Chapter 28

Suppose absorption of light by a diatomic molecule \(A-B\) in the lowest vibrational level of the ground state always resulted in dissociation into \(\mathrm{A}\) and \(\mathrm{B}\) atoms. Would this necessarily mean that the molecule could not exist in an excited state in which the atoms were bonded together? Explain.

The fluorescence of many substances can be "quenched" (diminished or even prevented) by a variety of means. Explain how concentration, temperature, viscosity, and presence of dissolved oxygen and impurities may affect the degree of fluorescence observed for solutions of a fluorescent material. Would you expect similar effects on phosphorescence? Explain.

The quantum yield in photochemical chlorination of hydrocarbons such as methane is quite sensitive to the experimental conditions. How would you expect \(\Phi\) to vary with (a) the intensity of the incident light, (b) the wavelength of the incident light from \(250 \mathrm{~nm}\) to \(450 \mathrm{~nm}\), (c) the presence of oxygen, and (d) the presence of alkenes? Explain.

The vapor-phase photochemical decomposition of 2 -propanone proceeds in the presence of iodine vapor, but the amount of carbon monoxide formed becomes very small. Explain how this result argues against one-step process, 2 -propanone \(\stackrel{h \nu}{\longrightarrow} 2 \mathrm{CH}_{3} \cdot+\mathrm{CO} .\) What do you expect the products to be in the presence of iodine?

What products would you expect in the photodissociation of 3 -methylpentanal?

Irradiation of benzophenone in isopropyl alcohol in the presence of oxygen gives no benzopinacol (the benzophenone is not consumed), but does give 2 -propanone (with \(\Phi\) equal to unity) and hydrogen peroxide (with \(\Phi\) nearly unity). The reaction does not occur readily in the absence of benzophenone. Explain how benzophenone acts as a photosensitizer for the oxidation of isopropyl alcohol by oxygen.

Write at least three possible reaction that \(\beta\) -carotene could undergo as a result of energy transfer from \({ }^{1} \mathrm{O}_{2}{\underline{\phantom{xx}}}^{*}\).

What color would you expect to perceive if white light were passed through a solution containing a substance that absorbed very strongly but only within the specified wavelength ranges? a. \(660 \pm 30 \mathrm{~nm}\) b. \(530 \pm 30 \mathrm{~nm}\) c. \(^{*} 560 \pm 300 \mathrm{~nm}\) \(\mathbf{d} . * 480 \pm 0.1 \mathrm{~nm}\)

The \(\pi \rightarrow \pi^{*}\) absorption spectra of trans,trans- trans,cis-, and cis,cis-1,4-diphenylbutadiene show maxima and \(E\) values (in parentheses) at about \(330 \mathrm{~nm}\left(5.5 \times 10^{4}\right), 310 \mathrm{~nm}\left(3 \times 10^{4}\right)\), and \(300 \mathrm{~nm}\left(3 \times 10^{4}\right)\), respectively. What is the difference in energy between the transitions of these isomers in kcalmol \(^{-1}\) ? Why should the trans,trans isomer have a different \(\lambda_{\max }\) than the other isomers? (It may be helpful to make scale drawings or models.)

Aqueous solutions of crystal violet turn from violet to blue to green to yellow on addition of successive amounts of acid. The color changes are reversed by adding alkali. What kind of chemical changes could be taking place ot give these color changes?

Classify the following groups as strong or weak, chromophores or auxochromes: $$ -\mathrm{NO}_{2},-\mathrm{CH}_{3},-\mathrm{I},-\mathrm{O}^{\ominus},-\stackrel{\oplus}{\mathrm{N}}\left(\mathrm{CH}_{3}\right)_{3},-\mathrm{N}=\stackrel{\oplus}-\mathrm{O}, \text { and }-\mathrm{C} \equiv \mathrm{N} $$ Give your reasoning.