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The $\ce{-[NMe3]+}$ group is ranked almost at the first position in the -I effect series, which is often given at the very beginning of the teaching of general organic chemistry.

I know this can be justified by hyperconjugation, but I am interested in a rigorous justification by molecular orbital theory.

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    $\begingroup$ This group has a positive charge, any molecular orbital effect will be negligible compared to that. $\endgroup$ – Martin - マーチン Aug 1 '18 at 16:02
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$\ce{Me}$ is a pushing group which pushes electron towards the nitrogen atom. As a result electron concentration is towards the nitrogen atom.

The anti bonding electrons of nitrogen are shifted to the bonding orbital of nitrogen. This is because the bonding orbitals are closer to the more electronegative atom which in this case is nitrogen.

So nitrogen becomes more "electron hungry" and in order to stabilize itself tries to pull electron towards itself and thereby renders a -I effect.

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  • $\begingroup$ Will somebody explain what i have done wrong in this answer. $\endgroup$ – starunique2016 Jan 4 '18 at 6:19
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    $\begingroup$ This is super hand wavy and probably not what the OP wanted. $\endgroup$ – ringo Jul 2 '18 at 13:34

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