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Influence of Modification pH and Temperature on the Interaction of Methylacetoacetate with (S)-Glutamic Acid-Modified Ni{111} T. E. Jones, A. E. Rekatas, and C. J. Baddeley* EaStCHEM School of Chemistry, UniVersity of St. Andrews, St. Andrews, Fife KY16 9ST, United Kingdom ReceiVed: December 20, 2006; In Final Form: February 5, 2007
The adsorption of (S)-glutamic acid onto Ni{111} from solution was investigated with reflection absorption infrared spectroscopy as functions of modification temperature and pH. Modification at 300 K results in the formation of approximately one monolayer of adsorbate. The extent of protonation of the adsorbate decreases with increasing pH. Modification at 350 K produces a thicker adsorbate layer, which is assigned to nickel glutamate (low pH) and a mixture of nickel and sodium glutamate (high pH). Adsorption of methylacetoacetate onto the chirally modified surfaces occurs predominantly in the diketone tautomeric form when adsorbed glutamic acid exists in the protonated form. By contrast, methylacetoacetate exists primarily in the enol tautomeric form when glutamate (i.e., anionic) species are present. The implications of our findings for understanding the mechanism of Ni-catalyzed β-ketoester hydrogenation are discussed.
Introduction Amino acid adsorption on metal surfaces has been the focus of a considerable number of recent research articles.1-7 An important use of adsorbed amino acids is as chiral modifiers in enantioselective heterogeneous catalysis. One of the most heavily studied examples of this type of catalysis is the hydrogenation of β-ketoesters over Ni catalysts.8-10 Hydrogenation of the simplest β-ketoester, methylacetoacetate (MAA), results in a racemic product mixture of the (R)- and (S)enantiomers of methyl-3-hydroxybutyrate (MHB). However, adsorption of R-amino or R-hydroxy acids onto Ni catalysts from solution influences the hydrogenation reaction such that one enantiomer is produced in excess over the other (Figure 1a). Various proposals have been put forward to explain the overall mechanism of the enantioselective reaction. The enantioselectivity of the Ni-catalyzed reaction depends on a large number of preparation variables including the pH of the modification solution and the modification temperature.8-11 The behavior of (S)-glutamic acid as a chiral modifier is particularly interesting and unique in the sense that a switch is observed in the dominant enantiomeric product with increasing modification temperature (Figure 1b).8 Modification at 273 K and pH 5 results in a Ni catalyst that favors the hydrogenation of methylacetoacetate to (R)-methyl-3-hydroxybutyrate ((R)-MHB)). As the modification temperature increases, the enantioselectivity drops smoothly such that, following modification at 350 K, a racemic mixture of products is formed. Modification at temperatures above 350 K results in an excess of (S)-MHB. We have earlier reported that the interaction between MAA and (S)-glutamic acidmodified Ni{111} surfaces (created under UHV conditions) depends strongly on modification temperature.12 MAA can exist in either of two tautomeric forms, that is, as a diketone or an enol (Figure 2). We found that when (S)-glutamic acid is adsorbed onto Ni{111} at 300 K, subsequent MAA adsorption occurs primarily in the diketone form. By contrast, when (S)glutamic acid is adsorbed onto Ni{111} at 350 K, subsequent MAA adsorption occurs exclusively via the enol form.12 We * Corresponding author. E-mail:
[email protected].
concluded that the preferred tautomeric form of MAA depends on the extent of protonation of the modifier. For example, at 300 K, the modifier is in the zwitterionic form,13 which we believe interacts with the diketone form via H-bonding from the NH3+ functionality to the CdO of the ketone group. At the higher modification temperature, anionic glutamate is formed,13 which we propose is able to accept H-bonds from the enol form of MAA. Clearly, in UHV, we are unable to adequately model the effect of modifier pH on the nature of the modified surface. In this study, we use reflection absorption infrared spectroscopy to characterize the adsorption from solution of glutamic acid as functions of modification pH and temperature. In addition, we investigate the adsorption of methylacetoacetate from solution onto the chirally modified surfaces. The significance of our findings for understanding the mechanism of the Ni-catalyzed enantioselective hydrogenation reaction is discussed. Experimental Section All experiments were performed using a mechanically polished Ni{111} single crystal, and the IR data were collected using a Digilab FTS 7000 spectrometer with a liquid nitrogen cooled MCT detector. Prior to each experiment, the Ni crystal was annealed at 1273 K in a H2 stream for 8 h and allowed to cool, in hydrogen, to room temperature. The “cleanliness” was analyzed using PEM-RAIRS to detect the presence of observable surface molecular contaminants. A single reflection (non PEM) background spectrum (256 scans) was then acquired from the unmodified Ni{111} at 4 cm-1 resolution. The sample was then immersed in a 10 mM (S)-glutamic acid (Fluka g99.5%) solution for 900 s as a function of temperature and pH under constant agitation. The solution pH was altered by the addition of 1 M NaOH solution. After modification, the surface was dried in a flow of N2, and RAIRS data were taken. A second background spectrum, corresponding to the (S)-GA covered Ni surface, was taken prior to immersion under constant agitation in a 50:50 mixture of MAA (Fluka g99%) in tetrahydrofuran (THF) for 900 s. A further RAIR spectrum
10.1021/jp0687697 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007
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Figure 3. RAIR spectra following immersion of Ni{111} in 10 mM (S)-GA solution at 300 K as a function of pH.
