsorbed on a metal surface.6,7 Although no experimental information is available to distinguish between the C3V or C3 symmetry point groups for C-CH3 adsorbed on Ni(111), a C3V symmetry structure is likely for the following reasons. The sole difference between a C3V and a C3 structure is the torsional angle of the three H atoms with respect

4960 J. Phys. Chem. B, Vol. 102, No. 25, 1998 to the metal surface. The torsional angle of the equilibrium structure is determined by the torsional potential, which itself is determined by the interaction of the CH3 group with the symmetric substrate. If the H atoms are aligned either with the adjacent Ni atoms or bridge sites, then C-CH3 belongs to the C3V point group, and either equilibrium structure corresponds to the global minima on the torsional potential energy surface, which has three maxima and three minima. Any other orientations of the H atoms correspond to C3 symmetry structures and require the cyclic torsional potential to have at least six minima and six maxima. Although the latter is possible and cannot be excluded on the basis of our experiment, it seems rather unlikely that the interaction of the methyl group on C-CH3 with the surface results in such a complicated torsional potential. For example, it is known that the torsional potential for methyl groups in compounds such as Cl3C-CH3 is very well approximated by a cyclic potential surface of 3-fold symmetry with three minima and maxima. Based on the above considerations, a C3V structure for C-CH3 on Ni(111) is the most reasonable one. A C3V structure has also been proposed for C-CH3 on Pt(111)53 and Rh(111)11 based on LEED data and for (CO)9Co3(-C-CH3)56 based on X-ray crystallography. Note, however, that although these diffraction techniques in principle provide structural information, they also do not unequivocally determine the positions of the hydrogen atoms and therefore the symmetry group. 4. OVertones. Although C-CH3 does not have any fundamentals between 1450 cm-1 and the C-H stretching region at 2800 cm-1, several prominent loss features are apparent at 1740, 1880, 2252, and 2646 cm-1 in Figure 1. The features at 1740 and 1880 cm-1 have already been assigned as overtones of coadsorbed H by comparison to the hydrogen spectrum in Figure 2. The feature at 2252 cm-1 is attributed to the hydrogen overtone at 2240 cm-1 unresolved from the overtone of the C-C stretching mode, 2ν(C-C), and/or the double loss of the C-C stretching mode of C-CH3. This assignment is based on the observation that the frequency of the corresponding feature in the 13C-13CH3 spectra in Figure 4 decreases to about 2162 cm-1. As in the 12C analogue, the low intensity of the 2162 cm-1 feature makes its resolution from the hydrogen overtones and combination bands difficult and, in turn, precludes its differentiation as an overtone, 2ν(13C-13C), or a double loss feature. However, in the case of C-CD3 in Figure 5, the feature at 2285 cm-1 is assigned to the overtone, 2ν(C-C), as opposed to a double loss, because its observed frequency is consistent with a small amount of anharmonicity. This feature is more intense than the corresponding feature at 2252 cm-1 in the C-CH3 spectra, likely for the same reason that the intensity of its fundamental mode is enhanced, as discussed in section IV.B.2, and because of its anharmonic character. The assignment of the 2ν(13C-13C) overtone of 13C-13CD3 is discussed in the next section. Finally, a feature at 2646 cm-1 in the C-CH3 spectra in Figure 1 is assigned to the overtone of the symmetric CH3 deformation mode, 2δs(CH3), which is clearly anharmonic, since the overtone of a harmonic mode should occur at 2672 ( 4 cm-1. The corresponding feature in the 13C-13CH3 spectra in Figure 4 is observed at 2624 cm-1 and is harmonic to within the experimental resolution. While unmarked on the spectra in Figures 1 and 4, there is persistent intensity at 2800 cm-1 for both C-CH3 and 13C-13CH3. This feature likely represents the overtone of the antisymmetric CH3 deformation mode, 2δa(CH3), rather than a double loss because a similar feature has been identified in the IR spectrum of C-CH3 on Pt(111).6 For

