Scott Flancher

Review of halogen bonding




σ-hole
Applications
Homo-halogen bonding hypothesis
Experiments / Data
Kinetics
 19F-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


Voth A. R. et.al. PNAS 2007;104:6188-6193
Crystal engineering
Macromolecular engineering
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
 19F-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:
F
F
F
C
F
F
C
F
C
h
F
C
I
F
C
F
F
C
F
F
C
F
C
F
C
F

+
I
F
F
F
x2
F
F
F
F
F
F
C
F
C
F
C
F
F
+
I
I
Measured absorbance every 10 minutes to
check iodine production
Time 20min
Neat
X=0.2
Time 30min
Neat
X=0.2
Time 45min
Neat
X=0.2
Time 60min
Time 90min
Time 18hrs
Neat
Neat
Neat
X=0.2
X=0.2
X=0.2

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

Two possibilities:


Halogen bond protects the C-I bond from breaking
Geminate pair recombination
F
F
F
F
C
F
F
C
F
F
C
I
F
C
I
F
C
F
F
C
F
F
h
F
C
F
F
C
F
F
C
I
F
C
I
F
C
F
F
C
F
F
F
F
F
C
F
C
F
C
F
F
F
I
F
F
F
F
C
F
F
C
I
F
C
F
F
F
GPR
F
F
C
F
F
C
F
F
C
I
F
C
I
F
C
F
F
C
F
F
F

Also saw less I2 production when diluted with
pyridine
F
F
F
C
F
F
C
I
F
C
F
h
N
F
C
F
F
C
I
F
C
F
F
F
F
F
C
F
C
F
C
F
F
I
N
GPR
F
F

N
F
C
F
F
C
I
F
C
F
N
F
1-iodo behaved differently

Dilution with hexane showed minimal difference in rate
of iodine production

Compare boiling point
difference of nonfluorinated to fluorinated:


Compare melting point
difference of nonfluorinated to fluorinated:


12°C difference compared
to 1°C difference
11°C difference compared
to 37°C difference
Skeptical of melting points
for perfluorinated
compounds
Compound Boiling
Point (°C)
Melting
Point
C3H7I
(1-iodo)
102
-101
C3H7I
(2-iodo)
90
-90
C3F7I
(1-iodo)
41
-95
C3F7I
(2-iodo)
40
-58


C-αF 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

 19F-NMR


α-peak and β-peak behavior
Measures amount of electron shielding
Less shielding
Halogen bonding
More shielding
Less shielding
More 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
Less shielding
Halogen bonding
More shielding
Less shielding
Halogen bonding
More shielding

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


Mixed in neutral solvent (cyclohexane)


Stronger halogen bond -> greater, negative chemical
shift
Showed shifting opposite to that of halogen bond
acceptor
Temperature studies
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


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