Figure 4. RAIR spectra following immersion of Ni{111} in 10 mM (S)-GA solution at 350 K as a function of pH. Figure 1. (a) Schematic diagram showing the modification temperature dependence of MAA hydrogenation over Ni/(S)-GA catalysts. (b) Graph of the optical rotation of the product verses the modification temperature (adapted from ref 8).
Figure 2. Schematic diagrams showing the molecular structure of the enol and diketo tautomers of MAA (most H atoms have been omitted for the purpose of clarity).
was subsequently acquired. THF has been previous studied as a solvent for the hydrogenation of MAA over modified Ni catalysts and was observed to be one of the more successful solvents used in the enantioselective reaction.8 Results Figure 3 shows the RAIR spectra following immersion of the Ni{111} single crystal in a 10 mM (S)-glutamic acid solution for 900 s at 300 K as a function of solution pH. At pH 3.3, bands are observed at 1721, 1647, 1514, and 1264 cm-1, with
the most intense band appearing at 1721 cm-1. At pH 4.0, there is a large change in the spectrum with an intense, broad, and asymmetric feature appearing between 1650 and 1500 cm-1 and an additional, relatively intense, band at 1408 cm-1. Treatment of the Ni{111} sample at pH 5-7 reveals similar spectra, although the 1721 cm-1 peak diminishes in intensity. Following treatment at pH 9, the 1721 cm-1 peak completely disappears. Figure 4 shows the RAIR spectra following immersion of the Ni{111} single crystal in 10 mM (S)-glutamic acid solution for 900 s at 350 K as a function of solution pH. There is much less variation in the intensity and band positions than is observed following treatment at 300 K. The spectra are dominated by bands centered at 1611 and 1410 cm-1. The 1611 cm-1 band is much narrower than that observed following 300 K modification, and there is little or no evidence for the 1721 cm-1 band at any modification pH. In addition, the bands are approximately 4 times more intense than those following modification at 300 K. Figure 5 displays the RAIR spectra following adsorption of methylacetoacetate (MAA) from THF solution onto the Ni{111} surface modified by (S)-glutamic acid adsorption from solution at 300 K. To deconvolve the IR bands associated with adsorbed MAA from those of the chiral modifier, each spectrum corresponds to a ratio of the single beam spectrum of the MAA exposed surface with respect to the single beam spectrum of
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Figure 5. RAIR spectra following immersion of Ni{111} modified with (S)-GA at 300 K in an MAA (in THF) solution at 300 K as a function of pH. (The MAA exposed surface is ratioed with respect to the chirally modified surface.)
Figure 6. RAIR spectra following immersion of Ni{111} modified with (S)-GA at 350 K in an MAA (in THF) solution at 300 K as a function of pH. (The MAA exposed surface is ratioed with respect to the chirally modified surface.)
