Consider this typical 1,2 methanide shift in a carbocation:

enter image description here

Which isotopic form of methanide ($\ce{CH3, CD3, or CT3}$) would migrate from secondary carbocation to form more stable tertiary carbocation?

This link explains the mechanism but I could not understand the abstract.

Assuming that the migration of $\ce{CH3-}$ takes place, the following facts can be considered:

  • The electronegativity of $\ce{CH3}$ is highest, thus, the formation of $\ce{CH3->CH3-}$ (methanide) is easiest. Consequently, the migration would be fastest in this case.

  • The transition state would be most unstable in this case due to highest electronegativity of $\ce{CH3}$.

  • The tertiary carbocation would be most stable in this case due to order of $\mathrm{+I}$ effect: $\ce{CT3 > CD3 > CH3}$

There are few exchanged comments below the second answer of this question and the proper explanation can be found in this link, which I can't access.

  • $\begingroup$ @Safdar So, the stability of T.S. is more important than that of final carbocation. In your comment, the numerator and denominator are same. $\endgroup$ – Apurvium Sep 25 '20 at 8:50

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