Bu¨rgi et al. the deuterated cases C-CD3 and 13C-13CD3, 2δs(CD3) and 2δa(CD3) are expected at around 1950 and 2000 cm-1, respectively. In both the C-CD3 and 13C-13CD3 spectra in Figures 5-7, there is some intensity at these frequencies that could be assigned to 2δs(CD3) and 2δa(CD3). 5. C-H Stretching Region. For C-CH3 with either C3V or C3 symmetry, a totally symmetric C-H stretching mode, νs(C-H), and a doubly degenerate antisymmetric C-H stretching mode, νa(C-H), of E symmetry are expected. These two modes are assigned to the loss features at 2883 and 2940 cm-1, respectively, in the C-CH3 spectra in Figure 1 and to the features at 2876 and 2920 cm-1, respectively, in the spectra of 13C-13CH in Figure 4. 3 Assignment of the corresponding modes in C-CD3 is not as straightforward. As shown in the spectra in Figure 6, four features at 2079, 2153, 2201, and 2285 cm-1 are apparent in the C-D stretching region. The feature at 2285 cm-1 has already been assigned as the 2ν(C-C) overtone. The features at 2079 and 2201 cm-1 are assigned to νs(C-D) and νa(C-D), respectively, while the feature at 2153 cm-1 is assigned to the combination band of the C-C stretching mode and the symmetric deformation mode, ν(C-C) + δs(CD3), in Fermi resonance with the νs(C-D) mode. The Fermi resonance manifests itself by both intensity borrowing and frequency shifts. For example, the combination band in C-CD3 is clearly observable as a result of intensity borrowing from the νs(CD) mode whereas it is absent in the C-CH3 spectra. The consequence of the intensity borrowing from the νs(C-D) mode is the loss of some of its intensity. For example, the intensity of the νs(C-D) mode is comparable to that of νa(C-D) as observed in the spectrum measured at the specular angle in Figure 6, while νs(C-H) is more intense than νa(C-H) in the corresponding spectrum of C-CH3 in Figure 2. The frequency shift due to the Fermi resonance can be seen by comparing the frequency difference between νa and νs in C-CH3 to that in C-CD3. If there were no Fermi interaction, the frequency difference would be smaller for the deuterated case due to the lower reduced mass. However, the opposite is observed. The difference in frequency between νa and νs is 57 cm-1 for C-CH3 and 122 cm-1 for C-CD3. The larger splitting in the deuterated case results from the Fermi resonance which pushes the frequency of the νs(C-D) mode down and the frequency of the combination band up from their unperturbed values. For 13C-13CD3 in the spectra shown in Figure 6, the situation changes qualitatively. Only three features are observed at 2062, 2113, and 2199 cm-1. The most intense feature at 2199 cm-1 is assigned to the νa(13C-D) mode, which has the same frequency as its analogue in C-CD3, but is now unresolved from the 2ν(13C-13C) overtone which has a lower frequency than in C-CD3. The feature at 2113 cm-1 is assigned to the combination band, ν(13C-13C) + δs(13CD3). The lower frequency of the combination band enhances its Fermi interaction with the νs(13C-D) mode, pushing νs(13C-D) farther down in frequency relative to the C-CD3 case. Therefore, the feature at 2062 cm-1 is assigned to νs(13C-D) where the frequency difference between the νs(13C-D) and νa(13C-D) modes has increased to 137 cm-1 as compared to 122 cm-1 in C-CD3. The lower frequency of the combination mode also results in greater intensity borrowing from the νs(13C-D) mode, making it comparable to the intensity of the νs(13C-D) mode. In contrast, the intensity of the combination mode at 2153 cm-1 in C-CD3 is much less than the intensity of νs(C-D) because the Fermi interaction is weaker.



TABLE 3: Geometry Used in the Normal-Modes Calculation of Ethylidyne bond lengths (Å) C-H C-C C-Ni

1.07 1.5 1.8

TABLE 4: Force Constants Used in the Normal-Modes Calculation of Ethylidyne in This Work and in Previous Worka

bond angles (deg) H-C-H H-C-C C-C-Ni Ni-C-Ni

107 111 127 88

The frequencies observed for the C-H stretching modes of C-CH3 adsorbed on Ni(111) are very similar to those observed previously. Specifically, νs(C-H) and νa(C-H) for C-CH3 adsorbed on Pt(111) occur at 2890 and 2950 cm-1 and shift to 2080 and 2220 cm-1, respectively, for the deuterated species.2 For the (CO)9Co3(-C-CH3) cluster,42 νs(C-H) and νa(C-H) are observed at 2888 and 2930 cm-1, respectively, while for (CO)9Co3(-C-CD3) only the νa(C-D) mode was assigned at 2192 cm-1. B. Normal-Modes Analysis. 1. Comparison between Calculated and ObserVed Frequencies. Additional support for the vibrational mode assignments compiled in Table 2 comes from a comparison of the observed frequencies to those calculated from a fit of a valence force field to the observed spectra using a least-squares procedure. A normal-modes analysis is performed to extract the vibrational frequencies from the force field. The adsorbed ethylidyne is modeled in the calculations as a Ni3C-CH3 complex with C3V symmetry and with bond lengths and angles given in Table 3. The values for the C-C and the C-Ni bond lengths of 1.5 and 1.8 Å, respectively, are equal to the C-C and C-Co bond lengths in (CO)9Co3(-CCH3) as measured by X-ray crystallography.56 The C-C-Ni angle of 127° was selected so as to leave the Ni-Ni separation equal to the bulk value. Because no C-H bond lengths for C-CH3 have been measured, a C-H bond length of 1.07 Å was chosen, which is between the values of 1.0657 and 1.0958 used in previous normal-modes calculations of ethylidyne. Specifically, the normal-modes analysis is performed by diagonalization of the mass-weighted force constant matrix in Cartesian coordinates, which is calculated numerically from the valence force field and the geometry, to yield the harmonic frequencies and displacement vectors. The normal-modes routine is incorporated in a nonlinear χ2 fitting routine, based on the Levenberg-Marquardt method.59 The experimental errors in frequency tabulated in Table 2 serve as weighting factors in the χ fitting. For modes that are not observed, the weighting factor is set at 200 cm-1. The valence force field is refined until the frequencies calculated from the normal-modes analysis fit the observed frequencies for all four isotopomers C-CH3, 13C-13CH3, C-CD3, and 13C-13CD3 simultaneously. The reliability of the fitting routine and its convergence are checked by repeating the calculation using different initial force fields. A total of 11 force constants, five of which are coupling terms, are adjusted to refine the force field. Their values, as determined by the best fit to the spectra, are shown in Table 4. The force constant for the methyl torsion, Rz, is not included in the force field, resulting in a frequency of zero for this mode. This condition has no influence on the frequencies calculated for other modes since Rz is the only mode of A2 symmetry in C3V and hence cannot couple to other modes within the framework of a normal-modes analysis. Table 5 compares the frequencies obtained from the best fit to the measured frequencies for C-CH3, 13C-13CH3, C-CD3, and 13C-13CD3. The agreement between calculated and

