Typical applications of MARY spectroscopy: Radical ions of substituted benzenes

Abstract

The paper describes the underlying principles and discusses the most important advantages and limitations of the experimental technique of magnetically aŸ reaction yield spectroscopy as developed in the authors' laboratory and guides the reader step by step through a typical experimenta[ sequence using as example the problem of short-lived radical cations of a series of methyl-substituted benzenes in X-irradiated nonpolar solutions. For two of the eight target substances -benzene itself and mesithylene -the paper reports the first unequivocal observation of their radical cations in liquid alkane so[ution at room temperature and provides a lower estimate of about 10 ns for their relaxation times in low magnetic field.

Specific MARY spectrum from radical anion of pentafluorobenzene

“…A detailed description of MARY spectroscopy as applied in this work was published recently [7,8], and we only briefly summarize the method here. A MARY spectrum is essentially a conventional stationary magnetic field effect (MFE) curve, the dependence of the yield of recombination fluorescence from radical ion pairs in irradiated solutions on extemal static magnetic field, with sharp lines in zero and low field due to the degeneracy of the spin energy levels of ttle pair.…”

MARY spectroscopy in the presence of coordination compound Zn(hfac)2(PPO)2

“…The implementation of MARY spectroscopy that is used in this work is a steady-state method based on the continuous cycle of generating spin-correlated radical ion pairs by X-irradiation of non-polar solution of suitable charge acceptors, spin evolution of these pairs in applied static magnetic field, and singlet state recombination of the pairs producing electronically excited molecules that are observed by fluorescence, provided one of the chosen acceptors is also a luminophore [5]. Sweeping of the applied magnetic field in the region of weak fields produces MARY spectra -the dependencies of fluorescence intensity on magnetic field, with characteristic features reflecting the magnetic constitution of the partners of the recombining radical ion pair.…”

“…The experimental setup for MARY spectroscopy was described in [5]. The samples containing about 1 ml of solution in a quartz cuvette are degassed by repeated freeze-pump-thaw cycles and placed in the magnetic field of a Bruker ER-200D CW ESR spectrometer equipped with an X-ray tube for sample irradiation (Mo, 40 kV Â 20 mA), a pair of coils with a separate current source to provide the constant 'negative' shift of the field required to sweep through the zero of the field, and a PMT (FEU-130) for fluorescence detection.…”

“…The method was formulated for organic radical ions, and has proved useful in detecting several important classes of hitherto unseen radical ions with lifetimes of just several nanoseconds, such as primary solvent radical cations (holes) in irradiated alkanes [3] and radical ions of several fluorinated [4] and methylated [5] benzenes. The method is also useful for studies of reactions involving short-lived radical ions, such as the reactions of ion-molecular charge transfer [6,7], dimerization and higher aggregation [8], and monomolecular chemical decay [9].…”

Low field photo-CIDNP in the intramolecular electron transfer of naproxen–pyrrolidine dyads

“…In the region of intersection in region (2), the system spends less time than in region (1) It is worth noting that a similar situation also occurs in the optically detected EPR spectra and MARY spectra in non-polar systems. 34 In this case for the theoretical description of the magnetic field influence on the recombination of charged particles generated by radiolysis in the Coulomb field hyperfine interaction is always sufficient, i.e. the exchange interaction does not manifest itself due to the short time that the system spends in the term crossing region 2.…”