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J. Phys. Chem. 1992, 96,4154-4156
> silica; for ethanol, alumina > magnesia > silica; for ethanal, silica >> alumina 2 silica. These product sensitive support effects indicate that hydrogen adsorption is not a controlling factor in the reaction of CO hydrogenation. In summary, *HN M R studies of D2 adsorbed on Rh over different supports yield electronic and dynamic information on the NMR-active species Dm and Dw. The electronic structures of adsorption sites for Dm and Dw are essentially independent of support. However, the relative proportions of Dm and Dw
depend on both support and overpressure. Furthermore, the intrinsic motions of Dm and Dw depend also on support. In the non-SMSI state, these results indicate clearly that the more useful directions to investigate effects of support are the numbers of active sites and the motions of adsorbed species. Acknowledgment. We thank the National Science Council (NSC-8 1-0208-M-007-84 and NSC-8 1-0208-M-007-6 1) of the Republic of China (Taiwan) for financial support.
Inhibition of Near-Infrared Photoinduced Electron Transfer between Free Hexacyanoferrate( II)and Hexacyanoferrate(III)by Tetramethylammonium Ions Dimitrij E. Khoshtariya,+Anna M. Kjaer,t Tamaz A. Marsagishvili,f and Jens Ulstrup*J Institute of Inorganic Chemistry and Electrochemistry of the Georgian Academy of Sciences, Jikiya 7, 380086 Tbilisi, Georgian Republic, and Chemistry Department A, Building 207, The Technical University of Denmark, 2800 Denmark (Received: February 11, 1992; In Final Form: April 7, 1992)
A recently reported weak 800-nm intervalence absorption band in mixtures of potassium hexacyanoferrate(II1) and -(II) was interpreted as photoinduced electron transfer between the free hexacyanoferrate ions. We have investigated the effect of replacing potassium ions by tetraalkylammonium ions on this band using mixtures of the appropriate hexacyanoferrates rather than by adding tetraalkylammonium halides to the potassium hexacyanoferrates. The intervalence band appears to vanish on full replacement. This suggests that the phototransition is likely to be in an ion pair bridge configuration.
Introduction
Correlations between optical and radiationless electronic transitions involving a given pair of Bom-Oppenheimer electronic states in molecules and solids have long been recognized.’ In molecular electron transfer (ET) such correlations involve the rate constant of the thermal process and the molar absorption coefficient or spectral emission distribution for the optical process. The correlations here acquire the particular perspective of providing a clue to the elusive parts of the rate constant such as the electron-exchange factor and free energy relations in the inverted free energy region. Photoinduced ET between separate molecular centres is well known for mixed-valence c ~ m p l e x e s ~and - ~ ion The thermal rate constants for such systems are, however, seldomly available. Optical transitions involving kinetically well-characterized mobile donor and acceptor ET molecules, on the other hand usually cannot be detected due to the small steady-state concentrations of the donor-acceptor “collision complexes” and the large average donor-acceptor separation. We have recently reported the analysis of a new near-infrared (800 nm) absorption band in concentrated (0.3-0.6M) aqueous and D 2 0 solutions of [Fe(CN),I3- and [Fe(CN)6]4-.8,9 This transition was interpreted as photoinduced ET between free [Fe(CN),]’- and [Fe(CN),]& ions, as opposed to a mixed-valence transition in a binuclear complex such as [(CN)5FeCNFe(CN)5]6. The interpretation was supported by the following observations: (A) The absorption intensity is proportional to the concentration of each of the two ions separately. (B) The band maximum is quite different from that of known3349’091 I binuclear [Fe(CN),]3-/4- complexes. (C) The weak band was only observed for high concentrations where the average interreactant distance only slightly exceeds the sum of their crystallographic radii. (D) The bands are stable in alkaline solution where the hydrolysis of [Fe(CN)6]W -preceding binuclear complex formation is suppressed.I2 ‘Georgian Academy of Sciences. ‘The Technical University of Denmark.
