1 / 30

Homo-halogen bonding in 2-iodo perfluoropropane

Homo-halogen bonding in 2-iodo perfluoropropane. Scott Flancher. Overview. Review of halogen bonding σ -hole Applications Homo-halogen bonding hypothesis Experiments / Data Kinetics 19 F-NMR IR Future research. Halogen Bonding: The σ -hole. Similar to hydrogen bonding

marcus
Download Presentation

Homo-halogen bonding in 2-iodo perfluoropropane

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Homo-halogen bonding in2-iodo perfluoropropane Scott Flancher

  2. Overview • Review of halogen bonding • σ-hole • Applications • Homo-halogen bonding hypothesis • Experiments / Data • Kinetics • 19F-NMR • IR • Future research

  3. Halogen Bonding:The σ-hole • Similar to hydrogen bonding • Electron density pulled into bond • Exposes area of positive potential on extension of bond axis (the σ-hole)

  4. Halogen Bonding:Applications • Biochemistry • Protein recognition • Drug design • DNA • Material Science • Crystal engineering • Macromolecular engineering Voth A. R. et.al. PNAS 2007;104:6188-6193 Resnati et.al. J. Fluorine Chem. 2004;104: 271

  5. Project History • Originally studied hydrogen bonding using the I(2)CARS method • Pyridine a good candidate for studies • Strong signal • Vibrational modes well characterized • Prime choice for the foray into halogen bonding • Perfluorinated compounds good for halogen bonding • Electron withdrawing nature of fluorines • I > Br > Cl > F

  6. Project History • Summer of 2009 • I(2)CARS with several iodo-perfluoroalkanes • Established presence of strong halogen bonding • Thermodynamic studies also shed more light on liquid structure • Ultimately led to the homo-halogen bonding hypothesis for 2-iodo perfluoropropane

  7. Hypothesis • α-fluorine directed halogen bonding • Thought to be more likely in 2-iodo perfluoropropane • In 1-iodo perfluoropropane the electron density “split” by two α-fluorines • Focused on the 2-iodo perfluoropropane

  8. Summer 2010 Strategy • To test the homo-halogen bonding hypothesis utilized several techniques • Analysis of physical properties • 19F-NMR • IR • Noticed photochemical dissociation when left in room lights • Suggested a kinetics study

  9. Kinetics • Let cuvettes sit in room light and observed their color change via the following reaction: • Measured absorbance every 10 minutes to check iodine production

  10. Time 20min Time 30min Time 45min Neat X=0.2 Neat X=0.2 Neat X=0.2 Time 60min Time 90min Time 18hrs Neat X=0.2 Neat X=0.2 Neat X=0.2

  11. Kinetics • Different rate constants observed • kobs= 0.0755min-1 in hexane (after correction for mole fraction) • kobs= 0.0019min-1 when neat • Iodine production nearly 40x faster in hexane • Protection of iodine • Dissociation and geminate pair recombination

  12. Kinetics • Two possibilities: • Halogen bond protects the C-I bond from breaking • Geminate pair recombination

  13. Kinetics • Also saw less I2 production when diluted with pyridine • 1-iodo behaved differently • Dilution with hexane showed minimal difference in rate of iodine production

  14. Boiling and Melting Points • Compare boiling point difference of non-fluorinated to fluorinated: • 12°C difference compared to 1°C difference • Compare melting point difference of non-fluorinated to fluorinated: • 11°C difference compared to 37°C difference • Skeptical of melting points for perfluorinated compounds

  15. IR • C-αF stretch • Uncharacterized vibrational modes

  16. IR • Gives compelling evidence for presence of two species in neat 2-iodo perfluoropropane • Lack of complete mode assignment • Still shows peak broadening • Suggests a different species is present

  17. NMR • 19F-NMR • α-peak and β-peak behavior • Measures amount of electron shielding

  18. Halogen bonding More shielding Less shielding

  19. More shielding Less shielding

  20. NMR • When diluted with pyridine, α-fluorine becomes more shielded • Electron density from pyridine pulled to α-fluorine • Chemical shift remains relatively stagnant when majority of solution is pyridine

  21. Halogen bonding More shielding Less shielding

  22. Halogen bonding More shielding Less shielding

  23. NMR • Dilution studies • When diluted with cyclohexane, less proclivity for homo-halogen bonding, therefore less shielding • Temperature studies • Lower temperatures show greater shielding / greater structuring

  24. NMR Results • α-peak behavior consistent with hypothesis • Stronger halogen bond -> greater, negative chemical shift • Mixed in neutral solvent (cyclohexane) • Showed shifting opposite to that of halogen bond acceptor • Temperature studies

  25. Conclusions • Kinetics • Iodine production rates • Geminate pair recombination • Boiling and melting points Homo-halogen bonding • NMR • Shift in α-peak • Shielding levels based on temperature • IR • Shift in the α-peak • Peak broadening indicative of dual-species

  26. Future Studies • Conventional Raman to compliment IR • 1-iodo perfluoropropane

  27. Acknowledgments • Concordia Chemistry Department and Laser Facility • Craig Jasperse and MSUM NMR facility • NSF • Dreyfus Foundation • Concordia College Research Endowment • Undergraduate Research, Scholarly and Creative Activities Grant Program • Dr. Ulness, Dr. Gealy

More Related