Effect of solvent structure on electron reactivity: 2-propanol/water mixtures

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RUZHONG CHEN, YURIS AVOTINSH, and GORDON R. FREEMAN. Can. J. Chem. 72, 1083 (1994.The effective reaction radii a,, where R, is the reactive encounter radius and K is the probability of reaction per encounter, for e; with H: , NH,',,, Ag:, Cu?, and NOT, are all 0.7 f 0.1 nm in isobutanol containing 1G20 mol% water. The value remains at 0.7 f 0.1 nm for H: , Ag:, and Cu? in pure isobutanol, and for the two transition metal ions in pure water solvent.The value for NO;, reduces to 0.35 nm in pure isobutanol and pure water solvents, whereas for H: and NH;, in pure water solvent it is only 0.14 nm and 2.6 x nm, respectively. The low reactivity of NH,',, with e; in water is attributed to the symmetry of the hydrogen-bonded solvation structure of NH; in water, and the higher reactivity of H: (OH;,,) is attributed to the lower symmetry of its hydrogen-bonded solvation structure. The NH,' and OH; ions have no low-lying orbital for an electron to occupy, so either reaction occurs by proton transfer to the electron site or the neutral species must decompose. We suggest that the proton transfer or the decomposition of the neutral species is facilitated by an unsymmetrical solvation structure.Reaction of e; in A1(C104)3 solutions in water is due mainly to H: from hydrolysis of Alp, and partly to partially hydroxylated aluminum(II1) species. Reaction of e; with A12 itself appears to be negligible in water. The reactivity of the solutions of Al(C104)3 in isobutanol-rich solvents is 3-5 times greater than that in water.In pure CI to C4 1-alcanol solvents the value of k2(e; +NO,,) increases linearly with the dielectric relaxation time z, of the solvent. In these solvents the probability of permanent capture per encounter increases approximately as the square of the encounter duration. On attribue la faible rCactivitC du NH,',, avec les e; dans l'eau i la symCtrie de la structure de solvatation comportant des liaisons hydrogknes du NH,',, dans l'eau et on attribue la rCactivitC plus ClevCe des H: (OH;,) B la symCtrie plus faible de sa structure de solvatation comportant une liaison hydrogkne. Les ions NH; et OH; n'ont aucune orbitale de bas niveau pour accommoder un Clectron; la rkaction se produit donc par un transfert de proton vers un site dlClectron ou les espkces neutres doivent se dicomposer. On suggkre que le transfert de proton ou la dCcomposition d'espkces neutres est rendu plus facile par une structure de solvatation qui n'est pas symttrique.La rCaction des e; dans des solutions de A1(C104)3 dans l'eau est principalement due aux H: provenant de l'hydrolyse

Section: Activation Energies Ez and Eamentioning

confidence: 96%

Solvent effects on the reactivity of solvated electrons with ions in isobutanol/water mixed solvents

Chen¹,

Avotinsh²,

Freeman³

1994

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“…The reaction of e; with nitrobenzene is nearly diffusion controlled (7,15) and we use it as a reference (4a). Arrhenius (Fig.…”

Section: Diffusion and Reactionmentioning

confidence: 99%

Solvent structure effects on solvated electron reactions with ions in 2-butanol/water mixed solvents

Peiris¹,

Freeman²

1991

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The Smoluchowski-Debye-Stokes-Einstein equation for the rate constant kz of a bimolecular reaction between charged or polar species was used to evaluate effects of bulk solvent properties on reaction rates of solvated electrons with NOT,,, H: , TI:, Cu:' , and AI:' in 2-butanol/water mixed solvents. To explain detailed effects it was necessary to consider more specific behavior of the solvent. Rate constants k 2 , activation energies E2, and pre-exponential factors Az of these reactions vary with the composition of 2-butanol/water mixtures. The values of E2 were in general similar to activation energies of ionic conductance EAo of the solutions, except for much higher values of E2 of (e; + NOT,,) in alcohol-rich solvents and of (e; + ~1:') in pure water solvent.The solvent apparently participates chemically in the NOT, reaction, and the ~1:' reaction is multistep. Rate constant and conductance measurements of thallium acetate solutions in 2-butanol containing zero and 10 mol% water were complicated by the formation of ion clusters larger than pairs.

