Alcohol radiolysis

The composition of alcohol radiolysis products is very different in the presence and absence of 02, due to the reactions [65]... 

We have demonstrated a particularly clear and simple technique for evaluating the reactions of pyridinyl radicals in water through measurement of the pH changes resulting from radiolysis of pyridinium ions in unbuffered aqueous solutions containing isopropyl alcohol ( ). Radiolysis generates one proton for each pyridinyl radical, as set forth in Eq. 9. ( ). 

Owing to the reactions of the initial primary radiolysis products among themselves, as in Eqs. (11-58)—(11-62), it is usually necessary to add another reagent to remove the unwanted ones. For example, to study reactions of e alone, one must work in neutral or basic solution to avoid its destruction by HsO+ (see Problem 11-12). Also, hydroxyl radicals and hydrogen atoms are removed from the system by prior addition of terf-butyl alcohol to give noninterfering products,... 

Nishimura and coworkers57-59 studied the y-radiolysis of aqueous solutions of sulfoxide amino acids. Sulfoxide amino acids are the precursors of the flavors of onions (S-propyl-L-cysteine sulfoxide, S-methyl-L-cysteine sulfoxide and S-(l-propenyl)-L-cysteine sulfoxide) and garlic (S-allyl-L-cysteine sulfoxide). In studies on sprout inhibition of onion by /-irradiation it was found that the characteristic flavor of onions became milder. In the y-radiolysis of an aqueous solution of S-propyl-L-cysteine sulfoxide (PCSO)57,58 they identified as the main products alanine, cysteic acid, dipropyl disulfide and dipropyl sulfide. In the radiolysis of S-allyl-L-cysteine sulfoxide (ACSO) they found that the main products are S-allyl-L-cysteine, cysteic acid, cystine, allyl alcohol, propyl allyl sulfide and diallyl sulfide. The mechanisms of formation of the products were partly elucidated by the study of the radiolysis in the presence of N20 and Br- as eaq - and OH radicals scavengers, respectively. 

In contrast with irradiation of ACSO and PCSO, where volatile products were formed (sulfides, disulfides and alcohols), no volatile products were formed in the radiolysis of aqueous solutions of S-(cis- l-propenyl)-L-cysteine. Here the authors found that reactions of OH" radicals are responsible for the formation of propyl-1-propenyl sulfides (cis and trans). 

Dihydrodithiin sulphoxides, synthesis of 243 Dihydrothiophene dioxides, reactions of 653 /(,/( -Dihydroxyketones 619 Dimerization, photochemical 877, 884 Dimethyl sulphoxide anion of - see Dimsyl anion hydrogen bonding with alcohols and phenols 546-552 oxidation of 981, 988 photolysis of 873, 874, 988 radiolysis of 890-909, 1054, 1055 self-association of 544-546 Dimsyl anion... 

In pulse radiolysis studies it is feasible to eliminate undesired radical reactions by the use of "scavengers". For example, to produce a totally reducing system simple aliphatic alcohols are added and the ensuing reaction... 

Radiolytic reduction has been investigated as a means of producing transient Rh(II) porphyrin products, and as in the above study, the observed products were strongly dependent on pH and solvent. Radiolytic reduction of Rh(TMP)Cl in alcohol formed transient Rh(TMP)- which was prevented from dimerization by the bulky TMP ligand. In alkaline 2-propanol the product is [Rh(TMP)r. in weakly acidic 2-propanol the hydride Rh(TMP)H is formed, and in strongly acidic 2-propanol the alkylated rhodium(III) porphyrins Rh(TMP)CH3 and Rh(TMP) (C(CH 3)20H) are observed. The alkyl products result from reaction of Rh(TMP)-with CH3- and C(CH3)20H formed by radiolysis of the 2-propanol solvent. 

By pulse radiolysis of nitrous oxide-saturated aqueous solutions of ferricyanide (2 X 10 " M) and various alcohols (0.1 M), Adams and Willson " were able to obtain absolute rate coefficients for the ferricyanide oxidation of the radicals derived from the alcohols by attack of the solvent irradiation product, OH-. 

Sumiyoshi and coworkers studied the radiolysis of aqueous solution of methyl methylthiomethyl sulfoxide [ H3S(0) H2S H3 MTMSO] at various pH by pulse radiolysis. They found that the reaction of e with MTMSO (in the presence of 1M t-butyl alcohol to scavenge OH" radicals) leads to formation of a transient with a broad absorption band of = 375 nm. The absorbance at 375 nm as a function of pH are of S shape, indicating an equilibrium due to reaction with H. Similar to the finding in dimethyl sulfoxide they suggested the scheme. 

In these solutions, the hydrated electrons from the radiolysis of water produce CO by their reaction with COj, and the OH radicals attack the alcohol to form (CHjIjCOH radicals (see also footnote on page 117). 

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

A similar explanation of accelerative effects observed in grafting with other solvents such as the higher straight chain alcohols, DMF, DMSO, acetone, chloroform and cyclohexane (Table VI) has been advanced (6). It appears that there is a relationship between G(H) value of solvent and the extent to which the solvent participates in accelerated grafting. The radiolysis pathway thus contributes, but not exclusively, to the mechanism of the overall copolymerisation reaction. 

