Protonation of the reduced species in irradiated acetonitrile

0146-5724/85 $3.00+ .oO 0 1985Pergamon PressLtd.

Phys. Chem. Vol. 26,No. 2, pp. 205-209,1985 printedin GreatBritain.

Radior.

PROTONATION OF THE REDUCED SPECIES IN IRRADIATED ACETONITRILE” W. A. MULAC, A. BROMBERG and D. MEISEL Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A. Abstract-The rate of the reaction of the radiolytically produced reduced species in acetonitrile with various protic solutes (alcohols and water) was measured by the competition method and by directly following the rate of disappearance of the reduced species in pulse radiolytic experiments. The rate constants thus obtained were correlated with the pK, of the protic additives. This correlation leads to the conclusion that the reaction occurs via a proton transfer. Temperature effect studies on the rate of this reaction lead to the conclusion that the reactive species is the dimer anion radical (CH$ZN)i.

INTRODUCTION

EXPERIMENTAL

PULSE RADIOLYSIS studies”’ of liquid acetonitrile have established the identity of the reducing species produced upon its radiolysis as the monomeric CH3CN- . This species undergoes rapid equilibration with another CH3CN molecule to form the dimerit radical anion (reaction 1):

(1)

CH$N-

+ CH3CN * (CH&N);

The enthalpy change accompanying reaction 1 has been measured to be - 34.9 kJ mol-’ and thus the prevailing species at elevated temperatures is the monomeric radical anion.“’ It was also reported in the same study that in the presence of small amounts of protic solvents (Hz0 or alcohols), the yield of CH3CNdecreases. Protonation of CH&Nwas suggested as the cause of this decrease in yield. The dependence of the reaction of the protic solvent on its concentration was, however, rather complex. In the course of our studies of reactions of radicals produced radiolytically in acetonitrile, we became interested in the effect of alcohols on the reducing species in this solvent. This is particularly important since alcohols are often used as hole scavengers in aprotic solvents. In the present report, we seek to further elucidate the characteristics of this reaction. Correlation of the rate of the reaction of the reducing species with the pK, of the reacting protic donor indeed confirms the nature of the reaction to be a protonation reaction. The effect of temperature on this reaction, however, indicates that the protonation is primarily of the dimeric (CHxCN);, form. * Work performed under the auspices of the OffIce of Basic Energy Sciences, Division of Chemical Science, US-DOE under contract number W-31-109-ENG-38. 205

Reagent grade acetonitrile was further purified by passing through a 0.5 m column of activated alumina. The decay rate of the pulse-radiolytically produced reducing species in deaerated acetonitrile followed a first-order rate law and yielded rate constants of (1.0 2 0.2) x lo6 s-l. Since this decay is primarily due to reaction with impurities and since its rate is similar to that observed previously,“’ we consider this purification technique satisfactory. Triphenylmethanol was purified by recrystallization from benzene. All other materials were of highest purity commercially available and were used without further purification. Solutions were deaerated by bubbling argon using the syringe technique. The pulse radiolysis experiments were performed using the Argonne 15 MeV Linac. Electron pulses of 2-40 nsec halfwidth were used to produce 0.5-10 p,M of reducing species in acetonitrile. Concentration changes in 2 cm Suprasil cells were monitored spectrophotometrically, the signal was digitized and transferred to a VAX 1l/780 computer for analysis. Unless otherwise stated, experiments were performed at 26°C. For temperature dependence studies double-walled Suprasil cells were used and the temperature was controlled by flowing acetonitrile at the desired temperature in the outer volume.