(E) The band-shape parameters were cation specific and the overall nuclear reorganization free energy contains a large component from a low-frequency displaced and distorted nuclear mode representative of ion pair reorganization. This corresponds to cation specificity of both the redox potential,’3J4 and the homogeneo~s’~-’’ and electrochemicalI8 electron exchange kinetics of the [Fe(CN)6]3-/4- system. (F) The electron-exchange factor for the thermal exchange process could be estimated from the spectral band-shape data and indicated that this process belongs to the adiabatic limit of strong interreactant interaction? The estimated rate constant was 20-50 times lower than determined experimentally by N M R line (1) For a review, see: Itskovitch, E.; Ulstrup, J.; Vorotyntsev, M. A. In The Chemical Physics of Solvation. Part B. Spectroscopy of Solvation; Dogonadze, R. R., Kilmin, E., Kornyshev, A. A., Ulstrup, J., Eds.; Elsevier: Amsterdam, 1986; pp 223-310. (2) Hush, N. S . Electrochim. Acta 1968, 13, 1005. (3) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10, 247. (4) Creutz, C. Progr. Inorg. Chem. 1983, 30, 1. (5) Curtis, T. C.; Meyer, T. J. Inorg. Chem. 1983, 21, 1562. (6) Toma, H. E. Can. J . Chem. 1979.57, 2079. (7) Kjaer, A. M.; Kristjinsson, I.; Ulstrup, J. J. Electroanal. Chem. 1986, 204, 45 and references therein. (8) Marsagishvili, T. A.; Khoshtariya, D. E. Khim. Fiz. 1987, 6, 1511. (9) Marsagishvili, T. A.; Kjaer, A. M.; Khoshtariya, D. E.; Ulstrup, J. J . Phys. Chem. 1991, 95, 8797. (10) Glauser, M.; Hauser, U.; Herren, F.; Ludi, A.; Roder, P.; Schmidt, E.; Siegenthaler, H.; Wenk, F. J . Am. Chem. Soc. 1973, 95, 8457. (11) Felix, F.; Ludi, A. Inorg. Chem. 1978, 17, 1782. (12) Sharpe, A. G.The Chemistry of Cyano Complexes of The Transition Metals; Academic Press: London, 1976; p 108. (13) Hanania, G.1. H.; Irvine, D. H.; Eaton, W. A. J . Phys. Chem. 1967, 71, 2022. (14) Cheng, I. F. Anal. Chim. Acfa 1991, 251, 35. (15) Shporer, M.; Ron, G.; Loewenshtein, A.; Navon, G. Inorg. Chem. 1965, 4, 361. (16) Campion, R. J.; Deck, C. F.; King, P.; Wahl, A. C. Inorg. Chem. 1967, 6, 672. (17) Cho, K. C.; Cham, P. M.; Che, C. M. Chem. Phys. Left. 1990,168, 361. (18) Peter, L. M.; Durr, W.; Bindra, P.; Gerischer, H. J . Electroanal. Chem. 1976, 71, 31.
