J. Phys. Chem. 1989, 93, 5341-5345
534 1
Comparisons between Coverage-Dependent Infrared Frequencies for Carbon Monoxide Pt( loo), and Pt( 110) in Electrochemical and Adsorbed on Ordered Pt( 11l), Ultrahigh-Vacuum Environments Si-Chung Chang, Lam-Wing H. Leung, and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: April 11, 1989)
Infrared spectra measured for carbon monoxide adsorbed on Pt (1 1 l), (100) and (1 10) surfaces in aqueous 0.1 M HC104 as a function of CO coverage, Bco, and electrode potential, E , are compared with coverage-dependent infrared spectra for the corresponding interfaces in ultrahigh vacuum (uhv). The electrochemical Bco values at saturation on all three faces are similar to those in uhv. The terminal C - O stretching frequencies, uko, and band intensities are markedly dependent on the crystallographicorientation at a given electrode potential. Extrapolation of the electrochemical coverage-dependent uko values to electrode potentials corresponding to the appropriate work function for the anhydrous uhv Pt/CO surfaces yields similar u ~ o - d c o plots as are observed for the latter interfaces. This suggests that the well-known effects of the double-layer solvating environment upon uco are primarily electrostatic in origin.
An issue of central fundamental importance in electrochemistry concerns the possible influences of the solvent and electrolyte on adsorbate structure at metal surfaces. An enticing approach to this question involves comparisons between corresponding metal surface/adsorbate systems in electrochemical and ultrahighvacuum (uhv) environments. Several practical limitations have curtailed severely such comparisons in the past, among them the desire to examine well-ordered monocrystalline metals and the need to acquire experimental information that is common to both surface environments. The recent development of reliable flame-annealing methods for preparing ordered low-index faces of platinum’-2 and other transition metals2 is enabling such well-defined surfaces to be examined in in-situ electrochemical as well as uhv systems. In addition, the emergence of infrared reflection-absorption spectroscopy (IRRAS) as an in-situ probe of electrochemical interfaces3 provides an opportunity to compare the vibrational properties of electrochemical adsorbates with corresponding uhv systems, given the availability of such vibrational data from both IRRAS4 and electron energy loss spectroscopy (EELS)5 for the latter surfaces. Carbon monoxide provides an especially suitable “model” adsorbate, not only in view of the availability of extensive vibrational spectra and other structural data at monocrystalline metals in uhv but also since the C - 0 stretching frequency, uc0, is well-known to be sensitive to (and characteristic of) the surface bondinge6 Adsorbed C O in electrochemical environments has the additional significance of acting as a “poison” for the catalytic electrooxidation of small organic molecules.’ While most electrochemical IRRAS studies of adsorbed C O involve polycrystalline surfaces, reports involving single-crystal substrates are starting to appear.*-I2 Besides adsorption from (1) (a) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (b) Clavilier, J. J. Electroanal. Chem. 1980, 107, 21 1. (2) (a) Zurawski, D.; Rice, L.; Hourani, M.; Wieckowski, A. J . Electroanal. Chem. 1987, 230, 221. (b) Hourani, M.; Wieckowski, A. J . Electroanal. Chem. 1987, 227, 259. (3) For reviews, see: (a) Bewick, A.; Pons, S. In Aduances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley-Heyden: New York, 1985; Vol. 12, Chapter 1. (b) Foley, J. K.; Korzeniewski, C.; Dashbach, J. L.; Pons, S. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1986; Vol. 14, p 309. (4) For a recent review, see: Heyden, B. E. In Vibrational Spectroscopy of Molecules at Surfaces; Yates, J. T., Jr., Madey, T. E., Eds.; Plenum: New York, 1987; p 267. (5) For a recent review, see: Avery, N R. In ref 4, p 223. (6) For example: Sheppard, N.; Nguyen, T. T. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 5 , p 67. (7) For a recent discussion, see: Parsons, R.; VanderNoot, T. J. Elecrroanal. Chem. 1988, 257, 9. (8) (a) Kitamura, F.; Takahashi, M.; Ito, M. J. Phys. Chem. 1988, 92, 3320. (b) Kitamura, F.; Takeda, M.; Takahashi, M.; Ito, M. Chem. Phys. Let?. 1987, 142, 318.
