Dissociative photochemistry

Another area of research ia laser photochemistry is the dissociation of molecular species by absorption of many photons (105). The dissociation energy of many molecules is around 4.8 x 10 J (3 eV). If one uses an iafrared laser with a photon energy around 1.6 x 10 ° J (0.1 eV), about 30 photons would have to be absorbed to produce dissociation (Eig. 17). The curve shows the molecular binding energy for a polyatomic molecule as a function of interatomic distance. The horizontal lines iadicate bound excited states of the molecule. These are the vibrational states of the molecule. Eor... 

The spectroscopy, structure, photochemistry, and unimolecular reactions of allyl radical have been studied extensively and reviewed recently.145 Possible dissociation channels of allyl radical, their energetics, and the potential energy barriers of the C3H5 system are shown in Figs. 20 and 21.145,146... 

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

The photochemistry of ethylene is fairly well understood, but not the radiation chemistry. UV-photolysis shows that the excited states dissociate mainly by elimination of an H atom or a H2 molecule as follows ... 

One problem we have had to overcome in developing metal-cluster oxidation-reduction photochemistry is the tendency of excited clusters to dissociate into radical fragments (for... 

Since the primary photochemical process for nitrosoalkane involves the homolytic dissociation of the C—N bond to generate free radicals141, recent studies on the photochemistry of nitrosoalkanes pay more attention to radical reactions and to the methods of detection, such as spin trapping studies coupled with ESR techniques142. 

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

DR. GEOFFROY We obviously don t know why the photochemistry changes. We know that it is reasonable that phosphine dissociation does not occur when we have a chelating diphosphine ligand. But we don t understand why we see hydrogen loss in this system when we don t see it in the other cases. 

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