Photolysis duration

One of the most important teclmiques for the study of gas-phase reactions is flash photolysis [8, ]. A reaction is initiated by absorption of an intense light pulse, originally generated from flash lamps (duration a=lp.s). Nowadays these have frequently been replaced by pulsed laser sources, with the shortest pulses of the order of a few femtoseconds [22, 64]. 

Nonyl aldehyde (32.66 g, 0.23 mol) and furan (200 mL, 187.2 g, 2.75 mol) were mixed in a 250-mL photolysis flask equipped with a quartz immersion well containing a Vycor filter and a 450-W Hanovia Lamp. The system was kept at -20° C with an isopropyl alcohol bath cooled by a Cryocool Immersion Cooler (CClOO). Nitrogen was bubbled throughout the duration of the reaction, and the solution was stirred vigorously. Additional furan (150 mL, 140.4 g, 2.06 mol) was added during the course of the reaction. TLC analysis indicated completion of the reaction after 20 h. After evaporation of excess furan and NMR analysis of the resultant oil (48.70 g, ca. 100%) indicated the desired photoadduct had been formed, without contamination from unreacted nonyl aldehyde. 

Flash photolysis of benzoyl and naphthoyl diazomethane, which should exist in the s-cis conformation, led to ketene intermediates within the duration of the pulse ( 20 ns).241... 

A very interesting technique for radical generation is flash photolysis, which employs a very intense pulse of radiation (visible or u.v.) of very short duration. This produces a very high immediate concentration of radicals, which may be detected—and whose fate may be followed—by spectroscopy through one or more subsequent pulses of lower intensity radiation of suitable wavelength. This is, of course, primarily a technique for the study of radicals rather than for their use in preparative procedures. Radicals may also be generated, in suitable cases, by irradiation of neutral molecules with X-rays or with y-rays radiolysis. 

Vary fast reactions, both in gaseous and liquid phases, can be studied by this method. In flash photolysis technique, a light flash of very high intensity and very short duration ( 10 6 sec) is produced in the neighborhood of the reaction vessel. This produces atoms, free radicals and excited species in the reaction system. These species undergo further reactions which can be followed by spectroscopic means. The method is also known as kinetic spectroscopy. The first order rate constant as large as 105 sec-1 and second order rate constants as large as 1011 mol dm sec-1 can be measured by this technique. 

With the invention of the laser in 1960 and the subsequent development of pulsed lasers using Q-switching (Chapter 1), monochromatic and highly-collimated light sources became available with pulse durations in the nanosecond timescale. These Q-switched pulsed lasers allow the study of photo-induced processes that occur some 103 times faster than events measured by flash lamp-based flash photolysis. 

Fig. 3.16 Time resolved ir spectra obtained by uv flash photolysis of [CpFe(CO)2l2(14) (0.6 mM) and MeCN(6mM) in cyclohexane solntion at 25°. Only 5% of 14 is destroyed by the flash so that the concentration of 16 < 14. The spectra have been reconstituted from <=70 kinetic traces recorded at intervals of 4 cm from 1750 cm to 1950 cm . The first three spectra correspond to the duration of the firing of the flash lamp and subsequent spectra are shown at intervals of 10 ps. The negative peaks in the first spectrum (subsequently omitted) are due to material destroyed by the flash. Reproduced with permission from A. J. Dixon, M. A. Healy, M. Poliakoff and J. J. Turner, J. Chem. Soc.