RESULTS

AND DISCUSSION

The rate constant for the reaction of the reducing radical (monomer or dimer), denoted S- , was measured by two methods. The first method involves direct detection of the decay of the absorption of S- . For reasons of convenience, we chose 550 nm in these experiments. At this wavelength, the absorbing species is primarily the dimer radical

206

W. A. MULAC

anion, while the monomer absorbs further into the red. However, since these two species are at fast equilibrium with one another, the detection wavelength has no effect on the kinetic behavior. The second method is competition between the protic donor, ROH, and triphenylmethanol for S-. The latter method was primarily employed when the decay rate of R approaches the time resolution of the presently utilized experimental setup. The following reaction scheme is assumed to take place in these experiments. S-

(2) S-

according

1

[$3C]

=

k3

k4[43COHl

ii&F&

That S- produces +SC’ radicals in reaction (4) has been verified by recording the absorption spectrum of the product (A,,, = 338 nm, l338 = 3.6 x lo4 M-i cm-‘).(*) In the competition method, the yield 100

nsec pulse

=

=k2

’

,

I

+2

!

where [43C’]o and [43C’] are the concentrations of the radical produced by the pulse in the absence and presence of ROH, respectively. From plots of 1/[43C’] as a function of [ROH] at constant [4,COH], the ratio k2/k4could be extracted. k4 was determined directly by following both the formation of the absorption of 43C’ and the decay of S- in the absence of any protic donor. The same value, within experimental accuracy, k4 = (5.9 ? 0.4) x lo9 M-’ SK’, was obtained by both methods. k3 was measured by following the decay of S- in pure acetonitrile for each batch (but see below). An example of this procedure is shown in Fig. 1. In all cases,

+ &COH -+ &C’ + S + OH-.

I

to eqn (5)

kztROH1 + U43COHl

+ RO-,

+ ROH+SH

was analyzed 1

@)

S- (+ Imp.) -+ P,

(3) (4)

of&C’

et al.

5.7 x 10’

10 nsec pulse :/

E

0

/ 0

60

‘z

=k2

=

4.7 x IO’

mc /

/

pulse I

01

0.05

0.00

[EtOHl, M Fig. 1. Determination of the rate of protonation by competition between EtOH and &COH at three different doses (2, 10, 20 nsec pulse width). [$,COH] = 10m4 M.

Protonation

207

of the reduced species in irradiated acetonitrile

k2 measured by the competition method agrees within 225% with those measured by the direct method. Figure 2 presents an example of the results obtained from the direct method, following the decay rate of S- as a function of MeOH and i-PrOH. As can be seen in Fig. 2, a complication arises at low alcohol concentrations. At these low concentrations, the rate of decay of S- seems to decrease somewhat upon increase of [ROH]. The range of concentration that this effect is observable correlates with the reactivity of the protic additive towards S-. The faster the latter, the smaller is the range of concentration at which this effect is observable. This decrease in the decay rate of S- indicates removal of one of the decay channels available to S- upon addition of ROH. We attribute this process to scavenging of the radiolytically produced oxidizing equivalents, i.e., holes or their subsequent products, by the alcohols. At any rate, this observation poses some difficulty in deriving the right value of k3. As the extrapolation in Fig. 2 indicates, the level of impurities is quite low and their effect on the decay rate is rather small. Yet these impurities dominate the decay of S- in the pure solvent since it follows first-order kinetics. Further decrease of k3 upon addition of ROH will have only

a marginal effect. For the competition experiments we therefore take the lowest value observed for the decay of k3. It should be noted that for the efficiently reactive alcohols the choice of k3 has only a small effect on the determination of kt. On the less efficient alcohols (i-PrOH and t-BuOH) this effect is quite significant. It was also noticed that at concentrations higher than those reported here another scavenging reaction by the protic donor sets in and the rate constant measured by either method abruptly increases. We attribute this increase to a change in the scavenging reaction (e.g., scavenging of the monomer in addition to the dimer or scavenging of their precursor). We, however, report here only results from experiments in the low concentration regime. In Fig. 3 we present free energy correlation for the rate of the reaction of ROH with the pK, of ROH. Although the pK,, values have been determined in water,“’ the same order is expected to be maintained in acetonitrile. The type of correlation given in Fig. 3 provides a sound indication that the reaction is indeed a protonation reaction. The large value of k2, although well below the diffusion controlled limit, precludes the possibility of a direct reaction with Ht. It can be seen in Fig. 3 that the rate constant of the reaction of MeOH with S- is

20.0

0

16.0 F I co 7. E!

10.0

X Y

6.0

0.0, 0.00

0.02

0.04

[ROHI, M

0.06

1

0.08

Fig. 2. Rate of decay of S- monitored at 550 nm as a function of [MeOH] and [i-PrOH].

