= Solution
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.
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