Flashkinetics_spring 2014 Updated 2j472m

Experiment 3 Flash Kinetic Spectroscopy I. Introduction Light induced reactions, photochemistry, are some of the most fascinating and important chemical events. For example, photon absorption initiates chain reactions—photosynthesis—that lead to conversion of sunlight into stored energy and to CO2 uptake form the atmosphere and conversion to more complex molecules1. Photochemistry is also the driving force for atmospheric chemistry, leading to production of the ozone layer which shields the surface from high energy UV photons that would otherwise contribute to DNA mutations. One of the most important advances in the study of photochemistry was the development of flash photolysis by Norrish and Porter2 in the late 1940’s. Flash photolysis and related flash kinetic experiments employs a pulse of light (from a laser, lamp, etc.) to initiate a reaction (or sequence of reactions) with a well defined time zero. After excitation, changes in the sample can be investigated using the tools of the analytical chemist (of course tailored to the time scale of the events following the flash). Time resolved spectroscopies are at the core of the modern physical chemistry laboratory because they are capable of following processes on the time scale of fundamental reactive events. The 1999 Nobel Prize in Chemistry was awarded to Ahmed Zewail for his work in ‘Femtochemistry’3, which is in essence an extension of flash kinetic spectroscopy on the time scale of a molecular vibration (~10-15sec). N

O

N

N

NH

N O

O

N

NH

N O

Figure1: Resonance Structures of 4-anilino-4'nitroazobenzene

This experiment was suggested by a J. Chem. Ed. article written by Hair et al.4 The molecule that will be studied is 4-anilino-4’-nitroazobenzene (4A4N). The cis configuration of this molecule is shown in Figure 1. It is a “push-pull” azobenzene with electron donor (-anilino) and acceptor (nitro) groups at different ends of a conjugated aromatic system. The presence of these groups results a resonance structure with an N-N single bond. Contribution from this resonance structure results in a weakening of the N=N double bond.

Energy

2 4

1

3

Ea cis trans o

o

0

180

Isomerization Coordinate Figure 2: Potental energy surfaces of 4A4N

In the ground state, trans-4A4N is most stable due to the steric repulsion between the substituents. Upon irradiation with visible light (the flash) the molecule undergoes a * transition. Following excitation, the molecule is very rapidly (<<1 sec) converted to the ground state cis isomer. Subsequently, it slowly relaxes back to the more stable trans isomer. The entire process follows the scheme depicted in figure 2 and detailed below (* denotes an excited state):

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Excitation:

trans-4A4N + h

Excited state isomerization: trans-4A4N* Relaxation:

cis-4A4N*

Ground-State isomerization: cis-4A4N

trans-4A4N* cis-4A4N*

(R1) (R2)

cis-4A4N

(R3)

trans-4A4N

(R4)

While the first three steps of this process occur very quickly, the ground state isomerization (essentially a rotation about the N=N bond) occurs on a time scale of several minutes. Solvent and temperature effects on the rate of this isomerization will be studied by flash photolysis.

II. Theory The rate of ground state isomerization can be written d [cis] dt

(1)

k[cis]

where k is the reaction rate constant. This can be integrated to give a familiar exponential decay expression [cis]t = [cis]0 e

kt

(2)

where [cis]0 is defined as the concentration of cis-4A4N at time zero. Beer’s law can be used to relate concentration and absorbance A(cis)t = b[cis]t

(3)

By substitution this gives A(cis)t = A(cis)0e-kt

(4)

Therefore, by fitting the decay of cis-4A4N absorbance to an exponential, the rate constant for isomerization can be easily determined.

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In practice, it is much easier to follow the absorbance of trans-4A4N because cis-4A4N does not contribute to the absorbance at the wavelength of the trans-4A4N * transition, while trans4A4N does contribute at the wavelength of the cis transition. In order to obtain the isomerization rate constant, kisom, using this absorbance, it is understood that [trans] = [trans]t + [cis]t

0.16

(5)

Flash

0.15

Absorbance

0.14 0.13 0.12 0.11 0.10 0.09 -200

0

200

400

600

800

1000

Time (s)

Figure 3: Bleach and subsequent recovery of the absorbance of trans-4A4N in cyclohexane at 40oC. Zero time immediately follows the abrupt change in absorbance due to the flash.

Note that the concentration of trans-4A4N after the reaction is completed, [trans] , will equal the concentration before the flash. Substituting Eq. 5 into Eq. 4 and applying Beer’s law: A(trans) -A(trans)t = A(trans) -A(trans)0e-kt

4

(6)

Observation of the exponential recovery of the absorbance of trans-4A4N will also give the rate constant for cis- to trans- isomerization. This isomerization reaction is strongly dependent on temperature. The rate constant of this reaction follows the Arrhenius rate equation: k

Ae

Ea RT

(7)

where A is the frequency factor and Ea is the activation energy. 1

G. R. Fleming and R. Vangrondelle, Physics Today 27, 48 (1994). R. G. Norrish and G. Porter, Nature 164, 164 (1949). 3 A. H. Zewail, J. Phys. Chem. 100, 12701 (1996). 4 S.R. Hair, G.A. Taylor, and L.W. Schultz, J. Chem. Ed. 67, 709 (1990). 2

