²H/¹H Isotope Shifts for ⁶Li‐NMR: First Experimental Detection and Applications to Structural Problems in the Field of Organolithium Compounds

Abstract

Deuterium‐induced isotope effects on 6Li chemical shifts, transmitted over two bonds, 2Δ(6Li)(2H/1H), have been measured for the first time for methyllithium and the system methyllithium/lithium iodide. The observed shifts are to low field and range from 15 to 20 ppb per CD3 group. The multiplicity and intensity distribution of the 6Li signals resulting for mixtures of CH3‐ and CD3‐containing samples in the limit of slow inter‐ as well as intra‐aggregate exchange yield information about the size of the various clusters present in diethylether solution. It is expected that these isotope shifts can facilitate structural investigations of organolithium compounds. In some cases, the results of such measurements are expected to be less ambiguous then the conclusions based on multiplets that arise from scalar 13C,6Li spin‐spin coupling.

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

“…3 , CH 3 , Br]Br). Although one of the signals 2 and 3 is therefore without doubt due to cluster D, especially since D (X D I) was also found for the system H 3 CLi-LiI, 7 it is not clear if it is signal 2 or 3. The other signal should then result from dimer E. The concentration of both species D and E can be estimated from the signal intensity relative to signal 4 as about 1.5%.…”

“…3 , Br, Br], respectively. Since deuterium in the CD 3 groups leads to high-frequency shifts, 7 the intensity distribution in the multiplets reflects the decreasing NOE effects for 6 Li with the decreasing number of CH 3 which indicates a deuterium-induced isotope shift over four bonds. Thus, under the conditions used in this study, we find that clusters A, B and C (Scheme 1, X D Br) are present in a molar ratio of 7 : 66 : 27.…”

“…7 Owing to deuterium-induced two-bond isotope shifts of the 6 Li resonance, 2  2 H, 1 H 6 Li, in aggregates with suitably labelled organic ligands, 1 : 1 mixtures of deuterated (d) and non-deuterated (h) compounds yield in the region of slow inter-and intra-aggregate exchange, which is usually accessible at low temperatures, typical multiplets for the 6 Li resonances which constitute fingerprints of the 6 Li environment. Thus, a dimer, to describe a typical result, † Dedicated to Professor Wolfgang von Philipsborn on the occasion of his 75th birthday.…”

“…3). Since the pioneering work of Waack et al 4 and Novak and Brown, 5 NMR spectroscopy of 1 H, 13 C, 6 Li and 7 Li has played a major role in these type of structural investigations (for a review, see Ref. 6).…”

“…Earlier we used this approach to study aggregation in the system H 3 CLi-LiI (1 : 1) in diethyl ether (DEE). 7 In this work, we investigated aggregate formation between H 3 CLi and LiBr in DEE and tetrahydrofuran (THF) (for a preliminary report, see Ref. 8) and, in addition, the system H 3 CLi-LiI in THF.…”