0022-365419212096-4154$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, NO. 11, 1992 4155
Letters TABLE I: Gaussian Analysis of the Intenalence Absorption Band"
0.1/0.3 0.2/0.3 0.3/0.3
[Fe(II)I / [Fe(III)I 0/0.3 0.1/0.3 0.2/0.3 0.3/0.3
i.8jo.3 1.5/0.6 1.2/0.9
0.083 0.074 0.060
2.1/0 1.7/0.4 1.3/0.8 0.9/ 1.2
0.096 0.08 1 0.059 0.043
12.26 f 0.03 12.09 f 0.03 11.76 f 0.03
[(Me)d'%[Fe(CN)d /&[Fe(CN),I 12.31 f 0.04 12.25 f 0.04 12.47 f 0.05 12.23 f 0.05
4.2 f 0.1 4.4 f 0.1 4.7 f 0.1
21 f 1 24 f 1 26 f 1
1.04 0.92 0.73
4.2 4.2 3.9 3.9
21 f 21 f 18f 18 f
1.20 1.02 0.73 0.53
f 0.1
f 0.1 f 0.1 f 0.1
1 1 1 1
" [Fe(CN),13- = [Fe(cN),]& = 0.3 M. D 2 0 solution; 60 OC. The column to the left shows the potassium and tetramethylammonium (Me4N+) salt concentrations (M) and the next column the cation concentrations (M). The parameters of the band are the Gaussian absorption maximum A,,, the corresponding frequency umPx (10' cm-I), the molar absorption coefficient emax (L2 M-2 cm-I ), and the Gaussian width A (10' cm-I). The overall reorganization free energy, ER(lo' cm-I) is calculated from A = 2(~??RkeT)''~. br~adening.'~.'~ A likely cause of this difference could be minor inhomogeneous spectral b r ~ a d e n i n g . ~ In the present work we report the effect of added (CH,),N+ ions on the new near-infrared absorption band features. Previous cation specificity investigations of [Fe(cN),I3-/+ have always rested on addition the appropriate salts to solutions of K4[Fe(CN)6] and K,[Fe(CN),]. Large amounts of K+ have therefore been present. We use here mixtures of the potassium and tetramethylammonium salts of [Fe(CN)61f and [Fe(CN)61e. In this way the cation composition is much less restricted. The surprising result emerges that the absorption band vanishes as K+ is gradually replaced by (CH,),N+. This supports that the 8Wnm intervalence transition involves extensive ion pairing. Replacement of K+ by the larger (CH3),N+ might thus increase the distance between [Fe(CN),],- and [Fe(CN),]" in the photo-ET configuration, leading to a corresponding absorption decrease. Experimental Section Milli-Q (Millipore) water and AnalaR grade reagents were used throughout. [(CH,),N],[Fe(CN),] was prepared by mixing 3.79 g of K,[Fe(CN),] with 4.4 g of (CH,),NCl, each dissolved in 25 mL of water and cooling to 0 O C for 15 min. The precipitate was washed with ice-cooled water and methanol and dried in vacuum over silica gel; yield 3.55 g. No trace of K+ in the product could be detected by atomic absorption spectroscopy. [(CH3)4]4[Fe(CN)6]was prepared by reducing a solution of 6.03 g of [(CH3)4N]3[Fe(CN)6] and 3.79 g of (CH3)4NOH in 175 mL of water by excess H202added dropwise until a color change from yellow to colorless was observed. The solution was evaporated in vacuum and cooled. [(CH,),N],[Fe(CN),] was precipitated by addition of ethanol, washed with ethanol, and dried in vacuum over silica gel. None of the tetramethylammonium salts showed any trace of Prussian blue or other mixed-valence compounds. Preparation of solutions, spectral data acquisition and processing, and spectral band-shape analysis were all undertaken as described previo~sly.~ Results and Discussion Figure 1 shows the near-infrared intervalence absorption band for equimolar mixtures of [Fe(CN)6l3- and [Fe(CN),]+ (0.3 M) but various combinations of the potassium and tetramethylammonium salts. K+ could be substituted to an extent corresponding to the total amount of either K,[Fe(CN),] or K4[ FeCN)6]. Further substitution caused precipitation. Lower concentrations of [Fe(CN),I3- and [Fe(CN),]+ was not feasible due to the weak intervalence absorption and substantial background absorption. The data were analyzed9 using either a Gaussian or a harmonic two-mode band-shape function. In the latter model one low-frequency mode is displaced and represents the solvent, while the other one is both displaced and distorted and represents ion pairing. The physical basis for both models is discussed e l s e ~ h e r e . l * ~ * ~ The band-shape parameters are summarized in Table I, where the following observations are appropriate:
01 8
I
I
10
12
I
I '
16
1L J lo3
-
d
Figure 1. Absorbance of the intervalence transition, A, in the frequency (v) range (9-16) X 10' cm-' (600-900 nm), corrected for background and individual [Fe(CN),]'- and [Fe(CN),]+ absorption. [ [Fe(CN),]'-] = [[Fe(CN),]&] = 0.3 M. D 2 0solution; 60 O C . The spectra also show the effect of substitution of K3[Fe(CN),] by the corresponding tetra-
methyl ammonium salt. The ratios [K,[Fe(CN),]]/[ [(CHJ4NI3[Fe(CN),]] are as follows: (-), 0.3 M/O; (.-.) 0.2 M/0.1 M; (---) 0.1 M/O.2 M; ( - - - )0/0.3 M. [Fe(CN),I4- added as K,[Fe(CN),].
