Bond Photodissociation Reactions

One of the simplest photochemical reactions is the dissociation of molecular chlorine. 

Absorption spectrum of CI2 in the gas phase. (Adapted from reference 204.) 

To a first approximation there is no net bonding energy. A more detailed analysis would revea 1 some repulsion. 

More precisely, the excitation is believed to cause a vertical transition from the ground state to an excited state with a negligible energy minimum. Calvert, J. G. Pitts, J. N. Photochemistry John Wiley Sons New York, 1966 pp. 184,226. 

The photodissociation of toluene (104) to the benzyl radical (105) and a hydrogen atom (equation 12.74) ° is an example of a a- bond dissociation in an organic compound. 

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It is clear that the analysis outlined is only the first step in developing a satisfactory theory of photochemical reactions. For example, the energy spectrum studied is not typical of all photodecomposable molecules, it is possible that direct photoexcitation of the bond that breaks is most important in some reactions, and that intramolecular energy transfer is important in others, etc. Much more work will be required before we have complete understanding of even the simplest photodissociation reactions. 

It has already been mentioned that one of the key points in the theory of photodissociation reactions is understanding how, as a bond stretches and breaks, there is established a continuous connection between the translational coordinate along which fragment separation occurs and the vibrational motions of the molecule. There is an analogous problem in... 

Incident Wavelengths above 1910 A. The observed photodissociation products must originate from reactions of an electronically excited state since photon energies are not sufficient to break the bond. The reaction products at 2144 and 2265 A irradiation are N2, N02, N20. The quantum yields are N2 = 0.19, [Pg.26]

Propanone (acetone) vapor undergoes a photodissociation reaction with 313-nm light with somewhat less than unity. Absorption of light by 2-propanone results in the formation of an excited state that has sufficient energy to undergo cleavage of a C-C bond (the weakest bond in the molecule) and form a methyl radical and an ethanoyl radical. This is a primary photochemical reaction ... 

Figure 6.13 Examples of photoinduced catalytic reactions (a) alkene hydrogenation by the [Fe(CO)3(alkene)f photocatalyst and (b) the double bond migration by the [Fe(CO)3(1-pentene)] photocatalyst both catalysts are generated in photosubstitution and photodissociation reactions of the [Fe(CO)5] precursor [28]...

The time-resolved method was applied to another photodissociation reaction [124], Upon the photoexcitation of diazo compounds (Fig. 16), nitrogen is dissociated to yield the singlet carbenes. In alcoholic solvents such as methanol, the O—H bond is inserted into the carbene part quickly (within 40 ps for diphenylcarbene), and an ether is formed. The AH and AV values for the ether formation from DPDM in methanol were measured by the time-resolved method. A similar TG signal to the DPCP case was observed after the excitation of DPDM in methanol. The signal rises within 50 ns after the excitation and decays monotonously. The time profile of the signal was found to be expressed well with a tri-exponential function... 

The simplest way to model a photodissociation reaction is by using a one-dimensional picture, such as the dissociation of a diatomic molecule into two atoms. The molecule starts out in the bound ground-state potential with the bond distance constrained near the equilibrium separation. One can also consider the dissociation of a polyatomic molecule in this way instead of using an interatomic distance, one can use either the length of the breaking bond or the distance between the centers of mass (the line of centers) of the two fragments as the dissociation coordinate. Two examples of such a model are shown in Figure 1 for methyl iodide and ketene photodissociation. 

For each of the following reactions, propose a mechanism in which the first step is a cr bond photodissociation. 

Photodissociation, photolysis or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons . Studies on molecular photodissociation reactions aim at understanding the chemistry of bond cleavage induced by irradiation with light. More than 500 papers with photodissociation (or photolysis, or photodecomposition) in the title have been published in 2012 and 2013. Theoretical calculations play an important role in most of these publications. The studied systems are carbonyl compounds, aromatic compounds, water, aliphatic and aryl halides, ozone and so on. The theoretical study on the photodissociation of NO3 is impressive, and it is selected here as an example. 

Crim F F 1996 Bond-selected chemistry vibrational state control of photodissociation and bimolecular reaction J. Phys. Chem. 100 12 725-34... 

Transient, or time-resolved, techniques measure tire response of a substance after a rapid perturbation. A swift kick can be provided by any means tliat suddenly moves tire system away from equilibrium—a change in reactant concentration, for instance, or tire photodissociation of a chemical bond. Kinetic properties such as rate constants and amplitudes of chemical reactions or transfonnations of physical state taking place in a material are tlien detennined by measuring tire time course of relaxation to some, possibly new, equilibrium state. Detennining how tire kinetic rate constants vary witli temperature can further yield infonnation about tire tliennodynamic properties (activation entlialpies and entropies) of transition states, tire exceedingly ephemeral species tliat he between reactants, intennediates and products in a chemical reaction. 

Easy availability of ultrafast high intensity lasers has fuelled the dream of their use as molecular scissors to cleave selected bonds (1-3). Theoretical approaches to laser assisted control of chemical reactions have kept pace and demonstrated remarkable success (4,5) with experimental results (6-9) buttressing the theoretical claims. The different tablished theoretical approaches to control have been reviewed recently (10). While the focus of these theoretical approaches has been on field design, the photodissociation yield has also been found to be extremely sensitive to the initial vibrational state from which photolysis is induced and results for (11), HI (12,13), HCl (14) and HOD (2,3,15,16) reveal a crucial role for the initial state of the system in product selectivity and enhancement. This critical dependence on initial vibrational state indicates that a suitably optimized linear superposition of the field free vibrational states may be another route to selective control of photodissociation. 

The degree of vibrational excitation in a newly formed bond (or vibrational mode) of the products may also increase with increasing difference in bond length (or normal coordinate displacement) between the transition state and the separated products. For example, in the photodissociation of vinyl chloride [9] (reaction 7), the H—Cl bond length at the transition state for four-center elimination is 1.80 A, whereas in the three-center elimination, it is 1.40 A. A Franck-Condon projection of these bond lengths onto that of an HCl molecule at equilibrium (1.275 A) will result in greater product vibrational excitation from the four-center transition state pathway, and provides a metric to distinguish between the two pathways. 

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