Lithium chemical shift

The magnitude of the chemical shift anisotropy depends on the bonding situation and the nucleus gyromagnetic ratio. Since the bonds formed by lithium in organolithium compounds or other lithiated systems are mainly ionic, the anisotropy of the lithium chemical shift is generally small. It is more pronounced for Li than for Li. Li spectra are dominated by the quadrupolar effect and the CSA contribution to the Li lineshape is often negligible. Exceptions are compounds with poly-hapto bound lithium, such as... 

Figure 1. Chart of lithium chemical shifts, (A) salt solutions and (B) organic compounds/ " 200-203)...

The situation is similar to the exchange between the monomer and the micellar states. Usually, the exchange between the monomer and the micellar states is fast. The spectra at surfactant concentrations above CMC, therefore, consist of a single set of peaks whose chemical shifts are averaged between the monomer and micellar states. Such an example is shown by spherical micelles formed by lithium perfluoro-octylsulfonate (FOS )... 

The 3H, 6Li and 13C NMR chemical shifts of the cyclopropenium cation and its lithium derivatives 115 118 were calculated both at GIAO-DFT and GIAO-HF level using B3LYP optimized geometries.143... 

Typical NMR data were compiled in CHEC-II(1996) <1996CHEC-II(8)2> for 3.77-pyrrolizine 1, its lithium salt, the pyrrolizin-3-one 2, and its regioisomer (pyrrolizin-2-one). More recently, Kissounko et al. <1998JOM(556)145> reported the H chemical shifts of the parent pyrrolizine anion and anion 21a as well as those of their silylated or stannylated derivatives 22-26. 

Lithiation shifts obtained by comparison of 13C chemical shifts in lithium dialkylamides and the corresponding amines have been found to be quite substantial and decrease in the order a > p > 7 > 8 (439). 

Finally, Al (/= 5/2) and Co NMR spectroscopy have been used to probe AP+ in Al-doped lithium cobalt oxides and lithium nickel oxides. A Al chemical shift of 62.5 ppm was observed for the environment Al(OCo)e for an AP+ ion in the transition-metal layers, surrounded by six Co + ions. Somewhat surprisingly, this is in the typical chemical shift range expected for tetrahedral environments (ca. 60—80 ppm), but no evidence for occupancy of the tetrahedral site was obtained from X-ray diffraction and IR studies on the same materials. Substitution of the Co + by AF+ in the first cation coordination shell leads to an additive chemical shift decrease of ca. 7 ppm, and the shift of the environment A1(0A1)6 (20 ppm) seen in spectra of materials with higher A1 content is closer to that expected for octahedral Al. The spectra are consistent with a continuous solid solution involving octahedral sites randomly occupied by Al and Co. It is possible that the unusual Al shifts seen for this compound are related to the Van-Vleck susceptibility of this compound. 

Although much of the V NMR has been performed on model systems or catalytic materials containing vanadium, 29 >30 compounds such as V2O5 or VOPO4 are used in both the catalysis and lithium battery fields, and many of the results can be used to help elucidate the structures of vanadium-containing cathode materials. V NMR spectra are sensitive to changes in the vanadium coordination number and distortions of the vanadium local environments from regular tetrahedra or octahedra. >33 5>V isotropic chemical shifts of between —400 and —800 ppm are seen for vanadium oxides, and unfortunately, unlike... 

Unfortunately, it cannot be applied to pyridine securely because of the indeterminate effect of charge delocalization on the para chemical shift. Nonetheless the sign and magnitude of the effects in phenyl-lithium would go a long way towards removing the discrepancies in the pnmr spectra of pyridine and pyridinium ion. 

In the DEE complex a chemical shift of —5.6 indicates that the ring current has a much more profound effect in this complex. However, the solid state structure was not known at that time. In the TMEDA complex, the lithium cation is postulated to be positioned above the central five-membered ring, and the Li chemical shift is —7.5 ppm, i.e. in the range of CIPs of cyclopentadienyllithium. In the THE complex, a shift of —2.6 ppm was observed. Again, no effect from the ring current is observed. However, based on the quadrupolar interaction this system was assigned as an SSIP, as discussed below. 

From these investigations it is clear that the Li chemical shift gives a clear indication of the lithium cation position when there are direct effects from ring currents in delocalized anions. However, as shown for the quinuclidine CIP and THF SSIP fluorenyllithium complexes, the correct assignment cannot be reached solely based on the chemical shifts. Furthermore, there is no clear-cut information about solvation to be gained from the chemical shifts. As we discuss in the following Section, the quadrupolar interaction is much more sensitive to these effects. In order to obtain maximal structural information from Li NMR spectroscopy, the chemical shift should be determined and used in combination with the quadrupolar coupling constant. 

The jT-type interaction to a lithium cation was proposed in the 1960s for delocalized carbanions. The basis for this was the observation of high-held Li chemical shifts ° As mentioned earlier, (Sections LB.2.a and II.B.2), the chemical shift is strongly affected by the ring current of delocalized carbanions. 

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