(1) A Gaussian band shape for a symmetric transition implies that humax= A2/4k0T = ER where h is Planck's constant, k0 Boltzmann's constant, T the temperature, v, the maximum absorption frequency, A the Gaussian bandwidth, and ER the total reorganization free energy. Table I shows that a Gaussian fits the data well, the variation in A or ER, and,,Y with the nature of the cation being small and hardly conspicuous enough to be significant. A notable drop in the maximum absorbance A,,, or molar absorption coefficient emax, is, however, observed when K+ is replaced by (CH3)4N+. The drop amounts to a factor of 2 or so when the K+/(CH3)4N+substitution corresponds to the total amount of one of the Fe(III)/Fe(II) components. (2) The parameters in Table I do not, however, correspond to the relation between v, and A expected for a displaced set of symmetric harmonic modes. The more detailed model which incorporates a harmonic mode which is both displaced and distorted must therefore be introduced. The four parameters in such a m0deP9~are A,, the ratio of the ion pair hindered translational frequencies when the cation is bound to [Fe(CN),]'- and to [Fe(CN)6l4-, y, vmaX, A. A full four-parameter analysis is, however, not feasible due to strong parameter correlati~n.~ Also,these parameters only crudely represent the real ion pair configurations. We therefore use a simpler approach than in our previous report.9 We note that the band shape is still approximately Gaussian close to the band maximum' but the maximum and width are humax= E , E;+(y y-') (1) y4(A2/kar) = E, + E,'.p.(y2 + y-4)
+
+
E, is the solvent reorganization free energy and EJ.P.the ion pair reorganization free energy around a single absorbing ion corre-
J. Phys. Chem. 1992,96, 4156-4159
4156
0
Figure 2. A,, I.
1 [k*l 2 M plotted against [K+].Conditions and data as in Table
sponding to the lower of the two vibrational frequencies (Le., around [Fe(CN),]*). The total ion pair reorganization free energy around both [Fe(CN),13- and [Fe(CN),]" is thus E;p.(tot) = Eri,P,(l Y - ~ ) . Equation 1 indicates that '/4(A2/ker) > hvmax (y < 1 ) such as observed. (3) E, and I3li.p. or Eri,P(tot) can be estimated separately from eq 1 at given y. Physically reasonable values are obtained for y = 0.5-0.65. Corresponding values, approximately independent of the cation distribution, are E, = (10.5 f 1 ) X lo3 cm-' and Eri.p.(tot) = (3.5 f 1 ) X lo3 cm-I for y = 0.5, while E, = (9.6 f 1 ) X lo3 cm-' and Eri.p.(tot) = (4.7 f 1.5) X lo3 cm-I for y = 0.55. At the larger y = 0.63, E, increases from (6 f 1) X lo3 cm-l to (8 f 0.5) X lo3 cm-I in the [K+] range 0.9-2.1 M while Eri+.(tot)decreases from (10 f 1 ) X lo3 to (7 f 0.5) X lo3 cm-' in the same range. Overall an approximately constant value of the sum E, Eri,P~(tot) = 15 X lo3 cm-' emerges. All the three parameters may, however, of course vary as the cation distribution is changed. ( 5 ) When A, or ,e is plotted against [K+], an approximately linear relation which extrapolates well to the origin is obtained (Figure 2). The intervalence band thus appears to vanish on
+
+
