No. 7, 1978 757
Pulse Radiolysis of Aqueous Cyanamide Solution
The Journal of Physical Chemistry, Vol. 82,
value due to the buildup of unsaturated species during the photolysis process.
A. A. Scala and P. Ausloos, J . Chem. Phys., 49, 2282 (1968). K. Shibuya, K. Obi, and J. Tanaka, Bull. Chem. SOC.Jpn., 48, 1974 (1975). A. Dhingra and R. D. Koob, J . Phys. Chem., 74, 4490 (1970). K. Dees and R. D. Koob, J. Phys. Chem., 77, 759 (1973). G. J. Collin, J . Chim. Phys., 74, 302 (1977). A. Deleon and R. D. Doepker, J . Phys. Chem., 75, 3656 (1971). C. K. Tu and R. D. Doepker, J . Photochem., 3, 13 (1974-1975). P. Ausloos and S. Lias, J . Chem. Phys., 44, 521 (1966). P. Ausloos, R. E. Rebbert, and S. Lias, J. Photochem., 2, 267 (1973-1974). F. P. Lossing, Can. J. Chem., 50, 3973 (1972). R. Leasleux, S. Searles, L. Wayne Sieck, and P. Ausloos, J. Chcm. Phys., 54, 3411 (1971). F. P. Lossing, D. G. Maroden, and J. B. Farmer, Can. J. Chem., 34, 701 (1956). R. D. Doepker, unpublished results. G. J. Coilin and P. M. Perrin, Can. J . Chem., 50, 2400 (1972). G. J. Collin, P. M. Perrin, and C. M. Gaucher, Can. J . Chem., 50, 239 (1972).
Acknowledgment. The authors acknowledge the National Science Foundation which aided in the support of this investigation in the early years of effort through Grant No. NSF-GP-20878. The authors also thank Dr. Guy Collin for timely correspondence. References and Notes (1) R. D. Doepker, J. Phys. Chem., 73, 3219 (1969). (2) K. L. Hill and R. D. Doepker, J . Phys Chem., 76, 3153 (1972). (3) (a) E. Lopez and R. D. Doepker, manuscript in preparation; (b) J. B. Binkewicz and R. D. Doepker, in preparation: (c) T. S. Pendleton and R. D. Doepker, study in progress. (4) C. L. Currie, H. 0. Okabe, and J. R. McNesby, J . Phys. Chem., 67, 1494 (1963).
A Pulse Radiolysis Study of Aqueous Cyanamide Solutions I. G. Dragan%,*+ 2. D. Draganib,+and K. Sehested Accelerator Department, Risd National Laboratory, DK-4000 Roskilde, Denmark (Received October 19, 1977) Publication costs assisted by Risd National Laboratory
The radiolysis of oxygen-free,aqueous solutions of cyanamide was studied by fast kinetic spectrophotometry. Computer simulation of the reaction mechanisms was used to evaluate the experimental data. Four different species are identified: (1)the radical anion (NH,CN)- absorbing light in the UV with, , A e240 nm and = 1500 M-l cm-l; the disappearance is a second-order process with 2k = 1.3 X lo9 M-' s-l; ( 2 ) the hydrogen adduct, NH2C(H)=N (or NH,C=NH), with A,, 300 nm and €300 = 150 M-' cm-l decaying by second-order kinetics with 2k = 3.1 X lo9M-l s-l; (3) the hydroxyl radical preferentially adds to the cyano group, NH2C(OH)==N 325 nm and (or "&=NOH). This species rearranges in the submicrosecond scale to NH,C(=O)NH (A, e325 = 1900 M-l cm-') and disappears by a second-order process with 2k = 6.3 X lo9 M-l s-l . (4) It is estimated that about 10% of OH radicals attack the substituent group and by H abstraction produce the NHCN radical (Amm 370 nm and €370 = 1800 M-l cm-l); it disappears by a pseudo-first-order process attributed to a hydrolysis reaction. At increasing acidities, protonation of this radical takes place, NHCN + H+ +NH,CN; the protonated form decays faster and absorbs more strongly. In a cyanamide solution containing S20a2-,the SO4-- radicals react with cyanamide, k = 1 x loa M-l s-l , producing 'NH2CN radicals. The dependence of the optical density at 325 nm on the dose rate and solute concentration are quantitatively consistent with the assumption that the OH radicals react with the NH,C(=O)NH species with k = 4 X lo9 M-l s-'. It is concluded that the cyano group in cyanamide, a N-cyano compound, is the main point of attack by ea;, H, and OH as was the case with previously studied nitriles with a C-cyano group and various cyanides.
-
Introduction Recent investigations1of the radiolysis of simple RCN compounds have suggested that the cyano group is the main target of attack by the primary free radicals from irradiated water. The substituent R was CH3, CzHj, CH,CN, or (CH2)&N, and they were found mainly to influence the amount of a given chemical change.l All these nitriles contained the +C-C=N group. The present work concerns the early stages of the radiolysis of cyanamide, H,NCN. This compound represents another type of nitriles, the N-cyano compounds. No published data on the radiolytic behavior of >N-C=N compounds in aqueous solutions are available. A recent study of the y radiolysis of aqueous cyanamide solutions2 gives the following rate constants for the primary reactions 'Permanent address: Laboratory of Radiation Chemistry, Boris K i d r i c Institute of Nuclear Science, P.O. Box 522, Beograd 11001, Yugoslavia.
