8.1.2 5-methyl-2-hydroxyphenol cation radical [a018]

The protons of the hydroxyl groups exchange quickly with the environment and therefore they do not contribute to the hyperfine structure, $a_{OH}$ = 0. Three sets of equivalent protons are observed: a) the proton in position 2, b) the two protons in position 6 and 4, and c) the three protons of the methyl group. Consequently, three different hyperfine splittings are obtained.

Figure 17: Successive splittings and EPR spectrum of the 5-methyl-2-hydroxyphenol cation radical.

Maximum number of lines (Eq. (9)): N = 2 × 3 × 4 = 24 lines.

Hyperfine splittings:
$a_H^2$ = 0.045 mT (doublet, 1:1). Distance between 1$^{st}$ and 2$^{nd}$ line.
$a_H^{CH_3}$ = 0.215 mT (quartet, 1:3:3:1). Distance between 1$^{st}$ and 3$^{rd}$ line.
$a_H^{4,6}$ = 1.065 mT (triplet, 1:2:1). Distance between 1$^{st}$ and 9$^{th}$ line.

Length of the spectrum (Eq. (8)): L = 0.045 $\cdot$ 1 + 0.215 $\cdot$ 3 + 1.065 $\cdot$ 2 = 2.820 mT.
If it coincides with the measured value (within a reasonable deviation) the interpretation will be correct.

Reconstruction of the spectrum. It is presented in the upper part of the Fig. 17. The numbers over the lines indicate the relative intensities (or relative heights).

This radical is a resolved example that you should try to understand. Load the simulator and introduce the data corresponding to this spectrum to obtain a correct simulation.

Simulator

The six following spectra (subsections 7.1.3 to 7.1.8) are not resolved. They have two groups of equivalent nuclei. Proceed in the following way for their interpretation:

• Observe the type of multiplet according to its structure and the shape of the splittings.
• Load the EPR simulator by clicking on the corresponding link.
• Check the scale with the mouse. Measure the value of the two hyperfine splittings in the experimental spectrum and fill the table of the simulator. Reload the spectrum.
• If your interpretation is correct, overlap both spectra. Refine the values of the hyperfine splittings and/or DHpp (Appendix A) to improve their coincidence.
• Calculate the total length of the spectrum using the Eq. (8).
• Measure the length of the experimental spectrum with the mouse and compare it with the calculated one.
• Print out the simulated and sperimental spectra with the parameters used in the simulation.