J. Phys. Chem. 1992, 96, 5213-5217 even at 4.2 K and recombine with trapped electrons, which are also suggested to move via t ~ n n e l i n g ,and ~ , ~selectively produce the isolated H atoms (reaction 6). The isotope effect for the isothermal conversion of trapped electrons to hydrogen atoms (H, D) was estimated to be a > lo3. This extremely large effect strongly suggests that the 0-H (0-D) bond dissociation of the thermalized H D 2 0 in reaction 6 occurs via a quantum tunneling process. A simple calculation predicts a large effect of a = 103-1063at 77-4.2 K, if the dissociation occurs via tunneling and if it is assumed that the H / D isotope ratio of the reaction rates depends only on the difference of the zero-point energies of @H and 0-D stretching vibration modes.15 On the other hand electrons produced in the high H content mixtures will rapidly lose their energies by collisions with the lattice molecules containing OH bonds and cannot be metastably trapped. They will be caught by the strong Coulomb potential with the countercationic species before migrating (and before complete thermalization) and form excited H3-,,D,,D* or H2-,,,D,0* species by geminate recombination. As a result a small isotope effect a = 2.5 might be observed via 0-H (0-D) bond dissociation of these excited species. Conclusion The present work gives direct and probably the first spectroscopic evidence for the conversion of electrons to hydrogen atoms. This reaction is suggested and is deduced from a number of studies of water radiolysis.’-I2 The large isotope effect observed for the conversion reaction may suggest a possible isotope separation
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method providing that reaction systems and temperatures can be found that are suitable for producing thermalized electrons and reactions with hydronium ions. References and Notes ( I ) DTaganic, I. G.;Draganic, 2.D. The Radiation Chemistry of Water; Academic Press: New York, 1971. (2) Matheson, M. S.; Smaller, B. J . Chem. Phys. 1955, 23, 521. (3) Box, H. C.; Budzinski, E. E.; Lilga, K. T.; Freund, H. G. J . Chem. Phys. 1970, 53, 1059. Box, H. C. Radiation Effects, ESR and ENDOR Analysis; Academic Press: New York, 1977. (4) Knight, L. B., Jr.; Steadman, J. J . Chem. Phys. 1983, 78, 5940. (5) Hase, H.; Kawabata, K. J . Chem. Phys. 1976, 65, 64. (6) Chernovitz, A. C.; Jonah, C. D. J . Phys. Chem. 1988, 92, 5946. (7) Bartels, D. M.; Craw, M. T.; Han, P.; Trifunac, A. D. J . Chem. Phys. 1989, 93, 2412. (8) Ohno, S. Bull. Chem. SOC.Jpn. 1968, 41, 1301. (9) Anbar, M.; Meyerstein, D. Trans. Faraday SOC.1966, 62, 2121; J . Phys. Chem. 1965, 69, 698. (IO) Boyd, A. W.; Willis, C.; Lalor, G. C. Can. J . Chem. 1972, 50, 8 3 . (11) Han, P.; Bartels, D. M. J . Phys. Chem. 1990, 94, 5824. ( I 2) Ausloos, P. Fundamental Process in Radiation Chemistry; Interscience: New York, 1968. (13) Judeikis, H. S.; Flournoy, J. M.; Siegel, S. J . Chem. Phys. 1962,37, 2272 and references therein. (14) Weingartner, H.; C-Dreismann, C. A. Nature 1990, 346, 548. (15) Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall: London, 1980. (16) Nunome, K.; Muto, H.; Toriyama, K.; Iwasaki, M. Chem. Phys. Lett. 1976, 39, 542. (17) Muto, H.; Nunome, K.; Iwasaki, M. J . Phys. Chem. 1980,84, 3402. (18) Muto, H.; Matsuura, K., to be submitted for publication. (19) Heinzinger, K.; Weston, R. E., Jr. J . Phys. Chem. 1964, 68, 744.