“…There seems to be an additional effect in the latter reaction, and we suggest it is that water, being a stronger acid than is the amine (17), is a better protonating agent for the phenylide ion in reaction [5].…”

Section: Rate Constantsmentioning

confidence: 83%

“…The increase of k2(es-+ RC6HS) on going from 99.0 to 99.5, 99.8, and 100 mol% water is attributed to the RC6HS being progressively less "coated" by amine molecules, thereby facilitating reactions [4] and [5]. The compressibility of solutions of butylamine in water has a minimum at 98.3 mol% water, which implies optimal packing of the molecules (23).…”

Section: Rate Constantsmentioning

confidence: 99%

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Solvent effects on the reactivity of solvated electrons with organic solutes in 1-butylamine–water mixtures

Zhao¹,

Freeman²

1996

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The values of the rate constants of the reactions of e,-with the efficient scavengers nitrobenzene and acetone are 2 2 x lo6 m3 mol-I s-' in the whole range of 1-butylamine-water mixtures at 298 K; the reaction rates in the mixed solvents vary approximately as the solvent fluidity. In pure butylamine at 298 K, kz(e,-+ nitrobenzene) = 84 x lo6 m3 mol-I s-I and kz(es-+ acetone) = 7.3 x lo6 m3 mol-' sK'. The values of the rate constants of the reactions of es-with the inefficient scavengers phenol and toluene are < 2 x lo5 m3 mol-I s-I in the whole range of l-butylaminewater mixtures at 298 K and have a maximum at 50 mol% water and a minimum at 99 mol% water. In pure 1-butylamine at 298 K, k2(es-+ phenol) = 1.0 x 10' m%olol-' s-I and k2(es-+ toluene) = 0.28 x lo4 m%olKl s-I. The reaction rates with inefficient scavengers show strong dependence on the solvent composition and selective solvation of electron and scavenger. In the amine-rich region (G30 mol% water), the rate constants increase with the itzcrease of viscosity, indicating the chemical participation of solvent molecules in the reaction. In the water-rich region from 50 to 99 mol% water, the decrease of the rate constants indicates the nonhomogeneous solvation of the electrons by water and of the organic solutes by 1-butylamine. From 99 mol% to pure water the rate constant increases rapidly, which we attribute to insufficient I-butylamine to coat the phenol or toluene molecules. The variation of the activation energies E2 for the efficient scavengers, 14-27 kJ mol-I, are similar to the variation of E, in the mixed solvents. The values of E1 for the inefficient scavengers are from 15 to 38 kJ mol-I for phenol and from 6 to 21 kJ mol-I for toluene. Both k2 and E2 for the inefficient scavenger reactions show a correlation with the temperature coefficient -dEA,,,/dT of the optical absorption of es-in the mixed solvents, but the reason is obscure.Key words: 1-butylamine-water solvent, solvated electron, organic solutes, reactivity, solvent effects.RCsumC : Les constantes de vitesse pour les reactions des es-avec les capteurs efficaces que sont le nitrobenzkne et l'acttone sont 2 2 x lo6 m3 rnolK' s-I pour I'ensemble des concentrations de la I-butylamine-eau, h 298 K; la variation des vitesses de rCaction dans les solvants mixtes suit approximativement celle de la fluidit6 du solvant. Dans la butylamine pure, B 298 K, k2(es-+ nitrobenzkne) = 84 x lo6 m3 mol-' s-I et k2(es-+ acCtone) = 7,3 x lo6 m h o l -I s-I. Les valeurs des constantes de vitesse des rtactions des es-avec les capteurs inefficaces que sont le phCnol et le tolukne sont <2 x lo5 m3 mol-I s-I pour l'ensemble des concentrations de la I-butylamine-eau h 298 K et elles prCsentent un maximum h 50 mol% d'eau et un minumum h 99 mol% d'eau. Dans la I-butylamine pure, h 298 K, k2(e,-+ phtnol) = 1,O x lo4 m3 mol-I s-I et k2(e,-+ tolukne) = 0,28 x lo4 m"01-l-l s-I. Les vitesses de rtaction avec les capteurs inefficaces prtsentent une forte dipendance sur le composition et une solvation selective de...