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The formation of enantiospecific phases on a Cu{110} surface |
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PhysChemComm,
Volume 2,
Issue 9,
1999,
Page 41-44
Q. Chen,
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摘要:
The formation of enantiospecific phases on a Cu{110} surface Q. Chen,* C. W. Lee,† D. J. Frankel and N. V. Richardson School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife, UK KY16 9ST. Fax: (+44) 1334-467285 Received 23rd July 1999, Accepted 24th August 1999, Published 27th August 1999 Both R- and S-phenylglycine and a quantitatively well defined mixture of the two enantiomers have been studied on Cu{110} surfaces. At saturation coverage, the molecules form well ordered, chiral structures with a chiral unit cell which ensures that the low energy electron diffraction (LEED) patterns of the two enantiomers are readily differentiated. Comparison is made with analogous structures observed for the non-chiral species glycine and R-, S-alanine adsorbed on Cu{110}.ø è In this paper, we present the results for the ordered structures of R-, S-phenylglycine on Cu{110}. background in the temperature-programmed desorption (TPD) experiments. Clean Cu{110} surfaces were obtained by Ar+ sputter and anneal (800 K) cycles. The cleanliness of the Cu{110} surface was assessed by the appearance of a sharp (1 � 1) LEED pattern. The chemicals are sublimed several times on a gas line, attached to the chamber through a gate valve. The doser is made of glass tube with heating wire and thermal couple sensor, so the dosing temperature is well controlled and reproducibility is ensured. 1. Introduction The study of enantiomeric surface structures is of significance since it is directly related to the fundamental mechanism of enantiomeric differentiation on modified heterogeneous catalysts1 and many aspects of molecular recognition relevant to biosensors and biocompatibility.2 Enantiomeric recognition is a fundamental property of many biological molecules3 and a characteristic of many enzyme systems, for example, is their ability to distinguish between enantiomers in catalytic reaction processes.4 We are interested in this enantioselectivity using well defined surfaces which might themselves form the basis of enantiospecific biosensors through the host–guest mechanism or of stereodirecting catalysts.Our previous work has focused on the structure and properties of the prochiral molecule glycine at monolayer coverage on the Cu{110} surface: a system which exhibits p(3 � 2)g periodicity.5 The surface species has both oxygen atoms bonding to the surface and the C–C backbone almost parallel to the surface.This conclusion, derived from IR data, is supported by X-ray photoelectron diffraction (XPD)6 and near edge X-ray absorption fine structure (NEXAFS)7 results. The oxygen atoms are found to be aligned along the <110> direction with bonding of the carboxylate group in a short bridge site i.e. both O atoms and indeed the N atom close to on-top sites. An analogous but chiral amino acid, S-alanine has also been studied on a Cu{110} surface with scanning tunnelling microscopy (STM), FTIR and low energy electron diffraction (LEED).8,9 At room temperature, the surface species forms an ordered structure, noted in matrix form as ç2 - 2÷ æ5 3 ö , with a carboxylate–Cu bonding similar to glycine.2. Experimental The experiments were performed in a UHV system equipped with a rear view LEED and quadrupole mass spectrometer. The diffraction pattern recorded from the rear view LEED optics corresponds directly to the unit cell via the standard relationship linking real and reciprocal space unit cell.10 The quadrupole mass spectrometer is terminated with an outlet of 5 mm id which can be moved towards the sample surface to increase sensitivity and reduce the PhysChemComm, 1999, 9 3. Results In order to understand enantiomeric differentiation on the (110) surface, which has a non-chiral p2mm space group, both R- and S-phenylglycine and a quantitatively well defined mixture of the two enantiomers have been studied