force constants

Ni(111) -CCH3

Co3(CO)9 -CCH3

Pt(111)b -CCH3

HCb -CCH3

C-H str C-C str C-M str M-M str H-C-H bend H-C-C bend C-C-M bend C-H/C-H C-C/H-C-H M-M/C-C-M M-M/M-M H-C-H/H-C-H reference

4.63 4.68 0.74 0.83 0.44 0.65 0.57 0.04 -0.29 0.33 0 -0.04 this work

4.65 4.63 1.51 1.10 0.52 0.62 0.59 0.08 -0.30 -0.38 -0.13 0 42

4.9 3.54 2.45 0.20 0.48 0.59 1.50 0.11 -0.27 -0.1 0 0 57

4.86 4.45 0.55 0.64 0.04 0 0 58

a The last five force constants are coupling constants between the two modes shown. Units are mdyn/Å for bond stretches and stretch/ stretch coupling terms, mdyn Å for bond bends and bend/bend coupling terms, and mdyn for stretch/bend coupling terms. M ) Ni, Co, or Pt. b Force constants beyond those listed are used in these studies.

TABLE 5: Comparison of Observed Vibrational Frequencies in cm-1 at the Peak of Each Feature to Those Calculated for the Four Isotopomers of Ethylidynea C-CH3 obs 362 ( 3 457 ( 5 1025 ( 5 1129 ( 4 1336 ( 3 1410 ( 8 2883 ( 3 2940 ( 8

calc

13C-13CH

obs

362 351 ( 5 463 431 ( 7 1036 1013 ( 5 1135 1080 ( 4 1342 1313 ( 9 1408 1406 ( 12 2882 2876 ( 9 2944 2920 ( 5

3

C-CD3

calc

obs

356 449 1024 1093 1334 1407 2877 2932

357 ( 7 436 ( 5 836 ( 11 1152 ( 7 987 ( 9 obscured 2079 ( 6 2201 ( 4

calc

13C-13CD 3

obs

351 353 ( 15 428 420 ( 5 834 829 ( 3 1146 1116 ( 3 976 971 ( 2 1007 obscured 2083 2062 ( 16 2193 2199 ( 10

calc 346 416 819 1107 966 1005 2076 2176

a Observed frequency values represent averages from 4 to 42 separate spectra, and uncertainties indicate 90% confidence limits of the standard deviation of the mean.

observed frequencies is very good, and in particular, the calculation reproduces the peculiar, but experimentally observed, increase in frequency of the ν(C-C) mode upon deuteration. However, the criteria for the validity of the model calculation extend beyond the agreement between the observed frequencies and those calculated from the normal-modes analysis, since a force field with 11 parameters might be flexible enough to produce frequencies in reasonable accordance with an incorrect assignment. The physical reasonableness of the refined force field must also be considered. This task is accomplished by comparison of the refined force field to force fields derived for other C-CH3 species as shown in Table 4. Included for comparison are the force constants derived from the IR spectrum of (CO)9Co3(-C-CH3),43 from the IR and EELS spectra of C-CH3 on Pt(111),57 and from spectra of hydrocarbons containing a C-CH3 unit.58 While the agreement between the force constants that describe the C-CH3 species is very good, there are discrepancies in the force constants which describe the interaction of the C-CH3 moiety with the metal atoms. For example, the C-M stretch force constant varies from 0.74 to 1.51 and 2.45 mdyn/Å for C-CH3 on Ni(111), in (CO)9Co3(-C-CH3), and on Pt(111), respectively, but these differences are expected because the bonding of C-CH3 to Ni, Co, and Pt is not identical. More importantly, the favorable comparison between the force field obtained here and other force fields for the C-CH3 moiety provides additional support for the identification of this adsorbate as an ethylidyne species.


Bu¨rgi et al.

Figure 8. Pictorial representation of ν(C-C) and δs(CH3) (δs(CD3)) for C-CH3 and C-CD3, respectively, illustrating the extensive mixing of the C-C stretching motion and the symmetric bending motion in C-CD3. The dark bars attached to the atoms represent the displacement vectors (eigenvectors) which correspond to the indicated eigenvalues in cm-1 as calculated from the normal-modes analysis.

