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 4 1 st Order2 nd Order

Properties of Light Wavelength =  (lambda) the units depend on the region of the spectrum Frequency = (nu) the units are s -1, also known as Hertz (Hz) Relationship between frequency and wavelength:  c For energy, think of light as particles called photons where: E = h We can combine these two ideas to get energy vs. wavelength: E = (hc) /

Types of Spectroscopy

UV-Visible Spectroscopy 7

Beer’s Law 8 The Absorbance is directly proportional to the concentration of the light- absorbing species: A =  bC Working range of Beer’s Law usually 0.1 – 1 au

9 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

10 How does the spectrometer work?

11 Proportionality Factor Measured from the center of the signal g = For a free electron – For organic radicals –Typically close to free- electron value – For transition metal compounds –Large variations due to spin-orbit coupling and zero-field splitting –

12 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 = 2 NI + 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

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 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