Nuclear magnetic complex splitting

As indicated in the previous discussion, Mossbauer spectroscopy provides information that when coupled with results using other structural techniques assists in determining the structure of the complex under analysis. The relationships between the various techniques are summarized in Table II. The Mossbauer chemical shift provides information about the 4 electron contribution to the bond between the metal and the ligands in a complex. Similar estimates can be obtained from the results of measurements on the fine structure in the x-ray absorption edge and nuclear magnetic resonance data. The number of unpaired electrons can be evaluated from magnetic susceptibility data, electron spin resonance, and the temperature coeflScient of the Mossbauer quadrupole splitting (Pr). 

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... 

Nuclear magnetic resonance spectra may be so simple as to have only a single absorption peak, but they also can be much more complex than the spectrum of Figure 9-23. However, it is important to recognize that no matter how complex an nmr spectrum appears to be, it involves just three parameters chemical shifts, spin-spin splittings, and kinetic (reaction-rate) processes. We shall have more to say about each of these later. First, let us try to establish the relationship of nmr spectroscopy to some of the other forms of spectroscopy we already have discussed in this chapter. 

Treatments to interpret the frequeney dependenee of Tie and Tje have become inereasingly sophisticated. The multifrequency studies done in solution define a funetion of zero-field splitting (ZFS) or the crystal-field interaction cfi) parameters, denoted as A. The relationship between the parameters D and E, whieh account for the quadrupolar part of the cfr, and is defined as A = 2I3)D + 2E. The cfi parameters for Gd(III) complexes are dealt with in mueh more detail below in Section 3, where we discuss more reeent results on frozen glassy solutions. Here we describe efforts to understand eleetronic relaxation in solution and how this impacts nuclear magnetic relaxation and MRI contrast agents. 

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