Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 52 / b

Useful isotope-selection factors include spin and quadrupole moment, gyromagnetic ratio, natural abundance or enrichment, sensitivity at the available field, line widths and relaxation, chemical-shift dispersion and the couplings needed for structural assignment. Both lithium isotopes are quadrupolar, but ${}^6$Li has a much smaller quadrupole moment and often much narrower lines, allowing inequivalent sites and carbon-lithium couplings to be resolved. Enrichment compensates its low natural abundance and removes isotope-mixture complexity.

Fully enriched ${}^7$Li would generally be more sensitive but broader in these aggregates. Its chemical-shift pattern would remain chemically similar, although isotope shifts and dynamics can make small differences. Resolved H-Li couplings increase in frequency approximately with ${\gamma }_7/{\gamma }_6=2.67$ and have different multiplicities because $I$ changes from $1$ to $3/2$: $n$ equivalent Li nuclei give $2nI+1$ lines. If such long-range proton couplings remain unresolved, the main visible difference can instead be broadened lines. One should not invent resolved multiplets absent from the printed spectra.