On the Formation of the Perchlorate Radical during Radiolysis of Perchlorate Systems

V. N. Gelovskii, I. K. Skorokhodov, L. T. Bugaenko

(Presented by the scientific seminar of the Department of Radiation Chemistry)

As is known, the possibility of the formation of \(\mathrm{ClO_4}\) radicals has been postulated for various processes in which the perchlorate ion takes part. Thus, Gomberg \([1]\) suggested that the \(\mathrm{ClO_4}\) radical arises in the reaction of \(\mathrm{I_2}\) with \(\mathrm{AgClO_4}\) in anhydrous ether. Its formation as an unstable intermediate product has also been assumed in a number of processes: in anodic oxidation of chloric acid (accompanied by liberation of \(\mathrm{ClO_2}\) and \(\mathrm{O_2}\) \([2,3]\)), in the thermal decomposition of perchlorates \([4,5]\), in a number of chain gas reactions of chlorine oxides \([6]\), and in the ionization of perchlorate ion under the action of ionizing radiation.

However, up to the present time no unambiguous proof of the formation of this radical has been obtained. ESR data likewise do not give a definite answer. In work \([7]\) it was reported that the \(\mathrm{ClO_4}\) radical was observed in the ESR spectrum during electrolysis of a \(\mathrm{NaClO_4}\) solution in acetonitrile. The spectrum observed by the authors of that work, however, apparently belonged not to the \(\mathrm{ClO_4}\) radical but to the product of its decomposition—the \(\mathrm{ClO_2}\) radical—since the obtained value of the splitting \(A_{\mathrm{iso}} = 42\) G, i.e., 15 G, corresponds precisely to the isotropic constant for \(\mathrm{ClO_2}\) \([8]\). Hiratsuka and Nikaido \([9]\), using a flow technique and a computer, observed during pulsed electrolysis of \(\mathrm{NaClO_4}\) in acetonitrile an isotropic quartet with \(A_{\mathrm{iso}} = 25\) G, attributed by them to the \(\mathrm{ClO_4}\) radical. The hyperfine constant obtained by the authors differs noticeably from the hyperfine constant for \(\mathrm{ClO_2}\).

In studies by the ESR method of the radiolysis of perchlorate systems \([10–20]\), \(\mathrm{ClO_4}\) radicals were not detected, although their formation was pos-

was discussed in considering the mechanism of radiolysis. True, Morton [21] reported the identification of the radical \(ClO_4\) in a single crystal of \(HClO_4\) irradiated with X-rays at \(77^\circ K\). The EPR spectral parameters obtained by him \((A_{\mathrm{iso}} = 160\ \mathrm{MHz},\ \tau \cdot \theta = 57\ \mathrm{G}\) for \({}^{35}Cl\) and \(g_{\mathrm{av}} = 2.05)\) differ strongly from the value \(A_{\mathrm{iso}} = 25\ \mathrm{G}\), assigned to the radical \(ClO_4\) in [9]. The most probable particles formed during radiolysis of \(KClO_4\) may be \(ClO_4\) and \(ClO_4^-\); however, Morton chooses \(ClO_4\) from among them without sufficient grounds.

Although in all works devoted to the radiolysis of perchlorates and perchlorate systems the \(ClO_4\) radical was not observed in the EPR spectrum (except for [21]), nevertheless a number of data indicate that this radical does arise during radiolysis. These include results obtained for \(\gamma\)-irradiated frozen solutions of \(HClO_4\) [16] and \(NaClO_4\) [18] at \(77^\circ K\), potassium perchlorate, and nitrosyl perchlorate [20]. In all these matrices, the dependence of the change in the concentration of \(ClO_2\) on the temperature of warming after irradiation has a similar form. At the temperature of liquid nitrogen only a small fraction of \(ClO_2\) is formed; the main amount of \(ClO_2\) appears upon heating above \(-50^\circ C\). In this temperature region the amount of \(ClO_2\) increases for different matrices by a factor of 2–15, as shown in three examples in Fig. 1. This increase occurs in one and the same temperature interval (\(-50 \div -30^\circ C\)) in all the matrices studied. For some of the systems studied (solutions of \(HClO_4\), acid solutions of \(NaClO_4\)), in the region where \(ClO_2\) radicals appear, no other radicals are manifested in the EPR spectrum.

Special experiments showed that the appearance of the \(ClO_2\) radical is not connected with any processes of decomposition or disproportionation of stable transformation products of the perchlorate ion—for example, chlorite and hypochlorite ions, etc. The only possible explanation is the assumption that chlorine dioxide appears during the thermal decomposition of the \(ClO_4\) radical, which was not observed in the matrices studied in the EPR spectrum.

