EPR Spectrum of the ClO$_3$ Radical in Irradiated Sodium Chlorate

L. V. Serikov, A. A. Vasil’ev, Yu. A. Zakharov

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

In constructing a mechanism for the radiolysis of solid chlorates of alkali metals, the formation of the ClO$_3$ radical is usually assumed as an intermediate decomposition product.

However, direct experimental evidence for the validity of this assumption had not been obtained up to the present time, despite the fact that the parameters of the EPR spectrum of this radical are known from many works [2, 3, 4] on the radiolysis of perchlorates.

Only in Gamble’s work [5] on the interpretation of the EPR spectrum in NaClO$_3$, irradiated at room temperature, was a group of lines attributed to this radical with parameters substantially different from those given by the authors of works [2, 3, 4].

In the present work, performed, unlike [5], under conditions of irradiation and recording of EPR spectra in NaClO$_3$ at −196°C, a signal was detected for the first time whose parameters make it possible to assign it to the ClO$_3$ radical.

Experiment

The work was carried out on an R1301 spectrometer combined with a 1.6 MeV electron accelerator [1]. The dose absorbed by NaClO$_3$ was varied within the range 0.02–0.05 Mrad.

Single crystals of NaClO$_3$ were grown by the method of slow cooling of an aqueous solution of chemically pure salt, recrystallized three times in distilled water.

The obtained single crystals had the form of rectangular parallelepipeds with dimensions $4 \times 4 \times 2$ mm. Irradiation and measurements were performed at temperatures of 77–300°K. Cooling was carried out by blowing the sample with a stream of nitrogen at the corresponding temperature, which was continuously monitored throughout the entire experiment.

The experiments were carried out either in complete darkness or with illumination of the sample by light from an incandescent lamp (8 V, 20 W).

Results

The electron paramagnetic resonance spectrum of a NaClO$_3$ single crystal is shown in Fig. 1.

Fig. 1. EPR spectrum of a NaClO$_3$ single crystal irradiated with electrons. Irradiation and measurement at a temperature of 77°K. The magnetic field H0 is parallel to the (001) axis. Cl$^{37}$O$_3$—lines A, Cl$^{35}$O$_3$—lines A′.

In the spectrum obtained, two quartets can be distinguished, the line intensities of which are in the ratio 1:3 (groups of lines A and A′), which corresponds to the contents of the isotopes Cl$_{37}$ and Cl$_{35}$ in natural chlorine (24.47% and 75.53%), respectively. The ratio of the hyperfine-splitting constants of these quartets is 0.83 and is equal to the ratio of the magnetic moments of the Cl$_{37}$ and Cl$_{35}$ nuclei, 0.68329 nuclear magnetons and 0.82089 nuclear magnetons, respectively.

The number of lines corresponds to the magnitude of the nuclear spins of the isotopes,

\[ I_{\mathrm{Cl}_{35}} = I_{\mathrm{Cl}_{37}} = \frac{3}{2} \left( n = 2I + 1 = 4 \right). \]

The \(g\)-factor is the same for both quartets and has axial symmetry. Its principal values are \(g_{\parallel} = 2.0107\), \(g_{\perp} = 2.0135\).

When the crystal is rotated in the magnetic field about the (100) axis, each of the eight lines splits into two, and two groups of lines are formed, with two quartets in each (groups A + A′ and B + B′, Fig. 2).

Fig. 2. EPR spectrum of the ClO$_3$ radical in NaClO$_3$; H0 field perpendicular to the (100) axis, the angle between H0 and the (001) axis is 30°, groups of lines A A′ and B B′ belong to different orientations of the radical.

This is apparently caused by stabilization of the radical in two different orientations relative to the crystallographic axes.

Table 1 gives the constants of the EPR spectra of the \(\mathrm{ClO_3}\) radical according to data of various authors and the parameters obtained in our investigations.

Table 1

From comparison of the parameters of the spectra of the \(\mathrm{ClO_3}\) radical obtained in substances with different crystal structures it is seen that these parameters depend only weakly on the environment of the radical.

Therefore it may be assumed with sufficient justification that the spectrum found by us also belongs to \(\mathrm{ClO_3}\). As for work [5], the HFS constants of the spectrum obtained there differ by an order of magnitude from the data of all other works; as a result, the conclusions drawn in it, as already indicated above, appear to be unfounded.

We checked the thermal stability of the radical obtained. It turned out that at a temperature of \(77^\circ\mathrm{K}\) the intensity of the lines decreases by half in 10 min.

When the single crystal was illuminated with visible light from an incandescent lamp, the signal disappeared completely in 15 min. (Fig. 3).

[scale: 100 G]

Fig. 3. Spectrum shown in Fig. 1 after heating the single crystal to a temperature of \(110^\circ\mathrm{K}\)

Upon heating the sample, the intensity of the lines decreases rapidly, and the signal disappeared completely at a temperature of \(-160^\circ\mathrm{C}\). Disappearance

of the \(\mathrm{ClO_3}\) radical did not lead to the appearance of new lines in the spectrum or to the intensification of lines that had previously existed.

It is still difficult to draw any definite conclusions concerning the mechanism of the destruction of the \(\mathrm{ClO_3}\) radical; however, it may be supposed that it either captures an electron and is converted into \(\mathrm{ClO}\) according to the reaction \(\mathrm{ClO_3 + e = ClO}\), or forms \(\mathrm{Cl_2O_6}\)

\[ \mathrm{ClO_3 + ClO_3 = ClO_6} \]

It is, of course, possible that both of these reactions proceed as parallel ones.

REFERENCES

1. Yu. Molin, A. T. Koritskii, A. G. Semenov, I. Ya. Buben, V. N. Shamshev. PTE No. 6, 73, 1960.
2. T. Cole, J. Chem. Soc. (1160) 1962.
3. P. W. Atkins, J. J. Brivati, Chem. Soc. (4785) 1962.
4. Yu. M. Boyarchuk, A. V. Dubovitskii et al. Kinetics and Catalysis. 5, 823, 1964.
5. F. T. Gamble, J. Chem. Phys. 42, No. 10 (3542) 1965.

Noted Misprints