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Adsorption of Chiral Alcohols on “Chiral” Metal Surfaces Christopher F. McFadden Department of Chemistry, University of Illinois at UrbanasChampaign, Urbana, Illinois 61801
Paul S. Cremer Department of Chemistry, University of California, Berkeley, California 94720
Andrew J. Gellman* Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received May 4, 1995X Kink sites on high Miller index surfaces are either left- or right-handed and can be thought of as chiral, when the step lengths on either side of the kink site are unequal. A silver single crystal was oriented and cut to expose the Ag(643) surface on one side and the Ag(6 h 4h 3h ) surface on the other side. A system is proposed for naming these surfaces as Ag(643)S and Ag(643)R, respectively , in analogy with the CahnIngold-Prelog rules used in the nomenclature of organic stereoisomers. The left hand/right hand relationship of the two surfaces was manifested by the direction of the splitting of the low-energy electron diffraction (LEED) spots. The interaction of the enantiomers of a chiral alcohol ((R)-2-butanol and (S)-2-butanol) with each surface was studied using temperature-programmed desorption (TPD) measurements in order to ascertain the magnitude of the effect of surface chirality on the heats of adsorption. Desorption of the alcohols following exposure to the clean surfaces was molecular and exhibited first-order kinetics. No difference was observed between (R)- and (S)-2-butanol in either desorption temperature (225 K) or peak shape. Upon exposure to preoxidized surfaces, the alcohols deprotonated to form (R)- and (S)-2-butanoxide, both of which decomposed upon heating via β-hydride elimination. The decomposition product, 2-butanone, desorbed at 282 K. Again, no difference in the reaction kinetics of the enantiomeric alkoxides was observed on the two surfaces. From these results it can be concluded that the difference in (a) the heat of adsorption of the enantiomeric alcohols and (b) the difference in the energy barrier to β-hydride elimination for the enantiomeric alkoxides is less than 0.1 kcal/mol.
1. Introduction Kink sites on the high Miller index surfaces of metal single crystals lack symmetry (other than translational) when the step lengths or step faces on either side of the kink are unequal. In such cases a new surface may be generated by reflection through a plane normal to the surface. Hence it is possible to prepare single crystal surfaces which possess kink sites with intrinsic (left or right) handedness. Two such surfaces, although related by reflection, are not superimposable and can be thought of as chiral. For example, when the (643) surface of an fcc lattice is reflected through the yz plane (as in Figure 1) its enantiomorph, the (6h 43) surface, is generated. It should be noted that for crystal lattices with cubic symmetry, a kinked surface with Miller indices (hkl) is equivalent by symmetry to the surfaces with Miller indices (hkhhl), (h h klh), and (h hk h l). The enantiomorphs to these surfaces form a set composed of the (h hk hhl), (h h kl), (hkh l), and (hklh) surfaces, all of which are equivalent. Because of the existence of chirality in kinked single crystal metal surfaces, it should be possible to observe enantiomerically selective adsorption and/or chemistry of chiral organic molecules at kink sites. This would result if steric considerations allowed one enantiomer to interact more intimately with the kink site than the other, in the same way that the left hand can interact more intimately with the left glove. By studying the interaction of chiral molecules with well-characterized metal surfaces with the techniques of ultrahigh vacuum surface science, a more * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, October 15, 1995.