Photolytic. Mathew and Khan (1996) studied the photolysis of metolachlor in water in the presence of kaolinite, montmorillonite, and goethite and fulvic acid under neutral and acidic conditions at 22 °C. Metolachlor degraded in all the treatments at both pH conditions. The rate of photolysis and degradation products formed was dependent on the duration of UV exposure, the initial pH of the solution, and the composition of the suspended/dissolved material. The following photoproducts identified included 2-hydroxy-A-(2-ethyl-6-methylphenyl)-A-(2-methoxy-l-meth-ylethyl)acetamide, 4-(2-ethyl-6-methylphenyl)-5-methyl-3-morpholine (major product forming at 74-84% yield), 8-ethyl-3-hydroxy-A-(2-methoxy-l-methylethyl)-2-oxo-l,2,3,4-tetrahydroquino-line, 2-chloro-A -(2-(l-hydroxyethyl)-6-methylphenyl)-7V-(2-hydroxy-l-methylethyl)acetamide, and 2-chloro-A -(2-ethyl-6-hydroxymethylphenyl)-A-(2-methoxy-l-methylethyl)acetamide. 

The fact that several lasers can generate very short light pulses (down to 10 sec duration) with high peak powers (up to 10 watts) which can be used to investigate short term transitions (e. g. lifetime measurements, flash photolysis, etc.). 

At very short times, very little motion of reactants has occurred so that little, if any, reaction will have taken place. But the manner of creation of the mixture of A and B reactants should be considered. A very simple means of preparing a reaction mixture is by photolysis. For instance, consider a solution of anthracene and carbon tetrabromide. Photostimulation of anthracene with an extremely short duration light pulse produces excited singlet (and triplet) states. The carbon tetrabromide quenches the excited singlet state fluorescence very efficiently. Just before the photostimulation event, the quencher (i.e. B) is randomly distributed throughout the system volume and for a short time after photostimulation, it remains randomly distributed. With the exception of the location where the fluorophor A is, there is no preferred location of the quencher B. No... 

Triplet states for naphthalene, anthracene, and other aromatic compounds had been identified by absorption spectroscopy mainly with the aid of flash photolysis by G. Porter and his co-workers.22 Although a triplet state of benzene had been identified in a glassy matrix and had been associated with a long-lived emission of 10 sec or more duration,5 no evidence for the existence of this state by spectroscopic means had been produced until recently.23 Thus it has been known for some time that benzene in a glassy matrix when irradiated at wavelengths around 2500 A produces molecules which cross over to a triplet state with a relatively high probability. 

A pulsed-laser photolysis study of [Cr(en)3]3+ illustrates quite dramatically the enhancement in reactivity that can result upon populating a ligand field excited state.26 Thus a significant fraction of the primary photoproduct, [Cr(en)2(enH)(OH2)]4+, is formed within the 20 ns duration of the laser pulse and is thought to arise from reaction of the lowest excited quartet state, Q° (see Figure 6). This observation establishes that the pseudo-first-order rate constant for this excited state... 

Photolysis of 413 afforded a trace of dibenzotrithiepin 47 along with 13% yield of 48 (Scheme 87). The sodium borohydride reduction of 413 afforded 414. Photolysis of 414 in CH2CI2 for different time durations afforded different amounts of 47 and 48, the amount of 47 decreasing with increasing irradiation time for example, when the irradiation was done for 12, 24, and 72 h, the ratio of 47 and 48 was found to be 34 32%, 10 63%, and 8 22%, respectively <2000TL1801>. 

When trans-RhCl(CO) (PPhQ in degassed benzene was subjected to flash photolysis (X3rr > 315 nm, pulse duration about 20 ysec), transient absorption was observed with the spectral characteristics illustrated in Figure 1. This transient (when monitored at Xmon 410 nm) decayed via second order kinetics over a period of several ms (Figure 2). When the solution was flashed under CO (1.0 atm,... 

Flash photolysis of RhCl(C0(PPh3>2 under ethylene (0.01 atm, 0.0011 M) led to immediate spectral changes consistent with the formation of the ethylene complex(3) RhCl(H2C=CH2)(PPhj), within the duration of the flash. This observation provides a lower limit of 2 x 107 M- -s-- - for the second order rate constant for the reaction of RhCl(PPli3)2 with ethylene. The back reaction of the ethylene adduct with CO to reform RhCl(CO) (PPl was also rather rapid and occurred within a period of a few milliseconds. 

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