Solution, flash photolysis

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

Evidence for the existence of a ketene intermediate was first obtained by Nakamura et al. (1972) in a study of the photolysis of 2,1-diazonaphthoquinone-5-sulfonic acid by flash photolysis in aqueous solution. An intermediate with a strong absorption at 350 nm and a lifetime of approximately 2 ms was found. 

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

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... 

This brings us to U(VI) as an electron acceptor in MMCT transitions. A few examples were mentioned above already. Krol et al. have shown and used the U(V)-U(VI) MMCT transition in oxygen-deficient uranates [78]. It is situated in the near-infrared. This transition plays an important role in the quenching of the luminescence of uranates. The luminescence of U(VI)02 in solution is quenched by Mn(II) and it has been shown by flash photolysis that this is due to an excited MMCT state Mn(III)U(V) [79]. 

Carbon tunneling in a second singlet chlorocarbene has also been proposed. It has proved impossible to observe noradamantylcarbene 73 spectroscopically, either by solution laser flash photolysis or with matrix isolation at low temperatures. It has been suggested that the carbene rearranges too rapidly, possibly via carbon tunneling, to adamantene (74). 

Chiang, Y. Kresge, A. J. Hellrung, B. Schunemann, P Wirz J. Flash photolysis of 5-methy 1-1,4-naphthoquinone in aqueous solution kinetics and mechanism of photoenoli-zation and of enol trapping. Helv. Chim. Acta 1997, 80, 1106-1121. 

Chiang, Y. Kresge, A. J. Zhu, Y. Flash photolysis generation and study of p-quinone methide in aqueous solution. An estimate of rate and equilibrium constants for heterolysis of the carbon-bromine bond inp-hydroxybenzyl bromide. J. Amer. Chem. Soc. 2002,124, 6349-6356. 

Porter and Wilkinson(56) measured the rates of quenching for a variety of triplet donors with triplet acceptors at room temperature in fluid solution by flash photolysis. The appearance of the triplet-triplet absorption spectrum of the acceptor and the simultaneous disappearance of the donor triplet-triplet absorption spectrum provided unequivocal evidence for the triplet-triplet energy transfer process. Table 6.5 provides some of the quenching rate constants reported in this classic paper. 

Flash photolysis has been used to study the triplet-triplet absorption spectra (T1 -> Tq) of a number of aromatic molecules both in solution and in the gas phase.<34) A disadvantage associated with the flash technique for obtaining triplet-triplet absorption spectra is that the transient absorption can occur for only a short time after the initial flash (determined by the lifetime of triplet... 

Analysis of the products of the photolysis of Zr (benzyl) 4 in toluene at 30°C in the region 300-450-nm with mass spectrometry shows that the predominant peak is due to p-benzyl toluene with small amounts of dibenzyl, and 4,4 -dimethyl diphenyl. Rate measurements (Fig. 5a) suggest that in this region of the spectrum the polymerization proceeds through radical intermediates (29). This concept is supported by the fact that the spectrum of zirconium tetrabenzyl has a maximum at 317 nm and is identical with that found for the benzyl radical using flash photolysis techniques (30). Also zirconium tetrabenzyl on irradiation in toluene solution produces... 

It is not our intention in this section to provide a comprehensive review of flash photolysis of organometallic species rather, we summarize some key experiments which establish the timescales of different types of reactions. Understandably, much more work has been done on the flash photolysis of metal carbonyls in solution than in the gas phase, and so we begin with solution experiments. 

Flash photolysis of metal carbonyls in solution was pioneered by Kelly and Koerner von Gustorf 30). The most complete of these studies has been carried out on the photolysis of Cr(CO)6 30). The historical development of these experiments, which forms an intriguing story in its own right, has been recently retold (2). The salient features are as follows ... 

Flash photolysis has now been applied to a wide range of metal carbonyl species in solution, including Mn2(CO)10 (37), [CpFe(CO)2]2 (38), and [CpMo(CO)3]2 (39). In almost every case, interesting data have emerged, but, as with Cr(CO)5, the structural information is usually minimal. Thus, the radical Mn(CO)5 has been generated in solution by flash photolysis (37), the rate constant for its bimolecular recombination has been measured, but the experiments did not show whether it had Z>3h or Qv symmetry. Some experiments have been unsuccessful. Although the fragment Fe(CO)4 is well known in matrices (15), it has never been... 