the chirally modified surface. Following modification at pH 3.3, MAA adsorption reveals a main band at 1744 cm-1, a weak feature at 1642 cm-1, and a number of bands in the 14501150 cm-1 range. As the modification pH is increased to pH 5, MAA adsorption results in an increase in the intensity of all of the main bands in the spectrum. Above pH 5, the 1744 cm-1 band decreases in intensity and is replaced by the band at 1250 cm-1 as the most intense feature. Figure 6 displays the RAIR spectra following adsorption of methylacetoacetate (MAA) from THF solution onto the Ni{111} surface modified by (S)-glutamic acid adsorption from solution at 350 K. The spectra as presented correspond to the MAA exposed surface ratioed to a background of the chirally modified surface. Following modification at pH 3.3, the most intense band is the 1250 cm-1 band. The relative intensity of this band relative to the 1747 cm-1 band increases with increasing modification pH. Discussion As shown in Figure 7, in aqueous solution, glutamic acid has pKa values of 2.2, 4.3, and 9.7 corresponding to removal of
Jones et al. protons from the carboxylic acid of the amino acid, the aliphatic carboxylic acid, and the NH3+ group, respectively. RoddickLanzilotta and McQuillan14 reported the IR spectra of 0.1 M glutamic acid solutions in water at a range of pH values to enable the identification of the key bands associated with the cationic, zwitterionic, anionic, and dianionic glutamate species. Essentially, Roddick-Lanzilotta and McQuillan14 interpreted their data as follows: the aliphatic -COOH group, present in both the cationic and the zwitterionic species, may be identified by the carbonyl stretching band at 1722 cm-1. The symmetric and asymmetric ν(OCO) frequencies of the amino acid carboxylate group appear at ∼1400 and ∼1600 cm-1, respectively. The symmetric and asymmetric δ(NH3) bands appear at ∼1520 and 1620 cm-1, respectively. Prior to immersion of the Ni{111} sample into the glutamic acid solution, no effort was made to avoid contact with air in keeping with the catalytic modification procedure.8 Exposure of the Ni{111} surface to air results in the formation of a passivating oxide surface terminated by hydroxyl species.15 Adsorption of (S)-Glutamic Acid at 300 K as a Function of Solution pH. The bands observed in the vibrational spectra following glutamic acid adsorption on Ni{111} are tabulated in Table 1 and compared to bands observed by argon matrix spectroscopy supported by DFT calculations for the zwitterionic form of (S)-glutamic acid16 and the non-zwitterionic form.17 In the spectra shown in Figure 3, it is clear that the RAIRS data for pH 3.3 differ strongly from those measured at higher pH. Little or no evidence is found for the presence of carboxylate bands; the most intense peak occurs at 1721 cm-1, which we interpret as the carbonyl group of the aliphatic acid, and at 1647 cm-1, which we believe contains contributions from δasym(NH3+) and ν(CdO) of the amino acid carbonyl functionality by reference to DFT calculations by Ramirez and Navarrete.16 Under these conditions, it appears that the adsorbate is primarily in the cationic state. The RAIR spectra measured following modification at pH 4, 5, 7, and 9 are similar with the exception that the 1721 cm-1 band becomes weaker at the higher pH. Growth of a relatively broad feature centered at 1647 cm-1 and a narrower band at 1408 cm-1 is evidence for the presence of carboxylate groups. We interpret the broadness of the former feature as being due to contributions from the NH3+-related bands mentioned above. Thus, at pH 4, the zwitterionic species is dominant, while by pH 7, the majority species is the anionic species. It is interesting to note that the intensity of the 1408 cm-1 band is ∼0.25-0.30%, which is similar to the intensity of the analogous band when (S)-glutamic acid is deposited onto Ni{111} under UHV conditions.13 This suggests that the glutamate layer is essentially in the monolayer coverage regime. Adsorption of (S)-Glutamic Acid at 350 K as a Function of Solution pH. Immersing the Ni{111} sample in (S)-glutamic acid at 350 K results in quite different RAIR spectra in Figure 4. At pH 9, the main features are bands at 1611 cm-1 (with a shoulder at 1561 cm-1) and 1410 cm-1. Similar bands have been recently reported for monosodium glutamate (NaOOC‚CH2‚CH2‚CH‚NH3+‚COO-) by Pieca et al.18 Essentially, the band at 1561 cm-1 is assigned to ν(COO-Na+), and the other, larger, bands are assigned to the νas(OCO) and νs(OCO) of the amino acid carboxylate functionality. At lower modification pH, the 1561 cm-1 shoulder is less prominent, and, between pH 3.3 and 5.0, there is evidence of a weak carbonyl band at ∼1720 cm-1. The band at 1611 cm-1 is consistently narrower than the feature in the 300 K modification experiments shown in Figure 3. Our interpretation of this is that there is little or no contribution from the δ(NH3+) bands. In addition,
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Figure 7. Speciation of glutamic acid (adapted from ref 14).
