[nj_bartel]

Once a methyl group has a hydrogen that's properly aligned for hyperconjugation with a carbocation's p orbital, does the energy to rotate that methyl group increase and if so, is it substantial?

Thanks.

[spirochete]

Yes, and I'd guess the energy cost would be equal to the energetic value of the stabilization it provides. Rotation would require a transition state where the orbitals of interest are orthoganol and do not benefit from overlap at all.

The stabilization for a single hyperconjugation interaction in a carbocation should be available in an undergraduate organic chem text, I don't know it off the top of my head.

[nj_bartel]

Thank you