Sanjeev Gupta,

Did you record the first two mentioned spectra at 77 K? Do I get it right, that EPR spectrum of TEMPO in benzene at 77 K in your specimen looks like singlet (see fig. 1), whereas TEMPOL in benzene at 77 K gives three separate lines (see fig. 2)?

If so, I could explain the first case as molecules of TEMPO have formed a separate phase while the solution was cooled. This leads to a huge broadening of a spectrum so that it eventually looks like an unstructured singlet.

Still, there's no reasonable explanation for the fact that at such low temperature TEMPOL would give three separate lines because it basically means very fast rotation of TEMPOL molecules (comparable to that at room temperature).

Hi, you can follow:

https://web.nmsu.edu/~snsm/classes/chem435/Lab7/

Toluene solvent favors to bring the glassy nature of the solution at your experimental condition, may favor the splitting. Although you should expect splitting, I don't think you will get the exact 1:1:1 line splitting (as in observed at room temperature). Therefore either one line or partial splitting only you should expected. You can also follow:

Biophysical Journal 1995,68, 2350-2360.

Regarding the spectrum of 3 line splitting of TEMPOL (instead of 1 line), the intermolecular H-bonding between the O-radical and OH group may effects the relaxation process to observe the experimental differences.