Pulse Radiolysis of the Cyanate Anion
803
Pulse Radiolysis of the Cyanate Anion in Aqueous Solution J. 6. Leopold and M. Faraggi" Atomic Energy Commission, Nuclear Research Centre-Negev, P.0.6. 900 1, Beer Sheva, Israel (Received August 5, 1978)
The reaction of OH radicals with NCO-(X-) has been investigated by the pusle radiolysis technique. Irradiation of cyanate anion solutes produced a transient species absorbing at ,A, 330 nm (e lo00 M-' cm-'1. The spectrum has been assigned to a complex form of the cyanate radical which arises from a reaction of OH with the solute. The second-order rate constant was established via competition with alcohols and was found to be OH + NCO(H') HNCOOH (l),k = 5.9 f 0.7 X lo7M-' s-' . The HNCO; radical (XOH-) complexes with a solute anion according to HNCOzH + NCO- s (NCO); + H20 (5), K = 100 f 20 M-' with k5 = 4.5 X lo6 M-'S-' and k-5 = 4.4 X lo4s-'. The decay of (NC0)i is of second order and depends on solute concentration. A rdical-radical recombination set of reactions is proposed.
-
Introduction Pulse radiolysis and flash photolysis oxidation of halides and some pseudohalides (X-) produce transients absorbing in the 330-500-nm region. The mechanism of the oxidation reaction has been interpreted by the production of the corresponding radical anion (XL). The simple mechanism adopted assumed the formation of the radical (X) by an electron transfer reaction from the halide anion.'-5 In pulse radiolysis, OH radicals are generated and the electron transfer reaction is X-+ OH-X+
OH-
(1)
Similarly, in flash photochemistry of some halide and pseudohalide anions an appropriate photon will cause the emission of an electron from an excited state: hv
X--+X-*+Xte,g
(la)
These reactions, producing by different pathways the same transient, are followed by the reaction:
xt
x-qx;
(2)
forming the radical anion (X;). Matheson et a1.6 studied the reaction of Br- with OH radicals and suggested replacing reaction 1by the sequence of reactions: X- + OH S XOHXOH- t; X + OH-
(3)
(4)
Recently Zehavi and Rabani7g8and Behar, Bevan, and Scholes$lo studying the oxidation of Br- and NCS-, concluded that the formation of the radical ion could be a result of an additional reaction: XOH-
+ X- f X2- t
OH-
(5)
Thus, the mechanism of X; formation could be described by reactions 2-5. Very recently Behar and F e ~ s e n d e n studied '~ the cyanate system by the radiolysis-ESR method and came to the conclusion that the transient is a proton containing radical (HNCO; or HNCOOH) produced via reaction 3. The absorbing transient species was suggested to be X,; however, it should be mentioned that other intermediates (XOH-) could absorb in the same wavelength region.'OJ3 In view of the above knowledge on the oxidation of halide and pseudohalide anions, we investigated the
* Present address: Chemistry Department, Ohio State University, Columbus, Ohio 43210.
cyanate system (NCO-) by pulse radiolysis. Unfortunately, due to the chemical properties of this anion, alkaline solutions (pH >9) could not be studied.
