Alkanes flash photolysis

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 reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... 

Rates of reaction of CO with (arene)Mo(CO)2 (alkane) and M(C0)5 (alkane), where M = Cr, Mo, W, have demonstrated that the reaction with Cr(CO)5 (alkane) involves an interchange mechanism while for Mo and W the mechanism is more associative. These resnlts are consistent with the known ability of the higher homologs to expand beyond a coordination number of 6. Flash photolysis and ultra high speed detection (0.4 ps, 4cm resolution) have allowed direct observation (Figure 6) of the vibrational cooling of CpCo(CO)(n-hexane), CpCo(CO)(n-hexene), and the group VI carbonyls. ... 

Parent radical cations derived from alkanes and alkyl chlorides can be directly observed in the nanosecond time domain by time-resolved spectroscopy such as laser flash photolysis and electron pulse radiolysis. Especially the latter one enables the direct ionization of the solvents independently on the optical properties of the sample and a well-defined electron transfer regime according to Eq. (2) or (3). Representative examples of the radiolyfic generation of solvent radical cations are given in Eqs. (4) and (5a) for the cases of 1-chlorobutane and -decane. ... 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

Two-laser two-photon results revealed photoisomerization of the cation E,E-11 to its stereoisomer Z,E-11, which undergoes thermal reversion with a lifetime of 3.5 ps at room temperature. Absolute rate constants for reaction of styrene, 4-methylstyrene, 4-methoxystyrene and /i-methyl-4-methoxystyrene radical cation with a series of alkanes, dienes and enol ethers are measured by Laser flash photolysis [208]. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. Reactivity of radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH30-jff-CH3, while the effect of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl < 2,2-dialkyl < tetraalkyl. 

Ermolenko, L. P. Delaire, J. A. Giannotti, C. Laser flash photolysis study of the mechanism of photooxidation of alkanes catalyzed by decatungstate anion. J. Chem. Soc. - Perkin Trans, mi, 2, 25-30. 

Other triplet state characterization studies involve phenazine phosphorescence in alkane solvents in the glass transition range, establishment of the cis-keto form of the triplet 2-(2 -hydroxyphenyl)benzothiazole in nonpolar glass , 1,5-diaminoanthraquinone and l-acetylaminoanthraquinone have been studied by laser flash photolysis in solution. The formation of the triplet state of oxazine and the consequent delayed fluorescence in acetonitrile has been sensitized by benzophenone. The intersystem crossing efficiency of molecules with small S,-T,... 

The photorearrangement of l-anilino-m-(p-nitrophenoxy)alkanes (78) to. AT-(/7-nitrophenyl)-ct>-anilinoalkan-l-ols is shownto proceed by way of a radical ion pair and a Meisenheimer complex, both of which are observed in flash photolysis experiments. In a simpler system, the Smiles rearrangment of the... 

The formation of diphenylcarbene by laser flash photolysis of diphenyldiazomethane was studied in SC-CO2 and other supercritical media. [18] A laser-flash induced ring-closure reaction of a bipyridyl complex (Scheme 2) revealed solvation properties. [19] Laser-flash impact to metal carbonyl complexes activated hydrogen and simple alkanes like CH4, C2H4, C2H6, and further inorganic reactions in supercritical fluids have been reviewed. [20]... 

Many studies have been done on unstable or transient H2 and other c complexes using techniques such as flash photolysis. As in the above cases, H2 displaces a weak ligand, which, e.g., can be an alkane solvent molecule63 ... 

Solution studies of transient alkane binding at room temperature were first performed in 1973 by flash photolysis of Cr(CO)6 in cyclohexane, which gave a UV/vis band at 503 cm-1 similar to that seen in CH4 matrices71 72 ... 

Intermediates (Alkane) M(CO)s are produced in predominant concentration in solution after flash photolysis of the metal carbonyls M(CO)6 (M = Cr,Mo, W). It has been proposed that the alkane (used as a solvent) is coordinated to the metal via a C-H Magostic interaction [21a,b] ... 

Laser flash photolysis of [CpIr(CO)2] results in the activation of alkanes in inert CgFg solvent. A study under these conditions revealed the anticipated preference for the activation of aromatic and unhindered C—H bonds on the basis of quantum yields (< ). It also implied the unexpected mechanism shown in Scheme 4. Added CO has virtually no effect on . Moreover the use of r/ -indenyl instead of Cp increases (f>. 

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