The normal-modes calculation both reproduces the experimentally observed increase in frequency of the ν(C-C) normal mode upon deuteration and demonstrates that the unexpected frequency shift arises from the mixing of the C-C stretching motion and the symmetric CD3 bending motion. This mixing can be visualized by comparison of the displacement or eigenvectors of the δs(CH3) and the ν(C-C) normal modes in C-CH3 to those in C-CD3, as calculated from the normal modes analysis and illustrated in Figure 8. Quantitatively, these displacement vectors are two rows of the £x-1 matrix which is the transformation matrix between normal and Cartesian coordinates as defined by

Q ) £x-1x and £x-1 ) £-1M1/2

(1)

where Q is the normal coordinate vector, x is the Cartesian displacement coordinate vector, and M is a diagonal matrix whose elements are the atomic masses.60 The elements of the £x-1 matrix are the coefficients with which the Cartesian displacement coordinates take part in a normal coordinate. As illustrated by the length of the displacement vectors in Figure 8, the ν(C-C) normal mode in C-CH3, at a calculated frequency of 1135 cm-1, appears to be an almost pure C-C stretching motion while the ν(C-C) normal mode in C-CD3, at a calculated frequency of 1146 cm-1, is a mixture of C-C stretching motion and a considerable amount of CD3 bending motion. Likewise, the δs(CH3) normal mode in C-CH3, at a calculated frequency of 1342 cm-1, appears to be pure CH3 bending motion while the corresponding normal mode in C-CD3, at a calculated frequency of 976 cm-1, contains a fair amount of C-C stretching motion. The larger mixing of the C-C stretching and bending motions in C-CD3 as compared to that in C-CH3 arises from the smaller absolute difference between the frequencies of their individual motions in C-CD3. Although the frequencies of the individual C-C stretching and bending motions are not known, the absolute difference between the calculated frequencies of the ν(C-C) and δs(CH3) normal modes, 170 cm-1 in C-CD3 and 207 cm-1 in C-CH3, serves

qualitatively to illustrate the point. A smaller frequency difference between two harmonic bond motions results in more effective mixing of them in the normal modes, as shown by an elementary classical treatment of coupled harmonic oscillators. In addition, the effective mixing results in repulsion of the frequencies of their normal modes. That is, the normal mode having the larger character of the higher frequency motion has a higher frequency while the normal mode having the larger character of the lower frequency motion would have a lower frequency than each would have if they were not mixed. Consider the ν(C-C) and the δs(CH3) normal modes of C-CH3 to be pure C-C stretch and bending motions, respectively, as suggested by the displacement vectors in Figure 8. Upon deuteration, the frequency of the symmetric deformation motion shifts down so that it now has a frequency closer to that of the C-C stretching motion. Consequently, in the normal modes of C-CD3, there is more effective mixing of these motions, as shown by the displacement vectors for C-CD3 in Figure 8. Furthermore, the more effective mixing of these motions in C-CD3 increases the frequency of the ν(C-C) normal mode beyond its frequency in C-CH3, resulting in the unexpected increase of the frequency of ν(C-C) upon deuteration. 2. Intensities and Bond Dipole Moments. This section demonstrates that the effective mixing of the C-C stretching motion and the symmetric bending motion of the CD3 group in C-CD3 is also responsible for the observed loss of dipole activity of its δs(CD3) normal mode and the increase in the dipole activity of its ν(C-C) normal mode. In short, it causes the cancellation of the dynamic dipole moments of the C-C stretching and the CD3 bending motions in the δs(CD3) normal mode and their enhancement in the ν(C-C) normal mode. This cancellation and enhancement can be seen by decomposing the dynamic dipole moments of the normal modes, ∂µ/∂Qi, into the contribution from each bond motion. That is, within the approximation of the additivity of bond dipole moments, the dynamic dipole moment is a linear combination of the derivatives of the dipole moments with respect to the internal symmetry coordinates associated with the jth bond motion, ∂µ/ ∂Sj,

∂µ ∂Qi

)

∂µ

∑j Lij ∂S

(2)

j

where the coefficients of ∂µ/∂Sj are given by the elements of the transformation matrix L between normal coordinates Qi and the internal symmetry coordinates Sj.61 The matrix L is calculated for all totally symmetric modes from the force constant matrix in Cartesian coordinates and from a definition of the internal symmetry coordinates.62 The dynamic dipole moments of the δs(CD3) normal mode Q2, of the ν(C-C) normal mode Q5, and of the symmetric ν(C-Ni) stretch Q4 of C-CD3 are given by

∂µ ∂µ ∂µ ) -0.50 (CD3 bend) + 0.09 (C-C) + ∂Q2 ∂S ∂S ∂µ -0.15 (C-Ni) (3) ∂S ∂µ ∂µ ∂µ ) 0.33 (CD3 bend) + 0.38 (C-C) + ∂Q5 ∂S ∂S ∂µ -0.21 (C-Ni) (4) ∂S


∂µ ∂µ ) -0.20 (C-Ni) ∂Q4 ∂S


(5)

Contributions of the C-H symmetric stretch and the C-Ni3 symmetric bending motions of the Ni3C-CH3 complex are not considered because they are insignificant and, in the case of the C-Ni3 symmetric bending motion, because its corresponding normal mode is not resolved from the elastic feature. The same quantities for C-CH3 are