TABLE 1: Vibrational Assignments of Adsorption Bands (in cm-1) of (S)-GA Adsorbed from Solution on Ni{111} by Comparison with the Spectra of Zwitterionic16 and Non-zwitterionic17 (S)-Glutamic Acid 300 K assignment acid
ν(CdO) ν(CdO)acid νasym(OCO-) δasym(NH3+) δsciss(NH2) ν(OCO-Na+) δsym(NH3+) δ(CH2) νsym(OCO-) ω(CH2) t(CH2), δsciss(NH2)
low pH
high pH
350 K
zwitterion16
non-zwitterion17
1721
1781, 1751 1659 1647
1611
1647
1641, 1631, 1615 1582
1635 1561
1514 1264
1452 1408 1346 1264
the intensity of the bands is ∼4-5 times greater than that following modification at 300 K. We conclude, therefore, that reaction of the hydroxylated NiO surface with glutamic acid produces glutamate salts. At pH 3, because no Na is added to the solution, it is clear that nickel glutamate has been produced. At pH 9, it is likely that the surface is covered by several layers of a mixture of sodium glutamate and nickel glutamate of unknown composition. It has been reported that following modification of Ni catalysts by tartaric acid (TA) at the most basic conditions (gpH 8.2), a nickel sodium tartrate species is produced.19 It was suggested that the complex can either remain on the support or diffuse into solution. The lack of NH3+ bands is likely to be caused by the coordination of the N atom (in the form of an NH2 functionality) to Ni2+ ions as is observed in salts of nickel with amino acids (e.g., nickel(II) aspartate20). Interaction of Methylacetoacetate with Glutamic AcidModified Ni{111}. The reactant molecule MAA exists in two tautomeric forms as shown in Figure 2, the diketo and enol tautomers. The CH3 group favors the formation of the enol, while the electron-withdrawing -OCH3 group favors the formation of the diketo tautomer. Some debate exists over which tautomeric form dominates in the liquid form.21 The 1H NMR spectrum of the liquid at 306 K is consistent with that of the diketo tautomer.22 Vibrational spectra are interpreted in terms of the enol form (Table 2).23 At 423 K, a free energy difference of -1.89 ( 1.61 kcal mol-1 has been derived22 with the diketo form being marginally favored. More recently, IR matrix spectroscopy and DFT calculations have been used to ascertain which bands in the IR spectrum are associated with the diketo form and which are associated with the enol form.21 Essentially, the three largest bands for the enol are observed at 1252 cm-1 (ν(C-OH), vvs), and 1635 and 1660 cm-1 (strongly coupled ν(CdC) and ν(CdO) bands). The diketo form is most easily recognized by a large band at 1760 cm-1 with a lower frequency shoulder corresponding to the ν(CdO) bands of the carbonyl functionalities.21 One may attempt to estimate the relative abundance of the diketo and enol forms of MAA on the modified surfaces by comparing the relative intensities of the ∼1740 (diketo) and ∼1640 cm-1 bands (enol), assuming that we are relatively insensitive to issues of molecular geometry. [The oxidation of
1516 1438 1421 1259
1452 1410 1352 1265
1405 1337 1279
TABLE 2: Vibrational Assignments of Adsorption Band (in cm-1) of Adsorbed MAA from THF Solution onto (S)-GA-Modified Ni{111} MAA (300 K) assignment ν(CdO)keto ν(CdO)keto ν(CdO)enol and ν(CdC)enol δsym(CH3) + δ(CH) + δ(CO-H)enol δsciss(CH2)keto ν(C-OH)enol + ν(CdO)enol ω(CH2)keto t(CH2) ν(C-OH)enol r(CH3) + δ(CH)
Ni{111}/(S)-GA 300 K Ni{111}/(S)-GA 350 K 1744 1713 1674, 1644
1747 1718 1670, 1642
1457
1459
1441
1442 1407
1361 1323 1250 1207, 1157
1360 1325 1250 1204, 1157
Ni{111} results in the formation of pyramidal facets of NiO. As a result, the surface is relatively rough so identifying molecular species as flat lying or upright on such facets is difficult because the RAIRS selection rules are sensitive to the orientation with respect to the Ni{111} surface, not the local geometry on an oxide microfacet.] In Figure 5, following adsorption of MAA onto the Ni{111} surface immersed in glutamic acid solution at 300 K and pH 3.3, the diketo peak is ∼10 times larger than the enol peak, indicating that the former is the majority species under these conditions. With increasing pH, the enol contribution increases until after modification at pH 9, where the two bands are of a relatively similar intensity. In Figure 6, following adsorption of MAA onto the Ni{111} surface immersed in glutamic acid solution at 350 K, it is generally the case that the ratio of diketone to enol is lower than that following modification at 300 K; indeed, at pH 9, the enol band is about twice as intense as the diketone peak. Following modification at 300 K, at low pH we believe that the glutamate coverage is relatively low (∼1 ML) and that the modifier is in a highly protonated form. Under such conditions, it seems reasonable that MAA is likely to act as a hydrogenbond acceptor in any 1:1 interaction with the modifier favoring the diketone form. As the modification pH is increased, the extent of modifier protonation decreases, which may cause the enol form to act as a hydrogen-bond donor. At higher modifica-
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Figure 8. RAIR spectra following immersion of Ni{111} modified with (S)-GA at pH 5, as a function of modification temperature, in a solution of MAA in THF at 300 K. (The MAA exposed surface is ratioed with respect to the chirally modified surface.)
