Acetonitrile laser experiments

Figure 11 Transient absorption spectra obtained following one-laser photolysis (355 nm ) and two-laser photolysis (355 + 590 nm ) of all-rrans-retinol in air-saturated acetonitrile. In the one-laser experiment, the spectrum was obtained 1.9 (xs following the 355 nm pulse. In the two-laser experiment, the spectrum was obtained 1.9 ps after the 355-nm pulse and 400 ns following the 590-nm pulse. The kinetic behaviors observed at 570 and 610 nm are also shown. (From Ref. 18.)...

Laser flash photolysis (LFP) of quinone diazide 2d in Freon-113 at room temperature produces carbene Id, which could be monitored indirectly by addition of trapping reagents.25 At 2.0 xs the lifetime of Id is slightly longer than that of la (1.65 xs), otherwise the reactivities of these carbenes are very similar. The Id —> 11 rearrangement is not observed in the LFP experiments. All trapping products with a variety of reagents (O2, acetonitrile, pyridine etc.) are derived from carbene Id. 

In the laser photolysis experiments the aromatic compound (4-10" M) and the nucleophile (0 04 M ) in acetonitrile-water (1 1) were irradiated with the frequency doubled pulse (100 mj, 6 ns, 347 nm) of a ruby laser. Only time-dependent absorption changes were measured (double pulsed xenon flash lamp with 10 /is continuous output as light source) absorption spectra were constructed from these measurements at 12 or 25 nm intervals. 

When irradiated at 360 nm, [Eu(terpy)3]3+ fluoresces at 595 nm while [Tb(terpy)]3+ fluoresces at 540 nm the other species do not fluoresce and hence the reaction kinetics could be followed by measurement of fluorescence intensity giving k = 1.9 x 10-3 s 1. The reaction was first order in [Eu(terpy)3]3+ and zero order in Tb, proceeding as shown in equations (3) and (4). The rapid nature of the second step was confirmed directly in separate experiments by similar means. Dye-laser excited emission and excitation spectra of [Eu(terpy)3](004)3 in the solid state and in acetonitrile solution have been measured168 and evidence has been obtained which was interpreted as pointing towards the presence of some mono- or bi-dentate terpyridyl ligands in addition to those which are tridentate as observed in the X-ray structural determination. 

Table I shows that in either dioxane or acetonitrile the quantum yield for degradation of I, is unaffected by the presence of 0.1 M of triplet quencher, either sorbic acid, naphthalene or cyclohexadiene. In ethanol, triplet quenchers reduce < >d from 0.34 to 0.14. Quantum yields for intersystem crossing, as determined by a laser opto-acoustic technique ( ), were 0.36 in ethanol and 0.59 in dioxane. These results agree with our earlier report (3), and indicate that significant reactivity occurs from St of I in protic solvents, and that reaction occurs exclusively from Sx in aprotic solvents. While triplet quenching experiments cannot rigorously exclude participation by short-lived higher triplet states, Palm et al (9) have obtained conclusive evidence from CIDNP experiments for singlet-state participation in a series of aryloxy-acetophenones. Note that the triplet state of I is formed in aprotic solvents, and that in deaerated solutions at room temperature it decays by first-order kinetics with a lifetime of 200 ns (3). Remarkably, despite having lifetimes about 100 times longer than other, differently-substituted, aryloxyacetophenones (the longer lifetimes may...

The potential of laser flash photolysis in the study of carbene reactions with heteroatoms has come to be recognized in recent years. A number of kinetic studies using this technique have been carried out with carbene precursors in nitrile solvents.122-127 An absorption band at 470 nm was observed in the laser flash photolysis of diazofluorene (246) in inert solvents. This band was assigned to triplet fluorenylidene (247). In acetonitrile, however, a second band was also detected at 400 nm and whose buildup is concurrent with the decay at 470 nm.122 Laser flash experiments in other nitrile solvents (i.e., benzonitrile and pivalonitrile) also produced a transient absorption band which is very similar to that observed in acetonitrile. The band at 400 nm was assigned to an intermediate nitrile ylide (248). This absorption could be quenched on addition of an electron-deficient olefin providing good support for its... 

