I. G. DRAG ANT^, Z. D. DRAGANI~, AND R. A. HOLROYD
608
Pulse Radiolysis of Aqueous Cyanogen Solutionla by I. G. Draganii,lb Z. D. Dragani6,lb and R. A. Holroyd” Chemistry Department, Brookhaven National Laboratory, Upton, L. I . , New York
11973
(Received September 10,1970)
Publication costs assisted by the U.S . Atomic Energu Commission
Pulse radiolysis has been used to study directly radiation-induced chemical changes in aqueous solutions of cyanogen. Reaction with the hydrated electron is fast, k(e,,(CN)L) = (2.1 XIZ 0.2) x lQIO M-l sec-1. The absorption spectrum of the cyanogen ion-radical (CN)z- shows two maxima at 290 and 440 nm, with respective molar extinction coefficients of 2100 and 360 M - l cm-l. The decay kinetics of the electron-adduct show a pseudo-first-order and a second-order process as two consecutive processes; probable reactions are discussed. The reactions of H and OH with cyanogen are slow in dilute solutions, and the rate constants were estimated as < l o 7 M - 1 sec-1. The formation of iminoacetonitrile radical N-CCNH and oxime radical NC-CNQH was considered.
+
Introduction The importance of the carbon-nitrogen triple bond in chemical evolution during the prebiological eja2t3 may explain the increasing interest in the chemistry and radiation chemistry of cyanogen compounds. The present paper describes our experiments with cyanogen, N=C-CaN. Gaseous (CN)2 has been studied by flash photoly~is,~J but there are no published data on the radiation chemistry of aqueous solutions of cyanogen. There is very little information on the chemistry of cyanogen dissolved in Like other cyanogen compounds it hydrolyzes and polymerizes in aqueous s ~ l u t i o n , but ~ ! ~ by application of fast reaction techniques the precursors of molecular products formed in irradiated aqueous solutions can be studied. Cyanogen is also very poisonous and pulsed beam experiments, with short irradiation and the possibility for direct observation of short-lived intermediates, minimise8 the amount of sample handling required.
booster which provided a square pulse of 200 A for 2.5msec duration. This increased the lamp intensity by approximately 60-fold in the ultraviolet. The optical arrangement allowed light path lengths through the cell of 4.0 and 6.1 ern in Febetron and Van de Graaff, respectively. High-intensity Bausch and Lomb gratings monochromators were used, and the light was detected by a photomultiplier (Amperex XP 1003) and oscilloscope (Tektronix 454) combination. The Febetron pulse and oscilloscope trace were delayed separately by 1.2 msec after the start of the light pulse. A t this time the light pulse was flat for -0.2 msec. Scattered light was reduced by suitable orientation of a quartz prism placed in the front of the monochromator. Cyanogen gas (Matheson) was used without additional purification. Triply distilled water was deaerated by bubbling with helium. The syringe technique‘l was used t o prepare samples with various cyanogen concentrations; a saturated solution was diluted and the cyanogen content calculated from the solubility datal2~l3 and the
Experimental Section The pulse radiolysis was carried out using a Febetron 705 system which provides a single pulse of 2-MeV electrons in a total time interval of less than 0.1 psec. Some irradiations were also performed with a Van de Graaff machine, 1.9-MeV electron pulses of about 60-psec duration. Dosimetry was carried out using two systems: (a) NzO-saturated 0.1 M thiocyanate solutions and measuring the (CNS)2- radical-ion at 500 nm, for which esO0 is 7600 M-’ ~ m - l ;(b) ~ NzO-saturated solutions of 0.001 M ferrocyanide containing a small amount of air. The ferricvanide formed was followed a t 420 nm and E420 is BOO0 A4-l cm-l.lo Both systems measure the free-radical yield and the value 5.4 was used for Gon -k Ge,,- in the dose Calculation. Absorbed doses varied from about 1 t o 40 krads per pulse. The analamp’ For Febelysing light source was a 450-w L r O n irradiations its output was increased with an arc
