Unimolecular photochemistry

The concept of absolute asymmetric synthesis using a chiral crystal was applied to unimolecular photochemistry, and now many fine examples are reported. Scheffer et al. reported elegant unimolecular absolute asymmetric transformations (Scheme 4). [19] This group demonstrated that the well-studied solution-phase di-ir-mcthane photorearrangement can also occur in the solid state. Of over 20 symmetrical and unsymmetrical dialkyl 9,10-ethanoanthracene-l 1,12-decarboxylate 22, only two compounds were found to undergo absolute asymmetric di-ir-methane... 

The concept of absolute asymmetric synthesis using a chiral crystal was applied to unimolecular photochemistry, and now many fine examples are reported of... 

All of the systems reviewed herein display little or no net unimolecular photochemistry, so we will neglect kp and define the lifetime in the absence of quenchers as t0. Therefore,... 

Nonradiative Deactivation Involving a Second Excited State. A somewhat different situation is presented by the pressure effects reported for the MLCT emissions from the ruthenium(Il) complex Ru(bpy)f+. At ambient temperature, in a fluid solution this species shows little unimolecular photochemistry and relatively small emission quantum yields (ff>r < 0.1) [32]. Initial pressure studies on the luminescence of this ion in 18°C aqueous solution detected little sensitivity to pressure [60], as might be expected for a weakly coupled nonradiative mechanism owing to the low compressibility of water. However, detailed studies by Fetterolf and Offen [32,61] painted a more complex picture. These workers probed the temperature dependence of AF and confirmed the small negative value at low temperature but also demonstrated a remarkable temperature dependence for this parameter. 

The photochemistry of alkenes and dienes has already been mentioned in connection with the principles of orbital symmetry control in electrocyclic and cycloaddition processes in Section 13.2. Cycloadditions are considered, from a synthetic viewpoint, in Chapter 6 of Part B. This section will emphasize unimolecular photoreactions of alkenes and dienes. 

The spectroscopy, structure, photochemistry, and unimolecular reactions of allyl radical have been studied extensively and reviewed recently.145 Possible dissociation channels of allyl radical, their energetics, and the potential energy barriers of the C3H5 system are shown in Figs. 20 and 21.145,146... 

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. 

The relative ease with which lasers can produce high concentrations of excited states can be important in initiating multi-molecular photochemistry. It is trivial to produce 0.1 M or greater photon "concentration" in a 1 y volume over a 10 ns period of time. Subsequent multimolecular reactions of excited states or labile photofragments are limited principally by the unimolecular lifetimes involved. 

The above processes describe the possible unimolecular reactions occurring after photoexcitation of aldehydes. For assessing the effect of solvation on the photochemistry, the following model systems have been used. Pinonic acid has been studied with one and five water molecules, and a pentanal cluster using five identical pentanal molecules has been built. 

D. Shemesh, S. L. Blair, S. A. Nizkorodov, R. B. Gerber. Photochemistry of aldehyde clusters cross-molecular versus unimolecular reaction dynamics, Phys. Chem. Chem. Phys., 16 23861-23868 (2014). 

Rate constants for photophysical unimolecular radiative processes can be obtained from spectral data and fcJSC and kjsc computed therefrom. The rate constants for radiationless processes are important parameters in photochemistry because the lowest singlet and triplet states are seats of photochemical reactions. 

In the first place, we shall take a look at the recent advances in fast reaction photochemical kinetics and spectroscopy, in particular at picosecond laser flash photolysis and femtosecond observations. Next, photophysics and photochemistry in molecular beams will be considered. Here observations are made under single molecule-single photon conditions, and these experiments provide insight into the most fundamental unimolecular gas phase reactions. 

The stilbenes have played a crucial role in the development of modern photochemistry. Direct or triplet sensitized irradiation of trans-stilbene (t-1) in dilute solution results in isomerization to cis-stilbene (c-1) as the exclusive uni-molecular photochemical reaction (1-3). Direct irradiation of c-1 results in isomerization to both t-1 and trans-4a,4b-di-hydrophenanthrene (2), which revert to c-1 both thermally and photochemically and can be trapped by oxidants such as iodine or oxygen to yield phenanthrene (3) (4-6). Triplet sensitized irradiation of c-1 yields only t-1. These unimolecular isomerization pathways are summarized in eq. 1. 

A true unimolecular reaction is induced by electromagnetic radiation. That is, only one molecule takes part in the reaction and the energy is provided by the electromagnetic field. In fact, chemical reactions induced by electromagnetic radiation form such an important subfield of chemistry that it has its own designation photochemistry. 

The photochemistry of 1,3-dienes can be highly dependent on the diene structure and reaction conditions. Important variables include the ground state conformation [22,23], the reaction concentration, the use (or not) and properties of a triplet sensitizer [14] or an electron acceptor [18], and solvent polarity. The simplest dienes also often yield the most complex chemistry. For example, 1,3-butadiene 3 undergoes unimolecular isomerization in dilute solution to give only cyclobutene 4 and bicyclobutane 5 (Sch. 2), and polymerization in concentrated solution [24]. At intermediate... 

The absorption of a quantum of light by a molecule in the gas phase may initiate a unimolecular decomposition or rearrangement process. The potentially intimate relationship between photochemistry and nonequilibrium unimolecular reaction theory has yet to be realized, since most of these photoprocesses take place on electronically excited poten-... 

New high-yield routes to frani -Cp Re(CO)2H2 have permitted a detailed examination of the photochemistry of this species (Scheme 1). Noble gas matrix and solution photolyses in cyclohexane and Xe establish the primary photoprocess of tra i -Cp Re(CO)2H2 to be unimolecular, nondissociative isomerization to the cis isomer. In solution, slow cis to trans isomerization is observed. Reductive elimination of H2 is only observed from the cis isomer. " ... 

In this chapter we will discuss the basis for the relationship between reactivity in a mass spectrometer and reactivity under thermal or photochemical activation. We will present an empirical guide which may be useful in predicting relationships between the three types of unimolecular reactivity. In the hght of this analysis, we will review the cases where the reactivity relationships appear to have broken down and finally, we will review the successful examples of the relationship between mass spectrometry and thermochemistry, and mass spectrometry and photochemistry. 

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