solution CO itself,8-’0*’2b there are several published examinations of adsorbed CO formed from dissociative chemisorption of organic species.’l~12Studies of the former type, however, have for the most part been restricted to spectral measurements of saturated CO layers on Pt( 11 1) and Pt( 100).8,9 We have recently undertaken detailed electrochemical IRRAS examinations of the adsorption and electrooxidation of C O on well-ordered Pt( 111) and Rh( 1 1 1) electrodes in aqueous media; these include an emphasis on the coverage, as well as the potential, dependence of the spectral frequencies and intensities.I0 Comparisons with corresponding coverage-dependent vibrational spectra in uhv should provide insight into the influence of the solvent (and other double-layer components) upon the electrochemical adsorption.I0 Described herein are some results from such a comparative study of the coverage-dependent infrared spectra for C O adsorbed on all three low-index faces of platinum in 0.1 M HClO,.
Experimental Section Details of the surface infrared measurements are largely as given in ref 13 and 14. The infrared spectrometer was an IBM-Bruker IR 98-4A vacuum Fourier transform instrument, with a globar light source and either an MCT narrow-band or an InSb detector (Infrared Associates). The spectral resolution was f 4 cm-I. The Pt(11l), Pt( loo), and Pt(ll0) crystals (a. 9-mm diameter, 4 mm thick) were obtained from the Material Preparation Facility of Cornel1 University; they are oriented correctly within f l O, as verified by X-ray diffraction. Electrical contact was made via a pair of 1-mm-diameter Pt wires spot-welded to the back of the crystal. The preparation of the ordered surfaces followed a procedure similar to that of Wieckowski et al.,z essentially as outlined in ref loa. Briefly, this entailed heating the surface to redness in an air-hydrogen flame and then transferring rapidly to a compartment with upward nitrogen flow, about 2 cm above an iodine crystal. This method yields an ordered surface with a protective iodine monolayer, as verified recently with scanning tunneling micro~copy.’~After transferral to an electrochemical (9) Furuya, N.; Motoo, S.; Kunimatsu, K. J. Electroanal. Chem. 1988, 239, 347. (10) (a) Leung, L.-W. H.; Wieckowski, A,; Weaver, M. J. J . Phys. Chem. 1988,92,6985. (b) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J . Chem. Phys. 1989, 90, 7426. (11) (a) Juanto, S.; Beden, B.; Hahn, F.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1987, 237, 119. (b) Beden, B.; Juanto, S.; Leger, J. M.; Lamy, C. J . Electroanal. Chem. 1987, 238, 323. (c) Sun, S. G.; Clavilier, J.; Bewick, A. J. Electroanal. Chem. 1988, 240, 147. (12) (a) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem., in press. (b) Leung, L.-W. H.; Weaver, M. J. J . Phys. Chem., in press. (13) (a) Corrigan, D. S.; Leung, L.-W. H.; Weaver, M. J. Anal. Chem. 1987, 59, 2252. (b) Corrigan, D. S . ; Weaver, M . J. J. Electroanal. Chem. 1988, 241, 143. (14) Corrigan, D. S . ; Weaver, M. J. J . Phys. Chem. 1986, 90, 5300.
0022-3654/89/2093-5341$01 .SO10 0 1989 American Chemical Society
5342
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 '
Letters
I
..::
* .
:
1 \
1 1 1 1 1 1 1 0
0.4
E/V
vs
10.002 a.u
0.8
SCE
Figure 1. Anodicnathcdic cyclic voltammograms for ordered Pt( 11l), Pt(100), and P t ( l l 0 ) (solid, dashed, and dotted traces, respectively) at 50 mV 8,starting from -0.25 vs SCE in 0.1 M HC104. Geometric electrode areas were 0.75, 0.75 and 0.8 cm2 for these three faces.
cell, the iodine layer is replaced by CO, the solution flushed, and the C O layer electrooxidatively removed. The electrode was then transferred rapidly to the infrared cell, voltammetric characterization being carried out both before and after this maneuver. The polycrystalline platinum surface was prepared as in ref 13b. Carbon monoxide (99.8%) was obtained from Matheson, and perchloric and sulfuric acids (double distilled from Vycor) were obtained from G. F. Smith. All potentials are quoted vs the saturated calomel electrode (SCE), and all measurements were made at 23 f 1 OC.