Experimental Section The pulse radiolysis apparatus, the experimental methods involved, and the analysis of the results have been described e1~ewhere.l~ Electron pulses of 0.2-1.0 ps (OH radical concentration from 1.4 to 9.4 X lo4 M) were used. Dosimetry was carried out using NzO saturated lo-' M Fe(CN)t- solutions assuming G(Fe(CN)z-) = 6.1." For calculation of the extinction coefficient of the transients, G(0H) = 2.7 for argon saturated solutions and G(0H) = 5.6 for NzO saturated solutions were assumed over all the concentration ranges used.4 The rate constants measured from the oscilloscopetraces are defied as Kobd= 1/ [X-] d In (Dms - D,)/dt. It is the observed second-order rate constant of formation when direct measurements of the absorbing species were investigated (Dms is the maximum optical density of the transient absorbing species; Dt is the measured optical density at time t). Potassium cyanate (KNCO, Fluka purum) was recrystallized at least three times from a water-methanol mixture.17 The purity of this compound was checked spectroscopically (A, 195 nm; tmax1.1 X lo3M-' cm-' ). All other materials were of analytical grade. Solutions were made up in triply distilled water deaerated by ultrapure gases (i.e., argon or NzO). Dilute analytical grade acid (HC104) or base (NaOH) was added to adjust the pH. Usually neutral solutions (pH 5.0 f 0.4) were studied. Results and Discussion When fast electrons are absorbed in water the effect may be described by the overall reaction H,O
4
eaq, H, OH, H,, H,O,, H,O+
(6)
and the ensuing chemistry occurring in aqueous solutions can be accounted for in terms of these initial entities; the yields of the products G (G = number of species per 100 eV) were as follows: G,, = 2.7, G H = 0.55, GOH = 2.7, GHz = 0.45, GHzOz = 0.7. When NzO saturated NCO- neutral solutions are irradiated by a single pulse, a transient species is formed with an absorption maximum at 330 nm (Figure 1). When oxygenated solutions were used, the absorption at 330 nm was exactly one half of that observed in the NzO saturated solutions. Addition of alcohols suppressed the absorption at 330 nm. The Journal of Physical Chemistty, Vol. 81, No. 8 , 1977
804
J. G. Leopold and M. Faraggl
I
I
I
10
50
100 CNC0-1~10~
Flgure 1. Absorption spectrum of transient obtained from the reaction of OH radicals with NCO-. N20 saturated solution of [NCO-] = lo-' M, [OH] = 9.7 X IO-' M at pH 5.0 f 0.2.
These results seem to indicate that OH radicals formed during the radiolysis of water react with NCO- to produce the transient absorbing species. In N 2 0 saturated solutions where all ea; are converted to OH N,O t e,;
-+
N, t OH t OH-
(7)
thus G(0H) = Gesq-+ GOH = 2GoH in oxygen saturated solution e,; t 0 ,
+
0,
(8)
and G(0H) = GoH in alcohol solutions, suppression of the transient is due to RCH,OH
+ OH
-+
RCHOH t H,O
(9)
The reaction between NCO- and OH could be the electron transfer-addition reactions similar to those observed in other halide and pseudohalide systems or, even though unlikely on thermodynamic grounds, an oxygen abstraction reaction resulting in the production of H202,followed by NCO- t OH HO, t HO,
+
-f
CN- t HO,
(10)
H,O, t 0,
(11)
or NCO- t OH
+ H'
.+ CN t H,O,
(12)
Flgure 2. The variation of the pseudo-first-orderformation rate constant (kbsd[NCO-])with NCO- concentration at pH 5.1 f 0.2. Each rate constant in the curve is a mean value calculated from values obtained at various doses (OH radical concentration of 2.8, 3.9, 5.7, 8.5, and 9.7 X IO-' M). The deviation in these rate constant was *IO% or better.
At lower concentrations of NCO- the recombination reaction should be taken into account. The possibility of the formation of the NCO radical from the HNCO; radical ion (the OH adduct intermediate, reaction 3) or ita protonated form HNCOOH, which differs from an activated complex by its lifetime and its ability to react with solutes, was investigated by the alcohol competition method as suggested by Zehavi and Rabani.'>' They showed that if XOH- is an intermediate reacting with X- to form X; via reaction 5, and equilibrium 2 is shifted to the X; side, the competition with alcohol on the OH radicals will effect the X; concentration as follows
absence of alcohol and in the presence of alcohol RCH20H, respectively. From eq I it seems that DOmaJDmax is a function of the relative concentration of alcohol to X- and of the absolute concentration of X-. The results of the alcohol competition reaction are shown in Figure 3. RefEdless of the alcohol used (methanol (k, = 8.6 X 10' M- s '), 2-methyl-2-propanol ( k , = 5.2 X 10' M-' s-' the absolute concentration of X- had no effect on the optical density ratio. From Figure 3 the rate constant of the reaction between OH radicals and cyanate was evaluated to be 5.9 f 0.7 X lo7 M-ls-'. These results seem to indicate that only one transient is formed during the oxidation reaction of the cyanate anion. In view of Behar and Fessenden'* finding that the transient is a proton containing radical ("COOor HNCOOH) the reaction proposed is
''
This last possibility was nevertheless investigated. As expected no increase in GVzoz(0.75) was found (both in y and pulse radiolysis). This eliminated the possibility of an abstraction reaction. The formation rate constant of the transient and its absorbance was found to depend on the cyanate concentration (Figure 2). Since this rate constant was found to be equal to 5.9 f 0.7 X lo7 M-' s-' (vide infra), it is readily shown that for the concentration range used (8 X 1 [OH] 1 2.9 1 [NCO-] 1 5 X lo3 M and 9.7 X X 104M) the interference of the OH recombination reaction ( k l ,= 1 X 10" M-'s-')" OH t OH + H,O, (13)
NCO-
is negligible.
