Spectroscopy EPR

EPR studies of S-N radicals were reviewed in 1990. Many radicals containing the S-N linkage are persistent for more than several hours in solution at room temperature. Perhaps the best known example is the nitrosodisulfonate dianion [0N(S03)] , named as Fremy s salt. In the solid state this radical dianion dimerizes through weak N 0 interactions, but it forms a paramagnetic blue-violet monomer in solution. Although most chalcogen-nitrogen radicals dimerize in the solid state, a few heterocyclic C-S-N systems can be isolated as monomers (Section 11.3). 

The N nucleus (/ = 1, 99%) has a moderately large magnetic moment and hyperfine splittings from this nucleus are a distinctive feature of the EPR spectra of chalcogen-nitrogen radicals. A-Arylthio-2,4,6-triphenylanilino radicals (3.16) are exceptionally persistent and oxygen-insensitive m solution. They exhibit a characteristic 1 1 1 

The EPR spectra of the related 1,2,4,6,3,5-thiatriazadiphosphinyl radicals (3.20) reveal a distinctly different electronic structure.The observed spectrum consists of a quintet of triplets consistent with coupling of the unpaired electron with two equivalent nitrogen atoms and two equivalent phosphorus atoms [Fig. 3.4(a)]. This interpretation was confirmed by the observation that the quintet collapses to a 1 2 1 triplet when the nitrogen atoms in the ring are 99% N-enriched [Fig 3.4(b)]. Thus the spin delocalization does not extend to the unique nitrogen atom in the phosphorus-containing system 3.20. 

An intriguing class of persistent radicals are those formed by the one-electron oxidation of the hexagonal prismatic clusters Li2[E(N Bu)3] 2 (3.21, E = S, Se). The air oxidation of 3.21 produces deep blue (E = S) or green (E = Se) solutions in toluene. The EPR spectra of these solutions consist of a septet (1 3 6 7 6 3 1) of decets (Eig. 3.5). This pattern results from interaction of the unpaired electron with three equivalent 7=1 nuclei, i.e., and three equivalent I = 3/2 nuclei, i.e., Ei. It has been proposed that the one-electron oxidation of 3.21 is accompanied by the removal of an Ei cation from the cluster to give the neutral radical 3.22 in which the dianion [S(N Bu)3] and the radical monoanion [S(N Bu)3] are bridged by three Ei cations. 

In order to be able to detect an EPR signal, a resulting overall spin S for the electron shell is necessary, provided by an odd number of electrons or, in the case of an even 

The condition for resonance in an EPR experiment is defined by Equation (3.8), in which V is the frequency, h is Planck s constant, jS is the Bohr magneton, B is the magnetic field and g is the so-called g factor  

Under conditions where isotropic tumbling of the molecules is restricted or annealed, as in frozen solutions, powders and other solid-state situations, anisotropic spectra are observed, i.e. additional splitting complicates the spectra, but also provides highly valuable information on the electronic nature and orientation of the ligand set. In the case of vanadyl complexes, the dominant V=0 unit defines the primary axis (the z direction). In this case of so-called axial symmetry, two sets of eight lines are observed with differing g and 

A values, denoted and and Ay and respectively. The parallel component is the one coinciding with the V=0 director and the direction of the magnetic field B. In a tetragonal complex (octahedral, square pyramidal), and Ay A. A typical 

A typical anisotropic axial EPR spectrum of an oxovanadium(IV) complex (inset) in frozen THF. The ligand is a Schiff base derived from vanillin and tyrosine. Diamonds indicate the eight components of A (the parallel hyperfine coupling constant parallel defined by the z direction, which is the direction of the magnetic field). Arrows indicate the five inner components of Aj. Abscissa magnetic field strength (G) ordinate, intensity (arbitrary units). 

The high sensitivity of the g-values of low-spin iron(III) to structural variations and their large anisotropy imply that the prediction of the EPR spectra must be based on highly accurate structures12071. The MM-AOM method for low-spin iron(III) complexes was tested on a number of examples involving bi-, tri- and hexadentate ligands with amine and pyridyl donor sets (Table 10.8). 

