Deceptively simple spectra

This spectrum is an excellent example of what have been termed deceptively simple spectra. If one were confronted with such a spectrum and did not realize that it is a special case of ABX (actually AA X, because the chemical shifts of A and B are equal), the doublet and triplet might mistakenly be interpreted as the components of a first-order A2X spectrum, with J x = 5 Hz. Actually, the observed splitting is the average of /AX and JA-X, rather than any coupling constant in the molecule. An experimental example is shown in Fig. 6.11. 

Deceptively simple spectra are not limited to ABX systems they may occur for specific combinations of parameters in any spin system. Caution should be exercised in interpreting spectra that contain accidental chemical shift equivalences and show apparent equivalences of coupling constants that one might intuitively think would not be equal. 

Also notice in Figure 5.30 that the AB spectra with AvU equal to 3, 6, and 15 all appear roughly first order, but the doublets observed in the range riv/J = 3 to 6 have chemical shifts that do not correspond to the center of the doublet (see Fig. 5.31). Unless the worker recognizes the possibility of second-order effects and does a mathematical extraction of the chemical shifts, the chemical shift values will be in error. Spectra that appear to be first order, but actually are not, are called deceptively simple spectra. The pattern appears to the casual observer to be first order and capable of being explained by the + 1 Rule. However, there may be second-order lines that are either too weak or too closely spaced to observe, and there may be other subtle changes. 

The question of a unique metal-ion position in lanthanide shift-reagent-ketone complexes has been discussed, particularly with respect to 4-t-butylcyclohexanone. The conformation of 1,4-dihydrobenzyl alcohol (103) has been investigated via its Eu(fod)3-expanded n.m.r. spectrum previous analyses have been hampered by the deceptively simple spectra which 1-substituted cyclohexa-1,4-dienes frequently exhibit. In the expanded spectrum protons H-4 and H-4 appeared as an AB quartet and H-4 was assigned to that absorption which moved downfield more. Since H3-H4 it W3S concluded that H-3, and also H-5, must... 

In Figure 8.19c the difference in the chemical environment of Ha and Hb is very sHght. The spectrum shown may be seen as intermediate between the limiting cases in Figure 8.19a and Figure 8.19b. Note that there is a Teaningin" of the doublets as the central members increase in intensity at the expense of the outer members. A full continuum of behavior may be expected with cases observed in which the outer members are lost in the noise and the central members take the appearance of a doublet. This would be one example of a class of spectra known as "deceptively simple spectra."... 

Deceptive Simplicity. Sometimes the combination of parameters is such that a deceptively simple spectrum results. Consider the spectrum of furan. 

Figure 10-28 Splitting pattern expected for Ht, in a propyl derivative when Jsb Jbc- Several of the peaks coincide, giving rise to a deceptively simple spectrum a sextet.

Evidently, we should be wary of drawing conclusions from what may be a deceptively-simple NMR spectrum measured at room temperature wherever fluxionality seems plausible, it must be suspected. Organometallic compounds involving the cyclopentadienide ligand... 

The 31P H NMR spectra of a number of heteronuclear gold cluster compounds are found to be deceptively simple and NMR studies have been used as a probe of the behavior of these species in solution. This is especially true of the higher nuclearity clusters, which often exhibit spectra that are much simpler than would be predicted on the basis of their solid-state structures. For example, [Pt(H)(PPh3)(AuPPh3)7]2+, which adopts the solid-state structure illustrated in Fig. 9 (137) in which the phosphine ligands occupy several different chemical environments within the molecule, shows only two resonances in the 31P 1H NMR spectrum. These are in a ratio of 7 1 and exhibit satellites due to coupling to the central platinum nucleus as Fig. 10 illustrates. 

It is tempting to treat the proton spectrum in a similar fashion to our treatment of fluoroacetone. What we have, however, is a higher-order system, AA GG MX, where X is a fluorine atom. The fluorine absorption is deceptively simple in being symmetrical the halves are mirror images. The A and A protons are not magnetic equivalent since they do not couple equally with the M proton or the M proton. Nor are the M and M protons magnetic equivalent since they do not couple equally with the A proton or the A proton. 

The AA XX spectrum often is observed as a deceptively simple pair of triplets, resembling A2X2. In this case, it is the A and A nuclei that are closely coupled (Av a = 0 Hz... 

If all 10 peaks are observed, the analysis of the AA XX spectrum is quite straightforward, as could be done, for example, with the spectrum of 1,1-difluoroethene in Figure 4-2. When fewer peaks are observed, as in the spectrum of 1,2-dichlorobenzene (which is AA BB rather than AA XX ) (Figure 4-3), the analysis is more difficult and normally requires the use of computer programs. Often, such spectra are termed deceptively simple. The analysis does not distinguish Aa from Ax or Ax from y x ... 

The n.m.r. spectra of a series of simple derivatives of 2-oxo-l,3,2-dioxa-phosphorinan have been reported, analysed, and discussed in terms of ring conformations. This paper draws attention to a type of deceptively simple ABX spectrum found in this type of compound. 

The NMR spectrum of the methylenic protons of syndiotactic polypropylene is deceptively simple [i]. The calculation of this spectrum requires examination of syndiotactic poly-trans-(or cis-)l,3,3,3 propylene. 

Timc-of-flight mass spectrometry (TOFMS) is deceptively simple in concept a packet of ions given the same kinetic energy fly down an evacuated, field-free tube and arrive at an ion detector in the order of their mass-to-chargc (w/z) values. The detector output current, measured as a fimction of time, can easily be converted into a mass spectrum. The physics and the instrumentation are both remarkably straightforward. The following inherent advantages of TOFMS are often listed by its advocates ... 

Anisotropy is observed when a substituent group bonds a carbonyl group directly to the benzene ring (Fig. 7.61). Once again, the ring protons fall into two groups, with the ortho protons downfield from the metalpara protons. Benzaldehyde (Fig. 7.62) and acetophenone both show this effect in their NMR spectra. A similar effect is sometimes observed when a carbon-carbon double bond is attached to the ring. The 300-MHz spectrum of benzaldehyde (Fig. 7.62b) is a nearly first-order spectrum (probably a deceptively simple AA BB C spectrum) and shows a doublet (He, 2 H), a triplet (Hb, 1 H), and a triplet (Ha, 2 H). 

Sometimes the ABX spectrum can be deceptively simple (i.e., one or more subspectra can degenerate into a single line), giving rise to one of the two following situations ... 

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