Contact vs solvent‐separated ion pairs: a ⁷Li and ¹³C solid‐state NMR study of organolithium compounds

NMR spectroscopy of organolithium compounds. XXVI—The aggregation behaviour of methyllithium in the presence of LiBr and LiI in diethyl ether and tetrahydrofuran

“…However, owing to competing exchange processes between LiBr and the other aggregates present in the mixture, which were slow but certainly not completely negligible, the population of the LiBr site in exchange with E cannot be determined and a more complete treatment was therefore not possible. The present estimate, based on a rate constant k of 0.2 at 173 K, yields with the Eyring equation a G ‡ (173) of 44 kJ mol 1 . In an independent approximation a value Table 2) and the coalescence temperature of 195 K. A linear regression of our data points indicates a high negative activation entropy for the exchange as expected for a highly ordered transition state.…”

Regio- and Stereoselective Lithiation of 2,3-Diphenylaziridines: A Multinuclear NMR Investigation

“…A complete assignment of the above protons has been made possible by a DQF-gCOSY analysis (see the Supporting Information), disclosing that the two H o of the lithiated intermediate have completely different chemical shifts (5.80 and 7.10 ppm) and the H p proton is less shielded than expected (6.20 ppm) . The different chemical shifts of the two ortho protons of the phenyl ring bonded to the C α carbon may be ascribed to a reduced mobility of this phenyl ring which rotates slowly around the C α −C i bond …”

13C and 23Na solid‐state NMR spectra of organosodium compounds

“…This finding may thus be compared with the shift difference of ca 3.5 ppm found in solution for 7 Li on going from cyclopentadienyllithium ( 8.37 ppm) 27 to lithiocene ( 11.05 ppm, both values relative to external aqueous 1 M LiCl) (H. Hausmann and H. Günther unpublished data). Compound 3 is also the only complex where a significant chemical shift anisotropy term has been found.…”

“…In particular, the quadrupolar coupling constant, ( 7 Li), turned out to be an indicator of the bonding situation around the lithium cation because its magnitude varies with the aggregation state, 3 the number and type of ligands, 4 the structural X-Li-X angle (X D N, C) 5,6 and the ion-pair structure. 7 In this context, it was of interest to evaluate if similar structure-NMR relationships exist for organosodium compounds where structural research for solids was so far exclusively performed by diffraction methods. 8 The nuclide 23 Na of spin I D 3/2, often studied by NMR in the case of inorganic compounds and materials science, 9 was expected to yield useful structural information, even if solid-state 23 Na spectra are more complicated than those of 7 Li.…”

“…7 In this context, it was of interest to evaluate if similar structure-NMR relationships exist for organosodium compounds where structural research for solids was so far exclusively performed by diffraction methods. 8 The nuclide 23 Na of spin I D 3/2, often studied by NMR in the case of inorganic compounds and materials science, 9 was expected to yield useful structural information, even if solid-state 23 Na spectra are more complicated than those of 7 Li. This is due to second-order quadrupole effects which are more important for 23 Na because of the larger quadrupole moment (Q D 10.06 ð 10 2 barn for 23 Na vs 4.01 ð 10 2 barn for 7 Li).…”