Results and Discussion Figure 1 shows typical cyclic voltammograms obtained at 50 mV s-l for the ordered Pt (1 1l ) , (loo), and (1 10) surfaces (solid, dashed, and dotted traces, respectively) in 0.1 M HClO,. Essentially identical results could be obtained both before and after electrode transfer from the "pretreatment" cell to the infrared cell, provided that this was accomplished rapidly and with a drop of electrolyte protecting the surface. The morphology of these voltammograms can provide a detailed, albeit empirical, monitor of the surface state.1v2*16These three characteristic voltammograms in 0.1 M HC104 are virtually identical with those obtained after using the Clavilier "annealing-water quenching" pretreatment procedure.I6 The positive potential limits were chosen so to avoid the surface disordering that is engendered by oxide formation.'^^' The distinctly different voltammetric features known to be produced from these well-ordered surfaces in 0.5 M H2SO4I8were also reproduced well by using the present pretreatment procedure [cf. ref 10a for Pt(111) and ref 19 for Pt( 111) and Pt(100)I. Dosage with solution C O by bubbling for 10 min, followed by argon purging yielded an irreversibly adsorbed C O layer. The ensuing voltammogram yielded a sharp anodic feature, peaking at about 0.46,0.45, and 0.40 V vs SCE for Pt( 11l), Pt( loo), and Pt(llO), respectively, in 0.1 M HC104. The anodic charges contained within these waves, 290,Ioa 350, and 220 pC cm-*, for the (1 1 l ) , (loo), and (1 10) faces when combined with the respective platinum packing densities, 1.5 X 10l5, 1.28 X 10l5,and ~
~
~
~
I
2100
I
2060
,
I
2020
v/ cm-' Figure 2. Infrared absorbance spectra in terminal C-O stretching region for saturated irreversibly adsorbed CO layer on well-ordered platinum faces and polycrystalline platinum, as indicated, in 0.1 M HC104 at -0.25 V vs SCE. Spectra were obtained by acquiring 100 interferometer scans sequentially at -0.25 V and then at 0.5 V (so to electrooxidize the CO layer rapidly to COz), the latter being subtracted from the former (see text). Resolution was h4 cm-'.
0.92 X lOI5 atoms cm-2,21-23yield estimates of the fractional C O coverage, Oc0 (Le., C O molecules per Pt atom), at saturation of 0.6,0.85, and 1.0 (f0.05),respectively. These Bc0 estimates are close to the corresponding values obtained for C O adsorbed on these platinum faces in uhv a t 160 K.24 Several variants of electrochemical IRRAS measurements were undertaken so to obtain surface infrared spectra as a function of both C O coverage and electrode potential in the frequency region 1800-2100 cm-' where the vco bands are located. These procedures are outlined in some detail for the Pt(1 11)/CO system in ref loa; the description will therefore be brief here. Figure 2 shows infrared spectra observed in the 2000-2100-cm-' (terminal vco) region for saturated C O layers on the three low-index platinum faces at -0.25 V in 0.1 M HClO,. A typical spectrum obtained on a polycrystalline platinum surface is also shown for comparison. These results were obtained by means of "singlepotential alteration infrared" (SPAIR) technique^,"^ involving the acquisition of 100 interferometer scans (consuming ca. 70 s) first at -0.25 V and then at a potential, 0.5 V, sufficient to electrooxidatively remove the irreversibly adsorbed CO, the latter being subtracted from the former. Comparable spectra were obtained in the presence of solution CO, although the vco peak frequencies are slightly higher: 10, 12-15, and 6-8 cm-I for Pt( 11l),loa Pt( loo), and Pt( 1 lo), respectively. Largely similar results were also observed when 0.5 M H2S04was used in place of 0.1 M HC104. These spectra demonstrate a marked sensitivity of the terminal vco band frequency to the crystallographic orientation. For Pt(loo), a weak band is also observed at about 1860 cm-', ascribed to bridge-bound CO. The feature disappears for potentials positive of ca. 0.1 V, consistent with a potential-induced site intercon-
~~~~
(15) Schardt, B. C.; Yau, S.-L.; Rinaldi, F. Science 1989, 243, 1050. (1 6 ) Clavilier, J.; Sun, S. G. J . Electroanal. Chem. 1986, 199, 471.