Generally, there is good agreement between the experimentally observed and calculated EPR spectra. Areas, where considerable errors resulted from a large variation of the g-values as a function of a specific distortion mode, were identified by model AOM calculations of the g-values as a function of the corresponding distortion mode. Two structural problems were addressed with the MM-AOM method applied to low spin iron(III)  

A combination of the two techniques was shown to be a useful method for the determination of solution structures of weakly coupled dicopper(II) complexes (Fig. 10.5)[165]. The MM-EPR approach involves a conformational analysis of the dimeric structure, the simulation of the EPR spectrum with the geometric parameters resulting from the calculated structures and spin Hamiltonian parameters 

Because of the approximations involved in this analysis the thermodynamic results have to be considered with caution. This is not only due to a rather crude analysis of the electrostatic effects but also, and this is a general problem, to the neglect of solvation in the molecular mechanics refinement. However, the structures presented in Fig. 10.7 are valuable because they are based not only on the structure optimization by molecular mechanics but also on spectroscopic data. This example is therefore instructive for two reasons first, it demonstrates that, depending on the study, the often-neglected electrostatic effects may be of considerable importance. Second, not only can experimental observables help to refine solution structures, they can also prevent a wrong conclusion. As in this example, the combination of experimental data with molecular mechanics calculations is often the only way to get reliable structural information. 

The MM-EPR approach has been used successfully in a number of recent studies [205,206 328 329]. The most novel is that of the solution structure refinement of a dicopper(II) compound of a cyclic octapeptide[205] which is only the second structure of a dicopper(II) compound of this type of biologically important ligand and the first of a metal compound of an artificial cyclic octapeptide. An important development in this area is a new method for the simulation of EPR spectra (SOPHE)[325,326], which allows the simulation of coupled EPR spectra of polynuclear species with more than two metal centers with any electron spin 0, based on sets of parameters similar to those discussed above. 

The volume of 250 pi of each sample solution was thoroughly mixed with 5.0 pi of DMPO spin trap prior to each experimental set carried out in a thin flat EPR quartz cell. The operational parameters of the equipment were adjusted as follows Centre field 3354 G, sweep width 100 G, time constant 81.92 ms, conversion time 20.48 ms, receiver gain 5e + 5, microwave power 10 mW, and modulation amphtude 2 G. 

The validity of the MM-EPR method is further demonstrated with the two structurally related dicopper(II) complexes A and B whose calculated structures (MM-EPR) are presented in Fig. 9.6. For both bis-macrocyclic ligands two identical 

As often is the case (see Chapter 2, Sections 2.2.6 and 2.7), the molecular mechanics analysis above does not include any electrostatic interaction energies. To include these, the charge distribution and the charge compensation by ion-pairing to counter ions (perchlorate) have to be known. Model calculations indicate that an effective charge of around +1.6 per copper site, a value that is expected from thermodynamic considerations, leads to electrostatic repulsion energies of ca. 17 kJ mol-1 and 10 kJ mol-1, respectively, for the folded and stretched conformers. In agreement with the experiment (EPR spectra), this qualitative analysis indicates a preference for the folded structure of A, and for the stretched structure of B1 201. 

Oxidation of the MM quadruply bonded center leads to a metal-centered radical that is easily detected by EPR spectroscopy. Both molybdenum and tungsten have various nuclei, the majority of which are spin inactive. However, Mo and Mo have 7 = 5/2 and very similar magnetic moments. Thus the isotropic EPR spectrum of the M02 cations consists of a central resonance, g- 1.9, flanked by a six-line hyperfine spectrum A 28 G. The isotope has I = 1/2 and 15% natural abundance and so the EPR spectrum of the W cation consists of a central 

Related spectra for the oxalate and terephthalate bridged Mo4-containing compounds supported by pivalate ligands are shown in Fig. 9. Here it is clearly evident that on the EPR time scale, 10 s , the oxalate cation involves complete delocalization of the positive charge over all four Mo atoms. The single electron in the HOMO sees equally all four Mo atoms. Here the presence of two spin active nuclei in the M center is more easily seen. In contrast, the EPR spectmm of the terephthalate radical cation is localized on one M02 center the mixed-valence ion is valence-trapped [34]. 

