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A facile method to determine the absolute structure of achiral molecules: supramolecular-tilt structures
Rosa María Tejedor1, Santiago Uriel, Sara Graus
1Centro Universitario de la Defensa, Academia General Militar, 50090 Zaragoza, Spain.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|March 7, 2013
Summary
This study reveals achiral compounds can crystallize into chiral forms, exhibiting enantiomeric excess. X-ray diffraction and vibrational circular dichroism confirmed their absolute structures, advancing chiral analysis methods.
Area of Science:
- Crystallography
- Spectroscopy
- Organic Chemistry
Background:
- Chirality is crucial in chemistry and biology.
- Determining absolute configurations of chiral molecules is essential.
- Achiral precursors can sometimes form chiral crystals.
Purpose of the Study:
- To determine the absolute structures of specific achiral compounds that crystallized in chiral forms.
- To validate vibrational circular dichroism (VCD) as a method for establishing absolute configurations.
- To investigate the phenomenon of achiral molecules forming chiral crystals.
Main Methods:
- X-ray diffraction at low temperatures was used to analyze crystal structures.
- Solid-state vibrational circular dichroism (VCD) spectra were measured and calculated.
- Comparison of experimental and calculated VCD spectra was performed.
- A known chiral compound, Tröger's base, was used as a reference for method validation.
Main Results:
- Compounds 4-methoxy-4-(p-methoxyphenyl)cyclohexanoneethylene ketal (2), 4-hydroxy-4-(p-methoxyphenyl)cyclohexanoneethylene ketal (3), and 3,5-dimethyl-4-nitropyrazole (4) were found to crystallize in chiral structures with enantiomeric excess.
- The absolute configurations of these compounds were successfully determined.
- Calculated and experimental VCD spectra showed good agreement, confirming the established absolute structures.
- The method was validated using Tröger's base ((R,R)-1).
Conclusions:
- Achiral molecules can spontaneously form chiral crystals.
- X-ray diffraction and VCD spectroscopy are powerful complementary techniques for determining absolute configurations.
- VCD spectroscopy is a reliable method for establishing prevalent absolute structures in chiral crystalline samples.
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