I n Situ Scanning Tunneling Microscopy Study of Iodine and Bromine Adsorption on Au( 111) under Potential Control N. J. Tao* and S. M. Lindsay Department of Physics, Arizona State University, Tempe, Arizona 85287- 1504 (Received: January 27, 1992; I n Final Form: May 5, 1992) We have studied the adsorption of iodine and bromine onto Au( 1 11) from NaI and NaBr solutions as a function of surface potential using scanning tunneling microscopy. We have found that at low potentials both iodine and bromine form a (d\/3xv‘/5)R3O0structure. At higher potentials a 3 X 3 adlattice was observed for iodine, and a hexagonal close-packed adlattice with a rotation angle of -20’ was observed for bromine. Adlattice orientations are determined by comparing the adsorbate lattice images and the images of bare Au( 11 1) and by simultaneously imaging the adlattice and the substrate lattice in the case of iodine. Introduction Adsorption from liquid electrolytes has been a topic of interest for many years. Direct structural information about adsorbates at solid-liquid interfaces has been obtained using ex situ techniques, such as LEED (low-energy electron diffraction) to examine surfaces in ultrahigh vacuum (UHV) after removal from contact with solution.’ Though these ex situ techniques have provided most of our current knowledge about the structure of adsorbates at solid-liquid interfaces at the atomic level, the removal of solvent and the lass of potential control might alter the adsorbate structure. Therefore, it is important to probe the structure of adsorbates using in situ techniques. Recently, scanning tunneling microscopy (STM),2 scanning force microscopy (SFM),2 and X-ray techn i q u e ~using ~ synchrotron sources have made it possible to study solid surface under liquid electrolytes at an atomic level. STM has been used to study iodine-pretreated Pt in 0.1 M HC102 and in air5 and adsorption of CO on iodine-pretreated Rh( 111)6 and ~t(100).7 Adsorption of halogen on Au from salt solutions has been studied by a number of in and ex situl0 techniques, including a recent ex situ STM study.” Gao and WeaverI2 have studied adsorption of iodine from 0.1 M HC104 + 0.5 mM KI while this
work was being submitted for publication. They have observed several adlattice structures at different potentials. In this paper, we report an in situ STM study of the adsorption of iodine and bromine from NaI and NaBr aqueous solutions as a function of potential. Determination of the orientations of adsorbate lattices relative to the substrate lattice has been a difficulty for STM. In this work, we determined the orientations by comparing the structure of the bare substrates to the adsorbates at various surface potentials. In the case of iodine, we have been able to image both the substrate lattice and adsorbate lattice simultaneously. Therefore, a direct determination of the orientation could be made. Experiments Au( 11 1) substrates were epitaxially grown on mica. Detailed preparation procedures have been described previ0us1y.l~ Ten millimolar NaI and NaBr were prepared using water from a Barnstead Nanopure (Bioresearch Grade) System and Ultrapure NaI and NaBr from Johnson Matthey. A Nanoscope I1 (Digital Instruments) with an electrochemistry base was used in the experiment. Our STM tips were prepared by etching 0.25-mm Pto,sIro,2wire and then coating them with Apiezon wax.I4 The typical Faradaic leakage current of the tip was a few picoamperes
0022-365419212096-5213%03.00/0 0 1992 American Chemical Society
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(Figure la), which shows that the adsorption occurs at --400 mV. As the potential is changed to -300 mV, a monolayer island &(~oo~v/soc.) of iodine adsorbates is formed, covering most of the frame of Figure 2B. (This image can be indexed relative to Figure 2A by G loo the small island on the lower right.) On bridging the potential back to -700 mV, a clean Au surface is recovered as shown in < = L o Figure 2C, showing that the process is basically reversible. -: (Streaks in Figure 2C may indicate some irreversibility.) In fact, -50 we have noticed that the gold surface becomes rough