Laser flash photolysis rate constants

At its best, the study of solvent kies by the formalism given can be used to learn about proton content and activation in the transition state. For this reason it is known as the proton inventory technique. The kinetics of decay of the lowest-energy electronic excited state of 7-azaindole illustrates the technique.25 Laser flash photolysis techniques (Section 11.6) were used to evaluate the rate constant for this very fast reaction. From the results it was suggested that, in alcohol, a double-proton tautomerism was mediated by a single molecule of solvent such that only two protons are involved in the transition state. In water, on the other hand, the excited state tautomerism is frustrated such that two water molecules may play separate roles. Diagrams for possible transition states that can be suggested from the data are shown, where of course any of the H s might be D s. 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

The formation of 7a was also observed in solution using laser flash photolysis (LFP) with nanosecond time resolution.25,26 In Freon-113 7a shows an absorption maximum at 470 nm, and a life-time of longer than 20 xs.25 The rate of 2.9 x 109 M 1 s-1 for this reaction is almost the diffusion limit and implies a very small or absent barrier. In aqueous solution the rate constant for the reaction of la with 3Oj is 3.5 x 109 M-1 s-1, and the absorption maximum of 7a was determined as 460 nm.26 This clearly demonstrates that the oxidation of carbene la in solid argon and in solution follows the same reaction pathway. 

For the sake of comparison and mutual validation of methods for measuring large follow-up reaction rate constants, it is interesting to apply different methods to the same system. Such a comparison between high-scan-rate ultramicroelectrode cyclic voltammetry, redox catalysis, and laser flash photolysis has been carried out for the system depicted in Scheme 2.25, where methylacridan is oxidized in acetonitrile, generating a cation radical that is deprotonated by a base present in the reaction medium.20... 

FIGURE 2.28. Comparison of high-scan-rate ultramicroelectrode cyclic voltammetry (A), redoc catalysis (A), and laser flash photolysis (x) for the determination of the rate constant of deprotonation of methylacridan cation radical by bases of increasing pKa. Adapted from Figure 6 in reference 20, with permission from the American Chemical Society. 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

According to results from laser flash photolysis, the p-(methoxyphenyl) sulfanyl radical adds exclusively to the central atom in of 2,4-dimethylpenta-2,3-diene (If) with a rate constant of 1.1 x 10s M-1 s-1 (23 1 °C) (Scheme 11.6) [45], A correlation between the measured rate constants for addition of para-substituted arylsulfanyl radicals to allene If was feasible using Brown and Okomoto s o+ constant [46], The p+ value of 1.83, which was obtained from this analysis, was interpreted in terms of a polar transition state for C-S bond formation with the sulfanyl radical being the electrophilic part [45]. This observation is in agreement with an increase in relative rate constant for phenylsulfanyl radical addition to 1-substituted allene in the series of methoxyallene lg, via dimethylallene Id, to phenylsulfanylallene lh, to ester-substituted 1,2-diene li (Table 11.2). 

The actual limit value of rr, below which the time constraint is met for a given transducer, is somewhat ambiguous. For a 0.5 MHz transducer (response time 2 xs), Mulder et al. [297] set this limit at 60 ns, based on the observation of a maximum of amplitude of the photoacoustic wave with the concentration of phenol and calculating rr from the rate constant of reaction 13.24, k = 3.3 x 108 mol-1 dm3 s-1 [298]. Later, Wayner et al. [293] empirically choose 100 ns as that limit and used laser flash photolysis results to adjust the phenol concentration until the lifetime of reaction 13.24 was less than that limit. In any case, the safest way of ensuring that the time constraint is being met is to verify it experimentally by varying the concentration of substrate until the observed waveform reaches a maximum (or, equivalently, until the final A0bs77 value reaches a maximum). 

Nanosecond laser flash photolysis was applied to study excited-state 2-nitrothiophene in polar and non-polar solvents76 the transient absorption at 545 5 nm was assigned to its lowest triplet state. The rate constants of the interaction of this triplet excited state, with a number of substrates such as cyanide and hydroxide ions, have been determined77. Similarly, the transient absorption at 490 5 nm was assigned to the lowest triplet excited state of 5-nitro-2-furoic acid78, and that at 500 5 nm to that of 7V-(n-butyl)-5-nitro-2-furamide79. 

An excess of the twisted form (B) was prepared from the photostable 2,2 -bis(trifluoromethyl)-129c by laser flash photolysis, and its decay to the folded form (A) was followed photometrically. From the rate constants at four temperatures, AH (B- A) = 16.1 0.7 kcal/mol was obtained. For this compound, AH° (A B) = 4.2 kcal/mol had been found, and the sum of these two values agrees well with AG (E- Z) = 21.5 0.3 kcal/mol obtained by band-shape analysis, which probably corresponds to a A// value of 20 to 21 kcal/mol, assuming AS to be -2 to -5 e.u. 

Laser flash photolysis (30 ns, 50 mj, 347 nm) of 4-nitrostilbene, 4,4 -dinitro-stilbene, 4-nitro-4 -methoxystilbene and 4-dimethyl-amino-4 -nitrostilbene permits the observation of transients 133) por the latter two compounds, the lifetime and the absorption spectra of the transients vary strongly with the polarity of the solvent used. First order decay rate constants are given in Table 4. 

The absolute rate constants for reaction of /t-tolyl(trifluoromethyl)carbene, generated by laser flash photolysis of the corresponding diazirine, with pyridine (4 x lO lmor s ... 

Ab initio and RRKM calculations indicate that the reactions of C, CH, and (H2C ) with acetylene occur with no barrier." Laser flash photolysis of the cyclopropanes (69) and (70) was used to generate the corresponding dihalocarbenes. The absolute rate constant for the formation of a pyridine ylide from Br2C was (4-11) x 10 lmoP s. The rates of additions of these carbenes to alkenes were measured by competition with pyridine ylide formation and the reactivity of BrClC was found to resemble that of Br2C rather than CI2C . 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

The absolute rate constants for oxygen and sulfur transfer to a range of carbenes (dialkyl, cycloalkylidene, alkylchloro, diaryl, arylchloro, arylalkoxy, and dialkoxy), generated by laser flash photolysis of diazirine or oxadiazoline precursors, were determined. No evidence was seen for ylide formation and a concerted mechanism via an ylide-like transition state was proposed. 

The kinetic data reported in this chapter have been determined either by direct measurements, using for example kinetic EPR spectroscopy and laser flash photolysis techniques or by competitive kinetics like the radical clock methodology (see below). The method for each given rate constant will be indicated as well as the solvent used. An extensive compilation of the kinetics of reaction of Group 14 hydrides (RsSiH, RsGeH and RsSnH) with radicals is available [1]. 

Absolute rate constants for the reaction of Et3SiH with aroyloxyl radicals 26 (Reaction 3.17) were measured by laser flash photolysis and were found to be in the range of (3.8-7.4) x 10 s at 24 °C [32]. The rate constants of t-BuO ... 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

The rate constants for the reaction of EtsSi radicals with dialkyl sulfides R2S were measured by the laser flash photolysis technique and decrease in the order R = primary > secondary > tertiary, viz., 1.1 x 10, 8.8 x 10 , and 3.3 x... 

---