Oxygen-17, chemical shift determination

Fig. 7.4(A) is indicative of the presence of strong CF ir-bonding, and this is supported by the reflection of this pattern in Fig. 7.4(B). The good linear correlation (by conventional linear regression analysis, r = -0.998, n = 6) between 5( O) and 5]Xxy> Ote lack of significant correlation with the ir(CO) bond order, suggest that the inductive effect dominates the determination of terminal oxygen chemical shift as appears also to be the case with the restricted data set for 5( F) [797] [1589b], whereas the chemical shift of the central carbon atom is determined by a balance of opposing a and x effects.

A few relatively recent published examples of the use of NMR spectroscopy for studying polymer degradation/oxidation processes will now be discussed briefly. At the early stages of degradation, the technique can be used to provide chemical identification and quantification of oxidised species for polyolefins, oxidation sites can be identified by the chemical shifts of -CH2- groups a and ji to carbons bonded to oxygen [85]. Spin-spin relaxation times may be determined by a pulse sequence known as the Hahn spin-echo pulse sequence. 

It has previously been concluded that even in strong acidic solution, the dioxotetracyanoosmate(VI) complex cannot be protonated to form the oxo aqua complex or even the corresponding hydroxo oxo complex. The pA i and pKa2 values have been estimated to be substantially less than -1, which is also supported by the relationship between pKa values and 170 and 13C chemical shifts (Table II). Extreme slow kinetic behavior, as expected in the case of a +6 charged metal center for a dissociative activation exchange process, has been observed, with only an upper limit for the oxygen exchange determined (Table II). 

The application of 170 NMR spectroscopy to obtain structural information about polyoxoanions, mainly in nonaqueous solution, had been examined and discussed in detail (99). A very useful finding was that chemical shifts are determined largely by metal-oxygen bond strengths. An inverse correlation exists between the 170 shift and the shortest bond length to a given metal. In aqueous solution the existence of [Mo7024]6 could be confirmed by the use of 170 and "Mo NMR spectroscopy (100-104). Evidence for the existence of the three... 

The isotropic 29Si chemical shifts of 88 and 89 in the solid state are as follows 88, <5 -69.8 89, S 98.8. As would be expected from the different electronegativities of sulfur and oxygen, these chemical shifts differ significantly from those determined for the corresponding oxygen analogs 53 (<5 -88.6) and 77a (8 -120.0). 

Despite its unfavorable NMR properties, the nO nucleus has attracted considerable interest, since its chemical shifts represent a discriminating probe for structural and molecular properties. In a study of some 5-membered heterocycles (furan and isoxazole methyl derivatives) (840MR(22)55) it was found that the nO chemical shifts are mainly determined by the p-electron density on the oxygen atom. A nO downfield shift of 222 ppm is observed on the formal aromatization of tetrahydrofuran to furan (61HCA865). 

Up to this time there has been no report of the experimental determination of the structure of the parent homotropenylium ion. The three simplest systems that have been studied are 18, 19 and the iron complex 20. Cations 18 and 19 each have an oxygen-containing electron-donor substituent and, as such, appear to have smaller induced ring currents than the parent ion. In fact 18 and 19 have almost identical chemical shift differences (A<5 = 3.10 ppm) between the two C(8) protons. In the case of 20, A<5 is very small and it was considered to be a non-cyclically delocalized model for the bicyclo[5.l.OJheptadienyl cation69. 

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