Pulse laser photolysis

Pulsed laser photolysis (PLP) has emerged as the most reliable method for extracting absolute rate constants for the propagation step of radical polymerizations,343 The method can be traced to the work of Aleksandrov el al.370 PLP in its present form owes its existence to the extensive work of Olaj and eoworkers 71 and the efforts of an 1UPAC working party/45"351 The method has now been successfully applied to establish rate constants, /rp(overall), for many polymerizations and copolymerizations. 

Cr(CO)5 interacts with solvent molecules and in solution cannot be considered as naked. The interaction is much weaker with fluorocarbon solvents than hydrocarbon 33). Using a pulsed laser photolysis source (frequency tripled NdYAG) and C7F14 as a solvent, Kelly and Bonneau 33) measured the rate constants for the reaction of Cr(CO)5 with C6H12, CO, and other ligands [Eq. (3)]. 

It was found that the azirine-nitrile ylide isomerization was a completely reversible process. The unlabeled nitrile ylide showed a prominent band at 1926 cm that underwent a 66-cm shift with N substitution. This shift was interpreted as being consistent with an allene-like skeleton (8) rather than the alternative pwpargyl-like stmcture (9). This conclusion was supported by the spectra from the C- and H-labeled variants. Warming the nitrile ylide in a xenon matrix from 12 to 82 K provided no new absorptions suggesting that the allene-like structure may also be adopted in solution. Some absorption spectra for benzonitriho benzylide (DPNY) and some substituted benzonitrilio methylides obtained via pulsed-laser photolysis of azirines are given in Table 7.1 (5). 

Much work has been done since the early 1980s on the detailed investigation of the azirine-nitrile ylide interconversion using pulsed-laser photolysis. Thus the azirines 103 (R =R =Ph, R =H R =Me, R = R =Ph R = p-napthyl, R = Me, R = H), on irradiation in isooctane, gave intense long-hved absorptions (250-400 nm) attributed to the nitrile ylides 104 (44). Quenching studies with electon-deficient alkenes led to the determination of absolute rate constants that were similar to those reported earlier for steady-state trapping experiments. The nitrile ylide-olefin reactions are discussed in more detail in Section 7.3.1. 

Pulsed-laser photolysis of the azirines 158a-c in the presence of electron-dehcient alkenes (44,66) allowed the determination of the bimolecular quenching rate constants (feq) for reactions with acrylonitrile (1.0-5.4 x 10 and... 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

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... 

Growth of long chains (n > 102) in mixed 1 1 crystals of ethylene with chlorine or bromine at 20-70 K has been studied in detail by Wight et al. [1992a, 1993]. Active radicals were generated by pulsed laser photolysis of Cl2 or Br2. The rate constant has been found to be kc = 8-12 s-1 below Tc = 45 K. The chain grows by the radical chain mechanism... 

In the presence of an atmosphere of air, reaction (1) has been studied by several groups using relative rate techniques under conditions of steady photolysis (18-21). and by Hynes et al. QJ) who employed the pulsed laser photolysis of H202 with LIF detection of OH. Inspection of Table I reveals that the direct time-resolved study (12) yielded a significantly lower result for k2. Hynes et al. (17) discuss the likelihood of there being a secondary reaction channel for DMS in the competitive kinetics experiments which leads to the high rate constants. Indeed, this discrepancy is repeated for the cases of OH + CH3SH and OH + CS2 (see Table II). For this reason the rate constants under atmospheric conditions obtained by the direct technique are recommended for all three of these reactions. 

By means of the sub-microsecond time resolution achieved by employing a pulsed laser photolysis/pulsed laser induced fluorescence technique, Hynes and wine are able to obtain k14a, k.14a and k and obtain an overall expression for kj4 as a function of temperature and [O2]- Their result in 700 torr of air is again substantially lower than the result from three competitive studies (Table IV)... 

Tyndall and Ravishankara employed the pulsed laser photolysis of DMDS at x = 248 nm to generate CH3S which was then detected by LIF at a > 371 nm. NO was observed as a product of reaction (18) by LIF at a = 226 nm. Furthermore, these authors observed the formation of a second NO molecule on a longer time-scale than reaction (18), which they attribute to... 