∂µ ∂µ ∂µ ) 0.79 (CH3 bend) + 0.12 (C-C) + ∂Q2 ∂S ∂S ∂µ -0.001 (C-Ni) (6) ∂S ∂µ ∂µ ∂µ ) -0.004 (CH3 bend) + 0.38 (C-C) + ∂Q5 ∂S ∂S ∂µ -0.24 (C-Ni) (7) ∂S ∂µ ∂µ (8) ) -0.21 (C-Ni) ∂Q4 ∂S These equations illustrate two major points. For ease of discussion, the ∂µ/∂S(C-Ni) term is excluded from the immediate discussion, as justified below. First, inspection of the coefficients reveals that the Q2 and Q5 normal modes of C-CH3 have significantly less mixing of bond motions than those of C-CD3, just as suggested by the displacement vectors in Figure 8. Second, the term that represents the contribution of the dynamic dipole moment from the CD3 bending motion to ∂µ/ ∂Q2 of the δs(CD3) normal mode is opposite in sign to that from the C-C stretching motion. If the ∂µ/∂S(bend) and ∂µ/∂S(CC) factors in each term had the same sign, then the contributions of the dynamic dipole moments from each bond motion to the normal mode would cancel, thereby leading to a loss of dipole activity of this mode in C-CD3. In contrast, the term that represents the contribution of the dynamic dipole moment from the CD3 bending motion to ∂µ/∂Q5 of the ν(C-C) normal mode has the same sign as that from the contribution of the C-C stretching motion. Again, if the ∂µ/∂S(bend) and ∂µ/∂S(C-C) factors in each of these terms had the same sign, then the contributions of the dynamic dipole moments from each bond motion to the normal mode would add, thereby enhancing the dipole activity of the ν(C-C) normal mode in C-CD3. Therefore, the key to understanding the changes in intensity upon deuteration lies in knowing the relative signs of ∂µ/ ∂S(bend) and ∂µ/∂S(C-C). To do so, a quantitative bond dipole moment analysis is carried out in analogy to many such treatments of infrared intensities of gas-phase molecules.63 That is, values for ∂µ/∂S(bend), ∂µ/∂S(C-C), and ∂µ/∂S(C-Ni) are obtained for C-CH3 and C-CD3 by simultaneously solving eqs 3-5 or 6-8, respectively, after substitution of values for ∂µ/∂Qi obtained from the intensities of the loss features, as discussed below. Equations similar to eqs 3-8 are used to solve for ∂µ/∂Sj of 13C-13CH3 and 13C-13CD3, but they are not shown. Since the values of ∂µ/∂Sj must be independent of isotopic substitution within the Born-Oppenheimer approximation, the attainment of equivalent values of ∂µ/∂Sj for the four isotopes would validate this analysis, and the attainment of ∂µ/∂S(bend) and ∂µ/∂S(C-C) with the same sign would substantiate the arguments for the cancellation and enhancement of the bond dipole moments in C-CD3. Specifically, a quantity proportional to ∂µ/∂Qi is calculated from the observed intensity, I, of the normal mode at frequency ωi as given by64

( )

I 1 ∂µ ∝ F(ωi) ωi ∂Qi

2

(9)

where F(ωi) is a factor that accounts for the effective decrease in the acceptance angle of the energy analyzer due to the increasing angular spread in the dipole loss intensity as the frequency increases.1 The intensity I is the loss intensity due to dipolar excitation only, that is, the intensity scattered at the specular direction minus that scattered at an off-specular angle, and is normalized to the incident intensity. The quadratic nature of eq 9 allows for either positive or negative values of ∂µ/∂Qi.64 Both sign choices and all possible sign combinations of the three ∂µ/∂Qi are then used to solve eqs 3-5, 6-8 or similar equations for the ∂µ/∂Sj of C-CH3, C-CD3, or 13C-13CH3 and 13C-13CD , respectively. The only sign combination of ∂µ/ 3 ∂Qi found to yield equivalent values of ∂µ/∂S(CH3 or CD3 bend) and ∂µ/∂S(C-C) for the four isotopomers is positive values of ∂µ/∂Q2 and ∂µ/∂Q5 for C-CH3 and 13C-13CH3 and a negative value of ∂µ/∂Q2 and a positive value of ∂µ/∂Q5 for C-CD3 and 13C-13CD . These values are shown in Table 6. As can be 3 seen, the agreement among the values of ∂µ/∂S(CH3 or CD3 bend) and the agreement among the values of ∂µ/∂S(C-C) for the four isotopes are excellent and within the precision of the measurements. Although the minor contribution of ∂µ/∂S(CNi) as derived from eqs 5 and 8 is included in the calculation of ∂µ/∂S(bend) and ∂µ/∂S(C-C), its value lacks precision because of the difficulty of measuring a loss intensity so close to the elastic feature and is not shown in Table 6. Note that the values of ∂µ/∂S are relative values. No attempt was made to calculate absolute values of the dynamic bond dipole moments. Note also that these relative values are for the dynamic dipole moments in the normal direction to the surface, since the normal dipole selection rule is operable in this system.1 However, the important point is that the signs of ∂µ/∂S(bend) and ∂µ/∂S(C-C) are the same, which leads to the cancellation of the dynamic dipole moment of the δs(CD3) normal mode in C-CD3 and 13C-13CD3 due to the extensive mixing of bond motions and, hence, to the loss of intensity of the δs(CD3) normal mode. Similarly, the sameness of the signs and extensive mixing leads to the enhancement of the dynamic dipole moment of the ν(C-C) normal mode and the enhancement of its intensity, as suggested previously.7 This excellent agreement between the values of the dynamic bond dipole moments among the four isotopomers corroborates the extensive mixing of bond motions in deuterated ethylidyne, which is the physical basis for the unusual frequency shifts and intensity changes upon deuteration in this system and very likely is the basis for similar observations in other ethylidyne-metal systems. C. Why Are the Spectra Not Compatible with Other C2 Hydrocarbons? The simplicity of the spectra shown in Figures 1, 4, 5, and 7 together with the observation of a single C-C stretching mode strongly suggests that the adsorbed product of the reaction of gas-phase H atoms with ethylene is a C2 hydrocarbon. In this section, the possibility of an alternative assignment to other C2 species, C2H6 (ethane), CH2-CH3 (ethyl), C2H4 (ethylene), CH-CH3 (ethylidene), CH-CH2 (vinyl), C-CH2 (vinylidene), C2H2 (acetylene), C-CH (acetylide), and C2, is discussed. Exclusion of C2 and CCH is straightforward on the basis of the observation of two C-H stretching modes. Also, four modes are observed in the C-C stretching and C-H bending region between 700 and 1600 cm-1, whereas at most three are possible for C-CH in this frequency range. Adsorbed ethane, ethylene, and acetylene are also readily excluded as the major reaction product because these spectra are known24,48,65 and are


