8.2.1 2,6-di-tert-butyl 4-amino-phenoxyl neutral radical [a024]

In this molecule, the unpaired electron does not interact with the $^{16}O$ ($I = 0$). The electron density accumulates preferably on the amine group and in positions 3 and 5 of the aromatic ring. The hyperfine splitting with the methyls of the terc-butyl is zero (or almost zero) and their hyperfine structure (19 equidistant lines) is not observed on the spectrum.

Figure 27: Successive splittings and EPR spectrum of the 2,6-di-tert-butyl 4-amino-phenoxyl neutral radical.

There are three hyperfine splittings. The two largest are very close and, therefore, there is an overlap of the central lines of its triplets. The intensities suggest that the triplet corresponds to the coupling between the unpaired electron with the nitrogen atom, lines 6, 14 and 22.

Hyperfine splittings:
$a_H^{3,5}$ = 0.061 mT (triplet, 1:2:1). Between 1$^{st}$ and 2$^{nd}$ line.
$a_H^{NH_2}$ = 0.385 mT (triplet, 1:2:1). Between 1$^{st}$ and 4$^{th}$ line.
$a_H^{NH_2}$ = 0.425 mT (triplet, 1:1:1). Between 1$^{st}$ and 5$^{th}$ line.

Maximum number of lines (Eq. (9)): N = 3 $\cdot$ 3 $\cdot$ 3 = 27 lines.

Length of the spectrum (Eq. (8)): L = 0.061 $\cdot$ 2 + 0.385 $\cdot$ 2 + 0.425 $\cdot$ 2 = 1.742 mT.

Reconstruction: It is presented in the upper part of the Fig. 27. The numbers on the lines show their relative intensities.

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

NOTES: Here it is not possible to assign the hyperfine splitting of the protons from the spectrum. However, there are different procedures to do that assignation:

• Isotopic substitution, for example, we can change the hydrogen of the $NH_2$ by deuterium. The obtained spectrum should be totally different because of the deuterium spin $I_D=1$ gives a different multiplicity and because the splitting $a_D$ is much smaller ( $a_D = 0.1535 a_H$).
• Theoretical calculations. It is known that a hyperfine splitting $a_i$ is proportional to the unpaired electron density, $\rho_j$ and $\rho_j \approx c_{kj}^2$, where $c_{kj}$ is the coefficient of the corresponding atomic orbital $\phi_j$.
• Qualitatively, in base of the stability of the different canonical structures in a specific radical. Thus in the biphenyl anion radical, analysed in the subsection 8.1.10, the unpaired electron is dislocated in both rings giving different canonical structures (Fig. 28) with the following stability order $para > ortho > meta$. Conjugation structures I and II have higher stability than structure III. in this last structure there is a strong repulsive effect owing to the proximity of the electron and the negative charge in a ring. This explanation agrees with the order assigned to the splittings: $a_H^{para} > a_H^{ortho}$.

  Figure 28: Canonical structures of the biphenyl anion radical.

  

Simulator

The nine spectra presented below (subsections 8.2.2 a 8.2.11) are unresolved. They have 2, 3 or 4 groups of equivalent nuclei with different value of I. Look carefully at the molecule structure to find out the number of hyperfine splittings and the nuclei that originates those splittings with the unpaired electron. Assign, if possible, the order (multiplicity) of the hyperfine splittings. For the interpretation, proceed like in section 8.1.2. Use the length of the experimental spectrum as an additional information to get the last hyperfine splitting using the Eq. (8). Print out the simulated and experimental spectra and the data used in the simulation.