I had been studying on the stable conformers of ethylene glycol and ethane-1,2-diamine.

Due to hydrogen bonding in the vicinal $\ce{-OH}$ groups, ethylene glycol prefers to show gauche conformer. The same can be said for ethane-1,2-diamine in neutral medium.

My query arises for ethane-1,2-diamine in pH<7, where I expect the $\ce{-NH2}$ group(s) to be protonated. Can we predict the most stable conformer now for these cases? Does protonation affect the extent of hydrogen bonding?

  • $\begingroup$ Wait, you really ask about diprotonate en? Well, yeah at such pH it would be, but it's not like any intramolecular hydrogen bond would be possible. $\endgroup$
    – Mithoron
    May 5, 2022 at 19:23
  • $\begingroup$ After @Geoff Hutchison 's answer I now asked for both cases without mentioning a specific pH, hope that's all right $\endgroup$ May 6, 2022 at 2:05
  • $\begingroup$ @KimDokja Generally, it is discouraged to expand the scope of a question after having received an answer. $\endgroup$
    – Tyberius
    May 6, 2022 at 13:33
  • $\begingroup$ @Tyberius, Will take care of that in the future . Thank you for pointing it out. $\endgroup$ May 6, 2022 at 14:00

1 Answer 1


I'm not 100% convinced that both amines will protonate at the same time, since protonation of a charged molecule is different than the neutral. At the moment, I don't have the time to calculate the $\mathrm{p}K_\mathrm{a}$ of the first and second protonation.

But let's go with your assumption - that you have two $\ce{NH3+}$ groups. Interestingly, there's much less known about conformational preferences of charged molecules.. which is a project currently underway in my group.

Here's my initial guess .. that the $\ce{NH3+}$ groups would function like $\ce{CH3}$ sterically, and the positively charged groups would want to move as far apart as possible due to electrostatics:

initial conformation of (NH3+)CH2CH2(NH3+)

We can test this using quantum chemical methods. A tool called CREST performs driven molecular dynamics on the charged species, and returns anything within $\pu{X kcal mol-1}$ (The default is $\pu{6 kcal mol-1}$, which is what I ran).

And .. that's the lowest energy conformer. Nothing else is within $\pu{6 kcal mol-1}$ .. not only by removing the possible hydrogen bonding, but also because the two charged $\ce{NH3+}$ groups repel.

  • $\begingroup$ Thank you very much, this was very helpful! As it is interesting, I have now edited my question regarding the number of protonated groups. I appreciate that this is an ongoing project, but what would you predict for the monoprotonated compound? $\endgroup$ May 6, 2022 at 2:01
  • $\begingroup$ The monoprotonated compound exhibits a strong hydrogen bond between the $\ce{NH3+}$ and the lone pair on the other $\ce{NH2}$ group. $\endgroup$ May 6, 2022 at 3:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.