Photochemistry, carbonyl

Over a pressure range from just above pc to 300 bar, this reaction yielded only a statistical distribution of products [55]. Thus, no cage effect (eidianced or otherwise) was observed for this reaction. 

This problem was recently discussed by Chateauneuf, Brennecke and coworkers who examined the decarbonylation of the phenylacetyl radical 

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There have been three primary motives behind the study of metal carbonyl photochemistry in the gas phase first, to discover the shapes of metal carbonyl fragments in the absence of perturbing solvents or matrices second, to probe the effect of uv photolysis wavelength on product distribution and third, to measure the reaction kinetics of carbonyl fragments. All three areas have already proved fruitful. The photochemistry of two molecules, Fe(CO)5 and Cr(CO)6, has been studied in detail. 

These IR kinetic experiments (75) were the first examples of vibrationally excited metal carbonyls to be observed. More detailed studies on the behavior of hot carbonyls should provide an intriguing insight into the photophysics of these molecules. We now look at metal carbonyl photochemistry in solution. 

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. 

Few would deny the importance of photochemistry -few would further deny that full exploitation of photochemistry demands an intimate knowledge of the identity, structure and behavior of the appropriate intermediates. In this article we review some of the methods used to obtain this information, and the techniques are illustrated with some detailed examples taken from Organo-metallic - mostly metal carbonyl - Photochemistry (1). Since this symposium looks towards the 21st century ( ) a few speculations on future experiments are included. 

In each case the weakest C-H bond in the R -H molecule is broken. A consequence of this reaction is that the range of suitable organic solvents for carbonyl photochemistry is somewhat limited. Cyclohexane is a relatively poor hydrogen donor although it has limited success in dissolving polar compounds, so acetonitrile and tertiary alcohols may be used, as is benzene. 

This question involves the analysis of carbonyl photochemistry using Dauben-Salem-Turro state correlation diagrams. 

Dougherty TP, Heilweil EJ. Dual track picosecond infrared spectroscopy of metal-carbonyl photochemistry. In Lau A, Siebert F, Wemcke W, eds. Proceedings in Physics. Berlin Springer-Verlag, 1994 136-140. 

The photochemistry of acid derivatives is not well understood. Since these compounds absorb in the far ultraviolet and rarely react with great quantum efficiency, they have not been studied at all systematically. Consequently, this review will emphasize aldehyde and especially ketone photochemistry. So much is known about carbonyl photochemistry and so much is added each year (e. g., over 200 pages in the Chemical Society s 1973 survey of photochemistry D), that this review had to be limited to but a few aspects of carbonyl photochemistry. Although the main kinds of carbonyl photoreactions have been adequately reviewed often 2>, several important generalizations have resulted from the past few years work. This review will attempt to summarize these new generalizations which may not yet be familiar to the entire chemical community. Several unresolved problems will also be outlined. 

Careful study of (S)- (+) -2-phenylpropiophenone reveals that approximately half of the radical pairs recombine before diffusing out of the initial solvent cage 50>. This conclusion follows from the 44% quantum weld of scavengable benzoyl radicals and the 33% quantum yield for racemization. Alkyl thiols are excellent radical scavengers in carbonyl photochemistry because they quench triplet ketones fairly slowly 51>. Lewis has shown that concentrations of thiol above 0.03 M generally trap all free benzoyl radicals as benzaldehyde 50>. Of course, the minimum concentration for complete scavenging depends on conversion. 

When one takes up a polyfunctional molecule which contains at least one carbonyl group, one knows which reactions of that carbonyl group are possible. One does not know how well these reactions will compete with various physical and chemical interactions of that excited carbonyl group with other functional groups in the molecule. It is this aspect of carbonyl photochemistry which requires and deserves extensive future research. The uniquely well understood chemistry of the carbonyl group can serve as a monitor for studying interactions in electronically excited polyfunctional molecules. 

Since the last reviews of carbonyl photochemistry (61,191), there have been several studies of the primary photodecomposition processes- The earlier studies have shown that there are two main primary photodissociation pathways, radical (I) and molecular (II) ... 

Formaldehyde can be used as a model for predicting carbonyl photochemistry and photophysics most successfully by exploring both the differences and similarities of the behavior of this molecule to that of the larger carbonyls. The "isolated molecule" processes by which the formaldehyde state is depopu-... 

An exception to this metal-carbonyl photochemistry is that irradiation of the quadruply bonded dimer, [RejClg] ", in CH3CN gives monomeric trans-[ReCl -(CH3CN)2J . Cleavage of the quadruple bond by one photon is not likely given the photoinertness of triple bonds (see 13.2.4.2) the process likely includes a large component of solvent assistance. 

Electronically excited carbonyl compounds serve as versatile intermediates in countless reactions. They not only operate as reactive substrates for intra- and intermolecular hydrogen abstraction and cycloaddition reactions as well as C-C cleavage steps, but many of them also are useful sensitizers for the generation of triplet excited compounds. This variety of reaction possibilities makes carbonyl photochemistry sometimes very complex,... 

The photochemical aspects of carbonyl photochemistry remain important subjects of research. Wagner and Thomas have used CIDNP to elucidate radical formation from a,a,a-trifluoroacetophenone. Irradiation of benzophenone and its derivatives in the presence of molecules with abstractable hydrogen atoms can give rise to intensely fluorescent compounds. This effect may interfere with the observation of nanosecond-domain kinetics.Quantum yields and kinetic isotope effects in nanosecond flash studies of the reduction of benzophenone by aliphatic amines have been measured by Inbar et Rate constant data are given in Tables 13 and 14. Winnik and Maharaj have studied the reaction of benzophenone with n-alkanes through hexane to hexatriacontane is 3.9 0.2kcal for all chain lengths.The effects of substituents on the benzophenone on these reactions have also been examined. The reactions of phenylacetophenone when used as polymerization initiator have been reviewed by Merlin and Fouassier. ... 

Much of the remaining interest has switched from the synthetic aspects of carbonyl photochemistry to more physical studies involving energy transfer and excited-state lifetime measurements. Typical of this area of study is the account by Zimmerman and his co-workers of the details of their studies of energy transfer in rod-like molecules (e.g., 1,2). A detailed study of the photochemical reaction of... 

Some review articles reported during the past year focus attention on material which is pertinent to this chapter. Dalton and Snyder1 have compiled a selective review of the 1972 literature, and Schaffner and Jeger2 have reviewed their own contribution in the area of carbonyl photochemistry. A review dealing with chemical methods for the generation of electronically excited states has also been published.3... 

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