(17) (a) Wagner, F. T.; Ross, P. N., Jr. J . Electroanal. Chem. 1988, 250, 301. (b) Aberdam, D.; Durand, R.; Faure, R.; El-Omar, F Surf. Sci. 1986, 171, 303. (18) Clavilier, J. ACS Symp. Ser. 1988, No. 378, 202. (19) Palaikis, L.; Zurawski, D.; Hourani, M.; Wieckowski, A. Surf. Sci. 1988, 199, 183. (20) Love, B.; Lipkowski, J ACS Symp. Ser. 1988, No. 378, 484.
(21) Norton, P. R.; Goodale, J. W.; Creber, D. K. Surf. Sci. 1982, 119, 41...1
(22) Norton, P. R.; Davies, J. A,; Creber, D. K.; Sitter, C. W.; Jackman, T. E. Surf. Sci. 1981, 108, 205. (23) Jackman, T. E.; Davies, J. A.; Jackson, D. P.; Unertl, W. N.; Norton, P. R. Surf. Sci. 1982, 120, 389. (24) Norton, P. R.; Davies, J. A.; Jackman, T. E. Surf. Sci. 1982, 122, L593.
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5343
Letters
TABLE I: Comparison between Infrared C - 0 Stretching Frequencies and Related Parameters for CO Adsorbed on Low-Index Platinum Surfaces at Selected Coverages in Electrochemical and uhv Environments at 300 K
surface
environmenta
E,b V vs SCE
Pt(l11)
elect
0.0
uhv
Pt( 100)
elect
0.0
uhv
Pt(ll0)
elect
uhv
0.0
A U ~ /cm-’ ~: 15
ecoc
ufCo,d cm-’
0.6 0.4 0.3 0.2
2066 2063 2058 2056
0.5 0.3 0.1
2095 209 1 2085
12
0.85 0.7 0.5 0.3
2055 2046 204 1 2042
15
0.8 0.5 0.2
2097 209 1 2087
20
1.o 0.6 0.3
2074 207 1 2063
12
0.9 0.6 0.3 (0.1)
2117 2104 2094 (2080)
12
18
15
(d&o/dE),’ cm-l V-l 34 38 44
40g
1830
1850
36 46 48 48
ref
25
(1 866) (1 855)
27, 28
30 30 30 29
12 14
elect = electrochemical environment (aqueous 0.1 M HCIO,); uhv = ultrahigh-vacuum environment. Electrode potential to which electrochemical uc0 values refer. ‘Fractional coverage of CO, calculated as number of CO molecules per surface Pt atom (see text for details). dPeak frequency of terminal (Le., on-top) uCo band corresponding to CO coverage listed alongside. Electrochemical values are from present study and ref loa; uhv values are extracted from data in references listed in far right-hand column. Value for Pt(llO)/uhv surface at Bco = 0.1 given in parentheses refers to separate band which appears only at such low coverages (see ref 29). ‘Approximate bandwidth (full width at half-maximum) of terminal uc0 band at given coverage. (If only one value is given, Au1/* was ascertained to be essentially independent of Oca.) /Electrode potential dependence of terminal uc0 frequency for electrochemical systems, obtained over potential range (-0.25 to ca. 0.25 V) below where significant CO electrooxidation occurs. See ref 10a for details. CPeak frequency . of bridge-bound CO at Oc0 values indicated. Band for Pt(100) in electrochemical environment obtained only for E 5 0.1 V (see text).
