7.
Intramolecular electron spin exchange as a function of temperature and solvent viscosity and polarity has been studied by
X-band electron paramagnetic resonance (EPR) spectroscopy in two rigid nitroxide biradicals existing in one spatial conformation
only. Temperature variations of the isotropic hyperfine splitting constant
a and exchange integral value |
J/a| were measured from EPR spectra and subsequently analyzed. The interaction of polar solvent molecules with >N-O fragments
of nitroxide groups led to a slight decrease of the |
J/a| value with the increase of temperature
T. In contrast, the interaction of polar solvent molecules with functional groups inside the bridge resulted in a noticeable
increase of |
J/a| vs.
T. In the last case, a coverse relationship between the values of |
J/a| and the hyperfine splitting constant
a has been observed for solvents with different polarity.
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