Unresolved splitting

Since the shifts produced by the cross-terms between ligand and central ion nuclei are often significant, the strikingly broad ENDOR lines ( 1 MHz) observed for transition metal ions61,62) may be traced back, at least in part, to unresolved splittings due to second order interactions of numerous ligands with the central ion. ENDOR frequencies up to second order for an I = 1 nucleus in the presence of a second ligand nucleus with a spin K = 1 are tabulated in Appendix B, Eqs. (B4). 

Figure 4. Mossbauer spectrum of Fe(III) (1 mole %) in CeOz taken with NSEC Model B spectrometer (> 200 points average of three spectra). The dashed line suggests that unresolved splitting may be responsible for the great breadth of the central peak...

The splitting pattern gives a crude sextet, but the second line has a shoulder on the left, and the fourth line shows an unresolved splitting. The other peaks are somewhat broadened. 

Ja,a, are now all H-D coupling and first order. These cause unresolvable splittings to occur which were removed by a strong irradiation at the deuterium resonance frequency (Bloom and Shoolery, 1955), Both of these studies of deuteriated cyclohexane are in agreement and suggest a virtually temperature-independent AG+ value of 10-3-10-5 kcal mole , JH+ = 10-9 + 0-6 kcal mole and AS+ = 2-9 + 2-3 e.u. A transmission coefficient of 0-5 is assumed in the inversion process. The r ults obtained by Harris and Sheppard (1961) have been recalculated and the values of JH+ and AS= now agree with the results on deuteriated cyclohexane. 

Chains of any length can exhibit this phenomenon, whether or not they consist solely of methylene groups. For instance, the spectrum of the protons in the second methylene group of propylbenzene is simulated as follows. The splitting pattern gives a crude sextet, but the second line has a shoulder on the left, and the fourth line shows an unresolved splitting. The other peaks are somewhat broadened. 

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