摘要:
Chemical Com m u n i cations NUMBER 3/1966 9 FEBRUARY The Isotope Effect in Hydrogen Formation by the Reaction between Two Aquated Electrons By M. ANBAR and D. MEYERSTEIN (The Weizmann Institute of Science and the Soreq Nuclear Research Centre, Rehovoth, Israel) THE photolysis of hydroxide ions at 1850 A was shown to produce hydrated electron~.l-~ This is consistent with the charge-transfer-to-solvent spectrum assigned to the 1920 A band of OH-.4 Hydrated electrons were shown to react with each other at a diffusion-controlled rates to give hydrogen as the product.s Hydrogen is therefore formed, in the presence of an efficient OH scavenger, as product of the photolysis of a hydroxide solution. The mechanism of the eiq 4- eiq reaction, which involves the formation of (eiq),' in the transition state or as an intermediate, has not yet been elucidated. H/D isotope effects in H,O-D20 mixtures have been shown to be helpful in the understanding of the eiq reactions with H,O+, NH;, and H,POT; .*p9 It was of interest, therefore, to measure the H/D isotope effect involved in the hydrogen formed from the photolysis of hydroxide solutions in H,O-D,O mixtures.Aquated electrons were produced in alkaline solution (pH = 14) , containing 0.025 M-CD,OH, by illumination with ultraviolet light at 185 mp, at 25" c. CD,OH was used as an OH radical scavenger to suppress the reaction of OH with H,. The fate of e;, under our conditions is deter- mined by two competing processes, eiq + eiq +H2; and eiq + H,O 3 H. The hydrogen atoms formed are converted into e a(l by the reaction H + OH,, + eiq, alternatively they may react with the CD,OH to give HD.This competition for hydrogen atoms depends upon the ratio [OH,,]/[CD,OH]. The overall yield of H,/HD from solutions of CD,OH in H,O is therefore dependent on the light intensity. A mercury discharge tube with a light output of the order of einstein/cm.2 min. was used. Owing to the high extinction coefficient of OH,, at 185 mp,4 99% of the light was absorbed in a layer of less than 0.02 mm., giving a high steady-state local concentration of eaq (of the order of M). Under these conditions only 2% of the hydrogen produced originated from hydrogen abstraction (CD,OH in H,O gave HD/H, = 0.02). The formation of hydrogen by deuterium abstraction from CD,OH introduces a small systematic error, slightly lowering the measured value of the isotope effect.The isotope effects were calculated by comparing the isotopic composition of the evolved hydrogen with that of the water.* The experimental procedure of the isotopic analysis was as earlier described. The H/D isotope effects measured for the eiq + eiq reaction were 3.9 0.2, 4.7 f 0.2, and 5.5 f 0.2 for solutions containing 25, 50, and 75 atom percent deuterium respectively. These values are the lower limits for the isotope effects, as the ratio R = [HD]2/[H,] x [D,] was higher than 4, the statistical value, in all the experiments. The values of R ranged between 4.7 and 6.0 for different samples. The increase in R cannot be accounted for by the H + CD,OH reaction alone, and it is58 probably due to the reaction between the OH radicals and the hydrogen molecules formed, the isotope effect of which causes an increase in R owing to a preferential depletion of the lighter isotope.The observed isotope effect is much higher than expected for the formation of hydrogen by a diffusion controlled reaction. The eiq + e& inter- action is diffusion controlled as measured by pulsed radiolysis,5 thus the observed rate is most probably that of the formation of the (e;q)2 centre. This is an intermediate with a finite lifetime which subsequently decomposes to give H, + 2 OHiq, with a preferential cleavage of H-0 as opposed to D-0 bonds. The involvement of several water molecules in the (e;q)2 centre may explain the change in the value of the isotope effect with the concentra- tion of deuterium.Hydrogen is one of the primary products in the radiolysis of water. In the presence of appropriate scavengers, the “molecular” hydrogen formed from water alone can be isolated. The e- + eiq has been suggested as a possible mechanism for its formation.8~f0-~ The H/D isotope effect in the formation of the “molecular” hydrogen was measured at pH = 14 by the radiolysis of the same water mixture, used in photolysis, and found to be 2.3 f 0.1 at 50% D a t 25” c. The same isotope as CHEMICAL COMMUNICATIONS effect has been previously observed in neutral and acid solutions.* The isotope effect in the formation of “molecular” hydrogen formed in the radiolysis of water is significantly smaller than that observed for the eiq -+ e& reaction.It has thus to be concluded that the e& + eib reaction is not liable for the formation of the major part of the “molecular” hydrogen. This conclusion is in accordance with previous arguments deduced from the measurements of other isotope effects8 and from the effect of scavengers on the yield of “molecular” hydr0gen.O The eis + e& reaction is, however, the main source of the “molecular” hydrogen according to the diffusion m0de1.l~~~~ It has also been demonstrated that the “residual” hydrogen atoms produced in neutral radiolysed solutions are not formed by the e& + H,O+ reactions as deduced in the diffusion mOdel.12~u Further, evidence was presented that a major part of the “molecular” hydrogen peroxide originates from a different reaction than OH + OH.16 The latter process is the only source for “molecular” H202 in the diffusion model.12 These findings imply the necessity for a revision of the diffusion model as presently conceived. The authors are thankful to Dr.A. Kuppermann for helpful and illuminating discussions. (Received, November 23rd., 1965; Corn. 731.) 1 M. S. Matheson, W. A. Mulac, and J, Rabani, J. Phys. Chem., 1963, 67, 2613. 3 F. S. Dainton and P. Fowles, Pvoc. Roy. Soc., 1965, A , 287, 313. 4 J. Jortner, B. Raz, and G. Stein, J. Chem. Phys., 1961, 34, 1455. 5 M. S. Matheson and J . Rabani, J. Phys. Chem., 1965, 69, 1324. 7 J. Jortner, Radiation Res., 1964, Suppl. 4, 24. * M. Anbar and D. Meyerstein, J. Phys. Chem., 1965, 69, 698. 9 D. Meyerstein, Thesis, Hebrew Univ., Jerusalem, 1965. 10 E. Hayon, Nature, 1962, 194, 737. 11 J. Rabani and G. Stein, J. Chem. Phys., 1962, 37, 1865. l2 A. Kuppermann, Radiatiolz Res., 1964, Suppl. 4, 69. 13 H. A. Schwarz, Radiation Res., 1964, Suppl. 4, 89. 14 T. J. Sworski, J. Amer. Chem. Soc., 1964,86, 5034, and T. J . Sworski in “Solvated Electrons”, American Chemica Society Advances in Chemistry Series, 1965, 50, 263. 16 A. Kuppermann, Radiation Res., 1965, 25, 101. 16 M. Anbar, S. Guttmann, and G. Stein, J. Chem. Phys., 1961,34, 703; M. Anbar, I. Pecht, and G. Stein, J. Chem. Phys., submitted for publication. J. Jortner, M. Ottolenghi, and G. Stein, J. Phys. Chem., 1964, 68, 247. L. M. Dorfman and I. A. Taub, J. Amer. Chem. Soc., 1963,85, 2370.
ISSN:0009-241X
DOI:10.1039/C19660000057
出版商:RSC
年代:1966
数据来源: RSC