Spin Exchange in Nitroxide Biradicals

Abstract

The dependence of the electron spin-exchange energy upon temperature and solvent has been studied for the carbonate, oxalate, and succinate diesters of tetramethyl-piperidinol-oxyl. An analysis is given of exchange effects on ESR line positions, intensities, and widths. For the carbonate diester the exchange energy is constant at low temperatures. As the temperature is raised this energy gradually decreases, passes through a minimum, and then increases exponentially. Interpretation suggests that at low temperatures exchange occurs through an indirect process involving the core of sigma electrons, whereas at high temperatures a direct exchange process is more important.

Peculiarities of Spin Exchange in Nitroxide Biradicals Containing Two para-Phenylene Groups in the Bridge: EPR Investigation and DFT Calculations

“…In the case when both radical fragments are identical and each carries only one nucleus with a nonzero nuclear spin I the following equation is valid [33,34]:…”

“…In low-viscous solvents, the tensor of the dipole-dipole coupling is averaged to zero by fast rotation of the biradical molecule [32,33]. For any individual conformation, the one value of |J| should correctly describe the position and integral intensities of all lines in the EPR spectrum.…”

“…For the two-confformational model [3,33,34], the Arrhenius plot of |J/a| should be linear for any biradical, and this allow determining the differences in enthalpies, ∆H, and entropies, ∆S, of these two conformations:…”

Five Generations of Nitroxyl-Functionalized Dendrimers

“…In solution, both classes of conformers will exist and the actual EPR spectrum depends on the values of J A and J B and the rate of interconversion between A and B, expressed by the lifetimes τ A and τ B . [20][21][22] When the exchange interaction is rapidly modulated, the broadening of the line widths due to modulation of J in time is given by 20 where j(2J h) is the spectral density and J h ) (τ A J A + τ B J B )/ (τ A + τ B ). This expression shows that of the total of nine nuclear spin functions in a nitroxyl biradical, the three lines corresponding to m 1 ) m 2 (i.e.…”

“…[20][21][22][23] Using the Hamiltonian (eq 1) and the usual singlet |S〉 and triplet states |T + 〉, |T 0 〉, |T -〉, the four energy levels for a symmetric nitroxyl biradical (n ) 2) in any nuclear spin state |m 1 m 2 〉 are described by Where θ is given by:…”