Inductive effect of alkyl ammonium groups
The inductive effect is both distance-dependent and &chi;-dependent. N-H bonds are 1.01 Å, yet the &chi; of H is only 2.1. N-C bonds are 1.42 Å, yet the &chi; of C is 2.5. At the extremes, where there are only N-C or N-H bonds, the ordering of -I is easy to rationalize.

However, the two, short N-H bonds in -NH2R+ outweigh the higher &chi; of N-C bonds, giving -NH2R+ a higher -I than -NHR2+.