HNCOO-
The Journal of fhysical Chemistry, Vol. 81, No. 8 , 1977
)
-- 1 + k-g [RCH20H1 (t + h4 +kh,[X-l -3 (1) Dmax k3 [x-I where Domaxand D,, are the absorption of X; in the DOmax
+ OH -+
HNCOO-
H'
HNCOOH
(3)
However the fact that the experimentally observed second rate constant of reaction 3 kowdecreases with the cyanate concentration (Table I) has to be explained. Baxendale et al.4 developed a solution for the halide system where the oxidation reaction is followed by an equilibrium reaction of the solute with the formed solute radical Le.
+ NCO- ?2
(NCO),' t OH-
(5)
805
Pulse Radiolysis of the Cyanate Anion 30 -
1.0
ly" l
!
.
'
"
.01
~ .05
'
'
'
alc. / NCO-
I
'
l
,lo
Figure 3. Competition for the OH radical by NCO- and alcohols plotted according to eq I. The upper and lower lines are for methanol and 2-methyl-2-propano1, respectively. Each point in the graphs is a mean of several determinatlonsat different NCO- concentrations at a constant ratio of [alcohol]/[NCO-1. N20 saturated solutions of M 5 [NCO-] I lo-' M. [alcohol] = 0 to lo-' M and [OH] = 9.7 X lom6M at pH 5.0 f 0.4.
or HNCOOH t NCO-
(NCO),-
+ H,O
(5')
([X;lm - [x;]t)/[XJo = Ck5'[(h5' + k 5 'exp(-k,'t) ) - k3' exp(-(k5' + k s ' ) t ) ] > / [ ( k 5+' k s ' ) ( k 5 ' + ks'- h3')]
(11)
-
(111)
or
+ k-5')t
(IV) Plotting log A(0D) vs. the time will give k5[NCO-] + k5' as the slope of the linear curve. This slope is in fact the observed slope determined experimentally from the oscilloscope traces. how [NCO-] = k,[NCO-] - Dt)/dt
+ &'
20
30
40 1/ INCOI
where k5'= k5[X-], k3'= k3[X-], [X;], and [XJ are the concentration of (NCO); at infinity (i.e., the concentration attained at equilibrium) and time t correspondingly; [X;], is the concentration of (NCO); which could be obtained if all the OH present yielded (NCO);. This equation holds on the condition when the initial concentration of OH radicals produced by the electron pulse is small compared to the NCO- concentration, i.e., the kinetics of (NCO); formation reactions will be first order. If k5[NCO-] + k5'> k3[NCO-], the rate of the overall reaction will be determined by JZ3[NCO-],and kohd will not be a function of NCO- concentration. However, if k3[NCO-] > k5[NCO-] + k+', then the rate at high concentration of NCO- will be determined by k5[NCQ-] + k5'. Under these conditions the above equation will be ([(NCO);] - [(NCO);I t)/[(Nco);I o = [k5[NCO-] exp(-(k,[NCO-] + h-5')t)]/(k5[NCO-] + k5')
10
= d(ln D,,
(VI
Figure 2 shows the curve obtained for k,b,,JNCO-] as function of [NCO-1. This line gives k5 = 4.5 X lo6M-' s-l, k5'=4.4 X lo4s-l, and k5 = k 5 / k 5 ' = 100 Me'. Hence the rate constant measure at low concentration of NCO- (
