Photoelimination of Carbon Monoxide

During the period covered by this review, the majority of studies of photodecar-bonylations have been either of small molecules such as ketene and phosgene, or of metal carbonyls. 

Similarly detailed studies have been made of the photodissociation of isocyanic acid (HNCO). With this molecule, three reaction channels compete fragmentation into NH and CO, with the NH in either of two electronic states, and fragmentation into H and NCO. Experiments in which HNCO was excited to the S state in expansion jets have shown that the competition depends on the photon energy and the excitation conditions. The photodissociation dynamics of 

HNCO at 248 nm have been investigated and, at this wavelength, the predominant reaction channel is direct formation of H atoms.Photolysis of DNCO at 193 and 148 nm has also been examined as a comparison with HNCO. Semiempirical (AMI) calculations have provided a potential-energy surface for dissociation of PhNCO.  

Other small molecules for which photodecarbonylation studies have been made include phosgene in both the gas phaseand as a solid,acetyl chloride, and oxalyl chloride though extrusion of CO is only one of the pathways observed for these species. Photodissociations of acetyl and propionyl radicals have been studied by IR emission spectroscopy of the vibrationally excited CO produced, and an ab initio study of the photodissociation of the formyl radical (HCO) has been made. Trifluoroacetylthiol (CF3COSH) and trifluoroacetyl-sulfenyl chloride (CF3COSCI) have been photolysed in noble gas matrices, to produce CF3SH and CO, and CF3SCI, CO and SCO, respectively.  

The formation of both Mn(CO)s and the CO-loss product, Mn2(CO)9, from UV photolysis of Mn2(CO)io has been investigated by ultrafast IR spectroscopy.Density functional calculations have been performed on the ground and excited states of MnCl(CO)5, as an aid to understanding the photochemistry of MX(C0)5 complexes (M = Mn, Re X = Cl, Br, I). The lowest energy excitations are to the Mn-Cl a orbital, and these states are dissociative for both axial and equatorial CO loss, but not for Mn-Cl bond homolysis. The photochemistry of manganese carbonyl complexes (58, R = Me, benzyl R = /-Pr, p-tolyl) depends critically on the R group.CO is released with R = Me, but with R = benzyl there is efficient homolysis of the Mn-R bond.  

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This base reaction does not depend on hydroxide and proceeds under mild conditions, thus replacing the often less-clean photoelimination of carbon monoxide. Certain cationic metal carbonyls are attacked even by water, a reaction that... 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

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