after a few :He -100 cycles between --700 and -200 mV. $. High-magnification images of the surface more negative than --600 mV reveal the hexagonal Au( 11 1) lattice as shown in Figure 3A. (Image was obtained at 1 nA and 100 mV tip bias.) The measured distance between two nearest neighbors is about 2.8 A. On bridging the potential above --400 mV, we observed 80 a hexagonal lattice with a nearest-neighbor distance of -4.8 A (Figure 3B). By comparing with the bare Au(ll1) lattice in (SOmVlsec.) Figure 3A, we can see clearly that the iodine adlattice is rotated by --30°. This means that the iodine adsorbate forms a 0 (d3Xd/S)R30° structure. The coverage is determined to be -0.34 (the ratio of the number of adatoms to the number of ,-20 substrate atoms per unit area). This result is in agreement with ex situ STM" and LEED measurements.1° The structure is stable -40 between -400 and --15 mV. Further increasing the potential above --15 mV, we observed a hexagonal lattice structure with a nearest-neighbor distance of 4.3 A and with the same orientation - 1 5 0 0 -1000 - 5 0 0 0 500 1000 as the Au(ll1) lattice (Figure 3C). The change from the V (mV, v.8. Ag/AgCI) (d/3Xv'/5)R3O0 structure to the new structure appears to be Figure 1. Voltammograms of Au( 11 1 ) in 10 mM NaI (A) and 10 m M instant, which suggests that the transition could be faster than NaBr (B). -20 s-the interval between two successive scans. We attribute the new structure to a 3 X 3 structure of the iodine adlattice or less when tested in 1 M NaOH under a bias of 0.16 V. Fresh Pt and Ag wires were used for the counter and quasi reference corresponding to a close packing of the iodine adsorbate (coverage electrodes, respectively. All the potentials described in this paper -0.42). A 3 X 3 iodine adlattice on Pt( 111) a t high potential are quoted against the commonly used AglAgCl reference has been identified by ex situ LEED.20,21However, in a recent electrode. (The Ag quasi reference was calibrated against this ex situ LEED study of iodine on Au(l1 l), Bravo et a1.I0 found scale in separate experiments.) Typical voltammograms of Authat the (d/5Xd/?)R3O0 structure of iodine adlattice transformed (1 11) in 10 mM NaI (A) and 10 mM NaBr (B) obtained in the into a 5 X v'/3 structure which corresponds to a compression of STM cell are shown in Figure 1. Adsorption and desorption peaks the (d/5Xd/3)R30° structure to a close packing in one direction. are clearly visible and are similar to those in the literaturea except Gao and Weaverlz observed the 5 X 47 structure in their in situ that our voltammograms show greater hysteresis between adsorption and desorption. STM experiment. We did not see the 5 X 4 7 structure. AlWe started the experiment two ways. The first was by setting though in the particular run shown in this paper, drift was small the potentiostat to a value on the negative sides of the adsorption enough for it to be possible to distinguish the (d5Xd/5)R3Oo peaks and then introducing the solution into the electrochemistry structure from the 5 X d/5;it is possible that we have missed the cell. The second method was by introducing the solution into the 5 X dj structure in many other runs when drift was severe. cell with the potentiostat off, turning on the potentiostat (preset to the value of the rest potential), and then decreasing the poThough only the iodine adlattice appears visible in the real space tentials below the adsorption peaks immediately. Both methods image (Figure 3B), the Fourier transform of the image (inset in gave the same results. STM images were obtained with a tunFigure 3B) clearly shows two sets of spots. One is due to the iodine neling current in the range 0.1-1 nA and a tip bias in the range adlattice, and the other one may correspond to the Au( 11 1) 30-100 mV with respect to the working electrode. Though the substrate lattice. (The nearest-neighbor distance is 2.8 A, pointed phenomena of adsorption and desorption were not affected by the to by arrows in the inset in Figure 3B.) The relative rotation tunneling parameters, the contrast of the adsorbates does depend between the two lattices is 30°, which is