on Cu{110} surfaces.The structures of this pair of enantiomers are presented in Fig. 1, along with those of glycine and alanine molecules. Fig. 1 Fischer projections of the molecular structure of the a- amino acids glycine, alanine and phenylglycine.Fig. 2 (a) The LEED pattern of æ5 -3ö periodicity of R- çè4 1 ÷øphenylglycine on Cu{110} surface with a primary energy of 26 eV. (b) Overlayer reciprocal lattice cell. An ordered surface overlayer of R-phenylglycine is achieved by dosing to saturation at room temperature followed by annealing to 450 K. LEED shows a ç4 1 ÷ æ5 -3ö è ø periodicity,11 shown in Fig. 2, with a unit cell spanning 17 Cu atoms. The periodic structures of both R(S)- phenylglycine and R(S)-alanine have a unit cell vector along the (5, –(+)3) direction.Although the periodicity, in each case, is commensurate with the Cu substrate, the LEED pattern does not show mirror symmetry. The corresponding Bravais lattice is oblique and the primitive vectors are not aligned along high symmetry directions of the underlying Cu lattice. The macroscopic 2D structures and the real space unit cells are therefore chiral. The packing arrangement of S-phenylglycine molecules in the unit cell appropriate to R-phenylglycine is anticipated to be substantially different, i.e. the S-modification requires the unit cell of opposite chirality. The matrix notation for the structure of S-phenylglycine on Cu{110} is therefore ö .This structure is confirmed by the LEED pattern of æ5 3 ÷ ç4 -1 è ø Fig. 3b which is the mirror image of that formed by Rphenylglycine shown in Fig. 3a. This demonstrates that each enantiomer forms distinctive 2D crystals with inverse optical response. We note that R-phenylglycine/Cu{110}, and R-alanine/Cu{110}, (Fig. 3c), have in common the 5, –3 primitive vector suggesting that inter-adsorbate interactions determined by the chiral, a-amino skeleton, common to the two molecules, is responsible for the ordering, at least in this direction. In contrast, glycine may exhibit chiral domains but these are necessarily matched, elsewhere on the surface, by domains of the complementary chirality.In any case, they are not distinguished in the LEED pattern since the Bravais lattice of the (3 � 2)-glycine/Cu{110} structure has mirror symmetry, although LEED I/V measurements can, in principle, distinguish the two domains. Two, C2 correlated, domains will exist for each enantiomer of phenylglycine which present the same LEED patterns and a real-space technique such as STM is necessary to distinguish these two domain types. 1 On the ordered p(3 � 2)g glycine/Cu{110}, the surface saturation coverage was found to be 0.33 monolayers (ML) (2 glycine/6 Cu in the unit cell). Based on the peak intensities of 27 (HCN), 44 (CO2) and 28 u (CO) mass fragments in thermal desorption experiments, glycine and phenylglycine have very similar saturation coverages, which, together with the unit cell size, suggests a surface coverage of 0.35 ML (6 molecules per 17 Cu atoms in the unit cell) for the ç4 æ5 -3ö periodicity of R-phenylglycine. In è æ5 -3ö Fig.3 ø è 4 -1ø è ÷ø The LEED pattern of (a) ç4 1 ÷ periodicity of Rphenylglycine with a primary energy of 26 eV. (b) æç 5 3 ö÷ periodicity of S-phenylglycine with a primary energy of 26 eV. (c) æ5 -3ö periodicity of R-alanine with a primary energy of 35 eV. (d) 2 çè2 ÷øA mixture of S- and R-phenylglycine with a ratio of 3 : 1. The beam energy is 26 eV. (e) A mixture of S- and R-phenylglycine with a ratio of 1 : 1. The beam energy is 26 eV. (f) The p(3 � 2)g periodicity after an annealing at 420 K.The diffraction spots on the vertical high symmetry axis are (0, ±1). The spots on the horizontal axis are (±2/ 3, 0). The primary energy is 30 eV. The glide plane is indicated by the missing spots at (0, ±1/2).the case of S-alanine, we suggest the coverage is most ö likely six molecules in the ç2 - 2ø æ5 3 ÷ unit cell spanning grave; atoms. For R-phenylglycine, in addition to the (0, 0) spot, there are six other intense spots which form a pseudohexagonal pattern with a periodicity of ç5/ 2 ö . We 2 / 3 3 / 1 3 / 4 æè ÷øconsider