III. Experimental In this experiment you will be looking at 4A4N which is 4-anilino-4’-nitroazobenzene. Noted that 4A4N is about 25% by weight of the dye Disperse Orange 1. Some of the salts in the dye will not dissolve in organic solvents, so it may be necessary to decant the solution. Exposure to ambient light should be minimized after preparation. In the hood prepare 20mL of a ~10-6 M solutions of 4A4N in cyclohexane, acetone and tetrahydrofuran (THF) from the ~10-5 M solutions provided by the ISF. Before you start, prepare three scintillation vials by covering them with foil and labeling each vial with the appropriate solvent. Keep lids on vials closed and store in your drawer when not in use to minimize exposure to light. Measure the absorption spectrum of each solution to determine the absorption maximum of the ground state trans isomer in each solvent. When performing the flash photolysis experiments, the spectrophotometer should be set at this wavelength. It may be necessary to dilute the solution to obtain an absorbance less than 1. Detailed instructions on using the SpectroVis can be found at the end of this lab. 1. Place a sample in the SpectroVis. 2. Begin the run and collect a few seconds of data to provide a baseline.

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3. Start the isomerization using a flash. Keep the location of the flash as consistent as possible each run. 4. Wait until the absorbance returns to the baseline value and end the run. (NOTE: the baseline level may change after the flash so wait until a plateau is achieved) 5. Save your data. 6. Complete this procedure a total of three times for each solvent. Repeat the experiment at two different temperatures (40ºC and 60ºC) in the solvent of your choice. Record the temperature of the water bath to two decimal places. Temperature dependent data will allow determination of the frequency factor and Ea for the reaction. 1. Set your cuvette in the temperature controlled water bath for 60 seconds. 2. Wipe off the cuvette with a kimwipe and place in the SpectroVis. 3. Immediately begin the run. Start the isomerization using a camera flash and wait until the absorbance returns to the baseline to end the run. Being efficient once the cuvette is removed from the water bath is essential to getting good data. 4. Complete this procedure a total of three times at each temperature.

IV. Analysis You will find it useful to use a program such as Origin or Excel in interpreting your results. 1. Plot the transient absorption traces and determine k for the reaction in each solvent at room temperature. Report these values in tabular form. 2. The polarity of the solvents increases in the following order: cyclohexane < THF < acetone. Explain how solvent polarity affects the isomerization rate? (Hint: What affect will solvent polarity have on the resonance structures of 4A4N? How will this affect the N=N bond?) 3. Qualitatively sketch the effect of solvent polarity on the ground state potential energy surface.

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4. Using the temperature dependent data, determine the activation energy and frequency factor for isomerization in your chosen solvent. Report these values. 5. How large is Ea compared to RT? Discuss the time scale of the reaction in of this answer.

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SpectroVis Instructions: Flash Kinetics Preparing the instrument: 1. Turn on the Vernier LabQuest. Use the provided power cable to plug it into the wall. 2. Connect the SpectroVis Plus Spectrophotometer to the LabQuest with the cable included (the USB side of the cable goes into the LabQuest2). Data Collection: Complete the following three sections completely for one solvent set before proceeding to the next solvent set. Spectrometer calibration 1. 2. 3. 4.

Click on (the Meter tab) to get to the correct screen. Click on Sensors with the stylus. Choose Calibrate Spectrometer. NOTE: recalibrate for each solvent. Place the “Blank” sample in the cuvette holder, making sure that the cuvette is placed so that the light source es through the clear side. 5. Follow instructions in the dialog box to complete the calibration. DO NOT skip the Lamp Warm-Up. (If you are recalibrating later, when the spectrometer has been on for a while, you may skip the lamp warmup.) 6. Click Finish Calibration. Absorption Spectrum 1. Place your cuvette containing a sample of ~10-6M 4A4N in the cuvette holder 2. Press the button to start recording data 3. Determine the wavelength that corresponds to the absorption maximum 4. Save your data. You will need to do this for each solvent. Kinetics Run Collection 1. Click on Sensors Data Collection 2. Under the Mode drop down menu select Time Based 3. Set the rate to 1 sample/s and the time to 2000s and press OK 4. Click on the large red area and select Change Wavelength. Enter in the wavelength you wish to monitor (the wavelength of maximum absorption). 5. Insert the cuvette and press the button to start recording data. 6. Collect a few seconds of data to provide a baseline then use a flash to start the isomerization. 7. Save each run with your drawer number and sample name. You will not export any data until the end of the lab

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Extracting Data to USB: 1. Plug in your USB drive 2. Go to File Open select one of your data files 3. Go to File Export and then click on the USB icon and name the file Be sure it is a .txt file or you will not be able to read it on anything but the LabQuest, and it may be impossible for you to retrieve your data after you leave the laboratory for the day. You should be able to open/import this .txt file into excel in order to graph and analyze the data.

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