For tungsten terephthalate bridged compounds the radical cations show delocalization over the four W centers, once again revealing the greater electronic coupling with the 5d element. 

A particularly interesting example of electron delocalization is seen for the 2,6-azulenedicarboxylate bridge, previously shown in XII. The W4-containing cation, [(r-BuC02)3W2]2(p--bridge)has a central resonance, g 1.81, and is flanked by 

Of the several less common spectroscopic methods to combine with electrochemical intermediate generation such as luminescence, Raman, NMR, or X-ray absorption spectroscopy, the EPR method is presented here because of its relative simplicity and pronounced selectivity. Only paramagnetic compounds with a certain, not too rapid relaxation rate from the spin-excited state give detectable signals for EPR spectroscopy, which helps to disregard many simultaneously present species. On the other hand, the rather slow time frame (At 10 s) and the sensitivity of the EPR method to electronic influences from the participating atoms via g-factor shift and hyperfine interaction can render EPR a very valuable method to determine the site of electron transfer (ligand or metal) as well as the spin and thus valence distribution. 

In contrast to the above, there can be situations when the EPR method is not applicable (diamagnetism, e.g. in tetranuclear complexes, even-electron situations) or when the signal is not observable even at low temperatures ( EPR 

Using mixed-valence dimolybdenum compounds, i.e. involving an element with only about 25% of the naturally occuring isotope mixture bearing a nuclear spin /= 5/2), one has to analyse the EPR spectra more carefully 

Two additional aspects may be mentioned briefly with respect to EPR analysis Hyperfine information on the symmetry of mixed-valence species can also be obtained through electron-nuclear double resonance (ENDOR). The main-group mixed-valence species 9 have thus been analysed despite the very 

Secondly, the EPR method is also appropriate to study inverse mixed-valency , involving metal-bridged ligands of different redox states.In those cases, the hopping of unpaired electrons may be noted through linewidth effects as for system [Ru(abpy)2(bpy)] (10).  

Whereas NMR spectroscopy is generally applied to the diamagnetic allyl, pentadienyl, or cyclopentadienyl cations or anions, this technique is generally not applicable to the analogous radical species, although CIDNP (chemically induced dynamic nuclear polarization) has been observed for allyl140 and pentadienyl radical species139. More useful for the radicals, naturally, is EPR spectroscopy. 

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In the previous chapters experiments have been discussed in which one frequency is applied to excite and detect an EPR transition. In multiple resonance experiments two or more radiation fields are used to induce different transitions simultaneously [19, 20, 21, 22 and 23], These experiments represent elaborations of standard CW and pulsed EPR spectroscopy, and are often carried out to complement conventional EPR studies, or to refine the infonnation which can in principle be obtained from them. 

In electron-spin-echo-detected EPR spectroscopy, spectral infomiation may, in principle, be obtained from a Fourier transfomiation of the second half of the echo shape, since it represents the FID of the refocused magnetizations, however, now recorded with much reduced deadtime problems. For the inhomogeneously broadened EPR lines considered here, however, the FID and therefore also the spin echo, show little structure. For this reason, the amplitude of tire echo is used as the main source of infomiation in ESE experiments. Recording the intensity of the two-pulse or tliree-pulse echo amplitude as a function of the external magnetic field defines electron-spm-echo- (ESE-)... 

ESE-detected EPR spectroscopy has been used advantageously for the separation of spectra arising from different paramagnetic species according to their different echo decay times. Furthemiore, field-swept ESE... 

Stehlik D, Bock C H and Thurnauer M 1989 Transient EPR-spectroscopy of photoinduced electronic spin states in rigid matrices Advanced ERR in Biology and Biochemistry ed A J Hoff (Amsterdam Elsevier) oh 11, pp 371 03... 

Barra A L, Brunei L and Robert J 1990 EPR spectroscopy at very high field Chem. Phys. Lett. 165 107-9... 