tion temperature, we believe that the chiral modifier is present as the metal salt. In this case, the N atom of the amino acid is unable to act as a hydrogen-bond donor because it is bonded to Ni in the metal salt complex. The lack of the NH3+ group may be a reason for the increased favorability for the presence of the enol at the higher modification temperature. Interestingly, and despite the vast difference in preparation conditions, this behavior mimics our findings for MAA adsorbed on (S)glutamic acid modified Ni{111} under UHV conditions12 where we drew correlations between the extent of protonation of the adsorbed modifier and the favored tautomeric form of MAA. It should be noted that the increase in the relative abundance of the enol tautomeric form correlates not only with pH, but also the concentration of Na+ in the modification solution. It is perhaps the case that the abundance of Na+ at high pH stabilizes the enolate form of MAA. It is difficult to distinguish between the enol and enolate species with RAIRS. Implications for the Heterogeneous Enantioselective Hydrogenation of β-Ketoesters. (S)-Glutamic acid-modified Ni catalysts display an unusual dependence of enantioselectivity with modification temperature.8 Modification at low temperature (i.e., 350 K displays a slight preference for the (S)-enantiomeric product. The optimum enantioselective catalytic performance occurs following modification at pH 5. In Figure 8, we show the RAIR spectra following exposure of chirally modified surfaces (created at pH 5) to MAA/THF. It is clear that the keto:enol ratio continues to drop with increasing modification temperature. This behavior implies that the diketone form of MAA is the precursor to (R)methyl-3-hydroxybutyrate when interacting with (S)-glutamic acid-modified Ni{111}. In contrast, the enol form appears to be the precursor to the (S)-enantiomeric product. Hence, it seems that control over the tautomeric form present on the surface may be the key to optimizing the enantioselectivity of the catalytic process. It has been postulated that the contrasting behavior of Pd and Pt catalysts toward enantioselective hydrogenation of R-ketoesters (e.g., methyl pyruvate) is related to the tendency of the reactant molecule to exisit in the diketone form (Pt) or the enol form (Pd). Modifying such catalysts with the chiral alkaloid cinchonidine results in a tendency to produce (R)-methyl lactate over Pt catalysts and (S)-methyl lactate on Pd.24
Jones et al. An interesting feature of tartaric acid and amino acid modification of Ni catalysts is that (R,R)-tartaric acid and (S)alanine favor the (R)-MHB product. In these cases, the absolute stereochemistry of the chiral center is reversed, yet the same enantiomeric product is favored. (S)-Glutamic acid, at 300 K, adsorbs on the Ni surface in the zwitterionic form. UHV work indicates a direct interaction between the NH3+ functionality and the diketone form of MAA.12 Under these conditions, one might expect (S)-glutamic acid to behave as a typical amino acid. At the higher modification temperature, (S)-glutamic acid reacts more strongly with the nickel oxide surface to form nickel glutamate. As such, the NH2 group is likely to be locked into an interaction with the Ni2+ ions and is unlikely to be involved in a direct H-bonding interaction with MAA. Also, (S)-glutamate is stereochemically similar to (S,S)-tartaric acid, a modifier that favors the (S)-MHB product. Hence, the switch in the preferred enantiomeric product with modification temperature for (S)glutamic acid may be related to its ability to