Electron Spin Resonance Spectra of the Anions of Benzene, Toluene and the Xylenes 1

The value of the rate constant kl for the reaction e-s,lv + RO-s,lv + H, [I], at 295 K and 1 bar is 51.4 X lo5 s-1 in methanol and 5 8 X 104 s-1 in ethanol. The respective volumes of activation averaged between 1 bar and 2 kbar are AVl* 2 -21 and 5 -22 cm3 mol-1. A high concentration of potassium hydroxide (1 M) or water (5 M) decreases the apparent value of kl somewhat but has little or no effect on the value of AV,". The effect of pressure on the rate constant of e-s,lv + S + product, [2], was also measured for a series of solutes that displays a wide range of reactivity. Experimental values of AV2* depend on the relative contributions of the effects of solvent density on the reactant diffusion rates, the concentrations of the actual reacting species, and the relative energies of the reactant and intermediate states. For reactions whose rates are near the diffusion controlled limit, k2 = 10'0 M-1 s-1 in methanol and ethanol, the values of AV2' are positive and similar to those for the diffusion of simple ions. AV* (e-s,!v diffusion) = 4 cml mol-1 in methanol and 6 cml mol-1 in ethanol. Cadmium chlor~de IS apparently not completely dissociated in alcohols, and k(e-s,lv + CdC12) < k(e-s,lv + CdCl+) < k(e-,,lv + Cd2+). For a series of compounds with lower rate constants there is a correlation between log k2 and AVz*, the latter being negative for very low values of k2. The products of electron capture by benzene, toluene, ethyl acetate, and possibly acetonitrile appear to be stabilized by protonation: e-,,,I,. + S = S-,,lv, 13, -31,; S-s,lv + ROH + SH + ,I,,[4]. The results indicate that the decomposition of e-s,lv in a pure alcohol occurs by protonatlon of the electron site, e-s,lv + ROH -t H + RO-,,,,, 14'1, rather than by electron transfer to an alcohol molecule followed by decomposition of the anion. [Traduit par le journal]

Section: Eflect Of Pressure On K2supporting

confidence: 82%

Reaction rates of electrons in liquid methanol and ethanol: effects of pressure

Bolton¹,

Robinson²,

Freeman³

1976

“…The large temperature coefficient of mobility at low temperatures is tentatively attributed to negative ion formation, by analogy with the reaction between solvated electrons and benzene in dimethoxyethane at low temperatures (28).…”

Section: Electron Mobilitiesmentioning

confidence: 92%

Electron Mobilities and Ranges in Liquid Hydrocarbons: Cyclic and Polycyclic, Saturated and Unsaturated Compounds

