Photolysis Experimental Procedure

Interestingly, photolysis of phenyl azide in liquid ammonia yields 3//-azepin-2-amine (39)35 (see experimental procedure in Houben-Weyl, Vol.4/5b, pi268). 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Experimental Procedure 2.2.1. Photolysis of a Chromium Carbene Complex 2-Benzyl-4-benzyloxy-4-methyl-2,3,4,4a,7,7a-hexahydro-li/-cyclopenta[c]pyri-din-3-one [294]... 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

The second type of experimental procedure to observe the spectra of transients uses flash photolysis as illustrated by the work of Porter and Windsor (122). The exciting light source consists of a flash resulting from the rapid discharge of a spark through a circuit having a large capacitance. This first flash pumps the molecules into the excited triplet... 

In a convenient experimental procedure, nitrogen heterocycles 3 are alkylated by a mixture of a carboxylic acid 4 and [bis(trifluoroacetoxy)iodo]benzene in boiling benzene or under irradiation in dichloromethane at room temperature (Scheme 2) [11, 12]. A similar procedure has been used for the stereoselective synthesis of C-nucleosides and their analogs via photolysis of the gulonic acid derivatives, (diacetoxy)iodobenzene, and the appropriate heteroaromatic bases [13]. 

Surprisingly, little new research on the prototypal tung-sten-arene complex see Arene Complexes), r] -Ceih)f, has appeared, possibly because of the low-yielding and elaborate experimental procedures required for its synthesis. Photolysis of W(CO)6 in the presence of ethyne leads to the formation of benzene and rf-CdRf) W(CO)3 (101), and the solid-state molecular structure of this complex was... 

Typical experimental procedures for a vapor-phase irradiation and for a solution-phase photolysis are given. Applications of the method that led to synthetically interesting yields or to products difficult to obtain by other methodology are summarized in Table 10. 

Although it may seem a trivial procedure, it is actually very difficult to determine the exact number of photons absorbed by the PAG, because there are other resist components that may absorb in the same region as the PAG, and more importantly, some of the photoproducts obtained through PAG photolysis absorb in the same region as the parent molecule. The second parameter needed to calculate the quantum yield is the number of moles of acid produced. There have been numerous reports on experimental procedures employed to detect and quantify the amount of acid produced. 

The experimental procedure was as follows. Isobutylene was expanded into a glass vacuum system and condensed into the reaction cell with liquid N2. FV relations were used to calculate the amounts taken. Photolysis was usually carried out for 120 minutes although times as short as 30 minutes were also used. After photolysis the cell was warmed to 0°C. and the isobutylene was expanded back into a vacuum system of known volume. The remaining high molecular weight products in the reaction cell were dissolved in 1 ml. of hexane and subjected to gas chromatography. 

The primaiy quantum yield is associated with the primary photochemical event in the overall photochemical proce.ss which may involve secondary events as well. An example that illustrates both kinds of events is the photolysis of HI described in Section 23.8(a). The primary quantum yield is defined as the ratio of the number of primaiy events to the number of photons absorbed (eqn 23.28) and its value can never exceed one. However, in reactions described by complex mechanisms, the overall quantum yield, which is the number of reactant molecules consumed in both primaiy and secondary processes per photon absorbed, can easily exceed one. Experimental procedures for the determination of the overall... 

Using the experimental procedures discussed for the simpler nitrites in the preceding sections, Zabarnick and Heicklen (1985c) measured the quantum yields of iso-CsHyCHO formed in the photolysis of mixtures of iso-C4H90NO (4.6Torr) and NO (30 mTorr) at 23°C. The measured quantum yield, o-C3H7Cho = 0.064 0.005, was essentially constant as the total pressure was varied from 4.6 to 150 Torr by nitrogen addition. There is no measurement of the rate coefficient ratio of ki/k2. 

While the relative importance of the various paths is not well established, it is expected that dissociation to the alkoxy radical, RO, and N02 will predominate. Luke et al. (1989) experimentally measured rates of photolysis of simple alkyl nitrates and compared them to rates calculated using the procedures outlined in Chapter 3.C.2. Figure 4.22 compares the experimentally determined values of the photolysis rate constants (kp) for ethyl and n-propyl nitrate with the values calculated assuming a quantum yield for photodissociation of unity. The good agreement suggests that the quantum yield for photodissociation of the alkyl nitrates indeed approaches 1.0. 

The reaction rate for the photolysis rate of H2O2 (R8) was calculated using data from previously published laboratory experiments of photolysis reactions of NOs and H2O2 in artificial snow for comparable experimental conditions (Table 1). Therefore, the obtained experimental rate constant of 0.48 h for the H2O2 photolysis was divided by a factor of 400 similar to the procedure for the photolysis rate of NOs as described in Jacobi et al. The HCHO photolysis reaction in snow is probably negligible under natural conditions and is not included in the reaction mechanism. 

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