Some obscure facets of this intricate picture have been unveiled by Filippi and Speranza who investigated the stereochemistry and the intimate mechanism of a model solvolytic reaction taking place in an ion-dipole pair in the gaseous phase. ° Adducts (7 )-56 and R)-51 are obtained in the gas phase by association of the relevant chiral alcohols, i.e., (/ )-(- -)-l-phenyl-ethanol ((I )-44) and (/f)-(- -)-l-(pentafiuorophenyl)ethanol K)-55), with the CHs OHJ ion, generated by y-radiolysis of CH3F/H2 0 mixtures (Scheme 24). As mentioned above, the absence of neutral nucleophile molecules, i.e., CH3 OH, in the reaction medium ensures that the 0-labeled ethers 45 and 58 of Scheme 24 arise exclusively from the intracomplex solvolysis of R)-56 and R)-51, respectively. 

The conductivity changes following pulse radiolysis of a mixture of 0.5 mM Co(acac)3, 0.04 mM HCIO4 and 0.1 M tert-butyl alcohol are shown in Figure 3 (the units of G x AA are (molecules/lOOeV) Q cm M ). The very first increase in conductivity also appears in solutions containing no Co(acac)j. The decreases in conductivity are speeded up in acid. Account for this behavior. 

Hydrogen abstraction from propan-2-ol and propan-2-ol- /7 by hydrogen and deuterium atoms has been studied by pulsed radiolysis FT-ESR. A secondary kinetic isotope effect was observed for H (D ) abstraction from the C—H (C—D) bonds. The results were compared with ab initio data. In similar work, the kinetic isotope effects in H and D abstraction from a variety of other alcohols in aqueous solvents have been measured. It was found that, compared with the gas phase, the reactions exhibit higher activation energies in agreement with the ability of solvation to decrease the dipole moment from the reactant alcohol to the transition state. 

Carbon dioxide itself can accept e. during radiolysis of water, giving rise to the carbon dioxide anion-radical. This anion-radical can add to carboradicals. Thus, aliphatic alcohols react with the radiolytically generated hydroxyl radicals, rupturing their C—H bonds RCHjOH + OH HjO + RCH OH. These radicals accept the radiolytically generated COj" forming a-hydroxycarboxylic acids RCHjOH + CO2 RCH(OH)COO (Morkovnik and Okhlobystin 1979). 

Rate constants for the protonation of radical-anions in dimethylformamide by added phenol can be determined by electrochemical techniques [8], Pulse radiolysis methods have been used to measure the rate constants in an alcohol solvent. This technique generates the radical-anion on a very short time scale and uv-spectroscopy is then be used to follow the protonation of this species to give the neutral radical with different uv-absorption characteristics [9]. In the case of anthracene, the protonation rate is 5 x 10 M" s with phenol in dimethylformamide and 5 x 10 s in neat isopropanol. Protonation by hydrogen ions approaches the diflusion-controlled limit with a rate constant of 10 M s in ethanol [9]. 

Additional experiments were done in mixtures of alcohol alkane [16,17]. The spectra and kinetics were measured in mixtures of 1-propanol n-hexane. Some experiments were done in cyclohexane, where the behavior was qualitatively similar however, the exact concentration where spectra and kinetics changed depended on the alkane [16]. Additional experiments observed the shift of the final spectrum of the solvated electron in supercritical ethane-methanol mixtures. These experiments were done using standard pulse radiolysis techniques and thus we were unable to observe the kinetics [19]. 

The stroboscopic pulse radiolysis with the single bunch electron pulse instead of pulse trains started in Argonne National Laboratory in 1975 [54]. The research fields have been extended by the stroboscopic pulse radiolysis with the picosecond single electron bunch, although most of researches had been limited to hydrated and solvated electrons in the aqueous and alcoholic solutions. This system was unable to study the kinetics of the geminate ion recombination in liquid hydrocarbons until the modification of the Argonne linac in 1983, which made possible the quality measurements of the weak absorption. 

Pulse radiolysis studies of the reactivity of 25-nm Agl particles with eaq in the presence of alcohol have shown that, first, the semiconductor spectrum at 360 nm is bleached, then silver atoms and clusters at 450-600 nm are formed [164,165]. The electron... 

In a novel kinetic approach, Dorfman et al. developed methods for rapidly generating very reactive carbanions such as the benzyl anion in solvent mixtures containing water and alcohols. With pulsed radiolysis techniques, they have been able to study the fast and very exothermic reactions of carbanions with these solvents. The studies have shown that despite the high exothermicity, the protonation is not diffusion controlled and depends on the nature of the carbanion s counterion. 

Irradiation with 7-rays was also used to synthesize bimetallic nanoparticles. Remita et al. synthesized poly(vinyl alcohol) (PVA)-stabilized Ag/Pt bimetallic nanoparticles by radiolysis of an aqueous mixture of Ag2S04 and K2PtCl4 at a concentration of 10-4 mol dm-3. A typical Ag plasmon absorption band is observed at —410 nm with only low intensity at the mole ratio of Ag Pt = 60 40, indicating the formation of Ag/Pt bimetallic nanoparticles. Polyfacrylic acid) was also used as the stabilizer, although the resulting UV-Vis spectra were quite different. 

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