fundamental understanding of such important applications as enantioselective heterogeneous catalysis or chiral separations may be attained. Enantioselectivity in heterogeneous catalysis is obtained by the adsorption of chiral modifiers onto the catalyst surface. The systems that have been studied most frequently are the hydrogenation of R-keto esters over supported platinum catalysts modified by cinchona alkaloids (cinchonadine)1-3 and the hydrogenation of β-keto esters over nickel catalysts modified by tartaric acid.4-6 Although enantiomeric excesses greater than 90% have been achieved, the mechanism of enantiodifferentiation is not well understood. Chromatographic methods of chiral separation also rely on the interaction of the two enantiomers with an achiral surface (stationary phase) modified by the adsorption of chiral molecules (chiral selectors).7-11 The miniscule difference in the interaction of the two enantiomers with the stationary phase is amplified by the thousands of (1) Sutherland, I. M.; Ibbotson, A.; Moyes, R. B.; Wells, P. B. J. Catal. 1990, 125, 77. (2) Minder, B.; Mallat, T.; Baiker, A.; Wang, G.; Heinz, T.; Pfaltz, A. J. Catal. 1995, 154, 371. (3) Singh, U. K.; Landau, R. N.; Sun, Y.; Leblond, C.; Blackmond, D. G. J. Catal. 1995, 154, 91. (4) Izumi, Y. In Advances in Catalysis, Vol. 32; Eley, D. D.; Pines, H.; Weisz, P. B., Eds. Academic Press: San Diego, 1983, p 215. (5) Webb, G.; Wells, P. B. Catal. Today. 1992, 12, 319. (6) Fu, L.; Kung, H. H.; Sachtler, W. M. H. J. Mol. Catal. 1987, 42, 29. (7) Whatley, J. A. J. Chromatogr. 1995, 697, 257. (8) Armstrong, D. W.; Tang, Y.; Chen, S. Anal. Chem. 1994, 66, 1473. (9) Armstrong, D. W.; Tang, Y.; Ward, T. Anal. Chem. 1993, 65, 1114. (10) Gubitz, G. Chromatagraphia 1990, 30, 555. (11) Pirkle, W. H.; Pochapsky, T. C. Chem. Rev. 1989, 111, 9222.
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Figure 1. (A) Three dimensional ball models of the Ag(643) and Ag(6 h4 h3 h ) surfaces. (B) Photographs of LEED patterns from the Ag(643)R and Ag(643)S surfaces. The vertical division (representing the yz plane) is meant to highlight the mirror symmetry of both the surfaces and their respective LEED patterns.
theoretical plates available with these techniques, allowing efficient separations. Because these methods of separation rely on the partitioning of analyte between stationary and mobile phases, the heat of adsorption of an analyte molecule on the stationary phase cannot easily be obtained.12 To determine the degree of preferential adsorption of an organic molecule on a chiral metal surface, we have investigated the surface chemistry of (R)- and (S)-2butanol adsorbed on the Ag(643) and Ag(6 h 4h 3h ) surfaces. This system was selected with the expectation that the surface chemistry would be similar to that of the normal alcohols adsorbed on the atomically corrugated Ag(110) surface.13-16 Temperature-programmed desorption (TPD) experiments of the straight chain alcohols methanol through pentanol showed that the alcohols desorb reversibly in the range 140 K (methanol) to 185 K (pentanol).1 These experiments indicated that the alkyl chains interact with the surface with a heat of 1.1 kcal/mol per methylene group, while work function measurements suggested that the alcohols adsorb through the oxygen atom. Quantitative X-ray photoelectron spectroscopy (XPS) of the straight chain alcohols indicated that the alkyl chains remain oriented roughly parallel to the surface. This surface chemistry can be altered by preoxidizing the Ag(110) surface by exposure to O2, which results in the formation of an adsorbed layer of atomic oxygen. Upon exposure to the preoxidized surface the alcohols deprotonate to form water (which desorbs at T < 200 K) and the alkoxide, which then decomposes via β-hydride elimination at temperatures in the range 270-300 K to form the aldehyde, which immediately desorbs.17 High-resolution (12) Novotny, M.; Soini, H.; Stefansson, M. Anal. Chem. 1994, 66, 646A. (13) Zhang, R.; Gellman, A. J. J. Phys. Chem. 1991, 95, 7433-7437. (14) Dai, Q.; Gellman, A. J. Surf. Sci. 1991, 257, 103-112. (15) Wachs, I. E.; Madix, R. J. Appl. Surf. Sci. 1978, 1, 303. (16) Wachs, I. E.; Madix, R. J. Surf. Sci. 1978, 76, 531. (17) Zhang, R. Ph.D. Dissertation, University of Illinois, 1992.