behave either as an amino acid (analogous to alanine) or as a chiral diacid (analogous to tartaric acid). Conclusions The modification of Ni{111} by the adsorption of (S)glutamic acid from solution is strongly dependent on modification temperature and pH. At room temperature and low pH, glutamic acid adsorbs in the cationic/zwitterionic form. As the pH increases, the adsorbed species becomes progressively deprotonated. After modification at 350 K, we find evidence for the formation of nickel glutamate (low pH) and a mixture of nickel and sodium glutamate at high pH. The adsorption of methylacetoacetate on glutamic acidmodified Ni surfaces is strongly dependent on modification pH and temperature. Essentially, the more protonated is the modifier, the greater is the tendency for diketone formation. Under the conditions when the (S)-glutamic acid-modified Ni catalysts achieve the most enantioselective production of (R)MHB, the pro-chiral reagent MAA exists predominantly in the diketo form. By contrast, under the modification conditions that lead to an enantiomeric excess of (S)-MHB, the dominant form of MAA is the enol form. Acknowledgment. T.E.J. acknowledges postdoctoral funding from the Engineering and Physical Sciences Research Council (GR/S86402/01). References and Notes (1) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201. (2) Zhao, X. Y.; Wang, H.; Yan, H.; Gai, Z.; Zhao, R. G.; Yang, W. S. Chin. Phys. 2001, 10, S84. (3) Zhao, X. Y. J. Am. Chem. Soc. 2000, 122, 12584. (4) Chen, Q.; Frankel, D. J.; Richardson, N. V. Surf. Sci. 2002, 497, 37. (5) Kuhnle, A.; Linderoth, T. R.; Besenbacher, F. J. Am. Chem. Soc. 2003, 125, 14680. (6) Kuhnle, A.; Linderoth, T. R.; Besenbacher, F. J. Am. Chem. Soc. 2006, 128, 1076. (7) Jones, T. E.; Baddeley, C. J.; Gerbi, A.; Savio, L.; Rocca, M.; Vattuone, L. Langmuir 2005, 21, 9468. (8) Izumi, Y. AdV. Catal. 1983, 32, 215. (9) Tai, A.; Sugimura, T. In Chiral Catalyst Immobilisation and Recycling; de Vos, D. E., Vankelecom, I. F. J., Jacobs, P. A., Eds.; WileyVCH: New York, 2000; Chapter 8, p 173. (10) Webb, G.; Wells, P. B. Catal. Today 1992, 12, 319. (11) Keane, M. A.; Webb, G. J. Catal. 1992, 136, 1. (12) Jones, T. E.; Baddeley, C. J. Langmuir 2006, 22, 148. (13) Jones, T. E.; Urquhart, M. E.; Baddeley, C. J. Surf. Sci. 2005, 587, 69. (14) Roddick-Lanzilotta, A. D.; McQuillan, A. J. J. Colloid Interface Sci. 2000, 227, 48.
(S)-Glutamic Acid-Modified Ni{111} (15) de Jesus, J. C.; Carrazza, J.; Pereira, P.; Zaera, F. Surf. Sci. 1998, 397, 34. (16) Ramirez, F. J.; Navarrete, J. T. L. Spectrochim. Acta, Part A 1995, 51, 293. (17) Navarrete, J. T. L.; Bencivenni, L.; Ramondo, F.; Hernandez, V.; Ramirez, F. J. J. Mol. Struct. (THEOCHEM) 1995, 330, 261. (18) Peica, N.; Lehene, C.; Leopold, N.; Schlucker, S.; Kiefer, W. Spectrochim. Acta, Part A, in press, corrected proof. (19) Keane, M. A. Catal. Lett. 1993, 19, 197. (20) Cabral, O. V.; Tellez, C. A.; Giannerini, T.; Felcman, J. Spectrochim. Acta, Part A 2005, 61, 337.
J. Phys. Chem. C, Vol. 111, No. 14, 2007 5505 (21) Belova, N. V.; Oberhammer, H.; Girichev, G. V. J. Phys. Chem. A 2004, 108, 3593. (22) Folkendt, M. M.; Weisslopez, B. E.; Chauvel, J. P.; True, N. S. J. Phys. Chem. 1985, 89, 3347. (23) Schiavoni, M. M.; Di Loreto, H. E.; Hermann, A.; Mack, H. G.; Ulic, S. E.; Della Vedova, C. O. J. Raman Spectrosc. 2001, 32, 319. (24) Hall, T. J.; Johnston, P.; Vermeer, W. A. H.; Watson, S. R.; Wells, P. B. Enantioselective hydrogenation catalysed by palladium. 11th International Congress on Catalysis - 40th AnniVersary, Pts A and B; 1996; Vol. 101, p 221.
