Secondary photolysis

How long should the irradiation be allowed to proceed Overirradiation may produce secondary photolysis products as a result of primary product absorption. It is best in cases where the products are likely to absorb to monitor the reaction as a function of time. 

Barrau and coworkers have synthesized a series of iron and ruthenium complexes by irradiation of Me2HGe(CH)KGeMe2H and Me2HGe(CH)K SiMe2H (n = 1, 2) in the presence of Fe(CO)5 and Ru3(CO)i293. In each case irradiation causes CO loss, with the formation of the M(CO)4 species (reaction 43). When n = 2 the products are photostable with n = 1 (65) a mixture of products (66-69) are obtained due to secondary photolysis (reaction 44). The mechanism, outlined in Scheme 23, is presented to explain these observations. 

Some of these problems can be overcome with a different calorimetric design (see later discussion). Other problems, which are more dependent on the chemistry and physics of the process under study than on the instrumentation, require careful attention. Unnoticed side reactions or secondary photolysis are examples, but one of the most serious error sources in photocalorimetry is caused by the quantum yield values, particularly, as explained, when they are small. Unfortunately, many literature quantum yields are unreliable, and it is a good practice to determine n for each photocalorimetric run. Errors in

inner filter effects, that is, photon absorption by reaction products. 

Other error sources discussed for the isoperibol instrument are not a problem in Teixeira and Wadso s microcalorimeter. For instance, as shown by equations 10.15 and 10.16, the radiation wavelength does not influence the precision or the accuracy of the final A rH result. However, the precision is still affected when the reaction quantum yield is low, because the experimental error will be divided by a small value of n. On the other hand, problems like side reactions or secondary photolysis, already mentioned, that are not related to the instrumental design may also lead to large errors. 

A mechanistic scheme involves intramolecular single-electron transfer (SET) from the enamino moiety toward carbon 5 of the ring, and subsequent transannular interaction in zwitter-ionic biradical 259 with formation of stabilized zwitter-ion 260. Opening of the oxetane ring in 260 leads to cyclobutene 256, while secondary photolysis of the latter gives olefin 257 and alkyne 258 (Scheme 100). 

The photosensitivity of all six forms was determined qualitatively, and the results are in Table III. The H2T and HT systems were difficult to work with because their high thermal lability made the dark reaction correction large. An additional complication was the secondary photolysis of cis isomer, which made it doubtful if the aquo product observed in the case of irradiated T was caused by primary photoaquation. The results in Table III are, therefore, qualitative and are displayed to provide a general picture of the situation. 

In this experiment, no carbon tetrachloride was detected but, on the basis of the mechanism proposed, carbon tetrachloride would have been an important reaction product, arising by the combination of chlorine atoms and trichloj-omethyl radicals. Further, the production of octachloropropane by the secondary photolysis of octachlorobutanone would involve the formation of decachlorobutane. In the presence of chlorine, only carbon tetrachloride was formed, whereas by interaction of the postulated CCI3CO- radical with chlorine, substantial amounts of trichloroacetyl chloride should have been observed. In the presence of oxygen, only carbon dioxide and phosgene were found and the yield of the latter was far too small to account for the loss of the radical products. 

Formic acid, methyl formate, and CO were detected when photoreduction was performed in Ti silicalite molecular sieve using methanol as electron donor.173 Mechanistic studies with labeled compounds indicated, however, that CO originates from secondary photolysis of formic acid, whereas methyl formate emerges mainly from the Tishchenko reaction of formaldehyde, the initial oxidation product of methanol. 

The intramolecular cyclization dominates other possible reactions even in solvents highly susceptible to free radical attack (110). Thus, irradiation of 4,5-octanedione in butanal gives 2-hydroxy-3-methyl-2-propylcyclobutanone in 92% yield (110). Irradiation of 1,2-cyclo-decanedione (Formula 264) gives 1-hydroxybicyclo [6.2.0 ]decan-10-one (Formula 265) in 74% yield and cyclooctanone (Formula 266) in 9% yield (111). The cyclooctanone is produced together with ketene by a secondary photolysis of Formula 265 (111). 