version.8b [A similar, yet more clear-cut, example of such an adsorption-site interconversion is observed for C O on Rh(1 1 1).’0bJ2b] The additional presence of a bridge-bound uco feature at 1825-1845 cm-’ on Pt( 11 1) has been discussed previousIy.loa Of primary concern here are the coverage-dependent terminal uco peak frequencies, uLo, in relation to corresponding results in uhv. Table I contains a summary of such vLo values for the three low-index forms at selected coverages, Bco, at 0 V in 0.1 M HCIO.,, along with the corresponding bandwidths, A V ! , ~ These . spectra for lower coverages were obtained as for Figure 2, but after electrooxidative removal of appropriate fractions of the irreversibly adsorbed CO. The corresponding Oc0 values were obtained by the intensity of the product C 0 2 band at 2343 cm-’ in the SPAIR spectra in proportion to that for electrooxidation of the saturated C O layer.lq [Essentially identical results were obtained by dosing with very low (micromolar) CO solution concentrations, so to yield intermediate C O coverages without electrooxidative removal.] Also listed in Table I are values of dvko/dE (cm-’ V-’), describing the electrode potential ( E ) dependence of the band frequency, for various CO coverages. These quantities were extracted from uko-E plots for potentials below those corresponding to the onset of CO electrooxidation, obtained by acquiring sequences of SPAIR spectra during slow positive-going potential sweeps as described in ref loa. Included also in Table I are selected coverage-dependent .Lo and Aul12values for the corresponding anhydrous uhv system^,^^-^^ extracted primarily from IRRAS measurements at 300 (See footnotes to Table I for details.) Given that the coverage-dependent vbo values in uhv necessarily refer to uncharged platinum surfaces in the absence of solvent, K.25927329
(25) Olsen, C. W.; Masel, R. I. Surf. Sci. 1988, 201, 444. (26) Hayden, B. E.; Bradshaw, A. M. Surf. Sci. 1983, 125, 787. (27) Crossley, A.; King, D. A. Surf. Sci. 1980, 95, 131. (28) Almost identical coverage-dependent uko values are also reported in: Banholzer, W. F.; Masel, R. I. Surf. Sci. 1984, 137, 339. (29) Bare, S . R.; Hofmann, P.; King, D. A. Surf. Sci. 1984, 144, 347. (30) Hofmann, P.; Bare, S. R.; King, D. A. Surf. Sci. 1982, 117, 295.
comparison with the combined electrode potential- and coverage-dependent & values for the electrochemical surfaces can yield information on the influence of the double layer on the adsorbate bonding. For these purposes, it is necessary to consider the relation between the work function of a metal surface in uhv, $JM, and the potential of zero charge of the corresponding metal-aqueous interface, EEc. This can be expressed as3I
where 6xMis the change in the “electronic” part of the work function as the metal is contacted with solution, and 6xs is the alteration in the solvent part of the surface potential when the solution is contacted with the metal.31 Estimates of $M for clean Pt( 11 l ) , Pt( loo), and Pt( 110) are 5.7,32 5.6,33 and 5.5 eV,34 r e ~ p e c t i v e l y . ~Combined ~ with the estimate for 4.5 V vs normal hydrogen electrode (E 4.25 V vs we can calculate (31) Trasatti, S. J. Electroanal. Chem. 1983, 150, I. (32) (a) A value of 5.7 f 0.2 eV for Pt( 111) has been reported by: Fisher, G. B. General Motors Research Publication No. GMR-4007/PCP-171, 1982. (b) Collins, D. M.; Lee, J. B.; Spicer, W. E. Surf. Sci. 1976, 55, 389. (c) Nieuwenhuys, B. E.; Sachtler, W. M. H. Surf. Sci. 1973, 34, 317. (33) Bonzel, H. P.; Fisher, T. E. Surf. Sci. 1975, 51, 213. (34) Value for Pt(ll0) estimated from other data collected in ref 35. (35) Nieuwenhuys, B. E. Ned. Tijdschr. Vacuumtech. 1975, 13, 41. (36) The accuracy of even the more reliable $M measurements appears to be only about 10.1-0.2 eV.35 However, we have somewhat more confidence in the relative @M estimates for Pt( 11I), Pt( loo), and Pt(l10) in the text, given the reliable Pt( 11 1) value32acoupled with the anticipated trend in @,‘ ( 1 11) > (100) > (1 lo), for the three low-index faces.35 (37) There remains some uncertainty in the absolute electrode potential @refi Gomer and Tryson obtained @& FS: 4.7 V vs NHE,38whereas others38q estimate a value of 4.44 V. An estimate closer to the latter value is utilized here, for reasons noted in ref 40. (38) Gomer, R.; Tryson, G. J. Chem. Phys. 1977,66, 4413. (39) (a) Trasatti, S. J . Electroanal. Chem. 1974,52, 313. (b) Frumkin, A. N.; Damaskin, B. B. J. Electroanal. Chem. 1975,66, 150. (c) Frumkin, A. N.; Damaskin, B. B. Dokl. Acad. Nauk SSSR 1975, 221, 395. (d) Also see: Trasatti, S. In Comprehensive Treatise of Electrochemistry; Bwkris, J. O’M, Conway, B. E., Yeager, E., Eds.; Plenum: New York, 1980; Vol. 1, Chapter 2.
