Frozen solution spectra

Most of the information content from ESR spectra of organometallic radicals and coordination complexes comes from dilute single-crystal spectra or frozen solution spectra. Nonetheless, there are some bits of information and applications that come uniquely from isotropic spectra, and we discuss those aspects in this chapter. 

The next section deals with the analysis of frozen solution spectra. 

Frozen solution spectra must be taken in solvents that form good glasses. A random but homogeneous distribution of the species to be studied is required to obtain a well-resolved spectrum. Notably, some solvents, such as toluene, can crack when frozen, thus giving rise to more features than predicted as spectra... 

Non-coincident matrix axis effects are seen in the frozen solution spectra of [(RCCR )Co2(CO)6] (3)18 and [SFeCo2(CO)9] (4),43 but the effects are rather more subtle than those discussed above. 

An experimental determination of the spin density distribution in these complexes was recently reported, based on EPR data on 77Se-enriched (at 100%) diselenolene analogs where the 77Se coupling constants and g tensors could be determined from frozen solutions spectra [60]. A 14% spin density was determined on each Se atom in 77Se-enriched [CpNi(bds)] while a larger 16% spin density on... 

Thus it is possible under favorable circumstances to extract spin Hamiltonian parameters from frozen solution spectra. The methods used here can,... 

Electron spin resonance studies of silver(II) pyridine complexes have proved to be extremely useful in determining the nature of the spedes in solution. Since natural silver has two isotopes, 107Ag and 109Ag, in approximately the same abundance, both of spin / = J, and since their nuclear magnetic moments differ by less than 15%, interpretation of spectra is often considered in terms of a single nucleus. The forms of the hyperfine splitting patterns for IN, cis and trans 2N, 3N and 4N, would be expected to be quite different and hence the number of pyridines can be readily assessed from well-resolved spectra. Spin Hamilton parameters obtained from both solid and frozen solution spectra are collected in Table 64.497 499 501-510... 

A range of nine Schiff Base and related complexes of cobalt(II) with up to seven phosphines or phosphites interacting in the axial position have been studied in CH2CI2 solution(6) (Table II). Their EPR spectra were recorded at 77 K and at about 200 K. The frozen solution spectra exhibited three well separated g-features at ca. g = 2.14-2.47, 2.11-2.18 and 2.02. The two g-features at higher field exhibited well resolved cobalt (hfs) and each line was split into two by further coupling to one phosphorus atom. Resolution on... 

Figure 4. Comparison of the ENDOR spectra of D " in LH(M160) in liquid and frozen solution. The four line pairs arising from methyl proton hfc s are indicated (a,a to d,d ). In the frozen solution spectra (bottom) the / -proton hfc s are broadened due to their larger anisotropy. The shoulders at line pair dd are attributed to hf anisotropy and not to an additional coupling. Note that even the smallest methyl proton hfc aa (AjgQ = 0.7 MHz) is clearly visible in the frozen solution spectrum.

Computer Simulation of Powder or Frozen Solution Spectra... 

In powder or frozen solution spectra, which are the most commonly observed for proteins, it is assumed that the molecules are randomly oriented with respect to the magnetic field the ESR intensity at any particular field value is superposition of spectra at that field over all possible orientations. Most computer simulation methods make use of the trapezoidal rule of integration which carries out summations over finite intervals small enough to avoid noise in the output. 

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