Dornhofer, Hack, and Langel (180), in a detailed study of the fluorescence that is induced by an ArF laser, have been able to show that an intense ArF laser can distort the observed vibrational distribution by photodissociating CS radicals with v" > 5. The ArF laser absorption by CS will also produce electronically excited CS which, when it emits, will redistribute the vibrational populations. Probing the CS quantum state population under these conditions could distort the CS ground state populations. The LIF measurements will underestimate the amount of CS radicals that are produced, while the direct detection methods will overestimate the amount of S(3p) atoms because of the secondary photolysis of CS. The vibrational distribution of Lu et al. (178) will be less prone to this secondary photolysis because very low laser powers (< 1 mj) were used. Dornhofer, et al. concluded from their results that the S(3p)/S(J-D) ratio was 3, which is in reasonable agreement with the LIF measurements of Lu et al. 

Solutions were flowed through a suprasil flat cell (0.1 mm thickness) at rates between 0.1 - S.O mL/min in order to minimize any interference from signals produced by secondary photolysis of products. Time-resolved polarization evolution profiles for the formation and relaxation of the polarized radicals were measured at a constant magnetic field and monitored by both a Hitachi 40 MHz digital oscilloscope and a Stanford Research Systems gated integrator/boxcar averager at 5 ns resolution, and both coupled to a 486 PC desk-top microcomputer for analysis. 

Methyl and 10-phenyl thioxanthenium salts can be deprotonated at the benzylic group either by base or photolytically to generate the stable deep orange ylide or thiaanthracene. Secondary photolysis results in the conversion to the 9-substituted thioxanthene (Scheme 132) <1997CC709>. 

Figure 3-19. Photodissociation of HI monomers and clusters. The solid traces indicate the substantial discrimination available when using polarized photolysis radiation note the high S/N. Under conditions of such minimal clustering, it is reasonable to assume that most of the clusters are binary. Peaks labeled v = 1 and v = 2 are due to inelastic H + HI collisions within the cluster. The superelastic peak ft is assigned tentatively to secondary photolysis of I HI complexes, in which the escaping hydrogen deactivates the nearby I, (a) Vertical and (b) horizontal polarization of the photolysis radiation relative to the molecular beam. The plenum pressure is 1900 torr.

These have been extensively studied by Neckers and coworkers [21]. Those which undergo y-hydrogen abstraction cleave cleanly to aldehyde or ketone, and thus provide a possible methodology for environmentally friendly oxidation of alcohols. The secondary photolysis of the hydroxyketene forms benzaldehyde and carbon monoxide, both of which could be considered nuisances, although it might be possible to trap the carbene intermediate in order to make a 1-phenylcyclopropanol. 

In another example, direct irradiation of compound 38 yields cyclopropane 39 and the thiopyrane derivative 40, arising from secondary photolysis of 39 (Sch. 13). The formation of 39 is rationalized based on a di-7r-methane rearrangement involving the participation of two benzo [6] thiophene rings present in 38 [29]. 

The ratio N20/03 was found by DeMore and Raper to approach zero at low percent reaction. This is a very strong indication that secondary photolysis of ozone at 2537 A. dissolved in liquid nitrogen is responsible for N20 formation. 

The other products may contain CH4, whose quantitative analysis, however, was not performed. The same secondary photolysis was found to proceed in the irradiation of the C2H6/02 mixtures as reported before [1],... 

Peroxynitrous acid, ONOOH, forms in another photochemical channel at shorter wavelengths but is absent at k > 300 nm. The O-atoms generated in reaction 99 may react with 02([02 Water 0.3 mM) via reaction 100 or, preferably, with nitrate via reaction 101. Nitrite (smax = 22.5 M 1 cm1 at 360 nm) will undergo secondary photolysis, reaction 102, and oxidation by OH radicals, reaction 103 ... 

The photoelimination reactions seem to present the cleanest progress the products mostly do not absorb at the irradiation wavelength. These reactions are the prime candidates for verification of the kinetic relations [12,40]. In many other cases, photoracemization, side reactions or, if the primary products absorb at the irradiation wavelength, secondary photolysis occur and obscure the kinetics of CD development. The isomerization of E-cyclooctene 42 also is a well-defined reaction. The a vs. time plots show a maximum [103], but a photostationary state will be reached at long times as the Z — E isomerization is also active under irradiation. 

Benzyltrialkylstannanes, for example, quench the fluorescent photoexdted singlet state of 9,10-dicyanoanthracene (DCA) by PET from the stannane to the DCA [33]. Eaton has proposed that the observed products arise from secondary photolysis of an initial adduct between DCA and benzyltrimethylstannane through... 

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