Figure 2. Three dimensional ball model of (R)-2-butanol (left) and (S)-2-butanol (right) adsorbed through the oxygen atom at kink sites on Ag(643)R. For the sake of clarity, all hydrogen atoms have been omitted except for the one attached to the chiral carbon. All bond distances have been drawn to scale. It should be emphasized that these are only two of many possible orientations and do not necessarily represent the true orientation of the molecules adsorbed on the Ag(643)R surface.
electron energy loss spectroscopy (HREELS) of the primary alkoxides suggested that the C-O bond axis was oriented normal to the surface only in the case of methoxide, while the C-O bond axis was oriented parallel to the surface for the higher alkoxides ethoxide to pentanoxide.2 The difference in the heat of adsorption of (R)- and (S)2-butanol can be estimated in the following way. The increase in heat of adsorption of the straight chain alcohols on Ag(110) of 1.1 kcal/mol/CH2 gives a reasonable estimate of the Ag-alkyl interaction energy. Consider the result if steric considerations allowed one enantiomer to “fit” better than the other at a given kink site. This might occur, for example, if the orientation of the alcohols adsorbed on the surface were as shown in Figure 2. At
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left an (R)-2-butanol molecule is adsorbed at a kink site on the Ag(6 h4 h3 h ) surface through the oxygen atom, with the ethyl and methyl groups oriented along the steps (the hydrogen is oriented away from the kink site). At right an (S)-2-butanol molecule is adsorbed in an identical kink site such that the ethyl and hydrogen are oriented along the steps (the methyl group oriented away from the kink site). The difference in the heat of adsorption in these cases would then be expected to be approximately 1.1 kcal/ mol. This would result in a difference in desorption peak temperature of approximately 20 K (assuming first-order desorption kinetics, a heating rate of 2 K/s, and a preexponential factor of 1013 s-1, all of which are relevant to the present study), which could be easily distinguished in TPD experiments. This estimate is probably an upper limit of what can be expected, as many other intermediate adsorbate orientations are possible. If the minimum resolvable difference in peak temperature in a TPD experiment is taken as 2 K, then a difference in the heat of adsorption of approximately 0.1 kcal/mol could in theory be measured. 2. Nomenclature for Chiral Surfaces Chirality in organic compounds is most often caused by a carbon atom tetrahedrally bound to four different groups. In order to unambiguously specify the stereochemistry of such compounds, the four groups bound to the carbon are ranked in priority according to the Cahn-Ingold-Prelog sequence rules, which may be found in introductory organic chemistry textbooks.18 Briefly, the four groups are ranked according to the atomic number of the atoms directly bound to the carbon. If two groups are attached to the chiral center through the same type of atom (as for methyl and ethyl in 2-butanol), priority is determined by working outward to the first point of difference. For 2-butanol (CH3CH2CH(OH)CH3), priority is assigned (high to low): 1, OH; 2, C2H5; 3, CH3, 4, H. After priority is assigned the molecule is mentally oriented such that the group of lowest priority is pointed directly away from the viewer. The three remaining groups then form a triangle in which group precedence can be traced in order (1 f 2 f 3) following either a clockwise or counterclockwise rotation. The chiral center is designated “R” (from the Latin rectus, right) if clockwise and “S” (from the Latin sinister, left) if counterclockwise. The R and S designation is determined by human convention and has no relation to the direction that a solution containing the molecule will rotate plane-polarized light. As noted previously, kinked metal surfaces will lack symmetry only when the step length along one side of the kink site is greater than the other. As seen in Figure 1, for the (643) surface of an fcc lattice one of the steps is two atoms long, while the other is one atom long (at least one step length must be monatomic). In analogy to the CahnIngold-Prelog rules, the long step will be given higher priority than the short step. Lowest priority will be given to the terrace below the step. Given the natural tendency to view the surface from above, one imagines spiraling down into the crystal in the order (long step f short step f terrace). We propose that the surface be designated (hkl)R if one spirals clockwise into the surface and (hkl)S if counterclockwise. Following this system of nomenclature the Ag(643) and Ag(6 h4 h3 h ) surfaces may be designated as Ag(643)S and Ag(643)R, respectively. As will be shown in section 4.1, the absolute stereochemistry of a given kinked metal surface may be determined by low-energy diffraction (LEED). (18) See, for example: McMurry, J. Organic Chemistry; Brooks/Cole Publishing Co.: New York, 1984; p 240.
Figure 3. Series of TPD spectra taken following increasing exposures of 2-butanol-d2 to the Ag(643)S surface. Inset is a plot of coverage (given as the area under the monolayer peak, in arbitrary units) vs exposure (given in Langmuir, without correction for the multiplication factor of the capillary array or the sensitivity of the ion gauge to 2-butanol-d2).