consistent with that the on the bias and tunneling current. All data are raw data (no image iodine adsorbate forms a (v'/5Xdj)R30° structure in this poprocessing) unless stated otherwise. tential range. On decreasing the tip bias from 50 to -20 mV, we observed images that seem to show only the Au(ll1) lattice Results in real space, but the Fourier transform reveals spots due to the Adsorptionof Iodine on Au(ll1). It is well-known that Au(ll1) iodine adlattice. Though this effect is obviously useful for idenreconstructs to a 23 X v'j structure in both UHVI5 and aqueous tifying the rotation between the adlattice and substrate lattice, solutions as observed directly by ex situ LEED,I6 in situ STM," we do not yet understand the mechanism in detail. It is certainly and X-ray techniques.Ia In aqueous solutions, the reconstruction an electronic effect which depends on the electronic states of the was found to transform to 1 X 1 above a certain p ~ t e n t i a l . ' ~ - ~ ~tip, the substrate, and the adsorbate. Further experiments are necessary to fully understand the phenomenon. In a previous STM We have previously studied the 23 X v'j reconstruction to the experiment in air, Yau et aL4 have found that they can see the 1 X 1 structure transition by in situ STM in a number of aqueous adlattice at high bias and the substrate lattice at low bias. solutions.19 We had difficulty imaging directly the 23 X v'? Note that the potential range for each structure determined reconstruction in NaI because the reconstruction was stable only in these experiments is subject to uncertainty, because a very small at very low potentials, and we have found that the contrast of the area (order of 5 nm X 5 nm) was scanned each time to ensure reconstruction stripes decreases as the surface potential decreases.I9 enough resolution for revealing lattice structure. The transition Figure 2A is a typical image of a bare gold surface a t -620 from one structure to another is a heterogeneous first-order phase mV under 10 mM NaI. It shows single atomic steps with no sign transition. In this regard diffraction techniques, such as LEED of adsorption of iodine. This is consistent with the voltammogram * .
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Figure 2. The 190-nm X 190-nm images of Au(lI1) in 10 mM NaI at potentials of -620 (A), -300 (B), and -700 mV (C). Tip bias and tunneling current are 100 mV and 1 nA, respectively. The images and following images are raw data unless stated otherwise.
Figure 3. The 3.4-nm X 3.4-nm images of Au(ll1) in 10 mM NaI at potentials of -620 (A), -400 (B), and -15 mV (C) obtained with 50-mV tip bias and I-nA tunneling current. The Fourier transforms of the images are shown as insets.
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5216 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
Figure 4. Au( 11 1) in 10 mM NaBr at potentials of -100 (A), 0 (B), 150 (C), and 120 mV (D) obtained with 40-mV bias and 1-nA tunneling current. Image sizes are 3.4 nm X 3.4 nm for (A)-(C) and 13.5 nm X 13.5 nm for (D). High- and low-frequency noise was filtered out in (D). The Fourier transforms of the images are shown as insets.
and X-ray, which probe properties of a very large area, are more suitable for determining the exact transition potentials. Adsorption of Bromine on Au( 111). In contrast to the case of NaI, here we can clearly observe the stripes on Au( 111) due to the 23 X dj reconstruction. Figure 4A shows an atomic resolution image of Au(ll1) at -100 mV. The inset shows the Fourier transform of the iamge. (The distortion from the 6-fold symmetry spots is due to drift.) Since the STM scan in the x direction (from left to right) is 200 times faster than t h e y direction (from top to bottom), the drift in the x direction is negligible. Thus, the periodicity in the x direction in the real space image or of the x coordinates of the diffraction spots is used to obtain the actual lattice constant. The measured lattice constant is -2.8 A for Au( 111). Raising the potential to 50 mV, we observed a hexagonal lattice with a lattice constant of 4.8 A (Figure 4B), which is the same as in the case of iodine (Figure 3B). The rotation angle of the adlattice relative to the Au(ll1) lattice is about 30°, and the ratio of the adlattice constant to the Au( 111) substrate lattice constant is about 1.7, which is close to 4 5 (as it should be for a (d5Xdj)R3O0 structure). The coverage is determined as -0.34, close to 0.33 for the (d%d5)R3O0 structure. Further