Bu¨rgi et al.

TABLE 6: Relative Values for the Dynamic Bond Dipole Moments of the Symmetric Bend of the CH3 or CD3 Group, Dµ/DS(bend), and of the C-C Stretch, Dµ/DS(C-C), As Evaluated from the Intensities of the Loss Features Using Eqs 3-9a C-CH3 C-CD3 13C-13CH 3 13 C-13CD3

∂µ/∂Q2

∂µ/∂Q5

∂µ/∂S(bend)

∂µ/∂S(C-C)

+ + -

+ + + +

0.13 ( 0.04 0.13 ( 0.03 0.11 ( 0.04 0.13 ( 0.03

0.59 ( 0.10 0.61 ( 0.05 0.66 ( 0.10 0.58 ( 0.05

a The error bars for C-CH3 and 13C-13CH3 are 90% confidence limits of 3-4 spectra of the 13C-13CH3 isotopomer while the error bars for C-CD3 and 13C-13CD3 are 90% confidence limits of 3-4 spectra of the 13C-13CD3 isotopomer.

fundamentally different from those shown in Figures 1, 4, 5, and 7. Exclusion of the other C2 species is somewhat more problematic because their spectra have not yet been reported on Ni(111). However, the spectra of these species adsorbed on other metal surfaces have been measured where the spectra and characteristic frequencies are expected to be similar. An adsorbed CH2-CH3 species has been identified on Pt(111).66 From this study and from the vibrational spectra of halogenated ethanes,67 an intense CH2 scissors mode is expected at 1450 cm-1, and the C-C stretching mode is expected at around 1000 cm-1. Neither mode is apparent in the spectra shown in Figure 1, and hence an adsorbed ethyl species is excluded. Another possible assignment is an adsorbed vinyl species. Several studies have reported the vibrational spectrum of CHCH2 adsorbed on a metal surface or of organometallic complexes containing the CH-CH2 moiety.68-73 The C-C stretching mode of vinyl adsorbed on Ni(100)68 and Pt(111)69 is observed at 1555 and 1600 cm-1, respectively, which is a much higher frequency than that observed for the reaction product associated with the spectra shown in Figure 1. Additionally, a C-H wag mode is expected below 800 cm-1 whereas no C-H modes appear below 1000 cm-1. It is therefore concluded that the spectra in Figure 1 are not associated with CH-CH2. Similar counterarguments hold for the assignment of the observed spectra to adsorbed C-CH2 which has been spectroscopically characterized on Pt(111)71 and Ru(001)74 and in organometallic complexes.72 The frequency of the C-C stretching mode in these systems ranges from 1306 to 1435 cm-1, which again is much higher than that in the present system. In addition, there is no evidence for the dipole-active scissors mode of C-CH2 that is expected to appear at 1400 cm-1 or above. On these bases, C-CH2 is also excluded as a possible reaction product of H atoms with adsorbed C2H4. The incompatibility of the remaining species, ethylidene CHCH3, with the spectra in Figure 1 is the most difficult case to demonstrate because of ethylidene’s structural similarity with C-CH3. In fact, an early study4 erroneously assigned the spectrum of C-CH3 adsorbed on Pt(111) to CH-CH3.5,42 More recently, ethylidene has been reported to be the decomposition product of C2H4 on a potassium-covered Pt(111) surface,75 and the spectra of organometallic complexes containing a CH-CH3 moiety have been studied.76 The frequency of the C-C stretching mode in these systems has been reported at 900 and 989 cm-1, respectively, considerably lower than the frequency found for the species associated with the spectra shown in Figures 1, 4, 5, and 7. Compared to C-CH3, CH-CH3 has three additional degrees of freedom which are one C-H stretching mode and two C-H bending modes. Furthermore, the highest symmetry CH-CH3 can have is Cs, and hence there

