Photoproduct distributions

If the rates of intersystem crossing vary with conformation, the problem of correlating photoproduct distributions and the relative abundances of equilibrated BR conformers is far more complex. Scaiano favors this view and theoretical considerations offer some support for it [262], However, Caldwell and co-workers [263] have collected inferential experimental evidence to the contrary. Wagner [264] has advanced the notion that bond formation and intersystem crossing are not discrete steps, but are, in fact, coupled in one process. The question remains unsettled but renders tentative any rationalization of the effects of confining media on Type II reactions offered by ourselves or others. 

Recently, Ramamurthy and Weiss and their coworkers reported the photo-Fries rearrangement of three 1-naphthyl phenylacelates (Fig. 36) in cation-ex-changed zeolite Y and high-density polyethylene films [193], When the substrates were irradiated in hexane to <30% conversion, the eight photoproducts in Fig. 36 were detected. The photoproduct distributions from polyethylene or a Y-zeolite are drastically different from those in solution. Cage-escape products (54 and 55) are absent in both constrained media, and in zeolite Y, only 49 was detected. The... 

First attempts to investigate the photodissociation dynamics of Fe(CO)5 used molecular beam technology coupled with high intensity femptosecond lasers [47, 48]. It is important to note that these experiments relied on multiphoton absorption to populate the electronic excited states of Fe(CO)5. This work built on the results of earlier experiments using nanosecond pulsed lasers which provided information on the photoproduct distribution and their energies [37-40, 49-56]. The energies of the various dissociation processes for Fe(CO)5 are presented in Fig. 18 for comparison with the excitation photon energies and the absorption profile of Fe(CO)5. 

TABLE 13.2 Photoproduct Distributions from Irradiations of 2-NA and 2-NM in Hexane and in PE Eilms at Room Temperature... 

FcAb depends not only on k and / dAB but also on co- Central to the results from the steady-state and laser-pulsed determinations of the cage effect. Equation 13.21 predicts that the tme cage effect (Fcab) and that obtained from photoproduct distributions (Fc) will be the same only when special conditions are met. In principle, there are two practical limiting conditions for Fc. ... 

Further studies on the the solution behavior have been carried out on p-PDA Et37 395. In a concentrated solution of p-PDA Et in acetonitrile, the photoproduct distribution varies with irradiation wavelength375, that is on irradiation at > 300 nm the photoreaction results in a diversity of products due to the excitation of the complexes and isolated p-PDA Et molecules385. On the other hand, the distribution is controlled by irradiation with ligth of wavelength > 340 nm that excites only the ground-state complexes. These complexes are only formed in highly concentrated monomer solutions and facilitate p-PDA Et excimer formation in the excited state via which stereoselective photodimeriza-... 

In addition to the spectroscopic changes observed for cations 1 and 2 in the AOT reversed micelle, significant differences in the distribution of photoproducts are observed relative to that in homogeneous solution (Tables 2 and 3). At large co values, the photoproduct distribution is similar to concentrated aqueous acid solution with the cis isomer and the a dimer as the principle photoproducts. However as a> is decreased, the p dimer becomes the predominant photoproduct and the ratio of p/a reaches a maximum value at 10 for 4-stilbazolium and at -15-20 for 2-stilbazolium. Formation of the 8 dimer from cation 1 follows the same trend as the p dimer the amount of 8 dimer formed increases as the water pool size is reduced. Decreasing the ratio of [HStz]/[AOTl appears to have an effect similar to increasing on the photoproduct distribution. These results clearly demonstrate that the size of the water pool and/or... 

The absence of or low levels of p dimer formed from cations 1 and 2 in the film was disappointing. Attempts to increase or reduce the hydration of the film had little effect upon the photoproduct distribution. Although more than 90 percent of cation 1 is extracted by the pyridinium solution, it is possible that some of the dimers formed within the film are not being removed. A control study was done in which a known distribution of dimers was added to the support and then re-extracted. Greater than 75 percent of the dimers were extracted and the distribution of dimers remained unchanged. The presence of stilbazolium "excimer"... 