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increasing the potential above 100 mV, we observed another hexagonal lattice with a lattice constant of 4.4 A (Figure 4C). This corresponds to a close-packed structure of bromine on Au(1 11). The coverage is found to be -0.41. The rotation angle of the close packed bromine adlattice with respect to the substrate lattice is about 20 f 3O; therefore, the structure may be the (d?Xd?)R19O which has been observed for the iodine adlattice on Pt( 111).20*21In the same potential range, we also observed a hexagonal superperiodic structure with a lattice constant of 9.4 A in some runs. Figure 4D shows both the close-packed bromine adlattice structure and the superperiodic structure. (High- and low-frequency noises were filtered out in this image.) The orientation of the superperiodic structure is the same as the closepacked structure as shown clearly by the Fourier transform shown as an inset in Figure 4D. We do not yet understand the superperiodic structure. Adsorption of bromine on Au( 111) has been studied by mass balance measurements: which indicated a close-packed structure for the bromine adlattice. Conclusions We have studied in situ the adsorption of iodine and bromine on Au( 1 11) from NaI and NaBr solutions as a function of po-
J. Phys. Chem. 1992, 96, 5217-5219 tential using scanning tunneling microscopy. For iodine on Au(1 1 l), we observed a (d/5Xd/5)R3Oostructure in the potential range --400 to --15 mV (vs Ag/AgCl) and a 3 X 3 closepacked structure above --15 mV. The adlattice orientation with respect to the substrate lattice is determined by comparing the bare Au( 1 1 1) lattice at low potential with the iodine adlattice or from the images that show both the adlattice and substrate lattice in some tip bias range. While the (d/5Xd3)R30° structure is in agreement with the ex situ LEED experiment, we observed a close-packed structure of iodine at high potentials instead of a semiclose-packed structure identified by the ex situ LEED experiment. In the case of bromine on Au( 1 1 l ) , we observed a (d/5Xd/3)R3O0 structure in the potential range -0 to 100 mV. A close-packed bromine adlattice with a rotation of -20° relative to the Au( 1 1 1) lattice was observed above 100 mV. In the similar potential range, we also observed a hexa onally superperiodic structure with a lattice constant of -9.4 superimposed on the close-packed structure. The superperiodic structure has the same orientation as the close-packed structure as determined from the images that show both the superperiodic structure and the close-packed structure. We found that the contrast of the iodine adlattice depends on both the tip bias and surface potential. The iodine adlattice can be “semitransparent” which allows us to “see” the underlying substrate lattice and, therefore, directly determine the orientation of the adlattice relative to the substrate lattice. We believe that the origin of the phenomenon is electronic, depending on the electronic states of the adlattice, substrate lattice, and the tip. In a similar potential and tip bias range, we did not see the effect on the bromine adlattice. This interesting phenomenon deserves further study. Acknowledgment. We thank H. Song, Y.Li, J. A. DeRose, P. I. Oden, and J. Pan for help in the lab, L. A. Nagahara for important discussions, M. J. Weaver for sharing his results with us prior to publication, and the referees who pointed out a mistake in our potential scale in the previous versions of the paper. Support was received from the N S F (Dir 89-20053), O N R (N00014-
