Chiral recognition in molecular clusters N. Borho, M. A. Suhm Institute of Physical Chemistry, University of Göttingen.

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Chiral recognition in molecular clusters N. Borho, M. A. Suhm Institute of Physical Chemistry, University of Göttingen

Study of hydrogen-bonded molecular aggregates production detection: FTIR-Spectroscopy supersonic expansion Filet-Jet FTIR-Spectroscopy advantages: wide spectral range (200 – 4000 cm -1 ) high reproducibility direct measurement of IR absorption

Filet-Jet FTIR-Spectroscopy

Chiral recognition The human handshake handshake: right-to-right left-to-leftleft-to-right right-to-left mirror images (enantiomers) R+RS+SR+SS+R not mirror images! (diastereomers) mirror images (enantiomers) molecular handshake: homochiral heterochiral

Spectroscopic evidence: Examination of OH S+R heterochiral SSRR SRRS homochiral S enantiopure racemic SS homochiral 1 : 1 dimer I monomer 2 : 0 difference I heterochiral homochiral I monomer dimer dimers OH spectrum

Chiral recognition of Methyl lactate positive: heterochiral negative: homochiral difference M DT Q M D T Q T D N. Borho and M. A. Suhm, OBC, 2003, 1 (23), chiral recognition for dimers D for oligomers T, Q relative cluster size of T, Q: pressure dependence studies ln(I 0 /I)/10 -3 R+S S

S-Methyl lactate: Pressure dependence M D T D M T

Rac. Methyl lactate: Pressure dependence M D T Q M D T Q

M DT Q M D T R+S S From cluster size to geometry? dimertrimertetramer monomer Q? ln(I 0 /I)/10 -3

Homochiral trimer: Information from jet FTIR spectra ? one OH-absorption one dominating conformer strong molecular recognition: one out of thousands of conformers 3 torsions per monomer + 2 lone pairs 2²x3³x3³x3³ = C 3 -symmetry (local C 3h )

Chemical substitution as a tool for structure determination methyl lactate 1  -C-H methyl  -hydroxyisobutyrate methyl glycolate 0  -C-H 2  -C-H

 -C-H is necessary for T N. Borho and M. A. Suhm, PCCP, 2004, 6, Methyl lactate racemic enantiopure M D T Q M D T D M M D TQ Derivatives

Size variation of the ester group ester groups at the edge of trimer spectra show similar pattern same cluster distribution enantiopure M D T M D T M D T M D size of ester group

* * * C3C3 Geometry of the homochiral trimer C 3 symmetry one OH band steric hindrance of methyl groups ester group at the edge B3LYP/6-31+G*: Minimum OH = 3564 cm -1, C=O = 1793 cm -1

Conclusions Filet-Jet FTIR-Spectroscopy well-suited for study of chiral recognition assignment of the homochiral methyl lactate trimer chemical substitution as a tool for structure determination specific self-aggregation of methyl lactate

Thanks to: GRK 782, SFB 357 and DAAD Suhm group cooperations: Th. Adler and M. Reiher U. Buck, M. Farnik, M. Weimann A. Zehnacker

60 cm 0.2 mm The Filet-Jet: FIne, but LEngThy