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Molecular self-assembly and chiral recognition of malic acid on a Cu(110) surface


Roth, C A. Molecular self-assembly and chiral recognition of malic acid on a Cu(110) surface. 2011, University of Zurich, Faculty of Science.

Abstract

The adsorption and two-dimensional self assembly of enantiopure and racemic malic acid (MA) on Cu(110) has been studied in ultra-high vacuum (UHV) with temperature programmed desorption (TPD), low energy electron diffraction (LEED), UV- and X-ray photoelectron spectroscopy (UPS, XPS), as well as scanning tunneling microscopy (STM). In order to get detailed informations on the chemical structure of the adsorbate complex, the first UHV surface infrared spectrometer (reflection-absorption infrared spectroscopy, RAIRS) in Switzerland has been installed. Depending on the coverage and the deposition conditions, eleven ordered structures of (R)- and (S)-MA were observed with LEED and STM. As previously reported for tartaric acid on this surface, MA shows two adsorbate modes in which either one (monomalate) or both carboxyl groups (bimalate) react with the surface under deprotonation. The high-coverage monolayer shows ”surface explosion”, a rapid decomposition phenomenon observed in TPD. As observed in STM the two-dimensionally ordered bimalate structures show features which are interpreted as reconstruction of the underlying Cu surface. This is the first direct STM observation of adsorption-induced reconstruction on Cu(110) for butanedioic acids and suggests that a pronounced chirality transfer via the substrate, rather than by intermolecular hydrogen bonding, occurs. Racemic MA shows ordered structures that have not been observed for the pure enantiomers. Our structure models consider therefore heterochiral pairs as building blocks. Surface reconstructions have been observed directly. Only at full monolayer coverage a well ordered c(2 × 4) phase covers the entire crystal surface. All other structures are only observed in small patches. The racemic c(2×4) shows an even better order than the enantiopure counterpart, indicating again a heterochiral arrangement of monomalate molecules.
We also investigated the effect of chiral conflict, that is, mixing of tartaric acid (TA) and malic acid (MA). Racemic TA shows a superposition of the two enantiomorphs in LEED that are observed for the pure TA enantiomers. Adding one enantiomer of MA at a concentration of 25-75% suppresses formation of one enantiomorph. (R)-MA allows thereby only the formation of the structure known for pure (R,R)-TA, while (S)-MA leads to the (S,S )-enantiomorph. The
suppressed enantiomorph forms quasi-racemic semi-ordered phases. Among other structure motifs, triplets of molecules have been identified in these regions. DFT calculations showed the most stable triplet to be formed from two MA molecules and one TA molecule. As driving force, strong lateral interactions via hydrogen bonding has been identified. The reverse mixing experiment, namely, rac.-MA mixed with enantiopure TA shows again induction of single enantiomorphism. A prolonged annealing time at elevated temperature needed to observe this effect is an indication for a large mass transport. This amplification effect is explained by breaking the balance of heterochiral trimer formation in rac.-MA.

Abstract

The adsorption and two-dimensional self assembly of enantiopure and racemic malic acid (MA) on Cu(110) has been studied in ultra-high vacuum (UHV) with temperature programmed desorption (TPD), low energy electron diffraction (LEED), UV- and X-ray photoelectron spectroscopy (UPS, XPS), as well as scanning tunneling microscopy (STM). In order to get detailed informations on the chemical structure of the adsorbate complex, the first UHV surface infrared spectrometer (reflection-absorption infrared spectroscopy, RAIRS) in Switzerland has been installed. Depending on the coverage and the deposition conditions, eleven ordered structures of (R)- and (S)-MA were observed with LEED and STM. As previously reported for tartaric acid on this surface, MA shows two adsorbate modes in which either one (monomalate) or both carboxyl groups (bimalate) react with the surface under deprotonation. The high-coverage monolayer shows ”surface explosion”, a rapid decomposition phenomenon observed in TPD. As observed in STM the two-dimensionally ordered bimalate structures show features which are interpreted as reconstruction of the underlying Cu surface. This is the first direct STM observation of adsorption-induced reconstruction on Cu(110) for butanedioic acids and suggests that a pronounced chirality transfer via the substrate, rather than by intermolecular hydrogen bonding, occurs. Racemic MA shows ordered structures that have not been observed for the pure enantiomers. Our structure models consider therefore heterochiral pairs as building blocks. Surface reconstructions have been observed directly. Only at full monolayer coverage a well ordered c(2 × 4) phase covers the entire crystal surface. All other structures are only observed in small patches. The racemic c(2×4) shows an even better order than the enantiopure counterpart, indicating again a heterochiral arrangement of monomalate molecules.
We also investigated the effect of chiral conflict, that is, mixing of tartaric acid (TA) and malic acid (MA). Racemic TA shows a superposition of the two enantiomorphs in LEED that are observed for the pure TA enantiomers. Adding one enantiomer of MA at a concentration of 25-75% suppresses formation of one enantiomorph. (R)-MA allows thereby only the formation of the structure known for pure (R,R)-TA, while (S)-MA leads to the (S,S )-enantiomorph. The
suppressed enantiomorph forms quasi-racemic semi-ordered phases. Among other structure motifs, triplets of molecules have been identified in these regions. DFT calculations showed the most stable triplet to be formed from two MA molecules and one TA molecule. As driving force, strong lateral interactions via hydrogen bonding has been identified. The reverse mixing experiment, namely, rac.-MA mixed with enantiopure TA shows again induction of single enantiomorphism. A prolonged annealing time at elevated temperature needed to observe this effect is an indication for a large mass transport. This amplification effect is explained by breaking the balance of heterochiral trimer formation in rac.-MA.

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Additional indexing

Item Type:Dissertation
Referees:Ernst K H, Baldridge K K
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Language:English
Date:7 March 2011
Deposited On:05 Mar 2012 10:22
Last Modified:05 Apr 2016 15:29
Related URLs:http://opac.nebis.ch/F/?local_base=NEBIS&CON_LNG=GER&func=find-b&find_code=SYS&request=006507357

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