Lithium has two useful nuclei. ⁶Li has a spin of one. Spins of greater than ½ are quadrupolar but ⁶Li has a low quadrupolar moment and yields sharp signals but has low sensitivity. On the other hand, ⁷Li is highly sensitive but has a higher quadrupolar moment so its signals are broader. Both nuclei have a moderate chemical shift range. Each type of signal has a characteristic chemical shift range (fig. 1) which is the same for both nuclei.

Fig. 1. Chemical shift ranges for lithium NMR

⁶Lithium

⁶Li-NMR (fig. 2) is less sensitive than ⁷Li even when enriched.

Fig. 2. ⁶Li-NMR spectrum of LiCl (1M) in D₂O at natural abundance

However, ⁶Li yields much shaper lines than ⁷Li, especially in asymmetric environments. As a result, it is possible to observe two-bond ¹H-⁶Li and ¹³C-⁶Li couplings of the order of 1 Hz in covalently bonded molecules. ⁶Li tends to have long T₁ relaxation times so a preacquisition delay of 5 to 10 seconds is usually required. The spectrum (fig. 3) shows a large number of signals arising from a supramolecular complex. Some signals are very sharply defined with 1 Hz line-width. However, sensitivity is limited even though the sample was enriched with ⁶Li.

Fig. 3. ⁶Li-NMR spectrum of Me₅C₆₀^5-/Cor^4-/9Li⁺ showing signals in three separate regions

(see I. Aprahamian, et al., J. Am. Chem. Soc., 127, 9581-7 (2005))

Properties of ⁶Li

(Click here for explanation)

Property	Value
Spin	1
Natural abundance	7.59%
Chemical shift range	28 ppm, from -16 to 11
Frequency ratio (Ξ)	14.716086%
Reference compound	9.7 m LiCl in D2O
Linewidth of reference	0.03 Hz
T1 of reference	~150 s
Receptivity rel. to 1H at natural abundance	6.45 × 10-4
Receptivity rel. to 1H when enriched	8.50 × 10-3
Receptivity rel. to 13C at natural abundance	3.79
Receptivity rel. to 13C when enriched	49.9
Linewidth parameter	0.033 fm4

⁷Lithium

⁷Li-NMR is more sensitive than ⁶Li but yields broader signals due to its higher quadrupolar moment. The broadening is not very evident for aqueous lithium ions because they are very small and symmetrical (fig. 4).

Fig. 4. ⁷Li-NMR spectrum of LiCl (1M) in D₂O

However, for organometallic systems with lithium bonded within a molecule, it has a short relaxation time, typically 0.2 s. In covalent and π-bonded systems this allows fast repetition times and high signal-to-noise. Coupling to ⁷Li is very rarely observable with very few cases of one-bond ¹H-⁷Li coupling (about 20 Hz) known. The ⁷Li spectrum (fig. 5) is much more sensitive than the ⁶Li spectrum (fig. 3). Low symmetry combined with the higher quadrupole moment of ⁷Li broaden the signals. As a result, the signals that were tall and sharp in the ⁶Li spectrum (fig. 3) are now low and broad (fig. 5).

Fig. 5. ⁷Li-NMR spectrum of Me₅C₆₀^5-/Cor^4-/9Li⁺ showing signals in three separate regions

(see I. Aprahamian, et al., J. Am. Chem. Soc., 127, 9581-7 (2005))

Properties of ⁷Li

(Click here for explanation)

Property	Value
Spin	3/2
Natural abundance	92.41%
Chemical shift range	28 ppm, from -16 to 11
Frequency ratio (Ξ)	38.863797%
Reference compound	9.7 m LiCl in D2O
Linewidth of reference	0.07 Hz
T1 of reference	36 s
Receptivity rel. to 1H at natural abundance	0.271
Receptivity rel. to 1H when enriched	0.29
Receptivity rel. to 13C at natural abundance	1590
Receptivity rel. to 13C when enriched	1721
Linewidth parameter	21 fm4

Safety note

Some of the materials mentioned here are very dangerous. Ask a qualified chemist for advice before handling them. Qualified chemists should check the relevant safety literature before handling or giving advice about unfamiliar substances. NMR solvents are toxic and most are flammable. Lithium salts are not toxic on account of the lithium under normal circumstances; however, the anion may be toxic.