5344
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 210
209'
Pt ( I I I ) 2 IO
209 0
-2
C
-a*---= pc.-
2111
c
c
-
, I
210
209 2081
Figure 3. Comparison of plots of the terminal C - O stretching frequency,
&, vs the fractional CO coverage, 8 ~ 0obtained , in electrochemical (filled symbols, solid traces) and uhv environments (open symbols, dashed traces) for Pt(l1 l ) , Pt(100), and P t ( l l 0 ) (A, B, and C, respectively). Latter data were extracted from ref 25, 27, and 29, respectively. Former values were obtained by extrapolating vko-electrode potential data for each Oc0 value to potentials, EM (eq 2), extracted from appropriate uhv work-function values, &", for Pt/CO (bo) interfaces as described in the text. Point for Pt(1 lO)/uhv interface at Beo = 0.1 is given in parentheses since it refers to a separate infrared band which only appears at such low coverages.29 EEc values that would be appropriate if (6xM- axs) = 0. Anticipating the discussion below, we may also define an electrode potential E M [= EEc - (6xM- 6xs)] such that simply EM= 4M- 4rcf
(2)
Irrespective of the detailed double-layer structure, setting E equal to EMestablishes the metal electrode potential (with respect to vacuum) equal to the appropriate work function. While EMwould only correspond to an uncharged interface when (6xM- 6xS) = 0, in the presence of net surface dipole orientation (especially water orientation, 6xs # 0), EMrefers to the point where such dipolar contributions to the overall ("macroscopic") surfaceptential drop are nullified by the potential component (EM- Epzc)due to free charges. It is therefore instructive to compare the corresponding coverage-dependent vko values in the uhv and electrochemical environments with the latter evaluated at E M . For this purpose, it is also desirable to correct E M for the change in work function, AbM,bought about by C O adsorption. The needed A $ V c o data are available for Pt( 11 1):' Pt( 100),4z and Pt( 1 [This correction turns out to be only small on Pt( 111) and Pt( 1lo), where lA$MI 5 0.1 V,4'*43although larger on Pt( loo), for which A$M < 0.4 V.42] Figure 3A-C consists of the resulting plots of vko against OCo for Pt(l1 l), Pt( loo), and Pt( 110) (top to bottom) in uhv (open symbols) and in the present electrochemical environment (closed symbols). The former were extracted from literature IRRAS data obtained at 300 K (Table I).2s27929The latter values were extracted from the electrochemical vko and dvLo/dE (40) Parsons, R. In Standard Potentials in Aqueous Solution; Bard, A. J., Parsons, R., Jordon, J., Eds.; Marcel Dekker: New York, 1985; Chapter 2.
(41) Norton, P. R.; Goodale, J. W.; Selkirk, E. B. Surf. Sci. 1979.83, 189. (42) Behm, R. J.; Thiel, P. A.; Norton, P. R.; Ertl, G. J. Chem. Phys. 1983, 78, 1437. (43) (a) Inbihl, R.; Ladas, S.;Ertl, G. Surf. Sci. 1988,206, L903. (b) Ishi, %-I.; Ohno, Y.; Viswanathan, B. Surf. Sci. 1985, 161, 349.
