Photochemically induced dissociation

Although additives to induce radical chemistry have allowed ligand substitutions of 18-electron complexes to be conducted under mild conditions, photochemical reactions provide a common and practical alternative. Photochemically induced dissociation of carbonyl ligands is most common, but photochemical dissociations of other dative ligands are known. Several examples are shown in Equations 5.36-5.40. These examples illustrate the dissociation of CO from homoleptic carbonyl compounds of iron - and chromium, the dissociation of CO from piano-stool carbonyl compounds, " ttie dissociation of N, and the dissociation of a carbodiimide to generate an intermediate that coordinates and cleaves the C-H bonds of alkanes. In some cases, like the formation of the two THE complexes, the products of the photochemical process are not isolated instead, they are treated in situ with a ligand, such as a phosphine, to form monosubstitution products selectively. 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

Molecular electronic spectroscopy can provide information on vibrational parameters (frequencies and force constants), rotational parameters (moments of inertia and therefore molecular geometries), electronic excitation energies, ionization potentials, and dissociation energies for ground and excited electronic states. Moreover, a knowledge of excited electronic states is important in understanding the course of photochemically induced reactions. 

Since many substitution reactions of (1) are done under thermal or photolytic conditions where Re-Re bond rupture may occur, Re(CO)s was first regarded as an intermediate in both types of substitution reactions. However, the mechanistic studies of photochemically induced substitutions suggest that (1) photochemical substitution may not occur through the monomeric M(CO)5 but from the longer-lived primary photointermediate M2(CO)9 (equation 2) (2) both primary photointermediates may give the same reaction products and (3) substitution of CO in M(CO)5 is most probably associative and not dissociative as in most substitution reactions of 17-... 

Photochemical reactions are often initiated by direct photodissociation or by collision-induced dissociation of a laser-excited molecule, where radicals are formed as intermediate products, which further react by collisions. The dynamics of photodissociation after excitation of the parent molecule by a UV laser has therefore been studied thoroughly [1063]. While the first experiments were restricted to measurements of the internal-state distribution of the dissociation products, later more refined arrangements also allowed the determination of the angular distribution and of the orientation of the products for different polarizations of the photodissociating... 

Photochemical homolytic dissociations of the oxygen-metal bond also induce various reactions [Eq. (19)] ... 

Dialkylplatinum complexes with the cod ligand undergo photolysis at Amax>320nm in GD2CI2 (Scheme 10). The dimethylplatinum complex releases 1,5-dimethylcyclooctane 78 and Pt metal, while the analogs with two isopropyl or benzyl groups produce chloro(alkyl)platinum complexes 79 under the same conditions. Both products are formed via photochemically induced homolytic dissociation of a Pt-C cr-bond. 

Vanadium (IV) Chloride. Vanadium(IV) chloride (vanadium tetrachloride, VCy is a red-brown hquid, is readily hydrolyzed, forms addition compounds with donor solvents such as pyridine, and is reduced by such molecules to trivalent vanadium compounds. Vanadium tetrachloride dissociates slowly at room temperature and rapidly at higher temperatures, yielding VCl and CI2. Decomposition also is induced catalyticahy and photochemically. This instabihty reflects the difficulty in storing and transporting it for industrial use. 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

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