Shinsaka¹,

Dodelet²,

Freeman³

1975

KYOII SHINSAKA, JEAN-POL DODELET, and GORDON R. FREEMAN. Can. J. Chem. 53,2714 (1975.Penetration ranges bGP of secondary electrons into 21 X-irradiated liquids were estimated from measured free ion yields. The density normalized ranges bcpd are independent of temperature. Increasing the molecular symmetry by creating one or more cycles in the molecule causes the normalized range to increase, provided that the amount of bond distortion due to strain remains small. Ranges are smaller in compounds that contain strained rings. Energy transfer from the secondary electrons to the molecules is enhanced by the presence of distorted bonds in the molecules. The ranges in olefins are affected by the same factors as those in saturated hydrocarbons, with the addition of a contribution of transient negative ion states to the energy transfer processes. Shielding the double bond by methyl groups is not effective; the effect of the added methyl groups on the molecular symmetry is a more important factor. The magnitude of the energy transfer interaction is an inverse function of molecular symmetry.The general correlation between bGp and thermal electron mobilities u. in liquids contains significant variations within it. For a given value of bGp, u, in different liquids increases in the order n-alkane < cycloalkane < cycloalkene < 1,4-cyclohexadiene or benzene. Arrhenius plots of u, in cyclic olefins curve downwards at low temperatures, due to the formation of a deeper trapped state of the electron. The trap is deepest in 1,4-cyclohexadiene (21 kcal/mol) and is attributed to an equilibrium between solvated electrons and anions at temperatures below about280K.KYOJI SHINSAKA, JEAN-POL DODELET et GORDON R. FREEMAN. Can. J. Chem. 53,2714Chem. 53, (1975. Les parcours des electrons secondaires, bGp, ont etk estimes d'aprts les rendements en ions libres mesurks dans 21 liquides irradits aux rayons X. NormalisCs pour la densite, ces parcours, bcpd, deviennent independants de la temperature. Une augmentation de la symktrie molCculaire par cyclisation simple ou multiple entraine une augmentation du parcours normalise, tant que la distorsion des liaisons demeure faible. Un important transfert d'energie des electrons secondaires est realise dans le cas de molCcules a cycle tendu, ce qui reduit le parcours des electrons dans de tels liquides. Les parcours electroniques dans les olefines sont fonction des mCmes facteurs que dans les alcanes, avec en plus la possibilite d'un transfert d'energie via la tendance a former un ion negatif CphCmere. L'addition de groupes mtthyles ne protege pas une double liaison, mais affecte la symetrie moleculaire. L'importance de la quantite d'energie transferee par collision est une fonction inverse de la symktrie moltculaire. La correlation gknerale entre bGP et la mobilitC de I'electron thermique dans les liquides presente une structure fine. Pour une valeur de bGp donnte, u. augmente selon I'ordre suivant: n-alcane < cycloalcane < cycloalctne < cyclohexaditne-1,4 ou benzene. La courbe obtenue en portant u, s...

“…The activation energies E,, of the mobilities are 4 5 1 kcaljmol, with the exception of that in benzene, 7.4 kcal/mol (Table 3) The anion is more stable at low temperatures, at least when formed by reaction between the hydrocarbon and solvated electrons in dimethoxyethane (27). Negative ions have lower mobilities than d o electrons, so the apparent mobility of electrons would be smaller the greater is the fraction of their lifetime that they Can.…”

Section: Thermal Electron Mobilitiesmentioning

confidence: 96%

“…Electrons attach less readily to the dimethylbenzenes (27), presumably because methyl groups donate electron density to the aromatic ring and decrease its electron affinity. Equilibria analogous to [7] would therefore contribute less to the temperature coefficient of electron inigration in the methyl-substituted benzenes.…”

Section: Thermal Electron Mobilitiesmentioning

confidence: 99%

Epithermal Electron Ranges and Thermal Electron Mobilities in Liquid Aromatic Hydrocarbons

Shinsaka¹,

Freeman²

1974

Radiolysis free ion yields were measured as functions of electric field strength and temperature in benzene, toluene, 1,2-, 1,3-, and 1,4-dimethylbenzene, 1,2,3-, 1,2,4-, and 1,3,5-trimethylbenzene, 1,2,3,4- and 1,2,4,5-tetramethylbenzene, pentamethylbenzene, hexamethylbenzene, naphthalene, and anthracene. Secondary electron ranges bGP were estimated from the yields. The density-normalized ranges bGPd were almost constant, 34−38 × 10−8 g/cm2, from benzene up to the tetramethylbenzenes, then increased to 47 × 10−8 in pentamethyl- and 52 × 10−8 in hexamethylbenzene. In naphthalene and anthracene the normalized ranges were 32 and ∼20 × 10−8 g/cm2, respectively. Electron mobilities in the liquids at 293 K, and their activation energies, expressed in (cm2/V s, kcal/mol) were: benzene (0.114, 7.4); toluene (0.063, 3.4); 1,2-dimethylbenzene (0.018, 4.4); 1,3-dimethylbenzene (0.057, 4.5); 1,4-dimethylbenzene (0.062, —); 1,2,3-trimethylbenzene (0.022, 4.8); 1,2,4-trimethylbenzene (0.035, 4.3); 1,3,5-trimethyl-benzene (0.17, 3.2); 1,2,3,4-tetramethylbenzene (∼0.02, 5). The mobilities reflect migration retarding influences of the aromatic ring and molecular asymmetry, and a slight ring-shielding effect of methyl groups that tends to enhance the mobility.