K+/(CH3)4N+substitution. This strongly supports the intervalence nature of the transition and that ion pair configurations are crucial elements of the absorbing molecular entity. For example, on replacement of K+ by the larger (CH3)4N+in a bridgelike configuration the ET distance would be increased and the ET probability therefore strongly lowered. Alternatively the organic part of (CH3)4N+might be envisaged to lower the electrostatic screening between [Fe(CN),I3- and [Fe(CN),I4- relative to the smaller K+. Interestingly, this is in fact opposite from the thermal ET behavior of [Fe(CN),]*-/&. The rate constant here increases more strongly for (CH3)4N+than for K+ when excess electrolyte is added.I6 These data refer, however, to much smaller reactant and electrolyte concentrations where the hydration patterns are not directly comparable to the nearly saturated solutions needed for the optical transitions. In conclusion these data substantiate that the near-infrared 800-nm intervalence [Fe(CN)6]3--/k transition is highly cation specific and involves outer-sphere interionic configurations. The size-specific absorbance could be indicative of bridgelike configurations and superexchange nature of the transition. Bridge group electronic and steric specificity would be important in such configurations. For example, longer distance and weaker electrostatic screening in the tetramethylammonium ion pairs compared to potassium ion pairs would lower the photo-ET transition probability. Electronic bridge group polarizability would be another important effect and is perhaps what is the cause of the Mn04-/2ET rate constant increase on K+ replacement by Cs+.I9 Observation of these effects and of possible similar effects in thermal E T rests on the use of the appropriate hexacyanoferrate salts, rather than addition of the appropriate halides to solutions of the more generally available potassium hexacyanoferrate(II1) and -(II). Acknowledgment. This work was supported financially by the Danish Natural Science Research Council. We are grateful to Professor H. E. Toma, Sao Paulo, and Dr. J. C. Reeve, Chemistry Department A, for helpful comments. (19) Gjertsen, L.; Wahl, A. C. J . Am. Chem. SOC.1959, 81, 1572.
Observing Surface Chemical Transformations by Atomic-Resolution Scanning Tunneling Microscopy: Sulfide Electrooxldation on Au( 111) Xiaoping Gao, Yun Zhang, and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: February 18, 1992; In Final Form: March 24, 1992)
The prospects of utilizing atomic-resolution scanning tunneling microscopy (STM) as a real-space in-situ probe of molecular transformations on electrochemical surfaces are illustrated for the specific case of sulfide electrooxidation on Au( 111) in aqueous solution. At relatively low electrode potentials, sulfide adsorption yields ordered ( d 3 X d3)R3Oo layers. The onset of sulfide electrooxidation produces marked changes in the adsorbate structure, arrays of rectangular close-packed structures being formed which consist predominantly of SBrings. The ring dimensions, including the S-S bond distances, differ significantly from bulk-phase polysulfur, being affected apparently by the gold substrate structure. Further reaction yields multilayer S, structures exhibiting- -greater disorder. The likely scope of reactive surface electrochemistry amenable io STM is noied. -
The increasing emergence of scanning tunneling microscopy (STM) as a means of obtaining atomic-level structural information at metal surfaces is promising to foster a new era in surface science.' The technique has recently been demonstrated to yield (1) For an insightful recent review, see: Ogletree, F.; Salmeron, M. Prog. Solid State Chem. 1990, 20, 2 3 5 .
true atomic resolution (i.e., identification of individual surface atoms) at in-situ electrochemical (metalsolution) interfaces as well as on metal surfaces in ultrahigh vacuum ( u ~ v ) . ~Besides -~ (2) Magnussen, 0. M.; Hotlos, J.; Nichols, R.J.; Kolb,D. M.; Behm, R. J. Phys. Rev. Lett. 1990, 64, 2929. (3) Yau, S.-L.;Vitus, C. M.; Schardt, B. C. J. Am. Chem. SOC.1990,112, 3677.
0022-365419212096-4156$03.00/0 0 1992 American Chemical Society
