Radiolysis methanol

Nelson54 studied the products of radiolysis of aqueous solution by variable-field CIDNP pulse radiolysis. On the basis of the chemical shifts the following products were identified methyl methanesulfinate, methanol, l,2-bis(methylsulfinylethane) [CH3S(0)CH2CH2S(0)CH3], dimethyl sulfone, dimethyl sulfide, methane and ethane. The high field polarization was used to study the mechanism of formation of polarized products. 

Arylperoxyl radicals (Ar02 ) (9-phenanthryl peroxyl, 1-naphthyl peroxyl, and 2-naphthyl peroxyl) were generated via the pulse radiolysis of arylbromides in methanol (see reactions below) (Edge 1998) ... 

Since charge conservation requires g(eh) = g(H30+), the latter yield will not be considered further. The chemical measurement of g(eh) uses Eq. (6.2) and the measurements of primary yields of H, H2, OH, and H202 in a suitable system. Various systems may be used for this purpose (see Draganic and Draganic , 1971). For example, in methanol solution radiolysis, H2 is produced by the reaction H + CH3OH—H2 + CH2OH. Therefore, in this system, G(H2) = g(H2) + g(H). If, in addition, there is excess oxygen, the H atoms would be removed by the reaction H + 02 H02. Therefore, from these two measurements, both g(H) and g(H2) may be obtained. 

Other complexities are revealed when frozen solutions of spin trap in methanol are irradiated, and the solution is then melted. The proportions of spin adducts are markedly dependent on radiolysis temperature. One contributory factor is undoubtedly the reaction of MeO with neighbouring methanol in the solid matrix, to produce HOCH2, before diffusion to reach spin-trap molecules is possible. 

An additional contributing factor to the mechanism of the present grafting reaction is the role of radiolytically produced hydrogen atoms. In the radiolysis of binary mixtures of aromatic and aliphatic compounds such as styrene-methanol, the concentration of aromatic strongly influences the G(H2) obtained from the methanol. In the most extensively studied binary mixtures of benzene-methanol (11) and pyridine-methanol (10), it is found that the yield of H atoms is important in determining product yields and types. Small additions (5%) of benzene and pyridine significantly reduce G(H2) from the methanol by scavenging H atoms. Above 5% additive, G(H2) is reduced further, but at a slower rate. These data for benzene-methanol and pyridine-methanol can be extrapolated... 

Further work (10) with acid effects in the radiolysis of binary mixtures such as benzene-methanol and pyridine-methanol indicates that the acid phenomenon is more complicated than the simple H atom model originally developed ( ). These more recent experiments (10) show that whilst increased hydrogen atom yields in the presence of acid enhance the overall grafting yield, other mechanisms also contribute to this acid effect. Thus the acid stability of intermediate radicals (I-III) and also analogous species involving the trunk polymer are important. With radicals (I-III), at low styrene concentrations in methanol, these intermediates (MR-) will predominantly react with other available... 

For the purpose of this discussion, styrene and pyridine may be considered to be structurally analogous since the aromatic ring of styrene is complemented by the electron rich -CH=CH2 group whilst pyridine has the nitrogen lone pair. Acid effects observed in the pyridine-methanol system may thus be extrapolated to styrene-methanol. In separate studies, it has been shown that inclusion of acid increased the quantities of scavenging products, pyridyl methanol and methylpyridine. In addition, one scavenging product, pyridylethanol, disappeared and was replaced by a number of unidentified products when acid was included in the radiolysis... 

The kinetics data on the reactions of silyl radicals with carbon-centred radicals are also available. The rate constant for the cross-combination of CHs with MesSi was measured to be 6.6 x 10 M s in the gas phase [19]. Studies on the steady-state and the pulse radiolysis of EtsSiH in methanol showed that the cross-combination of Et3Si with CH30 andHOCH2 occurs with rate constants of 1.1 x 10 and 0.7 x 10 M s , respectively [20]. 

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 gamma-radiation-induced oxidation of 2-propanol has been investigated. Acetone and hydrogen peroxide are the principal products and arise via a chain reaction in aqueous acid solutions at high concentrations of 2-propanol. In neutral solutions of 2-propanol and in solutions of methanol and ethanol, no such chain reactions are observed. The reasons for this are discussed along with the implications of the results for the hydroxyl radical yield in water radiolysis. 

