The conversion of compound 11.10 to its (Z)-isomer (see...

The conversion of compound 11.10 to its (Z)-isomer (see Section 6.11) is induced by a flash of light (flash photolysis). Isomer 11.10 absorbs light at $435 \mathrm{nm},$ and the $(Z)$ -isomer absorbs at $316 \mathrm{nm} .$ Figure 11.11 shows the change in absorbance at $\lambda=435 \mathrm{nm}$ for a solution of 11.9 in cyclohexane over a period of 1200 s during which time the solution is subjected to a flash of light. (a) Rationalize the shape of the curve in Figure $11.11 .$ (b) If the path length of the solution cell is $1 \mathrm{cm},$ and the solution concentration is $9.00 \times 10^{-6} \mathrm{mol} \mathrm{dm}^{-3},$ determine a value for $\log \varepsilon$ corresponding to the absorption at $435 \mathrm{nm}$ [Data from: S. R. Hair et al. (1990) J. Chem. Ed., vol. $67, p .709 .]$ CAN'T COPY THE FIGURE

The conversion of compound 11.10 to its
(Z)-isomer (see Section 6.11) is induced by a flash of light (flash photolysis). Isomer 11.10 absorbs light at $435 \mathrm{nm},$ and the $(Z)$ -isomer absorbs at $316 \mathrm{nm} .$ Figure 11.11 shows the change in absorbance at $\lambda=435 \mathrm{nm}$ for a solution of 11.9 in cyclohexane over a period of 1200 s during which time the solution is subjected to a flash of light. (a) Rationalize the shape of the curve in Figure $11.11 .$ (b) If the path length of the solution cell is $1 \mathrm{cm},$ and the solution concentration is $9.00 \times 10^{-6} \mathrm{mol} \mathrm{dm}^{-3},$ determine a value for $\log \varepsilon$ corresponding to the absorption at $435 \mathrm{nm}$ [Data from: S. R. Hair et al. (1990) J. Chem. Ed., vol. $67, p .709 .]$
CAN'T COPY THE FIGURE

Key Concepts

Beer–Lambert Law

The Beer–Lambert Law is a fundamental relationship in spectroscopy that relates the absorbance of a solution to the concentration of the absorbing species, the path length of the sample cell, and the molar absorptivity of the species. This law is used to quantify the concentration of a substance by measuring its absorbance at a specific wavelength, and adjustments in the log scale (log ?) can help in characterizing the absorbing properties of the compound.

Absorbance Kinetics

The shape of the absorbance versus time curve reflects the dynamics of the photochemical reaction. Initially, the absorbance associated with the reactant decreases as it is converted to the product upon exposure to light. Over time, the curve stabilizes as the system reaches a new photostationary state, indicating that the rate of formation of the product balances with any reverse reactions or secondary processes.

Flash Photolysis

Flash photolysis is a technique in which a brief, intense flash of light is used to excite molecules, initiating a photochemical reaction almost instantaneously. This method allows researchers to observe the reaction kinetics by tracking rapid changes in the reaction intermediates or products immediately after the flash, making it a valuable tool for studying short?lived species and transient processes.

Photoisomerization

Photoisomerization is the process in which a molecule absorbs light and undergoes a structural change, typically converting from one isomer to another. This is important because the different isomers often exhibit distinct absorption properties, enabling the monitoring of the reaction progress by following changes in their absorption spectra.

Transcript

00:02 Okay, so for the first question, it asks us to rationalize the shape of the curve in the figure, which shown here, this is the figure.

00:15 So let's read the question.

00:17 It says the conversion of the compound given in the figure is induced by flashlight.

00:26 And one, i saw some of the compound will actually absorb the light at 400.

00:34 And certify, right? and once it was flashed with light, the isosomer, right? isomer, where absorbed the light instead at the 316.

00:54 And this figure is actually the absorbance at 435.

01:01 So it's basically, right, this figure is looking as a change.

01:07 Of the original product.

01:10 And the question asked to rationalize what's happening during each step of the figures.

01:18 So we can see as the initial steps in this region, right? it goes flat, which means there's no conversion.

01:28 So which means here it's in the dark.

01:35 And here you can see the wavelength absorbence of the 434 dramatically decrease which means, right, the original product is decreased.

01:48 It was converted to the isomer, which means at this point, you shall show the light, shine the light, which induce the conversion from the original compound to its isomers.

02:07 And then you can see it slowly goes back, right? which means you only share the light for a very second...

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