Relatively little work has been done on the flash photolysis of gas phase metal carbonyls, partly because of the low volatility of many of the compounds. Early work by Callear (41,42) provided some evidence for Ni(CO)3 generated from Ni(CO)4 in the gas phase (41) and Fe atoms produced from Fe(CO)5 (42). This latter process has even been used as the basis of an Fe atom laser (43). More recently Breckenridge and Sinai (44) studied the flash photolysis of Cr(CO)6. Their results, interpreted largely on the basis of data from matrix isolation experiments, were in broad agreement with Kelly and Bonneau s solution work (JJ). In particular, they found no evidence for loss of more than one CO group [Eqs. (4) and (5)]. 

As with solution experiments, flash photolysis in the gas phase has produced evidence for the existence of intermediates but no information about their structure. In principle gas phase IR spectra can provide much more information, although the small rotational B value of gaseous carbonyls and low lying vibrational excited states preclude the observation of rotational fine structure. As described in Section II, time-resolved IR experiments in the gas phase do not suffer from problems of solvent absorption, but they do require very fast detection systems. This requirement arises because gas-kinetic reactions in the gas phase are usually one... 

In general, intramolecular isomerization in coordinatively unsaturated species would be expected to occur much faster than bimolecular processes. Some isomerizations, like those occurring with W(CO)4CS (47) are anticipated to be very fast, because they are associated with electronic relaxation. Assuming reasonable values for activation energies and A-factors, one predicts that, in solution, many isomerizations will have half-lives at room temperature in the range 10 7 to 10 6 seconds. The principal means of identifying transients in uv-visible flash photolysis is decay kinetics and their variation with reaction conditions. Such identification will be difficult if not impossible with unimolecular isomerization, particularly since uv-visible absorptions are not very sensitive to structural changes (see Section I,B). These restrictions do not apply to time-resolved IR measurements, which should have wide applications in this area. 

It had already been established by uv-vis flash photolysis (35) that Cr(CO)5 (solvent) was the first observable intermediate in the photolysis of Cr(CO)6. Figure 9 shows the IR spectrum (96) of the photoproduct Cr(CO)5(C6Hi2) in cyclohexane solution. The spectra were obtained using Cr(CO)5(13CO) (96). The extra spectroscopic information provided by the 13CO group was sufficient to show that the spectrum was consistent... 

Church and co-workers (77) have obtained time-resolved IR spectra of both Mn(CO)5 and Mn2(CO)9 by flash photolysis of Mn2(CO)I0 in solution. The spectra (Fig. 11) were in close agreement with the spectra of matrix isolated Mn(CO)5 (22) and Mn2(CO)9 (5,106). There was a bridging vc 0 band for Mn2(CO)9 showing that it has a CO-bridged structure in solution as well as in the matrix. Structural information of this type could not have been obtained from uv-vis spectroscopy. Similarly, the IR spectra indicated that Mn(CO)5 had the same C4v structure in solution (77) as in the matrix (22). In solution (77), the yield of Mn2(CO)9 was approximately equal to that of Mn(CO)5. Bearing in mind that one molecule of Mn2(CO),0 produces two molecules of Mn(CO)5 [Eq. (14)], CO loss from Mn2(CO)10 [Eq. (15)], must be the major process at these photolysis wavelengths (37,77). 

Fig. 12. Transient IR difference spectra showing changes in absorbance (a) 5 / seconds, (b) 25 seconds, and (c) 1.25 mseconds after the UV flash photolysis of [CpFe(CO)2]2 in cyclohexane solution under 1 atm pressure of CO. Bands pointing upward represent an increase in absorbance (i.e., formation of a compound) and those pointing downward a decrease [i.e., depletion of starting material, (A)]. The bands are assigned as follows A, [CpFe(CO)2]2 B, CpFe(CO)2 and C, CpFe(p.-CO)3Fe(Cp). Points marked were recorded with a 12CO laser and those marked + with a 13CO laser. [Reproduced with permission from Moore et al. (61).]...

Already a considerable number of transient organometallic species have been characterized by IR kinetic spectroscopy (see Table I). Like most other sporting techniques for structure determination, IR kinetic spectroscopy will not always provide a complete solution to every problem. What it can do is to provide more structural information, about metal carbonyl species at least, than conventional uv-visible flash photolysis. This structural information is obtained without loss of kinetic data, which can even be more precise than data from the corresponding uv-visible... 

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