Scott Flancher

 Review of halogen bonding  σ -hole  Applications  Homo-halogen bonding hypothesis  Experiments / Data  Kinetics  19 F-NMR  IR  Future research

 Similar to hydrogen bonding  Electron density pulled into bond  Exposes area of positive potential on extension of bond axis (the σ -hole)

 Biochemistry  Protein recognition  Drug design  DNA  Material Science  Crystal engineering  Macromolecular engineering Voth A. R. et.al. PNAS 2007;104: Resnati et.al. J. Fluorine Chem. 2004;104: 271

 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

 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

 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

 To test the homo-halogen bonding hypothesis utilized several techniques  Analysis of physical properties  19 F-NMR  IR  Noticed photochemical dissociation when left in room lights  Suggested a kinetics study

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

Time 20minTime 30minTime 45min Time 60minTime 90minTime 18hrs X=0.2NeatX=0.2NeatX=0.2Neat X=0.2NeatX=0.2Neat X=0.2Neat

 Different rate constants observed  k obs = min -1 in hexane (after correction for mole fraction)  k obs = min -1 when neat  Iodine production nearly 40x faster in hexane  Protection of iodine  Dissociation and geminate pair recombination

 Two possibilities:  Halogen bond protects the C-I bond from breaking  Geminate pair recombination

 Also saw less I 2 production when diluted with pyridine  1-iodo behaved differently  Dilution with hexane showed minimal difference in rate of iodine production

 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 CompoundBoiling Point (°C) Melting Point C 3 H 7 I (1-iodo) C 3 H 7 I (2-iodo) C 3 F 7 I (1-iodo) C 3 F 7 I (2-iodo) 40-58

 C- α F stretch  Uncharacterized vibrational modes

 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

 19 F-NMR  α -peak and β -peak behavior  Measures amount of electron shielding

More shielding Less shielding Halogen bonding

More shielding Less shielding

 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

More shielding Less shielding Halogen bonding

More shielding Less shielding Halogen bonding

 Dilution studies  When diluted with cyclohexane, less proclivity for homo-halogen bonding, therefore less shielding  Temperature studies  Lower temperatures show greater shielding / greater structuring

 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

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

 Conventional Raman to compliment IR  1-iodo perfluoropropane

 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