States photostationary

Equation (12-17) is called the photostationary state expression for ozone. Upon examination, one sees that the concentration of ozone is dependent on the ratio NO2/NO for any value of k. The maximum value of k is dependent on the latitude, time of year, and time of day. In the United States, the range of k is from 0 to 0.55 min T Table 12-5 illustrates the importance of the NO2/NO ratio with respect to how much ozone is required for the photostationary state to exist. The conclusion to be drawn from this table is that most of the NO must be converted to NO2 before O3 will build up in the atmosphere. This is also seen in the diurnal ambient air patterns shown in Fig. 12-2 and the smog chamber simulations shown in Fig. 12-3. It is apparent that without hydrocarbons, the NO is not converted to NO2 efficiently enough to permit the buildup of O3 to levels observed in urban areas. 

O3] Predicted tram Photostationary State Approximation as a Function of Initial [N02f... 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

The photoisomerization of aromatic rings has also been studied using 1,3,5-tri-t-butylbenzene. The composition of the photostationary state is shown below ... 

Compound 6 is a pivotal intermediate in Schreiber s synthesis. It was hoped that the conspicuous and strained bridgehead cyclobutene substructure in 6 would undergo a conrotatory electrocyclic ring opening upon thermolysis to give an isomeric 1,3-diene (8, Scheme 1). In the event, when a solution of cyclobutene 6 in toluene is confined to a sealed tube and heated to 180°C for 12 h, a stereoisomeric mixture of 1,3-dienes 7 and 8 is produced in an excellent yield of 95% (7 8 ca. 5 1). Finally, irradiation of the 5 1 mixture of cis-7 and trans-8, or of each independently, establishes a photostationary state in which the desired trans isomer 8 predominates (8 7 ca. 10 1). 

An equilibrium is established that gives rise to a so-called photostationary-state relation, which depends on the relative rates of the above reactions ... 

The relatively high concentrations of HONO observed during the initial period can be attributed to high OH and NO levels, with HONO being in photostationary state due to its rapid photolysis (11). 

The sudden consumption of the remaining UDMH, and the increased relative importance of N-nitrosamine formation at -30 minutes into the photolysis can be rationalized by assuming that at that time the [NO I/ENO] ratio, and thus the photostationary state [O3], has become sufficiently high that O3 may be reacting with the hydrazine directly, and that reaction (3) begins to dominate over reaction (10). This results in higher rates of UDMH consumption by the OH radicals formed in the UDMH + O3 reaction (1), and by the OH radicals generated by the reaction of NO... 

Any mechanism which involves isoenergetic, radiationless internal conversion from C, P, or T to a high vibrational level of the ground state would be expected to show a large deuterium isotope effect on the rate of internal conversion. In the direct photolysis of perdeuterio and perhydrostilbene, Saltiel<8a) found no isotope effect on the photostationary state or upon the quantum yields of cis-to-trans and trans-to-cis conversion. 

The 2,4-hexadienes (17)—(19) are of interest because, if a common triplet intermediate (like X, a 1,4 diradical) is involved, the sum of all six quantum yields should be equal to 2, and the individual quantum yields should be predictable from the composition at the photostationary state<83 64) ... 

Stilbene is known to undergo cis-trans isomerization in its triplet state to yield a photostationary state which is 60% cis and 40% irons,... 

Also, the photostationary state can be predicted from the ratio of the quantum yields ... 

The photostationary state composition for the benzophenone-sensitized isomerization of 2,4-hexadienes is given in Table 9.2. Table 9.3 gives the measured quantum yields for benzophenone-sensitized isomerization of 2,4-hexadienes along with the calculated quantum yields based on Eqs. (9.47)-(9.49) and the pss values given in Table 9.2. 

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

In one of the earliest applications of this type of process to complex molecule synthesis, Corey and Hortmann, in their synthesis of dihydrocostunolide 38, found that photolysis of 36 afforded a photostationary state of 36 and 37 (Scheme 9)19. Hydrogenation of this mixture then gave 38. A recent modification of this synthesis, which avoids the photostationary equilibrium between eudesmane (36) and germacrane (37) forms, was realized using a modified substrate, 3920. Irradiation of 39 provided a 77% yield of a mixture of diastereomeric ketones 41 these are produced via tautomerization of the intially produced trienol 40. Dienone 41 was then easily converted to 38 via a series of conventional steps (Scheme 9). 

Urocanic acid (2-propanoic acid 3-[lH-imidazol-4-yl] is located superficially in the stratum comeum. Metabolism of epidermal UCA does not occur in situ due to the absence of urocanase, resulting in the accumulation of UCA in the epidermis. Upon UV exposure, naturally occurring trans-UCA converts to the d.s-isomer, in a dose dependent manner, until the photostationary state is reached, when equal quantities of trans- and m-UCA are found in the skin.15 Based on an analysis of the action spectrum for UV-induced immune suppression, and the fact that no immune suppression was observed in mice whose stratum comeum was previously removed by tape stripping, De Fabo and Noonan suggested that urocanic acid was the photoreceptor for UV-induced immune suppression.16 Since the initial experiments many others have documented, the ability of ris-UCA to initiate immune suppression, documented its presence in the serum of UV-irradiated mice, and demonstrated that m-UCA plays a role in UV-induced skin cancer induction. (For a more complete review of the role of m-UCA in immune suppression see two excellent reviews by Norval and colleagues.1718)... 

Similar isomerizations of clopenthixol (354) and chlorprothixene (355) were observed but, unlike flupenthixol (353), the photostationary states of these mixtures differed significantly in their isomer ratios from those of the drugs as normally supplied. It was pointed out that the photoisomerizations could affect their potencies [195]. 

Explain the dependence of the composition of the photostationary state on the wavelength of radiation used for irradiation. 

This ratio is called the photostationary state composition. In the photostationary state, the rate of formation of each isomer from the nonvertical excited state is equal to its rate of removal by absorption of light. There is a roughly equal probability of the relaxation of the nonvertical excited state forming either the cis or the trans isomer and so the main factor influencing the photostationary state composition is the competition for absorbing light. This will, of course, depend on the relative values of the molar absorption coefficients of the two isomers at the particular wavelength chosen. 

Consider the absorption spectra of the two geometrical isomers, shown in Figure 8.3. The photostationary state obtained on direct irradiation depends principally on the wavelength of light used, because of competitive absorption by the two ground-state isomers. If a wavelength is chosen that is absorbed more strongly by the trans isomer than by... 

Consider the photosensitised cis-trans isomerisation of stilbene The two isomers have different triplet energy levels (247kJmoT1 for the cis and 205kjmol-1 for the trans). The proportion of the cis isomer in the photostationary state varies with the energy of the photosensitiser (ET), as shown in Figure 8.5. 

Figure 8.5 Effect of the sensitiser triplet energy on the percentage of cis-stilbene in the photostationary state...

This reaction can occur either thermally or photochemically, with the equilibrium position (thermal reaction) depending on the thermodynamic stability of the reactant and product, and the photostationary-state composition depending on the relative values of the absorption coefficients at the wavelengths used. 

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