Structural and optical Isomers of nonamethoxy cyclotriveratrylene: separation and physical characterization
| Autor: | Noga Friedman, Xiaolin Cao, Zeev Luz, Hailin Zheng, Herbert Zimmermann, Ellen J. Wachtel, Raphy Poupko, Teresa B. Freedman, Laurence A. Nafie |
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| Rok vydání: | 2007 |
| Předmět: |
Circular dichroism
Magnetic Resonance Spectroscopy Calorimetry Differential Scanning Molecular Structure Chemistry Crystallography Differential scanning calorimetry Isomerism Solubility X-Ray Diffraction Vibrational circular dichroism Solvents Structural isomer Thermodynamics Organic chemistry Pseudorotation Polycyclic Compounds Organic Chemicals Physical and Theoretical Chemistry Optical rotation Crystallization Racemization Isomerization Chromatography High Pressure Liquid |
| Zdroj: | The Journal of Physical Chemistry A |
| ISSN: | 1520-5215 1089-5639 |
| Popis: | The paper concerns the structural and optical isomers of nonamethoxy-tribenzocyclononene (compound 1). In the first part of the paper it is shown that 1 exists in two structural isomers: a rigid crown (c-1) with C3 symmetry and a flexible saddle (s-1) with C1 symmetry. The latter, not previously known, can be prepared from the as-synthesized c-1 by quenching a hot solution (or the melt) followed by HPLC separation. The crown/saddle equilibrium, isomerization kinetics, and associated thermodynamic parameters in various organic solvents are reported. Carbon-13 MAS NMR, X-ray diffraction, and differential scanning calorimetry (DSC) of polycrystalline c-1 and s-1 racemates are also reported. The different melting points of the isomers and their rapid isomerization in the melt result in unconventional DSC thermograms involving multiple endothermic and exothermic transitions. The second part of the paper concerns the chiral properties of 1. Both the saddle and crown isomers are structurally chiral, but due to the fast pseudorotation of s-1 in solution, it cannot be separated into its enantiomers. Those of c-1 were separated by HPLC using a chiral column. Their X-ray structure and melting points differ considerably from those of the racemate. This and their fast racemization in the melt lead to complex DSC thermograms with multiple transitions. Solutions of the neat enantiomers exhibit a relatively small specific optical rotation. In the UV they show circular dichroism for the B1u and B2u transitions, with the latter exhibiting a clear couplet structure. Infrared and vibrational circular dichroism spectra of the enantiomers in solution are reported. Comparison of these spectra with quantum mechanical simulations provides unambiguous identification of the enantiomers. |
| Databáze: | OpenAIRE |
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