8.1.10 Biphenyl anion radical [a020]

In this molecule, the unpaired electron is dislocated in the ortho, meta and para positions of the ring giving three groups of equivalent hydrogen. Nine quintets are observed in the spectrum (Fig. 25) which originate 45 experimental lines, a number smaller than the predicted one, $5\cdot 5 \cdot 3 = 75$ lines. This is because the value of the triplet (positions 4,4') hyperfine splitting is double of that of the quintet (positions 2,2',6,6'). If the situation were the reverse, the spectrum will have 11 quintets and therefore 55 experimental lines.

Figure 25: EPR spectrum of the biphenyl anion radical.

Simulator

[Exercise] Keeping in mind the previous observations:

• Load the EPR simulator.
• Verify the horizontal scale with the mouse and measure the three hyperfine splittings.
• Measure the heights (with the mouse) of the 9 central lines of each quintet and calculate the relation of heights taking as reference the first one. Compare with the theoretical intensities 1:4:8:12:14:12:8:4:1.
• Use, as additional data, the total length of the experimental spectrum and the value of DHpp (measured using a smaller range) to overlap correctly both spectra.
• Print out the results.

[Exercise]

• Fill out again the table of splittings on the simulator but changing the order of the two larger constants, that is, $a_H^{2,2',6,6'} = 2 \cdot a_H^{4,4'}$. Simulate the new spectrum (simulation 2).
• Measure the heights of the central lines of each quintet and the total length of the spectrum.
• Print out the following:
  Intensities Table 3
• Complete the Table with the intensities measured for both spectra.
• Enclose the result in the laboratory notebook.