this to be the unit cell of the adsorbate ignoring the substrate. Consistent with the above discussion of coverage, it contains one molecule and, hence, 2.83 (=17/6) Cu atoms. The intensity of this pattern suggests the coherent electron scattering by the molecules is strong and that all molecules are in very similar chemical environment, in terms of bonding to the substrate and molecular orientation.Mixtures of R- and S-phenylglycine have been dosed on the clean Cu{110} surface. Fig. 3d shows the LEED pattern of the mixture with a ratio of S/R = 3. Similarly, the LEED pattern, following dosing with a mixture of S/R = 1, is shown in Fig. 3e. The patterns present both the periodicities æ5 -3ö of R-phenylglycine and æ5 3 ö of S-phenylglycine of ç4 1 ÷øè ø ç4 -1÷ è strongly suggesting that phase separation of the two enantiomers occurs. The spots contributed by the Sphenylglycine phase are more intense than those from the R-phenylglycine phase in Fig.3d, simply due to the difference in the surface coverages of the two isomers. As the surface coverage of R- and S-enantiomers are equal in the case of Fig. 3e, the LEED intensities from the two molecular arrays are similar. Annealing this surface above 420 K causes a phase transition to the p(3 � 2)g periodicity; the same periodicity as glycine. At this stage, it is not possible to determine from the LEED data alone whether phase separation is maintained, although this phenomena has also been observed for the single enantiomer S-alanine on Cu{110}. Fig. 3f shows the LEED pattern of the p(3 � 2)g periodicity. 2 4. Discussion The proposed real space structure of the ç4 1 ÷ R- -3/ 2 3 / 1 2 / 5 3 / 4 ø æçècell contains only one molecule.However, the presence of the substrate causes imperfection in this æ5 -3ö è ø phenylglycine/Cu{110} is presented in Fig. 4a with a surface coverage of 0.35 ML (6 molecules per 17 Cu atoms in the unit cell). By analogy with glycine5 and alanine,8,9 the adsorbed species is considered to be in the anionic form with the displaced proton leaving the surface as H following recombination on the surface. The phenyl ring is directed away from the normal to limit steric interaction in the adsorbate plane. In this geometry, the molecule has a projected size on the surface plane similar to glycine consistent with the similar, saturation coverage, TPD intensities for the two species. The unit cell corresponding to the pseudo-hexagonal pattern with a periodicity of ö÷ is also marked on Fig.4a. We presume this unit -3/ 2 3 / 1 2 / 5 3 / 4 ö÷ø æçèperiodicity, because different adsorption sites are implied when projected into the bulk. This gives rise to the weak spots forming a ç4 1 ÷ æ5 -3ö pattern which is the truly periodic, ø è commensurate structure. In this model, all the carboxylate groups are aligned along the [ ] 1 1 0 azimuth, which is supported by our electron energy loss spectroscopy (EELS) impact scattering results.12 The structure is a compromise between strong adsorbate substrate bonding requirements, demanded by the carboxylate group and N atoms, and interadsorbate interactions, which probably originate from both hydrogen bonding and Pauli exclusion.The structure shown in Fig. 4a gives rise to favourable N–H···O bonding (linear with H···O distance of ca. 1.5 Å) in the adsorbate layer, whereas attempting to place the enantiomeric S-phenylglycine into the æ5 -3ö structure, reserved for R-phenylglycine, produces a less 1 çè4 (a) (b) è4 1 ø ÷øfavourable N–H···O interaction (nonlinear with H···O distance ca. 2.0 Å) and unfavourably short adsorbate– adsorbate distances along the 5, –3 direction leading to repulsive interactions. The favourable interactions for Sphenylglycine, and indeed S-alanine, are to be found on the mirror image 5, 3 direction. This is the likely origin of chirality in these unit cells and consequently the 2D ordered chiral overlayers.Fig. 4 The real space model of R-phenylglycine with periodicities of (a) æç 5 -3ö÷ and (b) p(3 � 2)g with the Cu{110} substrate mesh. The phenyl ring is directed away from the surface while carboxylate O atoms and the N atom of