208

W. A. MULAC et al. I

I

I

I MeOH 8.5

8.0 At?

g

7.5

7.0 OH 6.5

Fig. 3. Free energy correlation for the reaction of ROH with its pK,. Rate constants for reaction (2) vs the pK, of ROH.

approximately an order of magnitude smaller than the diffusion controlled limit. It has been shown for a large number of protonation reactions that the rate constant for proton transfer is between a factor of 2-10 smaller than the diffusion controlled limit when ApK, = 0 for the two acid-base couples.‘4’ From this consideration, we may conclude that the pK, of S- is -15. It is interesting to note that the reaction of Swith triphenylmethanol, and similarly with diphenylmethanol and benzyl alcohol is an electron transfer rather than a proton transfer reaction.‘5’ (6)

S- + Ar-

RI I C -OH--+Ar-CC’

RI I + OH-

+ S.

Since the pK, for benzyl alcohol is quite similar to that of MeOH (15.4),(3) the different pathway reflects the added resonance stabilization of the arylmethyl radicals. Yet, as discussed below, the pro-

ton transfer reaction occurs to the dimeric species while reaction (6) might occur with the monomeric radical ion. We may also note that the protonation site of CH3CN- in water has been shown by epr to be the nitrile carbon atom.@’ We now turn to the question of the identity of S- . As discussed in the introduction two reducing species in rapid equilibrium with one another have been identified in the radiolysis of acetonitrile [reaction (l)]. The protonation reaction could therefore proceed with either or both of these species. To elucidate this point we studied the effect of temperature on the rate of reaction (2). As can be seen in Fig. 4 the rate constant decreases by about an order of magnitude on increasing the temperature from -30°C to 63°C. This dependence, however, is non-Arrhenius. Qualitatively, the inverse dependence on temperature indicates that the reactive species is primarily the dimer radical anion, (CHsCN);. Assuming that equilibrium 1 is rapidly achieved as compared to reaction (2), the observed rate constant can be expressed as (7)

k obs

=

k*,K’/(l

+ K’),

Protonation

209

of the reduced species in irradiated acetonitrile

8.6

8.0

4.2

1000/T Fig. 4. The dependence

(K)

of k,,bs for the protonation on temperature. ROH = EtOH; dashed line: temperature dependence of klo (see text).

kza now specifies reaction (2) as the reaction of the dimer and K’ = KI[CH~CN]

where

prising and might indicate some more elaborate mechanism for the proton transfer reaction. Acknowledgement-Discussions

@a)

(CH3CN):

+ ROH + (CH3CN)zH

+ RO-.

The results of Bell et al. (I) clearly indicate that at low temperatures K’ % 1 and thus at -30°C kobs = kza = 4.0 x lo8 M-’ s-’ for EtOH. Using the ternperature dependence of K’ given in Ref. 1, we obtain from eqn (7) the temperature dependence of kza. This is presented in Fig. 4 as the dashed line. Considering the fact that the values of kza are approximately two orders of magnitude smaller than the diffusion controlled limit, the small activation energy thus obtained (2.5 kJ mol-‘) is quite sur-

with Dr. P. Neta are

gratefully acknowledged. The dedicated operation of the linac by D. Ficht and G. Cox is much appreciated. REFERENCES I. P. BELL, M. A. J. RODGERS and M. D. BURROWS,J.

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:. 5’ ’

74, 3362.