are no degenerate modes. Compared to C-CH3, eight additional modes are expected in the vibrational spectrum, five from removing the degeneracy and three from the additional H atom. The spectra in Figure 1 only show eight fundamentals of the 10 expected for C-CH3, while 18 are expected for CH-CH3. In addition, 10 of the 18 modes of CH-CH3 are dipole-allowed modes, while only three dipole active modes are apparent in the spectra shown in Figure 1. Clearly, the spectra are substantially more consistent with adsorbed ethylidyne as the reaction product rather than ethylidene. Finally, it should be noted that attempts to fit the observed frequencies to those calculated from several initial force fields for adsorbed ethylidene were not successful. Acknowledgment. This work is supported by DoE (DEFG02-89ER14035). References and Notes (1) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982; pp 326, 97. (2) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982, 117, 685. (3) Ibach, H.; Hopster, H.; Sexton, B. Appl. Surf. Sci. 1977, 1, 1; Appl. Phys. 1977, 14, 21. (4) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978, 15, 407. (5) Baro, A. M.; Ibach, H. J. Chem. Phys. 1981, 74, 4194. (6) Malik, I. J.; Brubaker, M. E.; Moshin, S. B.; Trenary, M. J. Chem. Phys. 1987, 87, 5554. (7) Malik, I. J.; Agraval, V. K.; Trenary, M. J. Chem. Phys. 1988, 89, 3861. (8) Chesters, M. A.; McCash, E. M. Surf. Sci. 1987, 187, L639. (9) Yagasaki, E.; Backman, A. L.; Masel, R. I. J. Phys. Chem. 1990, 94, 1066. (10) Ibach, H. Proc. Conf. Vibrations Adsorbed Layers, Julich 1978, 26. (11) Dubois, L. H.; Castner, D. G.; Somorjai, G. A. J. Chem. Phys. 1980, 72, 5234. (12) Koel, B. E.; Bent, B. E.; Somorjai, G. A. Surf. Sci. 1984, 146, 211. (13) Slavin, A. J.; Bent, B. E.; Kao, C. T.; Somorjai, G. A. Surf. Sci. 1988, 206, 124. (14) Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1983, 124, 68; 1981, 111, L747. (15) Kesmodel, L. L.; Gates, J. A. J. Electron Spectrosc. Relat. Phenom. 1983, 29, 307. (16) Marinova, Ts. S.; Kostov, K. L. Surf. Sci. 1987, 181, 573. (17) Barteau, M. A.; Broughton, J. Q.; Menzel, D. Appl. Surf. Sci. 1984, 19, 92. (18) Hills, M. M.; Parmeter, J. E.; Mullins, C. B.; Weinberg, W. H. J. Am. Chem. Soc. 1986, 108, 3554, 3563. (19) Fairbrother, D. H.; Peng, X. D.; Viswanathan, R.; Stair, P. C.; Trenary, M.; Fan, J. Surf. Sci. 1993, 285, L455. (20) Bertolini, J. C.; Rousseau, J. Surf. Sci. 1979, 83, 531. (21) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. (22) Yang, Q. Y.; Johnson, A. D.; Maynard, K. J.; Ceyer, S. T. J. Am. Chem. Soc. 1989, 111, 8748. (23) Yang, Q. Y.; Maynard, K. J.; Johnson, A. D.; Ceyer, S. T. J. Chem. Phys. 1995, 102, 7734. (24) Trautman, T. R. Ph.D. Thesis, Massachusetts Institute of Technology, 1996. (25) Bu¨rgi, T.; Haug, K. L.; Trautman, T. R.; Ceyer, S. T. Manuscript in preparation. (26) Thoms, B. D.; Russel, Jr., J. N.; Pehrsson, P. E.; Butler, J. E. J. Chem. Phys. 1994, 100, 8425. (27) Dea´k, P.; Giber, J.; Oechsner, H. Surf. Sci. 1991, 250, 287. (28) Biener, J.; Schubert, U. A.; Schenk, A.; Winter, B.; Lutterloh, C.; Ku¨ppers, J. J. Chem. Phys. 1993, 99, 3125. (29) Horn, A.; Schenk, A.; Biener, J.; Winter, B.; Lutterloh, C.; Wittmann, M.; Ku¨ppers, J. Chem. Phys. Lett. 1994, 231, 193. (30) Horn, A.; Biener, J.; Schenk, A.; Lutterloh, C.; Ku¨ppers, J. Surf. Sci. 1995, 178, 331. (31) Lutterloh, C.; Schenk, A.; Biener, J.; Winter, B.; Ku¨ppers, J. Surf. Sci. 1994, 316, L1039. (32) Mitchell, W. J.; Xie, J.; Jachimowski, T. A.; Weinberg, W. H. J. Am. Chem. Soc. 1995, 117, 2606. (33) Son, K.-A.; Gland, J. L. J. Am. Chem. Soc. 1996, 118, 10505. (34) Son, K.-A.; Gland, J. L. J. Am. Chem. Soc. 1995, 117, 5415. (35) Son, K.-A.; Mavrikakis, M.; Gland, J. L. J. Phys. Chem. 1995, 99, 6270.