Experimental Techniques A absorption CIMS = chemical ionization mass spectroscopy CK = competitive kinetics DF discharge flow EPR = electron paramagnetic resonance FP = flash photolysis FT = flow tube FTIR Fourier transform intra-red GC = gas chromatography, UF = laser induced fluorescence LMR = laser magnetic resonance MS = mass spectroscopy PLP = pulsed laser photolysis SC = smog chamber SP = steady (continuous) photolysis UVF = ultraviolet flourescence spectroscopy... 

Fig. 1. A schematic view of a pulsed laser photolysis experiment in which O (1Z ) atoms were generated by photolysis of O3 at 266 nm. The reaction...

We have employed a pulsed laser photolysis - pulsed laser induced fluorescence technique to carry out direct, real time studies of OH reactions with DMS and DMS-dfc in N2, air, and O2 buffer gases. Both temperature and pressure dependencies have been investigated. We find that the observed rate constant (kQbs = d[0H]/[0H] [DMS]dt) depends on the O2 concentration. Our results are consistent with a mechanism which includes an abstraction route, a reversible addition route, and an adduct + O2 reaction which competes with adduct decomposition under atmospheric conditions. 

A schematic of the apparatus is shown in Figure 1. OH was produced by 248 nm (or 266 nm in some experiments) pulsed laser photolysis of H2O2 and detected by observing fluorescence excited by a pulsed tunable dye laser. Fluorescence was excited in the 0H(a2e+ - X tt) 0-1 band at 282 nm and detected in the O-O and 1-1 bands at 309+5 nm. Kinetic data was obtained by electronically varying the time delay between the photolysis laser and the probe laser. Sulfide concentrations were measured in situ in the slow flow system by UV photometry at 228.8 nm. 

Rosenfeld et al. (144) have recorded the absorption spectrum of the fluorescent states of retinol, retinyl acetate, and retinyl-n-butylamine, using pulsed laser photolysis. Theoretical calculations (145) have closely reproudced the observed 435-nm band assuming that emission originates from the lowest - Ag state. Unfortunately, these results are not discriminative as far as identification of the lowest singlet state is concerned, since a strong absorption in the same region is also predicted for the 1B+ state (145). 

Isomerization due to ionic photodissociation. The (all-trans) + (cis) photoisomerization of ROH, RAc, and RBA is markedly enhanced in polar solvents. Using pulsed laser photolysis it was also shown that ionic photodissociation is responsible for the polarity-induced shortening of the corresponding fluorescence lifetimes. An analysis of both effects led to the formulation of the following scheme (e.g., in the case of retinol)... 

Donohoue et al. [31] has reported two other kinetic data sets for Cl and Br reactions using a pulsed laser photolysis-pulsed laser induced fluorescence spectroscopy. These data sets are obtained using pseudo-first order conditions with respect to halogens or mercury and experiments were performed at a broad range of temperatures. The authors of these studies indicate an uncertainty estimation of 50% in the rate coefficients due to the determination of absolute concentrations of chlorine and bromine atoms [31]. Sumner et al. [20] reinvestigated both reactions using a 17.3 m environmental chambers equipped with fluorescent lamps and sun lamps to mimic environmental reactions, and evaluated the rate constants... 

Kinetic studies of the reaction of 1-bromopropane with OH radicals have been previously performed [27,30,32,36,37]. Donaghy et al. [30], using the relative rate technique, found a rate constant of (11.8 3.0) x 10 cm molecule s while using C-C6H12 as the reference compound. The obtained atmospheric lifetime was 11-16 days. Teton et al. [32] via pulsed laser photolysis followed... 

Gilles and coworkers [37], via pulsed laser photolysis followed by laser induced fluorescence, found the temperature-dependent rate constant for the OH reaction at 50 torr pressure to be 6.6 x cm molecule s for... 

Icqjj = 18.0 X 10" cm molecule" S" by relative rate method Icqjj = 16.7 x lO cm molecule s by pulse laser photolysis-laser induced fluorescence and atmospheric lifetime calculated to be 16 h at 298 2 K measured range 263-372 K (Moriarty et al. 2003)... 

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