Solvent effects on the intramolecular spin exchange in biradicals at room temperature

Abstract

Solvent effects on intramolecular electron spin exchange in biradicals have been investigated in various solvents by electron spin resonance (ESR) spectroscopy at room temperature. Biradicals containing different radical fragments and bridges of different length and composition have been used. Interactions between solvent molecules and biradicals were found to take place at the radical fragment as well as at the functional group in the connecting bridge and have been proved to have an influence on the intramolecular spin exchange. The experimentally measured parameters, exchange integral (J) and characteristic time of an intramolecular movement (τ eff), have been compared with solvent viscosity and polarity parameters (macroscopic level) and longitudinal solvent relaxation time (microscopic level). The dependencies on viscosity and polarity were in good agreement with reported results. In addition, strong evidence of hydrogen bonding between solvent and different sites of the biradicals was found, in some cases even surpassing the other effects.

Polyradical PROXYL/TEMPO‐Derived Amides: Synthesis, Physicochemical Studies, DFT Calculations, and Antimicrobial Activity

“…Owing to the presence of two unpaired electrons in the dinitroxide radicals, EPR spectroscopy is an outstanding tool for the characterization of the intramolecular electron spin exchange and conformational dynamics . Analysis of a dinitroxide EPR spectrum can provide valuable information on intramolecular movements, thermodynamics, and spin exchange mechanism …”

“…In the case of tri‐ and tetranitroxide radicals, three ( J 12 , J 13 , J 23 ) or six ( J 12 , J 13 , J 14 , J 23 , J 24 , J 34 ) exchange couplings, respectively, define the spin exchange . The structure of the nitroxide fragments, along with the type of connecting bridge, has a substantial effect not only on the J value, but also on the mechanism of the spin exchange …”

“…For biradicals with flexible, long‐chain bridges, the EPR spectra consist of alternately broadened lines and exhibit a temperature dependence that can be interpreted by using a three‐conformation model . This model includes spin exchange between three dinitroxide conformations, namely, “elongated” conformation A , which is characterized by J A =0 and lifetime τ A , and two other “loop” conformations, B and C , with a combined lifetime of τ B+C , in which the nitroxide centers are close to each other in a “cage” of solvent molecules, with J B =0 and J C ≫ A N . Thus, relevant experimental EPR spectra represent a superposition of two signals: one corresponding to conformation A and the other to the mixed contribution from conformations B and C …”

Peculiarities of Intramolecular Motions in Ionic Liquids

“…Areas in RTILs (polar, non-polar), in which spin probes (hydrophilic, charged, hydrophobic) are localized were determined. Just recently, attention of the scientists was attracted to another type of molecular motions in the ionic liquids (Tran et al, 2007a(Tran et al, , 2009. Such processes as well as solvent effects on them can be examined in detail by EPR spectroscopy with the use of stable nitroxide biradicals (Parmon et al, 1977a(Parmon et al, , 1980.…”

“…Such processes as well as solvent effects on them can be examined in detail by EPR spectroscopy with the use of stable nitroxide biradicals (Parmon et al, 1977a(Parmon et al, , 1980. Solvent effects on intramolecular electron spin exchange in biradicals have been investigated in various solvents, including Bmim + PF 6 -, by electron spin resonance (ESR) spectroscopy at room temperature (Tran et al, 2007a). Using biradicals contained different radical fragments and bridges of various length and composition, it was demonstrated that interactions among solvent molecules and biradicals took place at the radical fragment and also at the functional group in the connecting bridge.…”