Photoionization is also an important (although not the major) decay pathway for the excited diphenylketyl radical (161) in polar solvents such as acetonitrile [137] (Scheme 22). This was confirmed with the observation of benzophenone radical anion in laser flash experiments following 515-nm photolysis of ground-state 161. The anion radical is apparently produced as a result of electron ejection followed by trapping of the electron by ground-state benzophenone. 

Radical recombination. The results p>ertaining to many of the reactions in Scheme I will now be presented, starting with (a). When a solution of the metal dimer is subjected to a laser flash, -25% of the dimer is dissociated in a typical experiment a 20 iM solution of [CpM(CO)3l2 (M = Mo, W) yields about 25 pM of the dimer. A slightly different procedure was used for M = Cr, but with comparable results. The recombination of the radicals occurs over about 300 ps and follows second-order kinetics. A typical experiment for the molybdenum radical and the fit to second-order kinetics is shown in Figure 1. The rate constants (fc/lO L mol s ) in acetonitrile at 23 °C arc Cr 0.27, Mo 2.16, and W 4.7. In other organic solvents the rate constants are comparable to these, reflecting the relatively small differences in viscosity. In aqueous solution (C5H4C02 )Mo(CO)3 has it/10 L mol s = 3.0. 

Figure 1 Time-resolved CIDNP experiment on the system xanthone/triisopropylamine in acetonitrile at 294 K amine concentration 10 mM. Shown are the CIDNP signals of the terminal protons of the amine as functions of the delay Atbetween the laser flash and the sampling rf pulse (duration, 1.0 ps).

Photoionization also occurs in the already mentioned photoreactions between frans-anthole and fumarodinitrile. In this case, the electron is immediately scavenged by the solvent to give a monomeric or dimeric anion of acetonitrile, which constitutes the second radical of the pair. The precursor multiplicity is again singlet, and CIDNP experiments with varying laser intensity show the ionization to be a two-photon process. 

Protonation of the olefin, or protonation and subsequent dehydration of the parent alcohol, gives cations which are then subjected to laser excitation or steady-state irradiation. Cations generated in this way were identified by their characteristic absorption spectra, which also indicated cation stability over the time scale of the individual experiments by the lack of change in their absorption spectra. Among the numerous cations generated in acidified solution for photochemical studies are the xanthyl and thioxanthyl [7-15], dibenzosuberenyl [10], triphenylmethyl [10,15], a,co-diphenylpolyenyl [16], and 1,1-diarylethyl [17] cations. Media included acetonitrile acidified with trifluoroacetic acid (TFA-ACN) or aqueous sulfuric acid [7-9,11,14,15], TEA in 2,2,2-trifluoro-ethanol (TFA-TFE) [10,12,13], n-heptane acidified with TFA [9], and BFj-etherate in methylene chloride [16]. The absorption spectral data for several cations have been previously reviewed [6]. Characterization of the cation excited states will be discussed in Section III. 

Additional work soon discredited this postulate. The 386 nm transient could not be detected in other solvents. No transient was detected upon LFP of 10 in benzene or hexafluorobenzene. In cyclohexane, a transient with X = 367 nm was formed 144 ns after the laser pulse. Only in nitrile solvents were transient absorption bands observed near 400 nm. The definitive experiment was performed by Linda Fladel who demonstrated that the pseudo-first order rate constant of formation of the 386 nm absorbing transient was linearly dependent on the concentration of acetonitrile. This proved that the transient is formed by reaction of an invisible species which reacts with acetonitrile. The observable transient was ylide 12 formed by capture of 11. The same transient was formed by LFP of azirine 13. 

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