3. Experimental Section All LEED and TPD experiments were performed in a stainless steel ultrahigh vacuum chamber ion pumped to a base pressure of 1 × 10-10 Torr. The chamber is equipped with a four-grid retarding field analyzer used to perform LEED and auger electron spectroscopy (AES). A quadrupole mass analyzer is used to perform TPD measurements. An ion sputter gun is used to clean the sample by Ar+ bombardment, and leak valves, both with and without capillary arrays, are used to introduce gases and organic vapors into the chamber. A silver single crystal rod of approximately 1 cm diameter (Monocrystals Inc.) was oriented using a goniometer and cut to within 0.5° of the (643) surface by electrodischarge machining. The other side of the sample was oriented and cut parallel to the (643) surface, which exposed the (6h 4 h 3h ) surface. The crystal was spot-welded between two tantalum wires on a sample holder, which in turn was attached to a manipulator. The crystal could be heated resistively to over 1000 K and cooled with liquid nitrogen to approximately 100 K. The temperature was measured using a chromel/alumel thermocouple junction inserted into a small hole near the edge of the crystal that was formed by electrodischarge machining. Desorption temperatures measured on the Ag(643)S surface were consistently measured to be 1 K higher than those measured on the Ag(643)R surface, which is probably due to the placement of the thermocouple and serves as an excellent indicator of the precision of our temperature measurements. This was quite reproducible and was observed for all desorption systems studied. The TPD spectra presented in Figures 3-5 have not been corrected for this systematic error, which is most apparent in Figure 4. The Ag(643) surface was cleaned by 1 keV Ar+ bombardment and annealing to 900 K, followed by cycles of oxygen adsorption/ desorption to remove carbon, as this is a common impurity that cannot be detected with AES due to interference from the AES signal from silver at 270 eV. The sample was judged clean when no sulfur or oxygen was detected with AES and the amount of CO2 desorbed following oxygen adsorption was minimized. Oxygen desorbed from the clean surface at approximately 540 K.
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4. Results/Discussion
Figure 4. TPD spectra of (R)- and (S)-2-butanol adsorbed on the clean Ag(643)R and Ag(643)S surfaces ((R)-2-butanol adsorbed on Ag(643)R is labeled r/Ag(643)R, etc.).
Figure 5. TPD spectra of (R)- and (S)-2-butanol adsorbed on the preoxidized Ag(643)R and Ag(643)S surfaces ((R)-2-butanol adsorbed on Ag(643)R is labeled r/Ag(643)R, etc.). (R)- and (S)-2-butanol (CH3CH2CH(OH)CH3, Aldrich Chemical, 99% nominal purity) and 2-butanol-d2 (an racemic mixture of CH3CH2CD(OH)CH3, Cambridge Isotope Laboratories, 99%), were subjected to cycles of freezing, pumping, and thawing to remove dissolved air or other high-vapor-pressure impurities. The fragmentation patterns of the (R)- and (S)-2-butanol in the mass spectrometer were identical. Vapors were introduced into the chamber using leak valves fitted with capillary arrays, with the crystal positioned approximately 1 cm away from the doser unless otherwise indicated. Exposures were calculated using pressures indicated by the ion gauge, uncorrected for the multiplication factor of the capillary array or the relative sensitivity of the ion gauge to the alcohols.