Letters values in Table I by extrapolating vko to the electrode potentials, E M ,obtained from eq 2 by using the Oco-dependent c#P values estimated as noted above. (This procedure entailed sizable extrapolations of the uko-E data to more positive potentials, since the EMvalues lie in the range 0.7-1.2 V vs SCE, depending on the platinum face and Oca.) Inspection of Figure 3 shows that the corresponding v~o-$co plots for the corresponding uhv and electrochemical systems are similar when compared on this basis, especially for the Pt (1 11) and (100) surfaces. Given the various assumptions employed in the analysis, the ca. 10-1 5-cm-' deviations between the uhv and electrochemical vko values obtained for Pt( 110) (Figure 3C) may not be sufficiently large to be significant. The comparable (10-15 cm-I) disparities between vko values often reported by different workers even for ostensibly identical platinum-uhv interfaces (e.g., ref 29) lend support to this contention. Indeed, uhv vk0 values that are closer to the electrochemical estimates in Figure 3C are observed by EELS.29,30 Irrespective of such details, however, the results in Figure 3 infer that adsorbed CO in these corresponding uhv and electrochemical environments exhibits vko values, or at least their dependence upon Oca, which are interestingly similar when the latter are evaluated at E M . This finding carries the significant implication that the detailed double-layer potential distribution brought about by surface water dipole orientation and free (electronic and ionic) charges has only a small influence on the CO bonding once EMis established at a given metal surface. This finding is perhaps most notable a t low CO coverages where the Helmholtz (or "inner") layer will be comprised primarily of water molecules; the 6xs term then should be substantial, perhaps ca. 0.4 V, associated with preferential water dipole ~ r i e n t a t i o n . ~ ~At, ~high ~ , ~CO ~ coverages, on the other hand, 62 should be markedly smaller since the solvent then has little direct access to the surface; in this case the combined quantity (6xM- 6xs) (eq 1) should also be small, so that E M z E:.. Despite these anticipated &-dependent changes in the double-layer potential profile, the dependencies of vko upon Oc0 for the uhv and electrochemical systems are very similar (Figure 3). Nevertheless, the significantly smaller increase in vko with Oc0 for the electrochemical vs the uhv Pt( 1 11) interface (Figure 3A) is worthy of note. Inasmuch as the lack of a dependence of vco upon Oc0 is indicative of adsorbate "island" formation, this result suggests that the double layer may encourage the formation of C O islands on P t ( l l 1 ) . Such structures appear to be absent on the uhv surface.z5 The very close similarities in the corresponding pairs of v!-o-t9co slopes for Pt(100) and P t ( l l 0 ) (Figure 3B,C) indicate the absence of such double-layer-induced effects on these faces. (We are grateful to a reviewer for this suggestion.) The ca. 30-cm-l downshifts in vk0 observed upon coadsorption of HzO with CO on Pt( 11 1) in uhv under some condition^^^-^^ are also consistent with these findings. The most clear-cut res u l t ~refer ~ ~ to , ~low ~ CO coverages (& < 0.3) where the coadsorbed water is anticipated to yield substantial decreases in 4M (cf. ref 4.9, especially since preferential HzO orientation should be aided by the low temperatures ( 0, so that EM> E*=. Given that dvkO/dE is positive, evaluated at.E ! (Le., at an uncharged electrode) should also be smaller than at EM. In the analysis employed in Figure 3, then, this electrostatic effect of coadsorbed water upon vko for the electrochemical system is nullified by evaluating vko at EMrather than E:., thereby yielding higher vk0 values in accord with those for the anhydrous uhv system.51
R
(5 1) It should be noted that the CO-covered electrochemical interface extrapolated to E M is in a sense a hypothetical state since rapid CO electrooxidation occurs at these relatively positive (0.7-1.2 V vs SCE) EM values.
5345
Nevertheless, the lack of marked specific effects of the double-layer environment upon the vco frequencies (or bandshape) is perhaps surprising. The coverage-dependent reconstruction of and Pt(1 known in uhv might well be different in Pt( the electrochemical systems. Quite apart from these effects of solvation on the CO surface bonding, one also might expect coadsorbed solvent to influence significantly the extent of adsorbate dipole-dipole coupling. The characterization of such coupling via CO isotopic substitution (cf. ref 25) would be clearly of interest. A detailed infrared study of the electrooxidation as well as adsorption of CO on low-index platinum surfaces will be submitted in the near future. Acknowledgment. This work is supported by the National Science Foundation.