Solution photolysis studies were also used to study the effect of a number of stabilizers on the efficiency of the photoinitiator during cure. Both the amount of the photoinitiator decomposed and the photoproduct distribution were analyzed (Table IV). In the absence of antioxidant, 47% of the photoinitiator is converted to photoproducts. Several of the stabilizers (HD-2, PD-2 and PD-3) decrease the amount of DMPA converted while three other stabilizers (HD-1, PD-1 and PD-4) increase the amount of DMPA decomposed. In all cases residual photoinitiator capable of catalyzing additional crosslinking or oxidation reactions remains. 

Photoproduct distribution is also affected by the stabilizers. A marked decrease in the methyl benzoate/DMPA ratio from 0.52 to less than 0.17 occurs in the presence of all of the... 

TABLE 7 Photoproduct Distribution from Irradiation of Naphthalene with Triphenylsulfonium Triflate 94)... 

Two basic types of experiments can be preformed with the TOF-MS that will reveal information about the correlated photoproduct distribution. One set of experiments involves the use of the TOF-MS as a velocity analyzer. In this chapter we call this technique as the extraction time-of-flight method. Another type of experiment uses the TOF-MS in its mass resolution mode in conjunction with a two-dimensional ion detector. This technique is known as ion imaging. Variations of these techniques have been developed and are discussed in Section III.B.l.c. Experimentalists have used these methods to measure scalar and vector correlations in many types of photodissociation reactions their results are presented in Section III.B.2. 

The photoproduct distribution is strongly affected by CD in the photo-Claisen rearrangement of m-alkoxyphenylallyl ethers 95 (Scheme 17), where significantly higher orthpselectivity was obtained as compared to water or... 

The photoproduct distribution of P-CD inclusion complexes of N,N-dialkyl a-ketoamides in the solid state exhibits stereoselectivity [293]. Regio-and enantioselectivity were observed in the case of the N-benzyl-iV-methyl-pyruvamide (R = CHj, R]=H, R2 = Ph, R3 = CHj in Scheme 23) [293]. 

Table I Photoproduct Distributions from Ex-situ GC Analysis...

Integrating both Equations we see that in the second case the chain growth smears out the photoproduct distribution for a certain amount. This effect should be stronger if LI A approaches 1. For the quotient of both polymer modulations we get the ratio d. 

The Norrish type II reaction of various phenyl alkanones in zeohtes has been studied by Ramamurthy and his collaborators. " The size and shape of zeohte reaction cavities control the behavior of triplet ketone as well as the 1,4-biradical intermediate that is consequently monitored as a difference in photoproduct distribution and triplet lifetimes. Norrish type I and type II reactions of 2-pentanone included within cavities of various types of zeohtes and the alkah metal cation-exchanged ZSM-5 zeolite have been investigated by experimental and theoretical approaches. " Ion-exchanged cations had significant effects not only on the adsorption state but also on the photochemical reactions of the ketone. The... 

We observed that the irradiation of a-iodoketones in hexane under a nitrogen atmosphere with a high-pressure mercury lamp (k > 300 nm) at room temperature yields the corresponding a,P-unsaturated ketones, accompanied by some deiodination products. Moreover, it was shown that the reaction was dependent upon the media used, and such changes resulted in different photoproduct distribution. 

In the case of hexane containing a small amount of water, the irradiation of 6 gave 6e, 6f, and an unexpected substitution product, 2-hydroxycyclododecanone (6a) (Table 56.12, entries 5 to 10). When 6 was irradiated under a commercial fluorescent lamp, the yields of photochemical products 6a, 6e, and 6f were almost invariable (Table 56.12, entry 10). Apparently, the photoproduct distribution is due to... 

Direct Measurement of Deprotonation Rates Relative Rates of Aminium Radical a-CH Deprotonation Based on Photoproduct Distributions Desilylation Reactions of a-Trialkylsilylaminium... 

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