4.1. Low Energy Electron Diffraction. In general, stepped and kinked surfaces are expected to exhibit LEED patterns derived from diffraction from the terraces, while the presence of evenly spaced steps causes energydependent splitting of those spots.19 The energy at which diffraction maxima split is different for each beam, which means that the splitting energy for the (0,0) spot is different from that of the (1,0) spot, etc. Following the two-dimensional kinematic treatment of Henzler,20 the terrace width can be calculated from the magnitude of the splitting while the step height can be related to the energy dependence of the splitting. Photographs of the LEED patterns for both the Ag(643)R and the Ag(643)S surfaces are presented in Figure 1. The primary beam energy in each case was 132.0 eV and the crystal temperature was 100 K. The diffraction patterns in Figure 1 show the basic hexagonal symmetry derived from the (111) terraces. The lattice parameter of the (111) terraces was found to be 4.1 Å, in good agreement with the bulk value of 4.09 Å.21 The terrace width was calculated to be 12.9 Å, which corresponds to three-atomwide (111) terraces. The right-hand/left-hand relationship of the two surfaces is apparent in Figure 1 from the direction of spot splitting. For the Ag(643)S surface the vector that connects the split spots is +23° away from the [1,1] direction, while for the Ag(643)R surface the direction of splitting is -23° from the [1,1] direction. This is in good agreement with the angle that the steps make with the [0,1] direction in real space, which is 21.8°. The sharpness of the split spots was found to be sensitive to surface cleanliness, with carbon-contaminated surfaces exhibiting diffuse diffraction spots from the terraces with streaks along the splitting direction. 4.2. Temperature-Programmed Desorption/Reaction. As described in section 1, the normal alcohols methanol to pentanol adsorb and desorb reversibly on the Ag(110) surface, with an increase in the heat of adsorption of the alcohols of 1.1 kcal/mol per methylene group in the alkyl chain. Preoxidizing the surface causes the alcohol to deprotonate to form water, which desorbs at low temperatures (T < 200 K), and leaves the corresponding alkoxide. The alkoxide decomposes by β-hydride elimination to form the corresponding aldehyde, which desorbs at temperatures in the range 270-300 K. As will be described below, qualitatively similar behavior was observed for (R)- and (S)-2-butanol adsorbed on Ag(643)R and Ag(643)S. 4.2.1. Alcohol Desorption from Ag(643)R and Ag(643)S. In the initial stages of the study TPD spectra were acquired following background (non-line-of-sight) exposure of 2-butanol-d2 (CH3CH2CD(OH)CH3) to the clean Ag(643)S surface at 125 K. Deuteration has no effect on the basic desorption characteristics, as determined by comparison with subsequent TPD of (R)- and (S)-2-butanol. Figure 3 shows a series of TPD spectra following increasing exposure of 2-butanol-d2 to the Ag(643)S surface taken with a heating rate of 2 K/s. At the lowest exposures (0.1 L) a single peak was observed at 226 K corresponding to desorption from the monolayer. The peak temperature was independent of coverage, indicating first-order desorption kinetics. The heat of desorption was calculated to be 14.1 kcal/mol by using the standard relation for firstorder desorption kinetics22 (Tp2 ) (βEd/Rν1) exp(Ed/RTp)) with an assumed preexponential factor (ν1) of 1013 s-1. In (19) Ertl, G.; Ku¨ppers, J. Low Energy Electron and Surface Chemistry, 2nd ed.; VCH: Weinheim, 1985; pp 246-248. (20) Henzler, M. Surf. Sci. 1970, 19, 159-171. (21) Kittel, C. Introduction to Solid State Physics, 6th ed.; John Wiley and Sons: New York, 1986; p 23. (22) Redhead, P. A. Vacuum 1962, 12, 203.
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this equation Tp is the peak desorption temperature, β the heating rate, and Ed the heat of desorption for an unactivated process. The area under the monolayer desorption peak increased linearly with increasing exposure until 4 L, at which point the monolayer was saturated and a second peak was observed at 177 K, which then grew without limit with increasing exposure. The peak temperatures increased with increasing coverage, indicating zeroth-order kinetics. The desorption behavior of 2-butanol-d2 on the Ag(643) surface is quite similar to that of 1-butanol on the Ag(110) surface, which exhibited monolayer and multilayer peaks at 215 and 170 K, respectively.1 Great care was taken in comparing the desorption of (R)- and (S)-2-butanol from the chiral surfaces, as any difference in peak temperature or shape would be expected to be subtle. Desorption spectra were taken following the exposure of (R)- and (S)-2-butanol (done separately) to both the Ag(643)R and the Ag(643)S surface. The alcohols were exposed to each surface at 175 K. This prevented multilayer formation, the desorption of which could obscure the leading edge of the monolayer desorption peak. Direct (line-of-sight) exposure using a capillary array allowed smaller apparent exposures (0.1 L) to be used to adsorb a saturated monolayer, minimizing the background pressure after exposure. TPD spectra were acquired over the temperature range 