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

The photochemistry of Eosin under both reductive and oxidative conditions has been studied by several groups [145-151], Photoreduction by amines such as tribenzylamine (R = CH2, R" = ) produces two leuco analogues, the dihydro derivative, and the cross-coupled product formed from the amine radical and the dye radical anion (2) [152], In addition, debromination of Eosin is reported during photobleaching with amines and phenols. The reader however is referred to the extensive studies of Rose Bengal dehalogenation by Paczkowski and Neckers [153]. Radiolysis of Eosin in methanol shows that debromination is a consequence of the photochemical decomposition of semireduced Eosin [154],... 

Hydrolysis of Pd(MeCOCHMe)2 in aqueous methanol is considered to involve Pd(0,0-MeCOCHCOMe)(0-MeCOCHCOMe)(MeOH) as an intermediate from which the monodentate acetylacetonate ligand is then solvolyzed.221 Subsequent studies on Lewis base complexes of palladium bis(diketonate) complexes provide ample support for the proposed intermediate. A pulse radiolysis study of the kinetics of aquation of M(MeCOCHCOMe) " (M = Cr, Co) indicates that an 17,-172 equilibrium involving one or more of the acetylacetonate ligands occurs, associated with an acid-catalyzed removal of the monodentate ligand.222 Treatment of Cu(MeCOCHCOMe)2 with picric acid in moist dichloromethane affords a partially hydrolyzed material, Cu(MeCOCHC-0Me)(H20)2[C6H2(N02)30], proposed to contain square pyramidal five-coordinate copper with the oxygen atom from the picrate moiety at the apex.223... 

Rate studies of the reaction between cesium and water in ethylenediamine, using the stopped-flow technique, have been extended to all alkali metals. The earlier rate constant (k — 20 NT1 sec.-1) and, in some cases, a slower second-order process (k — 7 Af"1 sec.-1) have been observed. This is consistent with optical absorption data and agrees with recent results obtained in aqueous pulsed-radiolysis systems. Preliminary studies of the reaction rate of the solvated electron in ethylenediamine with other electron acceptors have been made. The rate constant for the reaction with ethylene-diammonium ions is about 105 NCl sec.-1 Reactions with methanol and with ethanol show rates similar to those with water. In addition, however, the presence of a strongly absorbing intermediate is indicated, which warrants more detailed examination. 

Although the detailed nature of the reactions with methanol and ethanol remains unknown, it is clear from these studies that the reaction rate of solvated electrons with these alcohols is not very different from that with water. Pulse radiolysis studies (24) show minimum halftimes of 1.5 and 3 /xsec. for the disappearance of the electron peak in the pure alcohols but this does not represent reaction of the electron with the alcohol since no attempt was made to remove the counter ion or other radiolysis products. 

A review of radiolysis in liquid methanol has appeared which compares the relative abilities of methanol and water towards electron solvation.290... 

After thermalization, the electron may recombine with a positive ion or be captured by a molecule forming a negative ion, or it may be locked in a trap the role of which may be played by fluctuation cavities or structural disturbances in the medium, or by polarization pits that the electron digs when it interacts with surrounding molecules. Such captured electrons are called solvated electrons (in water they are sometimes called hydrated electrons).31,32 According to the data obtained in picosecond pulsed-radiolysis sets,33 34 the solvation time of an electron is 2 x 10-12 s in water and —10 11 s in methanol. 

Sherman WV (1967b) Light-induced and radiation-induced reactions in methanol. I. y-Radiolysis of solutions containing nitrous oxide. J Phys Chem 71 4245-4255 Sherman WV (1967c) The y-radiolysis of liquid 2-propanol. III. Chain reactions in alkaline solutions containing nitrous oxide. J Phys Chem 71 1695-1702 Silva C, Walhout PK, Yokoyama K, Barbara PF (1998) Femtosecond solvation dynamics of the hydrated electron. Phys Rev Lett 80 1086-1089... 

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