0022-3654/78/2082-0757$01 .OO/O
eaq- t NH,CN
H+
NH,CN
--f
+
OH t NH,CN
R ~ , ~ - h = 1.5 x 109 ~
RH
-*
ROH
h = 6.7
X
k = 8.5 X
-
s-1 1
10' M-' s-'
l o 6 M-'
S"
Read,RH, and ROHare the short-lived intermediates. Their absorption spectra and kinetic behavior under various experimental conditions are the subject of the present study. The experimental data were obtained by fast kinetic spectrophotometry, and computer calculations were used to determine concentrations of the intermediates and to derive rate constants which could not be obtained directly from experiments. Experimental Section Irradiations. The pulse radiolysis system is described e l ~ e w h e r e .Relative ~ dosimetry of the 10-MeV electron pulses was carried out by means of a setup registering the current induced in a coil surrounding the electron beam.
0 1978 American Chemical
Society
758
I. G. Draganic, 2 . D. Draganic, and K. Sehested
The Journal of Physical Chemistry, Vol. 82, No. 7, 1978
The absolute dose was measured with a hexacyanoferrate(I1) dosimeter, in which G,,, + OH) = 5.25 and t420[Fe(CN)2-]= 1000 M-l cm-l were used. The dosimeter solution was 1 X M. Solutions. All samples were prepared in fresh, tripledistilled water and deoxygenated by bubbling argon through it for at least 30 min. The cyanamide was a Fluka product, mp 44-46 "C, with 2 % boric acid as stabilizer. A Merck product without stabilizer, mp 43-46 "C, was used in some experiments without affecting the results. In preparing strongly acid samples HC104 was added to deaerated neutral solutions before irradiations. Computer Simulation. The reaction mechanism was analyzed by computer simulation. When the fitted rate constants were derived, a sequence of reactions was selected so that only one parameter had a dominant influence on the experimentally observed rate of chemical changes. Computer-calculated concentrations of the reactive intermediates were used for the determination of molar extinction coefficients. The contributions from various short-lived species were taken into account, when the calculated values were fitted to the experimental data at different times after the pulse, usually up to about 15 ,us. In the reaction model, the following reactions of primary radicals in pure irradiated water were considered: eaq- t H,O+ H t H,O k , = 2.1 X 10,' M" s-' (1) --f
k , = 3 x 10" M-' s-'
eaq- t O H - OHH O
ea< t H
H, t OH-
2H 0
-
ea< t eaq- 2H,
OH t OH
+ 20H-
(2)
k , = 2.5 X 10" M-ls-' (3)
k , = 5.5 X
l o 9 M-'s-'
(4)
k , = 5.5 x 109 M-, s-1 ( 5 ) 12, = 2 x 10" M-'S-' (6) k, = 7.75 X l o 9 M-'S-' (7 1
H,O,
OH t H - H,O HtH-H,
The rate constants for reactions 1-7 were taken from ref 4-6. The values of GoH, GH, and Geac were taken according to the reactivities of solutes from ref 7. The computations were performed on the Burroughs B 6700 computer a t Risa, using a B 6700 extended algol program language and a program developed for kinetic studies in radiation chemistry by Lang Rasmussen.8
Results and Discussion Reaction of the Hydrated Electron with Cyanamide. No hydrogen is produced by the reaction of the hydrated electron with cyanamidea2 Also, the measured rate constant2of this reaction satisfies Taft's empirical relation, which was previously found to be valid for a series of e,, + RCN reacti0ns.l This supports the assumption of eaq addition to the cyano group ea< t NH,CN
-
(NH,CN)-
k , = 1.5 X
l o 9 M-ls-'
(8)
Figure 1 summarizes the transient absorptions observed 1,us after the pulse in deoxygenated cyanamide solutions. Besides the addition product from reaction 8 the transient spectrum in Figure l a also includes the contributions of unscavenged OH radicals and the product from the reaction of cyanamide with hydroxyl radicals. When tert-butyl alcohol (t-BuOH) is present in the solution (0.2 M), hydroxyl radicals are efficiently removed by reaction K, = 5.2 X l o 8 M-' s-' OH t t-BuOH R, (9) I Z ,=, 1 x 105 ~ - s1 16 H + ~ - B ~ O HR, (10) k,, = 7 x 10'M-l s-' (11) R, + R9 '11
-
--t
+
9 and the absorption in Figure l b is thus due to the radical
I
I
I
a
\
h,nm
Figure 1. The absorption spectra of the transients in aqueous solutions of cyanamlde (2.5 X lo-' M), pH 5, 20 krd. (a) Argon saturated solution: e, 1 ,us after the pulse; x, 35 ,us after the pulse. (b) e, 0.2 M t-BuOH added to the cyanamide solution; x, 0.2 M t-BuOH. (c) Molar extinction coefficients of the (NH,CN)- radical anion.
anion, (NH2CN)-, and the tert-butyl radical, Rg. Scavenging of the hydrated electrons by N 2 0 leads to a decrease of the absorption in the UV region and supports the assumption that the product of reaction 8, the radical anion, is the species absorbing with A,