125-425 K using a heating rate of 2 K/s, giving a sampling rate of 1 data point/K. This is roughly equal to the temperature resolution (1.33 K) of the A-D converter used to digitize the thermocouple voltages. Desorption spectra for each of the four alcohol/ surface combinations were repeated several times following identical exposures. Peak desorption temperatures and peak areas were reproducible to better than 1%. Representative TPD spectra for each of the four alcohol/ surface permutations are presented in Figure 4. Aside from the 1 K temperature difference between the Ag(643)R and Ag(643)S surfaces mentioned in section 2, there was no enantiospecific difference in the peak temperature (225 K) or peak shape. TPD spectra were also taken at submonolayer alcohol coverages (not shown), and again no difference was observed. 4.2.2. Alkoxide Decomposition on Preoxidized Ag(643)R and Ag(643)S. The next comparison was of the decomposition kinetics of (R)- and (S)-2-butanoxide on the preoxidized chiral surfaces. To form the alkoxides the surfaces were exposed to 10 L of O2 at 300 K, followed by a 0.1 L exposure of the alcohol at 200 K (both exposures were performed with the sample directly in front of the doser). This recipe allowed reproducible desorption yields, prevented multilayer adsorption of the alcohol on top of the alkoxide monolayer, and left no residual oxygen on the surface. TPD spectra were acquired over the temperature range 200-400 K at a heating rate of 2 K/s, resulting in a sampling rate of 1.5 data points/K. A representative TPD spectrum for each alcohol/ preoxidized surface combination is presented in Figure 5. The desorbing species was identified as 2-butanone by monitoring the ratio of the intensities at m/q 45 and 43, which was determined to be approximately 3 for 2-butanol and 0.1 for 2-butanone. As can be seen in Figure 5, the desorption of 2-butanone following alkoxide decomposition occurs at 282 K on both the Ag(643)R and Ag(643)S surfaces. There is no detectable difference between the decomposi-
tion kinetics of (R)- and (S)-2-butanoxide on a given surface, as determined by comparison of desorption peak temperatures or shapes. 5. Conclusion A silver crystal was oriented and cut to expose the Ag(643)R surface on one side and the Ag(643)S surface on the opposite side. These surfaces are nonsuperimposable mirror images of one another composed of kink sites that are left- and right-handed, respectively. The primary result of this work is the demonstration through the mirror symmetry of the LEED patterns that these surfaces are indeed enantiomers of one another. We believe that this represents the first demonstration of the chiral nature of the kinked metal single crystal surfaces. In principle, there must be some enantiospecific difference in the reaction kinetics of chiral molecules on the chiral surfaces. The issue is its magnitude. We have presented the results of initial attempts to investigate the influence of chirality on surface reaction kinetics. The reactions studied were the desorption of (R)- and (S)-2butanol from the clean surfaces and the decomposition of (R)- and (S)-2-butanoxide by β-hydride elimination on the preoxidized surfaces. We argue that ∼1 kcal/mol difference in the heat of desorption of the alcohols and the energy barrier to alkoxide decomposition of the alkoxides is the maximum that could be expected. Unfortunately, neither of these reactions exhibited an influence of chirality on their reaction kinetics. Given the sensitivity of our measurements, this indicates that the effect of chirality on the energy barriers to these two processes is less than 0.1 kcal/mol. Clearly the demonstration of chirality in surface reaction kinetics awaits only the identification of a suitable combination of adsorbate and metal surface. As can be seen upon close inspection of Figure 2, it is apparent that the ethyl group could interact equally well with either the long or short step edge. In our laboratory we are pursuing the use of larger secondary alcohols that have one alkyl chain of greater length than the longer step edge of the Ag(643) surface. The hope is that this will induce a measureable degree of asymmetry in their interactions with the surface. Another approach is the identification of an adsorbate which will interact with the surface directly through its chiral center. An example would be a molecule containing iodine bound to a chiral carbon atom. In such a species one would expect the C-I bond to break at low temperatures, leaving the chiral carbon atom bound directly to the metal surface. In another tack we are pursuing the use of FT-IRRAS to try to differentiate the orientation of the (R)- and (S)-2-butanoxides on the Ag(643) surfaces. With the identification of suitable adsorbates for investigation we feel that the use of these chiral surfaces offers enormously exciting opportunities for the investigation of enantiospecific surface chemistry. Ultimately, they offer the opportunity to contribute to the fundamental understanding of chiral heterogeneous catalysis. Acknowledgment. This work was supported through a Fellowship in Science and Engineering awarded to A.J.G. from the David and Lucile Packard Foundation. LA950348L
