Photodissociation steps involved

An intermolecular mechanism involving a primary ligand photodissociation step has been proposed for the cis trans isomerization of the dicarbene complexes, M(C0) L2, (M = Cr,Mo,... 

The photodissociation of water has attracted considerable experimental and theoretical interest (Schinke, 1993 Lee, 2003) owing to the simplicity of its electronic structure and proposals that it played an important role in the evolution of the Earth s atmosphere, and other planetary atmospheres. Most of the laser-based studies have involved multiphoton excitation techniques, as the first absorption band lies in the VUV. However, a few studies have employed a VUV laser for the photodissociation step, and almost all recent... 

A schematic of this sequence is depicted in Figure27.23. The initial step involves selective photodissociation of O2 co-adsorbed with CO. This conclusion is supported by the fact that the wavelength dependence of the CO2 yield follows that for photodissociation of O2. Note that the photodissociation of O2 adsorbed on Pt(l 11) was red-shifted with respect to the gas-phase photodissociation the rather significant red shift is attributed to alteration in the O2 electric structure upon adsorption. In a second step, the photochemically produced hot O atom collides with a neighbouring CO molecule to form CO2. In a last step, the CO2 molecule desorbs from the surface. 

Reactions with molecular species above the arrow e.g. RIO) involve subsequent reactions with these species to produce the indicated products. In most cases the reactants shown to the left of the arrow participate in the slowest or rate-determining step]. The CH3O radical formed in Rll then follows reaction R7. The H02 radical formed in RIO is the other member of the family and is linked with HO in a variety of chain reactions. These radicals are produced following HO attack on hydrocarbons or by photodissociation of oxygenated hydrocarbons such as formaldehyde (RIO) and acetaldehyde ... 

Indirect photodissociation involves two more or less separate steps the absorption of the photon and the fragmentation of the excited complex. Resonances, which mirror the quasi-bound states of the intermediate complex in the upper electronic state, are the main features. They have an inherently quantum mechanical origin. If we consider — in very general terms — the inner region, before the fragments have obtained their identities, as the transition state, then the resolution of resonance structures in the absorption spectrum manifests transition state spectroscopy in the original sense of the word (Foth, Polanyi, and Telle 1982 Brooks 1988). 

The primary photofragment accompanying the formation of HC=CH is C2H2S. The structure for C2H2S is envisioned as c-CH=CHS (thiirene) [159]. The heat of formation for thiirene is estimated to be 87kcal/mol [157], which is higher than that of the most stable thioketene structure (CH2=C=S) by 48kcal/mol. If the photodissociation of thiophene involves the formation of the biradical, CH==CH—CH=CH—S% as the initial step, the rapid decomposition of this biradical may yield the biradical CH=CH—S. The CH=CHS biradical is estimated to lie 14 kcal/mol... 

The chain reaction is initiated by dissociation of CI2, a stable molecule, to form two highly reactive chlorine atoms (Cl). Since chemical bond fission requires the input of energy, initiation may be achieved by heating the sample (see Lindemann mechanism) or by ultraviolet irradiation (photodissociation). Following initiation, the two elementary bimolecular propagating reactions will continue until either or both of the reactants (H2 and CI2) are consumed, at which time the termination steps end the chain reaction. Termination typically involves termolecular recombination of two radicals to form a stable molecule in the... 

Photolysis of Mn2(CO)8(phen) in the presence of CCU, however, results in the formation of a mixture of MnCl(CO)5 and MnCl(CO)3phen. The reaction, which involves the initial photohomoloysis of the Mn-Mn bond followed by chlorine atom abstraction from CCU by the resulting Mn radicals, is discussed in more detail in a later section of this chapter. In other cases, however, photolysis leads to the dissociation of a carbonyl ligand rather than to homolysis of the intermetallic bond. The quantum yields of these particular reactions are dependent on both the metal and the ligands in the complex. The photodissociation of a carbonyl ligand has also been proposed as the initial step in the photochemical disproportionation reactions of these complexes/ In this case two electron donor ligands such as THF or a tertiary phosphine induce the photodisproportionation of the complex (CO)sM-M(C0)3 (diimine) (M = Mn, Re) into a cationic complex M(CO)3(diimine)L and an anionic complex M(CO)s (Refs. 99 and 100) ... 

R-Cl is a chlorinated hydrocarbon or chlorofluorocarbon which decomposes under the influence of UV light. Oxygen atoms involved in the second propagation step are abundantly present in the higher layers of the atmosphere, where they originate from photodissociation of O2 and NO2. Termination of the chain occurs if Cl reacts with hydrocarbons, or other H-containing molecules to HCl. 

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