Qualitative Mechanistic Photochemistry

The ESR study of solids at low temperatures can yield evidence for primary photochemical processes involving radical production. Svejda and Volman (353) have obtained ESR evidence for two primary photochemical processes of CH CN one involves a C-C bond scission, the other involves a C-H bond split. They further substantiated the C-H split mechanism by observing the ethylidenimino radical (CH )HC=N, resulting from the addition of the H atom to CH CN. The photochemistry of CH CN appears rather simple compared to the radiolysis of CH CN in which methyl radicals are produced by photochemical reactions in the y-irradiated sample (354). Recently Sprague and Williams (355) have demonstrated that methyl radicals produced from CH CN in a low-temperature glass can abstract hydrogen 

Direct observation of transient radicals in liquid photolysis has added a new dimension to mechanistic organic photochemistry. While the nature of the primary process in the photolysis of gaseous acetone is well known (364), the mechanism of the photochemical reaction in pure liquid acetone or in hydrocarbon solvent has not been established in detail. Zeldes and Livingston (216) have observed (CH3)2COH, C COCH, and some CH3 radicals when pure liquid acetone is photolysed and have suggested the following mechanism  

In the presence of an alcohol or ether the excited acetone abstracts an H atom from the carbon alpha to the 0 atom of the alcohol or ether. Similar photoreduction of perinaph-thenone (365) has been observed. However, the nature of the excited state of acetone for liquid photoreduction in these studies has not been established. Free radicals produced from photoreduction of acetaldehyde, biacetyl and acetoin in the presence of good H-atom donors have been observed by Zeldes and Livingston (216). These authors also studied the photolysis of oxalic acid and esters (366). 

Photolysis of acyl peroxides apparently leads to formation of alkyl radicals and carbon dioxide as observed by ESR (196)  

A further detailed ESR study of the photochemistry of aliphatic diacyl peroxides by Sheldon and Kochi (367) 

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Quantum chemical methods, exemplified by CASSCF and other MCSCF methods, have now evolved to an extent where it is possible to routinely treat accurately the excited electronic states of molecules containing a number of atoms. Mixed nuclear dynamics, such as swarm of trajectory based surface hopping or Ehrenfest dynamics, or the Gaussian wavepacket based multiple spawning method, use an approximate representation of the nuclear wavepacket based on classical trajectories. They are thus able to use the infoiination from quantum chemistry calculations required for the propagation of the nuclei in the form of forces. These methods seem able to reproduce, at least qualitatively, the dynamics of non-adiabatic systems. Test calculations have now been run using duect dynamics, and these show that even a small number of trajectories is able to produce useful mechanistic infomiation about the photochemistry of a system. In some cases it is even possible to extract some quantitative information. 

Thus far the discussion has centered on n-ir excited states and their reactions, t-t photochemistry is equally intriguing but more difficult to discuss from a mechanistic viewpoint. In the n-rr excited state the localized orbital from whence the electron is promoted and the ir system receiving the promoted electron are separated from one another and each is subject to qualitative valence bond description in fact, the ir system becomes that of a metal ketyl which is a well-known species in organic chemistry. Furthermore, in the ir system of the excited state there are no vacant low-energy molecular orbitals. 

Luminescence appears in mechanistic chemistry in two distinct contexts. The details of the emission process itself are important in photochemistry, which is considered in Section 8. In other applications, fluorescence is simply an assay for qualitative and quantitative analyses. Good selectivity and sensitivity have earned it a role in commercial apparatus. However, the sensitivity is compromised by the small windows of a high-pressure cell, which limit the solid angle available. Laser sources help, but they restrict luminescence methods to specialized facilities. It is important to understand the polarization artefacts which may be introduced by thick windows and to appreciate that they may change with pressure. Many optical elements, including monochromators, have transmission properties that depend on the polarization. 

The central mechanistic feature in most photochemical mechanisms is the conical intersection. Thus we hope to present some thoughts about how to predict and rationalize the molecular and electronic structure of such mechanistic features using VB ideas. It turns out that one can derive analytical results for n orbitals with n electrons so we shall develop the main ideas with reference to the photochemistry of some simple model systems such as the cycloaddition of two ethylene molecules and the radiationless decay of benzene. Once one allows zwitterionic systems, lone pairs and heteroatoms, the same principles apply but analytical results are not available so easily and one must be content with a qualitative analysis at the moment. 

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