Formation and Reactivity of the Amino Radical type of discharge. Under no circumstances can a Boltzmann distribution, or for that matter, any time-averaged distribution, be utilized in the evaluation of this discharge. These results suggest that a valid model for this type of discharge must take into account the extreme voltage gradient in the vicinity of the wall and, its consequences, i.e., the enhancement of this gradient due to dielectric properties of the dark zone. In the wall region the electron energy is constantly changing and the variation of inelastic cross sections with the electron energy must be included. The concept of a plasma sheath in the vicinity of the capacitor plates is erroneous. Also, the spatial electron density must reflect the fact that the peak density occurs in the region of maximum ionization, not the discharge tube center. Experiments which require more than one period to complete must be considered as time-averaged, nonrepresentative of the plasma processes occurring. The principal technique which becomes necessarily suspect is that of a Langmuir probe analysis which determines electron number density and electron temperature. In addition, this study indicates that the inelastic collision processes resulting in the production of excited states must be taken into account to produce a realistic model.
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Further, the assumption that electrons are not swept from any part of the discharge tube is invalid. The thermalization of electrons by inelastic collisions and the production of low energy electrons by ionizing collisions in the vicinity of the rf plates must be accounted for. A realistic model should be able to predict the spatial and temporal properties determined by this research. Acknowledgment. This work was supported in part by an American Chemical Society, Division of Analytical Chemistry Summer Fellowship sponsored by the Olin Corporation Charitable Trust and an ACS, Division of Analytical Chemistry Edwin Dowzard Summer Fellowship. The loan of rf plasma equipment by ICP Corp., Hayward, Calif. is also greatly appreciated.
References and Notes (1) S. C. Brown in "Handbuch der Physik", Vol. 22, S. Flugge, Ed., Springer-Verlag,Berlin, 1956,pp 531-574. (2) See, for example, K. G. Muller, Chem. Ing. Tech., 45, 122 (1973). (3) A. T. Bell, Ind. Eng. Chem., Fundam., 9, 160 (1970). (4) A. T. Bell and K. Kwong, AIChEJ., 18,990 (1972). (5) J. B. Thompson, Proc. R. SOC. London, Ser. A , 262, 503 (1961). (6) E. 0.Degenkolb, C. J. Mogab, M. R. Goldrick, and J. E. Grlffiths, Appl. Spectrosc., 30, 520 (1976). (7) W. L. Harries and A. von Engel, Roc. R. SOC.London, Ser. A , 222, 490 (1950).
Formation and Reactivity of the Amino Radical' P. Neta," P. Maruthamuthu, P. M. Carton, and R. W. Fessenden Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received April 20, 1978) Publication costs assisted by the Department of Energy
The amino radical has been produced by reaction of the OH radical with ammonia in irradiated aqueous solutions. At pH 11.4 the unprotonated radical, NH2, is produced with a rate constant h = (9 f 1) X lo' M-' s-l, while in acidic solution the corresponding reaction of OH with NH4+is too slow to be observed. The SO4- and' :OP radicals are found to react more slowly with NH3 than does OH. It is concluded that these radicals react with ammonia by hydrogen abstraction and not by electron transfer oxidation. The method is not useful for the production of the protonated NH3+radical. The subsequent reactions of NHz are followed by monitoring the kinetics of formation of the more strongly absorbing radical products. Rate constants for the reactions of NH2 with various substituted phenoxide ions have been measured; with phenoxide itself at pH >11, k = 3.0 X lo6 M-' s-'. These rate constants are strongly affected by the ring substituent. When the rates are plotted against the Hammett (T values, a slope corresponding to p i= -3.3 is derived. The mechanism is considered to be oxidation of the phenoxide ions by electron transfer. Although the NHz radical was found to be relatively unreactive toward addition to benzene, its adduct radical to the fumarate anion has been observed by ESR. The NH2 radical may also be produced by reaction of ea; with NHZOH. An attempt was made to generate the NH3+ radical in the corresponding reaction with the acid form of hydroxylamine, +",OH, which is especially fast. On the basis of several pieces of evidence, we postulate that NH3+cannot be the main product of this reaction, but that +NH30Hsplits preferentially into the OH radical and NH3.
Introduction The amino radical can be produced from ammonia by the reaction NH3 + OH NHz + HzO (1) which was studied by pulse radiolysis of aqueous solutions.2 A rate constant of hl = 1.0 X lo8 M-' s-l was determined, while the parallel reaction of OH with NH4+was found to be very much slower and could not be detected by pulse radiolysis. Several studies on the reactivity of the amino radical, by ESR3+ and p ~ l a r o g r a p h y ,have ~ , ~ utilized the redox system Ti"'-NH,OH for the production of NH2.
