Transcript Radical Recombination Kinetics

Radical Recombination Kinetics
Objectives
•
To synthesize a dimer, which upon irradiation, undergoes dissociation to a
radical
•
Determine the order and rate constant of the recombination reaction using
UV-visible spectroscopy
•
Confirm the presence of the radical by EPR spectroscopy.
2
Chemical Kinetics
•
Study of reaction rates or speeds
•
Used to study reaction mechanisms
– Ways in which reactants are transformed
into products
•
Rates dependent upon several factors
– Pressure
– Presence of a catalyst
3
Rates of Reactions
1st Order
2nd Order
4
Properties of Light
Wavelength = l (lambda)
the units depend on the region of the spectrum
Frequency = n (nu)
the units are s-1, also known as Hertz (Hz)
Relationship between frequency and wavelength: ln = c
For energy, think of light as particles called photons where: E = h n
We can combine these two ideas to get energy vs. wavelength: E = (hc) / l
Types of Spectroscopy
UV-Visible Spectroscopy
7
Beer’s Law
The Absorbance is directly proportional to the concentration of the lightabsorbing species:
A = ebC
Working range of
Beer’s Law
usually 0.1 – 1 au
8
ESR Spectroscopy
•
Electron Spin Resonance Spectroscopy
•
Also called EPR Spectroscopy
– Electron Paramagnetic Resonance
Spectroscopy
•
Applicable for species with one or more
unpaired electrons
•
ESR measures the transition between the electron spin energy levels in a
magnetic field
– Transition induced by the appropriate frequency radiation
•
Required frequency of radiation dependent upon strength of magnetic field
– Common field strengths 9.5 and 35 GHz (microwave region)
•
Important features of the spectrum
– Proportionality factor, g
– Hyperfine interactions
9
How does the spectrometer work?
10
Proportionality Factor
•
•
•
•
•
Measured from the center
of the signal
hν
714.48 ν(GHz)
=
g=
μB Bo
B(G)
For a free electron
– 2.00232
For organic radicals
– Typically close to freeelectron value
– 1.99-2.01
For transition metal compounds
– Large variations due to spin-orbit coupling and zero-field splitting
– 1.4-3.0
11
Hyperfine Interactions
•
EPR signal is ‘split’ by neighboring nuclei
– Called hyperfine interactions
•
Provides information about number and identity of nuclei
and their distance from unpaired electron
•
Coupling patterns (or splitting) same as in NMR
•
More common to see coupling to nuclei with I > ½
•
The number of lines = 2NI + 1
•
•
•
Only determines the number of lines--not the intensities
Relative intensities determined by the number of interacting nuclei
If only one nucleus interacting
– All lines have equal intensity
If multiple nuclei interacting
– Distributions derived based upon spin
– For spin ½ (most common), intensities follow binomial distribution
•
12
Overview of the Procedure
•
Synthesize dimer from 2,4,5-triphenylimidazole
•
Prepare solution of dimer in dry toluene
•
Record absorbance at 560 nm over time
– Use data to determine order of reaction, rate constant, and activation
energy
•
Obtain EPR spectrum of solution
– Remeasure EPR spectrum after 10-15 minutes, note changes
13
Safety/Tips
• Dispose of all waste in the appropriately labeled waste containers – do not
throw solutions down the drain
• Wear your safety glasses at all times
• If you should get any reagents on your skin, rinse with plenty of water and
tell your TA
• Triphenylimidazole must be completely dissolved before adding bleach
solution.
• Add bleach solution slowly with vigorous stirring
• Keep cuvette in the dark and in a water bath between UV-vis readings
14