(1) (a) Research performed under the auspices of the U. 9. Atomic Energy Commission; (b) on leave from Boris KldriE Institute of Nuclear Sciences, VinEa, Yugoslavia. (2) (a) G. W. Fox, Ed., “The Origins of Prebiological Systems,” Academic Press, New York, N. Y., 1965; (b) M . Calvin, “Chemical Evolution,” Oxford University Press, London, 1969. (3) R. M. Lemon, Chem. Rev., 70, 95 (1970). (4) D. E. Paul and F. W. Dalby, J . Chem. Phys., 37, 592 (1962). (6) N. Basco and K. K. Yee, Chem. Commun., 150 (1968). (6) T. K. Brotherton and J. W. Lynn, Chem. Rev., 59, 841 (1959). (7) R. Nauman, 2. Elektrochem., 18, 772 (1910). (8) R. L. Webb, “Hydrogen Cyanide and Cyanogen Polymers,” in “Encyclopedia of Polymer Science and Technology,” Vol. 7, Wiley, New York, N. Y., 1967, p 571. (9) J. H . Bsxendale, P. L. T. Bevan, and D. A , Scott, Trans. Faraday Soc., 64, 2389 (1968). (10) P. Pagsberg, H. Christensen, J. Rabani, 6. Nilsson, J. Fenger, and S.0 . ~ i ~J. Phys. l ~chern., ~ 73, ~ 1029 , (1969). (11) E. J. Hart, “The Hydrated Electron,” in “Actions Chimiques et Biologiques des Radiations,” Vol. 10, M. Haissinaky, Ed., Masson, Paris, 1966, p 11. (12) A. Beidel, “Solubilities of Inorganic and Metal Organic Compounds,” v a n Nostrand, Princeton, N. J., 1940, p 216.
The Journal of Physical Chemistry, Vol. 76, No. 6,1871
PULSE RADIOLYSIS OF AQUEOUSCYANOGEN SOLUTION dilution factor. The solutions studied were at a natural pH of about 6. Since added substances accelerated chemical chmges (see below), no buffer could be used. Some irre,diations (1-2-rnin durations) were made with :t laboratory j°Co source. I n such cases the optical densities were measured in a Cary 16 spectrophotometer a t various time intervals after irradiation. Norrirradiate4 Aqueous Solutions of Cyanogen. It is that concentrated Cyanogen solutions change chemically on standing for a few hours. The first visible result ie the appearance of a yellow color in the solution followed by a brownish precipitate. We have examined the optical density changes of dilute solutions in time conditions currently used in our irradiation experiments, Cyanogt:n was introduced directly into the optical cell, and the measurements were performed various times after [he end of bubbling. Dilute cyanogen solutions do not a,bsorb appreciably above 230 nm; below 230 nrn I he absorption increases with decreasing wavelength. Absorbancy increases with time and with the initial concentration of (CN),. The increase is especiarllypronounced in the far-uv ( 240 nm, where the nonirradiated solution does not absorb appreciably. This is not the case if some other substances are present in the solution. Depending on the nature and concentration, they often cause such an increase in abmrbancy that the use of scavengers is extremely delicate il” not impossible. The chemistry that takes place in such cases is not well understood. It is even more complex in the presence of radiation, as shown by the analysis of kinetic data. We have studied the following salutes known to be good free-radical scavengers: CB@W, W 2 0 2 , I C N S , HCOONa, KCN, The conclusion reached was that he exception of N20, unsuitable for compelition experiments in the presence of cyanogen. The effect of pP3 variations could not be studied since the presence of acids or bases in the aqueous solutions of cyanogen accelerates the hydrolysis and other chemical changes. ~
Figure 1 shows the absorption spectrum of transients observed in a helium-purged cyanogen solution (1.92 X M ) 0.2 psec after the beginning of the Febetron pulse (9.9 krads). The shape of the spectrum is independent of cyanogen concentration (0.37 X to 1 and independent of the dose in the pulse 8). The maximum absorption is a t 290 nm initially but shifi a to shorter wavelengths following the pulse. Our experiments have shown that cyanogen reacts
609
200
300 X,
400 nm
500
Figure 1. Absorption spectrum of transients observed in helium-purged aqueous solutions of cyanogen (1.92 X 10-8 M ) : 0, 0.2 psec after t,he beginning of the Febetron pulse; A, 6 gsec after; 0, 40 psec after; absorbed dose 9.9 krads/pulse.