Fig. 1. Change in the relative concentrations of $\mathrm{ClO_2}$ and $\mathrm{O_2}$ upon heating irradiated perchlorate matrices.

\[ \mathrm{ClO_4} = \mathrm{ClO_2} + \mathrm{O_2} \tag{1} \]

This decomposition, as was assumed [2, 3], also occurs during anodic oxidation of chloric acid. Such an assumption is also supported by the fact that, in the temperature region in which an increase in the concentration of $\mathrm{ClO_2}$ is observed, all the matrices studied become less rigid, rotation of the $\mathrm{ClO_2}$ radical is unfrozen, and its spectrum changes from anisotropic to isotropic, with $\Delta H_{\mathrm{iso}} = 16.5$ Oe; i.e., in this temperature region the decomposition process is facilitated.

If $\mathrm{ClO_2}$ and $\mathrm{O_2}$ are formed by reaction (1) as a result of decomposition of the $\mathrm{ClO_4}$ radical, then the kinetic parameters of the process of $\mathrm{ClO_2}$ accumulation and of $\mathrm{O_2}$ release into the gas phase should coincide. To test this assumption, we carried out parallel studies of the kinetics of formation of $\mathrm{ClO_2}$ and $\mathrm{O_2}$. Chlorine dioxide was determined by the ESR method on a serial radiospectrometer, as described in [6], and oxygen was determined mass-spectrometrically using an MI-1305 mass spectrometer.

Fig. 2. Linear anamorphoses of the curves for the appearance of \(ClO_2\).

The kinetics of the appearance of \(ClO_2\) was studied under isothermal conditions in the temperature interval from \(-55\) to \(-28^\circ C\) for nitronium perchlorate hydrate \((NO_2ClO_4 \cdot \tfrac{1}{3}H_2O)\) irradiated at \(77^\circ K\), described in [22], and for a 12.5 M aqueous solution of chloric acid, for which there are no phase transitions in the region where \(ClO_2\) appears.

The kinetics of the appearance of \(O_2\) was investigated in nitronium perchlorate hydrate irradiated at \(77^\circ K\), which is a sufficiently “loose” matrix, and oxygen is released from it even at very low temperature. The kinetics of \(O_2\) evolution was recorded at a constant heating rate (from 0.5 to \(2^\circ\) per min). Nitronium perchlorate hydrate was taken in the form of a fine powder. Oxygen evolution was studied in the temperature interval from \(-196\) to \(20^\circ C\); the evolution of oxygen from the unirradiated preparation may be neglected.

Typical kinetic curves for \(ClO_2\) evolution are given in Fig. 2 in first-order reaction coordinates. As can be seen, the kinetics of the appearance of \(ClO_2\) satisfies a first-order reaction. The dependence of the rate constant of the reaction for the appearance of \(ClO_2\) on temperature (see Fig. 3) is determined by the following expression:

Fig. 3. Dependence of the rate constant for the appearance of \(O_2\) on temperature for nitronium perchlorate hydrate.

\[ K_I = 10^{10.6}\exp\left(\frac{-13.9 \pm 2\ \text{kcal}}{RT}\right)\ \text{min}^{-1}. \]

A typical curve for the evolution of \(O_2\) is given in Fig. 1. As can be seen, the increase in \(O_2\) pressure occurs simultaneously with the growth of the \(ClO_2\) concentration in the sample volume, although the polythermal curves, owing to hindered diffusion, are shifted relative to the curves for \(ClO_2\) into the region of higher temperatures by \(10\text{–}15^\circ\). At the same time, the rate of \(O_2\) evolution at a constant heating rate has a maximum in the interval \(-40 \div -35^\circ\mathrm{C}\), which almost coincides with the maximum rate of \(ClO_2\) formation measured under the same conditions (in Fig. 1 the accumulation curves for \(ClO_2\) were obtained not at a constant heating rate, but with holding for 2 min at the temperatures indicated on the graph).

From the polythermal curves for oxygen pressure, one can estimate, assuming a first-order reaction, the activation energy for the rate of \(O_2\) evolution from the equation:

\[ \ln\left(\frac{dP_{O_2}}{dt}\right)_i = \ln K_0 + \ln\left(P_{O_2}^{\circ} - P_{O_2}^{i}\right) - \frac{E}{RT} \]

For the temperature interval \(-55 \div -20^\circ\mathrm{C}\), we obtain an activation-energy value of \(10\text{–}14\ \text{kcal/mol}\), while for the region \(-20 \div 0^\circ\mathrm{C}\) one observes—

...the activation energy is equal to 2–6 kcal/mole. The first value agrees with the value of the activation energy of the reaction of the appearance of \(ClO_2\) (13.9±2 kcal/mole); the second value, apparently, corresponds to the activation energy of oxygen diffusion in this matrix.

The experimental results presented make it possible to conclude that both products—chlorine dioxide and molecular oxygen—are formed in one and the same process, which agrees well with reaction (I), the decomposition of the perchlorate radical.

Thus, the data obtained by us confirm the hypothesis of the formation of the perchlorate radical under the action of ionizing radiation on the perchlorate ion.

The authors express their gratitude to R. Sagitov for carrying out the mass-spectrometric measurements.

Moscow State University
Faculty of Chemistry

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