Forbes M D E, Peterson J and Breivogel C S 1991 Simple modification of Varian E-line microwave bridges for fast time-resolved EPR spectroscopy Rev. Sc/. Instrum. 66 2662-5... 

Closs G L and Forbes M D E 1991 EPR spectroscopy of electron spin polarized biradicals in liquid solutions. Technique, spectral simulation, scope and limitations J. Phys. Chem. 95 1924-33... 

Levanon H and Mobius K 1997 Advanced EPR spectroscopy on electron transfer processes in photosynthesis and biomimetic model systems Ann. Rev. Biophys. Biomol. Struct. 26 495-540... 

Prisner T F, van der Est A, BittI R, Lubitz W, Stehlik D and Mdbius K 1995 Time-resolved W-band (95 GHz) EPR spectroscopy of Zn-substituted reaction centers of Rhodobacter sphaeroides R-26 Chem. Phys. 194 361-70... 

The standard monograph for those seeking an introduction to EPR spectroscopy. Frieboiin H 1993 Basic One- and Two-Dimensional NMR Spectroscopy (New York VCH) A basic introduction to NMR spectrai anaiysis. 

In PMD radicals, the bond orders are the same as those in the polymethines with the closed electron shell, insofar as the single occupied MO with its modes near atoms does not contribute to the bond orders. Also, an unpaired electron leads the electron density distribution to equalize. PMD radicals are characterized by a considerable alternation of spin density, which is confirmed by epr spectroscopy data (3,19,20). 

The reactions of cyanoisopropyl radicals with monomers have been widely studied. Methods used include time resolved EPR spectroscopy,352 radical trappingj53 355 and oligomer00 356 and polymer end group determination. 1 Absolute341 and relative reactivity data obtained using the various methods (Table 3.6) are in broad general agreement. 

Time resolved EPR spectroscopy and UV-visible spectophotometry have proved invaluable in determining the absolute rate constants for radical-monomer reactions. The results of many of these studies are summarized in the Tables included in the previous section (3.4), Absolute rate constants for the reactions of carbon-centered radicals are reported in Table 3.6. These include t-butyl374 and cyanoisopropyP2 radicals. 

Many nitrones and nitroso-compounds have been exploited as spin traps in elucidating radical reaction mechanisms by EPR spectroscopy (Section 3.5.2.1). The initial adducts are nitroxides which can trap further radicals (Scheme 5.17). 

ESI mass spectrometry ive mass spectrometry ESR spectroscopy set EPR spectroscopy ethyl acetate, chain transfer to 295 ethyl acrylate (EA) polymerizalion, transfer constants, to macromonomers 307 ethyl methacrylate (EMA) polymerization combination v.v disproportionation 255, 262 kinetic parameters 219 tacticity, solvent effects 428 thermodynamics 215 ethyl radicals... 

A Relaxation time measurement in the solid (Al) in solution (A2). B Mechanical spectroscopy. C Variable-temperature NMR spectroscopy (coalescence temperature measurement). D Variable-temperature EPR spectroscopy... 

Molybdenum enzymes a survey of structural information from EXAFS and EPR spectroscopy. S. P. Cramer, Adv. Inorg. Bioinorg. Mech., 1983, 2, 260 (137). 

EPR spectroscopy is usually used to calibrate the clock (i.e., to determine kc). The method described here uses EPR to detect the two radicals. These are the parent (R1 ) and the product (R2 ) of its reaction, be it cyclization, decarbonylation, decarboxylation, rearrangement, or whatever. The radical R1 is produced photochemi-cally in the desired inert solvent by steady and usually quite intense light irradiation of the EPR cavity. Typically, R1 and R2 attain steady-state concentrations of 10-8 to 10 6 M. 

With this expression, kjkn can be obtained by the measurement of one set of [RI ], [R2 ] values, at full light intensity only. As to kii itself, which is needed to evaluate kc, one must either do a separate experiment by time-resolved EPR spectroscopy (see Chapter 11) or, with less accuracy and reliability, one can assign it the value for the diffusion-controlled rate constant in that solvent. 

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