The tetrabutylammonium salts of the four halide ions ($\mathrm{Bu}{}_4\mathrm{N}{}^{+}\mathrm{X}{}^{-}$) are allowed to react with $\mathrm{CH}{}_3\mathrm{I}$ in dry DMF. The order of the rates of ${\mathrm{S}}_{\mathrm{N}}2$ substitution is

DMF is polar aprotic and does not hydrogen bond to anions, and the large $\mathrm{Bu}{}_4\mathrm{N}{}^{+}$ cation barely ion-pairs. The halides are therefore nearly 'naked' and their intrinsic nucleophilicity follows basicity: $\mathrm{F}{}^{-}$ is the most reactive and $\mathrm{I}{}^{-}$ the least. This is the reverse of the order in methanol.