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Effect of intramolecular dispersion interactions on the conformational preferences of monoterpenoids

This study investigates the impact of intramolecular dispersion interactions on the conformational preferences of monoterpenoids. The rotational spectra of perillaldehyde, limonene, and carvone were analyzed using a 2-8 GHz CP-FTMW spectrometer. The results provide valuable insights into the molecular mechanisms of odor recognition and the role of intermolecular interactions.

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Effect of intramolecular dispersion interactions on the conformational preferences of monoterpenoids

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  1. Effect of intramoleculardispersioninteractions on the conformationalpreferences of monoterpenoids DonatellaLoru, AnnalisaVigorito, Andreia Santos, Jackson Tang, M. Eugenia Sanz, Departmentof Chemistry, King’sCollege London, UK

  2. Outline • MOTIVATION • EXPERIMENTAL METHOD • MOLECULAR SYSTEMS • CONCLUSIONS

  3. Motivation Ourability to smelldepends on interactionsbetweenmoleculesthat are chiral… (R)-Carvone (S)-Carvone • Mirror images of eachother and onlydiffer in one stereogenic centre • Samephysical and chemicalproperties • They rotate plane-polarized light by equalamountbut in opposite directions

  4. Motivation Ourability to smelldepends on interactionsbetweenmoleculesthat are chiral… Smelldifferently Smell the same (R)-limonene (R)-fenchone (S)-fenchone (S)-limonene

  5. Motivation • How does the human olfactorysystemrecognize an odorant? • Isenantiomericdiscriminationrelated to the molecularflexibility? • What are the molecularmechanisms by whichodorants are identified? • Interaction with receptors? ? HO SIGNAL TRANSDUCTION SMELL

  6. Experimentalmethod 2-8 GHz CP-FTMW SPECTROMETER • Chirped pulse, 2-8 GHz • broadband acquisition • larger molecular systems • Supersonic expansion • only lower-energy levels populated • formation of complexes

  7. Experimentalmethod 2-8 GHz CP-FTMW SPECTROMETER 1. Chirpedpulse MW generation and polarisation 2. Molecularbeamchamber 3. FID detection

  8. Molecularsystems Enantiomers smell different Enantiomers smell the same Limonene Carvone Perillaldehyde

  9. Perillaldehyde • Enantiomers smell the same • Conformations? Previous studies • IR-Raman-VCD spectroscopy in liquid phase: three equatorial conformers observed. Juan Ramon Aviles Moreno, et al., Chemical Physiscs Letters, 473, 17-20, 2009 • FTMW spectroscopy in gas phase: two equatorial conformers observed. The third equatorial conformers was thought to relax in supersonic expansion F. PartalUreña, et al., J. Phys. Chem., A 112 , 7887, 2008

  10. Perillaldehyde Ax Eq A 1 C 3 2 4 C1-C2-C3-C3 C 0 A  +120 a  -120 a

  11. Perillaldehyde Ax Eq 12 possible conformers A 8 6 7 5 C5-C6-C7-C8 C 1 3 2 4 C1-C2-C3-C3 C 0 A  +120 a  -120 a

  12. Perillaldehyde MP2/6-311++G (d,p) Equatorial Eq-A Eq-C Eq-a 307 cm-1 377 cm-1 119 cm-1 Axial Ax-A Ax-C Ax-a 770 cm-1 0 cm-1 612 cm-1

  13. Perillaldehyde MP2/6-311++G(d,p) Predicted barrier for conformational interconversion Equatorial Axial Ax-a Ax-C Ax-A Eq-A Eq-C Eq-a

  14. Perillaldehyde Reinvestigation of the rotational spectrum of perillaldehydein the 2-8 GHz frequency region Experimental conditions Ne@5bar T = 133C 109.3k FIDs

  15. Perillaldehyde Reinvestigation of the rotational spectrum of perillaldehydein the 2-8 GHz frequency region What are the non-identified lines? Experimental conditions Ne@5bar T = 133C 109.3k FIDs

  16. Perillaldehyde Reinvestigation of the rotational spectrum of perillaldehydein the 2-8 GHz frequency region Experimental conditions Ne@5bar T = 133C 109.3k FIDs

  17. Perillaldehyde Reinvestigation of the rotational spectrum of perillaldehydein the 2-8 GHz frequency region Experimental conditions Ne@5bar T = 133C 109.3k FIDs

  18. Perillaldehyde Four conformers of perillaldehyde have been identified: three equatorial and one axial conformers. Eq-A Eq-C Ax-A Eq-a

  19. IiNi n Perillaldehyde Conformational relaxation? E E conf. 1 conf. 1 conf. 3 conf. 3 conf. 2 conf. 2 Expansion Spectrum n

  20. IiNi n Perillaldehyde Conformational relaxation? E E conf. 1 conf. 1 conf. 3 conf. 3 conf. 2 conf. 2 Expansion missingconformer Spectrum !! n

  21. Perillaldehyde Spectra with different carrier gasses He Ne Ar

  22. Perillaldehyde Conformational abundances in three different carrier gasses He 1 : 0.7 : 0.2 : 0.2 Ne 1 : 0.7 : 0.1 : 0.2 Ar 1 : 0.7 : 0 : 0.1

  23. Perillaldehyde Observation of isotopic species Many transitions show a S/N of 1000/1 and show a number of lines at lower frequency with about one hundredth of the intensity.

  24. PERILLALDEHYDE Substitution structurevs ab initio structure Eq-C Eq-A Eq-C MP2/6-311++G (d,p)

  25. Perillaldehyde Substitution structure vs ab initio structure Ax-A MP2/6-311++G (d,p) B3LYP/6-311++G (d,p) M062X/6-311++G (d,p)

  26. Perillaldehyde Substitution structure vs ab initio structure B3LYP-D3/6-311++G (d,p)

  27. Perillaldehyde Investigation of other terpenes Are these observation an oddity? Enantiomers smell differently Enantiomers smell the same Limonene Carvone Perillaldehyde

  28. Perillaldehyde Investigation of other terpenes in Ar, Ne and He Are these observation an oddity? Carvone Limonene Eq-A Eq-A Eq-C Eq-C Eq-a Eq-a Ax-C Ax-C

  29. Perillaldehyde, limonene and carvone • Four conformers of perillaldehyde have been observed • Conformational relaxation of higher-energy equatorial form • Observation of axial conformer • The axial form is considerably less well-described by theory since dispersion interactions play a larger role in its stabilisation Larger difference between experimental and ab initio rotational constants

  30. Perillaldehyde, limonene and carvone • Four conformers of perillaldehyde have been observed • Conformational relaxation of higher-energy equatorial form • Observation of axial conformer • The axial form is considerably less well-described by theory since dispersion interactions play a larger role in its stabilisation • Conformational abundances are not well described by any method

  31. Acknowledgements Dr. Maria Sanz Dr. Isabel Peña Jackson Tang AnnalisaVigorito Andreia Santos FUNDING

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