very efficiently with hydrated electrons, The rate constant for this reaction was derived from the firstorder decay of +eaq-in dilute solutions of cyanogen (5 X to 1 X lW4 M ) . Appropriate corrections for decay in the absence of solute were applied. The value calculated from the experimental data is (2.1 ztr 0.2) x 1O1O M-l sec-l. This value u’as confirmed by also following the buildup in absorption by the electron adduct in the uv range. The presence of nitrous oxide in the solutions causes a decrease in absorbancy and the disappearance of absorption maxima. I n these solutions the reaction of the
(CNZ)
+ eaq-
-
(CWICl = 2.1 X 1OlQ
sec-l
(1)
hydrated electron with cyanogen is in competition with its reaction with NzO. The effect observed can be seen NzO
+ eaq- --+OH + N2 4-OHkz
=
8.67 X lo9 M - l sec-l
l4
(2)
in Figure 2. The top curve gives the absorption spectrum in the uv range of a helium-purged dilute cyanogen M ) . The dashed curve represents solution (3.7 X the same solution saturated with nitrous oxide (2.4 X 10-2 M NzO), while the lower curve concerns heliumpurged water saturated with N20 (2.4 X lov2 M ) . The doses in the pulse were the same within experi(13).“Handbook of Chemistry and Physics,” Chemical Rubber Publishing Co.,Cleveland, Ohio, 1960. (14) S. Gordon, E.J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, Discussions Faraday SOC.,36, 193 (1963).
I. G. DRAGANI~, Z. D. DRAGANI~, AND R. A. HOLROYD
610 I
I
i
e
-P0.00
I--
A 25C
I
-
Figure 2. Absorption spectra of transients observed in 3.7 X 1 0 - 4 M cyanogen solutions: 0, top curve, helium-purged solutions; 13, dashed curve, 2.4 X M NzO present; X, lower curve, 2 4 x 10-2 M NzO in helium-purged water. Lower solid line through the experimental data was calculated assuming Gon = . 2.7 and using molar extinction coefficients for OH radical according to Pagsberg, et a2.lo Absorbed dose, 4.72 krads/pu’lse; ]Par lower curves OD = 0.50D,,,,,,ed.
mental erroy. It is significant that the lower curves differ very little. The solid line through the experimental data in Figure 2 was calculated assuming GOH= 2.7 and using pulolished molar extinction coefficients for the 013 radical.’* The difference between the two lower curves can be attributed to the incomplete conversion of the hydrated electron into OH in reaction 2 and to the presence of small amounts of the electronadduct, ‘The species giving rise to upper absorption curve could only be the electron-adduct (CN)2- plus either the hydroxyl radical-adduct or, if the reaction 3 is
-
CNOH
I
(3)
GN
slow, the hydroxyl radicals themselves. Analysis of the experimental data excludes the oxime radical, which is the eventual product of reaction 3, as the possible contributor in the above conditions. As seen in Figure 2, the experimental diata agree fairly well with the generally accepted trend for the hydroxyl radical.’O Also kinetic analysis ot the data indicates the presence of OH; a secoad-order decay is observed with a 2k N 1 X 1 0 1 0 M-’ sec-1. Similar observations were made in nitrous oxide saturated solutions containing larger cyanogen concentraand 1.92 X M). The conclusion tions (9.2 X The J O U T of ~ iPh.y8itm,! ~ Chewktry, Vol. 76,No. 6,1971
I
I
400
500
to3
0.0 200
300 X,nm
350
300 A , nrn
(cni), + ow
I
Figure 3. Absorption spectrum of cyanogen ion-radical (CN)zcalculated as molar extinction coefficients.
was that the reaction of cyanogen with O R is slow in dilute solution (kg 6 lo7 M-’ and the species contributing to the absorption shown in Figure 2 (top curve) are OH and (GN)2- only. The presence of oxime radicals and reaction 3 would require investigations with larger concentrations of cyanogen. Figure 3 shows the absorption spectrum of the electron-adduct, derived from the experimental data corrected for hydroxyl-radical ContribuLions. The data in Figure 2 and the corresponding dosea in pulse were used for correction calculations. The accuracy of such correction was considered sufficient as the O H contributions were low ( 10%); only at 260 and 240 nm were they somewhat more important, up to about 2070The decay of the absorbancy shown in Figure 1 is fairly fast; about 40 psec after a 9.9-krad pulse it is reduced to 20% of the initial value. Kinetic analysis of the experimental data was carried out at wavelengths where the OH contribution could be neglected (A 440 and 450 nm), or the correction was low 6 10% ( 290 nm) and the experimentally obtained corrections were considered as satisfactory. The two bands exhibit similar deca