-
0022-3654/78/2082-1875$0 1.OO/O
Ti"'
+ NHzOH
-
Ti"
+ NHz + OH-
(2)
Reduction of hydroxylamine by e,; has also been suggested to produce the amino r a d i ~ a l .Data ~ from a pulse radiolysis study of this latter systemg suggested that the pK, value for
NH3+ + NH2 + H+
(3)
was 6.7. However, an earlier studylo indicated a lower value of pK, = 3.65. In either case the reactivity of the amino radical is expected to be p H dependent as a result of this equilibrium and this change should occur in the 0 1978 American Chemical Society
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The Journal of Physical Chemistry, VoL 82, No. 17, 1978
region of pH 3-7. The ESR and polarographic experiments were carried out in acid media so that they dealt mostly with NH3+ radicals. The adducts of this radical with various olefins were detected by ESR3t5and the observation of the nitrogen splitting in these adducts confirms the occurrence of reaction 2. Relative reactivities of the amino radical with several organic compounds have been r e p ~ r t e d ~but f - ~the absolute rate constants for such reactions are unknown. The study of these rate constants by pulse radiolysis was one of the aims of this work. Unfortunately, the optical absorption of NH, is too weak2 to be used to follow the decay of this radical. One has to resort to observation of the buildup of product radicals. We have attempted to observe such radicals and to determine the rate constants for their formation, utilizing both ammonia and hydroxylamine as sources for NH, and NH3+, respectively. Experimental Section Aqueous solutions were prepared using Reagent grade water from a Millipore system. Ammonium hydroxide was a Baker Analyzed reagent and hydroxylamine was Fisher Certified in the form of (NHzOH)2H2S04.The perchloric acid and potassium hydroxide and phosphates used for pH adjustment and buffering were Baker Analyzed reagents. The alcohols, phenol, resorcinol, and benzoic acid were also Baker Analyzed, while the other organic compounds were obtained from Eastman and Aldrich and were of the purest grade available. The solutions were deoxygenated by bubbling with pure nitrogen or nitrous oxide. The latter was used when the reactions of OH were desired, since N 2 0 reacts with ea; rapidly to produce OH. In experiments with ammonia, the water was thoroughly bubbled before the ammonium hydroxide solution was added; after addition only slow bubbling was continued in order to minimize the loss of NH3. The solutions were irradiated with 5-11s electron pulses from an ARC0 LP-7 linear accelerator. The doses used produced radical concentrations of 2-4 pM. Other details of the experiments and the computer controlled pulse radiolysis apparatus were as described previous1y.l' Results and Discussion Formation of NH2 from Ammonia. Reaction 1 can be used to produce NH, from ammonia in irradiated aqueous solutions. Its rate constant was reported2 to be 1.0 X lo8 M-ls-l. We have redetermined this value using two pulse radiolysis competition methods, namely, with benzoate" and t h i ~ c y a n a t e 'ions, ~ both at pH 11.4. A value of hl = (9 f 1) x 107 ~ - 1 s - 1was obtained from both systems, in good agreement with the previous determination., The reaction of OH with NH4+was found to be too slow to be observed by pulse radiolysisa2 Reaction 1 is formulated as a hydrogen abstraction reaction. We considered that radicals, which are stronger oxidants than OH, might possibly react with NH3 more rapidly than OH and might even oxidize NH4+to produce the amino radical in acid solutions. An attempt to use SO, for this purpose14was.without success. The rate constants for the reactions of SO4-and PO?' radicals with NH3 and :4" were determined as described previ0us1y.l~The rate for SO4- + NH4+was measured at pH 7.0 and found to be only (3 f 0.5) X lo5 M-l s-l. The rate for So4- NH, was measured at pH 9.2, assuming that half of the total ammonia was present as NH,, and found to be only (1.4 f 0.2) X lo7M-l s-l, Le., considerably lower than the rate of the OH reaction. The phosphate radical reacted even more slowly than SO4-. The rate constant for P042;+ NH, at p H 11.0 is (2.2 f 0.3) X lo6M-l s-l and for HP04- + NH4+
+
Neta et al.
is only (4 f 1) X lo4 M-' s-l at pH 7.1. These findings indicate that all three, radicals react with ammonia by hydrogen abstraction and not by electron transfer oxidation.14 They also show that ammonia can only be used as a source of amino radicals in irradiated basic aqueous solutions so that one cannot study the properties of the NH3+ radical using solutions of ammonium salts. The Reactivity o f N H 2 Radicals. The NH, radical exhibits only a very low optical absorption in the visible region (Ams 530 nm, emax 81 M-l s-l),, while its absorption in the UV (