the amine group interact directly with surface copper atoms.We recognise also the possibility that Cu atom rearrangement may occur to make local bonding of molecules to the surface Cu atoms more equivalent than suggested by Fig. 4a. It may be, for example, that the surface reconstructs to the extent that an additional Cu atom is incorporated into the unit cell to give 18 rather 17 Cu atoms. This would then allow the pair of O atoms and the N atom in each molecule to bind to a different Cu atom in the most effective manner.A model for the p(3 � 2)g structure, observed after annealing the phenylglycine covered surface at 420 K, is presented at Fig. 4b. The proposed structure is very similar to that for p(3 � 2)g of glycine5–7 on Cu{110} surfaces. Two pseudo-glide planes, along the [001] direction, are indicated by the dashed lines. However, for a chiral molecule, the glide operation projects to its enantiomer, rather than itself, so a glide line is not a perfect symmetry operation. Nevertheless, they appear to be sufficiently well defined to remove intensity from the 1/2 order beams. 5. Conclusions LEED measurements, with support from TPD, have been used to characterise the adsorption of R- and Sphenylglycine on a Cu{110} surface. At saturation coverage, the molecules form well ordered, chiral structures with a chiral unit cell which determines that the LEED patterns of the two enantiomers are readily differentiated.The origin of enantiomeric specificity in the structures is thought to arise from the interadsorbate H-bonding which occurs in the saturated monolayer. The commensurate æ5 -3ö unit cell (R-phenylglycine) contains six adsorbate ÷ 4 1 ø çèmolecules most likely in anionic form by analogy with glycine and formally 17 Cu atoms, although reconstructions in the outermost Cu layer may well produce a structure with 18 Cu atoms, such that each O and N atom of the adsorbate is bonded to a separate Cu atom in equivalent local bonding geometries. Comparison is made with analogous structures observed for the non-chiral species glycine and R-, S-alanine adsorbed on Cu{110}.As also observed for both these species, the stable structure on annealing the surface is a (3 � 2) periodicity with a glide line, although we recognise that in chiral adsorption systems a glide line is formally forbidden. Footnote † Permanent address: Department of Physics, Kookmin University, Seoul 136-702, Korea. Paper 9/05986E References 1 Y. Izumi, Adv. Catal., 1983, 32, 315. 2 B. Kasemo, Curr. Opin. Solid State Mater. Sci., 1998, 3, 451. 3 S. Allenmark and S. Andersson, J. Chromatogr., 1994, 666, 167. 4 C. Ziegler, W. Göpel, H. Hämmerle, H. Hatt, G. Jung, L. Laxhuber, H. L. Schmidt, S. Schütz, F. Vögtle and A. Zell, Biosens. Bioelectron., 1998, 13, 539. 5 S. M. Barlow, K. J. Kitching, S. Haq and N. V. Richardson, Surf. Sci., 1998, 401, 322. Nilsson, M. Nyberg, L. G. M. Pettersson, M. G. Samant 6 J. Hasselstrom, O. Karis, M. Weinelt, N. Wassdahl, A. and J. Stohr, Surf. Sci., 1998, 407, 221. 7 N. A. Booth, D. P. Woodruff, O. Schaff, T. Giessel, R. Lindsay, P. Baumgartel and A. M. Bradshaw, Surf. Sci., 8 J. Williams, S. Haq and R. Raval, Surf. Sci., 1996, 368, 1998, 397, 258. 303. 9 R. Raval, C. J. Baddeley, S. Haq, S. Louafi, P. Murray, C. Muryn, M. O. Lorenzo and J. Williams, Reactiand the Development of Catalytic Processes, Elsevier Science B.V., Amsterdam, 1999. 10 We note also that care is required in the interpretation of LEED patterns of ordered, chiral systems since front view LEED reverses the image. 11 We have adopted the following matrix notation for the real space structures on the fcc (110) b a d c æçèø öæ ö , ÷çèj i ÷ ø surface. The translation vectors of the substrate unit cell, i and j, form a right-handed coordinate system directed along [ ] 1 1 0 and [001] respectively. For a commensurate overlayer, a, b, c and d are integers. 12 Q. Chen, C. W. Lee, D. J. Frankel and N. V. Richardson, to be published. PhysChemComm © The Royal Society of Chemistry 1999
ISSN:1460-2733
DOI:10.1039/a905986e
出版商:RSC
年代:1999
数据来源: RSC
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