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90-J-1455), and the Vice President for Research at ASU. References and Notes (1) Hubbard, A. T. Chem. Reu. 1988,88, 633. (2) Sonnenfeld, R.; Schneir, J.; Hansma, P. K. In Modern Aspects of Electrochemistry; Brockris, J. O M . , Ed.;Plenum: New York, 1990. Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (3) Samant, M. G.; Toney, M. F.; Borges, G. L.; Blum, L.; Melory, 0. R. J . Phys. Chem. 1988,92,220. Ocko, B. M.; Wang, J.; Davenport, A.; Isaacs, H. Phys. Rev. Lett. 1990, 65, 1466. (4) Yau, S.-L.; Vitus, C. M.; Schardt, B. C. J . Am. Chem. SOC.1990,112, 3677. (5) Schardt, B. C.; Yau, S.-L.;Rinelli, F. Science 1989, 243, 1050. Vogel, R.; Baltruschat, H. Surf. Sci. 1991, 259, L739. (6) Vitus, C. M.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J . Phys. Chem. 1991, 95, 7559. (7) Yau, S.-L.;Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J . Am. Chem. Soc. 1991, 113, 6049. (8) Deakin, M. R.; Li, T. T.; Melroy, 0. W. J . Electroanal. Chem. 1988, 243, 343. (9) Rodriguez, J. F.; Soriaga, M. P. J . Electrochem. Soc. 1988, 135,616. (10) Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P.; Villegas, 1.; Suggs, D. W.; Stickney, J. L. J . Phys. Chem. 1991, 95, 5245. (11) McCarley, R. L.; Bard, A. J. J . Phys. Chem. 1991, 95, 9618. (12) Gao, X. P.; Weaver, M. J. Submitted for publication. (13) DeRose, J . A.; Thundat, T.; Nagahara, L. A,; Lindsay, S. M. Surf. Sci. 1991, 256, 102. (14) Nagahara, L. A,; Thundat, T.; Lindsay, S. M. Reu. Sci. Instrum. 1989, 60, 3128. (15) Takayanagi, K.; Yagi, K. Jpn. Instrum. Methods 1983, 24, 337. Harten, U.; Lahee, A. M.; Toennies, J. P.; Woll, Ch. Phys. Reu. Lett. 1985, 54, 2619. Woll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Reu. 1989, B39, 7988. Barth, J. V.; Brune, H.; Ertl, G.; Behem, R. J . Phys. Reu. 1990, 42. 9307. (16) Nakai, Y.; Zei, M. S.; Kolb, D. M.; Lehmpfuhl, G. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 340. (17) Tao, N . J.; Lindsay, S. M. J . Appl. Phys. 1991, 70, 5141. Gao, X.; Hamelin. A.; Weaver, M. J . Chem. Phys. 1991, 95, 6993. (18) Wang, J.; Davenport, A. J.; Isaacs, H. S.; Ocko, B. M. Science 1992, 255, 1416. (19) Tao, N . J.; Lindsay, S. M. Submitted for publication. (20) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985, I , 66. (21) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J . Electroanal. Chem. 1987, 222, 305.
Concerted Hydroxyl Ion Attack and Pseudorotation in the Basecatalyzed Hydrolysis of Methyl Ethylene Phosphatet Carmay Lim* and Philip Tole Department of Molecular and Medical Genetics, Department of Chemistry and Department of Biochemistry, University of Toronto, I King’s College Circle, Toronto, Ontario M5S IA8, Canada (Received: March 4, 1992; In Final Form: April 16, 1992)
The gas-phase and solution free energy profiles for the base-catalyzed hydrolysis of methyl ethylene phosphate (MEP) were obtained using ab initio molecular orbital calculations and continuum dielectric methods. The correlation energy is estimated with second-order Mdler-Plesset theory and the 6-3 1+G* basis using fully optimized 3-21+G* geometries. In vacuum and in solution, OH- attack on MEP is concerted with pseudorotation to form intermediate 2a, which undergoes ring opening faster than exocyclic cleavage of intermediate 2b (Scheme 11); the apical attack of OH- and the apical departure of the ring oxygen in 2a are in accord with an in-line mechanism. This new mechanism is consistent with the experimental observation that MEP hydrolyzes exclusively with ring opening in dilute alkaline solution (pH 8-1 1).
Introduction Pseudorotation in pentacovalent species is defined as an intramolecular process where a trigonal bipyramid (TBp), which may be short-lived, is converted into another by deforming angles This work was supported by the Protein Engineering Network Center of Excellence.
so that the final TBP appears to have performed a 90° rotation relative to the initial state.’-3 The pseudorotation concept has been used to explain the rapid exocyclic cleavage with ring retention of phosphate esters and other kinetic data for phostonate and phosphinate hydrolyse^.^,^ A classic example of its application is in rationalizing the PH-Prduct profile for the hydrolysis of methyl ethylene phosphate (MEP)? In particular, the mechanism
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