Polarization Assignments in the 270-nm Band of the Adenine Chromophore Leigh B. Clark Department of Chemistry, University of California, San Diego, La Jolla, California 92093-0342 (Received: February 27, 1989)
Polarized electronic spectra taken from two crystal systems containing the adenine chromophore (9-methyladenine and 6-(methylamino)purine) indicate that the previous assignment of transition moment directions should be revised. The first UV absorption region (-270 nm) is dominated by a transition polarized 25' away from the short molecular axis while another weaker, overlapped transition at slightly lower energy is polarized close to the long molecular axis.
Introduction In 1963 Stewart and Davidson published the first electronic absorption spectra of single crystals of nucleic acid bases.' In that study (and its subsequent refinement2), 9-methyladenine (9-MA) was examined in both the neat crystal and the 1:l hydrogen-bonded complex with 1-methylthymine. After a complicated analysis it was concluded that the 270-nm region (267 nm in solution) was composed of two overlapping transitions. The great bulk of the intensity (97%, f = 0.28) arose from a transition polarized close to the short molecular axis (-3' according to the Tinoco-DeVoe convention as positive toward C6from the C4-C5 line). The existence of a weak, second band cf= 0.008, X = 255 nm) polarized along the long molecular axis was indirectly identified owing to a gradual change of the dichroic ratio found for this absorption region of the (100) face of 9-MA single crystals. Although there has been persistent discussion regarding the weak transition, the assignment of the strong band as short axis polarized has stood unchallenged for 25 years and has been "supported and confirmed" by numerous other s t ~ d i e s . Further, ~ these results constitute the basic optical parameters of the adenine chromophore that have been routinely used in calculations and interpretations of the optical properties of polynucleotides containing this chromophore. They have served as the experimental standard with which to judge the adequacy of theoretical calculations of the electronic structure of these complex molecules. In this Letter we present evidence that suggests that the original assignments ought be reconsidered. Results We have reexamined the crystal spectra of 9-methyladenine and, in addition, obtained new data from 6-(methy1amino)purine ( 1 ) Stewart, R. F.; Davidson, J. J . Chem. Phys. 1963, 39, 255. (2) Stewart, R. F.; Jensen, L. H. J . Chem. Phys. 1964, 40, 2071. (3) Callis, P. R. Annu. Rev. Phys. Chem. 1983, 34, 329.
0022-3654/89/2093-5345$01.50/0
(6-MAP) single crystals. The crystal absorption spectra reported here are obtained by Kramers-Kronig analysis of polarized reflection spectra. Although the data extend well into the vacuum-UV, we focus on the 270-nm (3.7 X lo4 cm-') region for the purpose of this Letter. Absorption spectra for radiation polarized along the b and c axes of the (100) face of 9-MA are given in Figure 1. This face is nearly parallel with the planes of the molecules (-9'). Stewart took the value for the dichroic ratio (b:c)of 5.1 f 0.5 from the low-energy portion of the band envelopes and found it could arise from the two possible in-plane transition moment directions of -3' or +45' for the dominant transition. The A:T dimer spectra were then shown to be more consistent with the -3' possibility than the +45' choice, so the -3' alternative was provisionally chosen. Since crystal field caused intensity borrowing among the vibronic components of a given electronic transition can lead to different contours for band components along different crystal axes, it is felt that the dichroic ratio taken from the region of the 0,O is not an appropriate characteristic of the transition as a whole. To minimize the effects of such intensity borrowing, we prefer to employ band areas or oscillator strengths. In this regard we find for 9-MA a value for fbfcof 4.8 and the corresponding in-plane polarizations of -3O and +46' which however are virtually coincident with Stewart's two possibilities. However, from the spectra taken along the a, b, and c axes of the (001) and (100) faces of 6-(methylamino)purine a different band pattern emerges (Figure 2), for there are what appear to be two significant bands in the 3.7 X 104 cm-' (270 nm3 absorption region. A lower energy, structured transition starting a t about 3.4 X lo4 cm-' (294 nm) appears dominantly along the c axis, while apparently another, more intense transition centered at 3.7 X lo4 cm-' (270 nm) appears strongly along the b and a axes. It should be noted here that the X-ray determination of the crystal structure of 6-MAP located all the protons in the structure and identified the 9-H tautomer as the form p r e ~ e n t . ~Given the
0 1989 American Chemical Society