Ethylidyne Adsorbed on Ni(111) (36) Biener, J.; Lutterloh, C.; Schenk, A.; Po¨hlmann, K.; Ku¨ppers, J. Surf. Sci. 1996, 365, 255. (37) Kim, J.-Y.; Park, Y.-S.; Lee, J. Surf. Sci. 1996, 357, 733; 1996, 363, 62. (38) Xi, M.; Bent, B. E. J. Phys. Chem. 1993, 97, 4167. Jenks, C. J.; Xi, M.; Yang, M. X.; Bent, B. E. J.Phys. Chem. 1994, 98, 2152. (39) Teplyakov, A. V.; Bent, B. E. J. Chem. Soc., Faraday Trans. 1995, 91, 3645. (40) Yang, M. X.; Bent, B. E. J. Phys. Chem. 1996, 100, 822. (41) Zhu, X. Y.; White, J. M. Catal. Lett. 1988, 1, 247; Surf. Sci. 1989, 214, 240. (42) Skinner, P.; Howard, M. W.; Oxton, I. A.; Kettle, S. F.; Powell, D. B.; Sheppard, N. J. Chem. Soc., Faraday Trans. 1981, 77, 1203. (43) Lee, M. B.; Yang, Q. Y.; Ceyer, S. T. J. Chem. Phys. 1987, 87, 2724. (44) Ceyer, S. T.; Gladstone, D. J.; McGonigal, M.; Schulberg, M. T. In Physical Methods of Chemistry, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds., Wiley: New York, 1993; Vol. IXA, p 415. (45) Beckerle, J. D.; Johnson, A. D.; Yang, Q. Y.; Ceyer, S. T. J. Chem. Phys. 1989, 91, 5756. (46) Yang, Q. Y. Ph.D. Thesis, Massachusetts Institute of Technology, 1989. (47) Johnson, A. D.; Maynard, K. J.; Daley, S. P.; Yang, Q. Y.; Ceyer, S. T. Phys. ReV. Lett. 1991, 67, 927. (48) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1981, 18, 625. (49) Ibach, H.; Bruchmann, D. Phys. ReV. Lett. 1980, 44, 36. (50) Lapinski, M. P.; Ekerdt, J. G. J. Phys. Chem. 1990, 94, 4599. (51) Crawford, B. J. Chem. Phys. 1952, 20, 977. (52) Kearley, G. J. J. Chem. Soc., Faraday Trans. 2 1986, 82, 41. (53) Starke, U.; Barbieri, A.; Materer, N.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1993, 286, 1. (54) Wander, A.; Van Hove, M. A.; Somorjai, G. A. Phys. ReV. Lett. 1991, 67, 626. (55) Wang, P.-K.; Slichter, C. P.; Sinfelt, J. H. J. Phys. Chem. 1985, 89, 3606.

J. Phys. Chem. B, Vol. 102, No. 25, 1998 4965 (56) Sutton, P. W.; Dahl, L. F. J. Am. Chem. Soc. 1967, 89, 261. (57) Johnson, W. G.; Buch, V.; Trenary, M. J. Chem. Phys. 1990, 93, 9167. (58) Duncan, J. L. Spectrochim. Acta 1964, 20, 1197. (59) Press, W. H. Numerical Recipes in FORTRAN: The Art of Scientific Computing; Cambridge University Press: New York, 1992. (60) Califano, S. Vibrational States; Wiley: New York, 1976. (61) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; Dover: New York, 1955; p 166. (62) Pulay, P.; Fogarasi, G.; Pang, F.; Boggs, J. E. J. Am. Chem. Soc. 1979, 101, 2550. (63) See for example: Vibrational Intensities in IR and Raman Spectroscopy; Person, W. B.; Zerbi, G., Eds.; Elsevier: Amsterdam, 1982. (64) Steele, D. Theory of Vibrational Spectroscopy, Saunders: Philadelphia, 1971; pp 120-132. (65) Hammer, L.; Hertlein, T.; Muller, K. Surf. Sci. 1986, 178, 693. (66) Lloyd, K.; Roop, G. B.; Campion, A.; White, J. M. Surf. Sci. 1989, 214, 227. (67) Shimanouchi, T. Tables of Molecular Vibrational Frequencies, Natl. Stand. Ref. Data Ser.; National Bureau of Standards: Washington, DC, 1962; p 39. (68) Zaera, F.; Hall, R. B. Surf. Sci. 1987, 180, 1. (69) Liu, Z. M.; Zhou, X. L.; Buchanan, D. A.; Kiss, J.; White, J. M. J. Am. Chem. Soc. 1992, 114, 2031. (70) Stuve, E. M.; Madix, R. J. J. Phys. Chem. 1985, 89, 105. (71) Zaera, F.; Bernstein, N. J. Am. Chem. Soc. 1994, 116, 4881. (72) Evans, J.; McNulty, G. S. J. Chem. Soc., Dalton Trans. 4 1983, 639. (73) Masetti, G.; Zerbi, G. Spectrochim. Acta 1970, 26A, 1891. (74) Hills, M. M.; Parmeter, J. E.; Weinberg, W. H. J. Am. Chem. Soc. 1987, 109, 597. (75) Windham, R. G.; Koel, B. E. J. Phys. Chem. 1990, 94, 1489. (76) Anson, C. E.; Sheppard, N.; Powell, D. B.; Norton, J. R.; Fischer, W.; Keiter, R. L.; Johnson, B. F.; Lewis, J.; Bhattacharrya, A. K.; Knox, S. A.; Turner, M. L. J. Am. Chem. Soc. 1994, 116, 3058.

Synthesis and Spectroscopic Identification of Ethylidyne Adsorbed on

Recommend Documents