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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

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Homo-halogen bonding in 2-iodo perfluoropropane

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  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

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