Spin-flip narrowing

In a nutshell, the function of the pulse sequence is to rotate the nuclear spins by large amounts in such a way that over a cycle (consisting of 4 to as many as 52 pulses at present) the average dipolar interaction vanishes. The reason it is possible to accomplish this is that the sign of the dipolar field from a magnetic moment depends on the position around the dipole. Such an averaging of the dipolar interaction is called spin-flip narrowing by Slichter (Appendix A) and we now paraphrase his simplified description. 

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

All three of these problems tend to be resolved when bands due to intraconfigurational transitions are used. In six-coordinate, approximately Oh complexes these are l2g t2g and eg -> eg transitions, quite often accompanied by a spin flip, and therefore forbidden. In contrast to t2g - eg transitions, these are usually numerous, narrow (because the excited state resembles the ground state), and simple each observed narrow line comprises just one component. The relative abundance of these sharp lines (8 for d3 complexes, for example, and as many as 14 for low-spin d4) is deceptive it is because the also numerous t2g —> eg components cluster under broad bands. 

Many transition metal ions are known as luminescent centers as for example Mn2+ in Zn2SiC>4 Mn with a broad emission band due to the electronic transition 4Ti (t eg) — 6A (t g e2). The emitted light is green. On the other hand, the emission band of Mn4+ in Mg4Ge05.5F Mn is narrow with some vibronic interaction structure. The emission in this compound is due to a spin flip transition in t g without any change in chemical bonding. 

From experiments on several different kinds of films, including films of a-Si H to be discussed in the next section, Thomas et al (1978) suggest that the qualitative behavior of the temperature-independent component of the linewidth as a function of spin density Ns is as exhibited in Fig. 17. At low spin densities the asymptotic behavior is controlled by g-tensor effects. The narrowing with increasing is thought to be caused by dipolar spin-flip interactions or exchange effects, and the increase at high Ns is attributed to the normal dipolar interaction as expressed in Eq. (3). 

The improved resolution in ENDOR spectra makes it possible not only to measure the hyperfine couplings with high accuracy, but also to observe splittings that are not resolved by ESR. An example is shown in Fig. 2.5 for the malonic acid radical, HCc (COOH)2 [24]. The ESR spectrum is a main doublet due to hyperfine coupling with the H at the Ca position. The resolution is limited by the line-width and the occurrence of forbidden and so-called spin flip lines discussed in Chapter 4. The ENDOR lines denoted V(t and Va are narrower than the ESR lines by more than an order of magnitude. As in the liquid state the intensities between the pair differ due to hyperfine enhancement and relaxation factors. The additional lines in the ENDOR spectrum were examined using the ENDOR Induced ESR (EIE) method described... 

A new method has been recently proposed for spin dynamic studies in conducting polymers [33]. It basically relies on the following. Collisions between moving spins (species A) and unlike spins (species B), instead of giving rise to a motional narrowing, can result in a broadening of the line. Such an effect can be observed in two cases. First, in the slow exchange case, i.e., when the spin flip-flop frequency Larmor frequencies of the two species < Soab = weA Wcb. Second, in the fast... 

In principle, multivalent counterions can physically crosslink polyelectrolytes, which cause a further complication in the description of the joint conformational distribution of the polymer chain and spatial distribution of the counterions. While FS with its two spatially close charges may not be well suited to study this phenomenon, the larger trivalent counterion TAM (Figure 15) does crosslink PDADMAC." Unlike the nitroxide FS, the triphenylmethyl radical TAM is not amenable to DEER experiments, as its ESR spectmm is too narrow. On the other hand, the narrow spectmm opens up the possibility to estimate distance distributions from dipolar line broadening, in analogy to the related analysis of nitroxide spectra described in Sertion 2.08.2.4. To avoid distortions by electron-proton spin flip transitions, the experiments were performed at W-band frequencies. 

Fig. 2. Different polarization transfer elements for the 1Hn-15N spin pair, INEPT (a), CRIPT (b), and CRINEPT (c) elements. Narrow and wide bars correspond to 90° and 180° flip angles, respectively, applied with phase x unless otherwise stated.

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