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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?

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  • $\begingroup$ 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 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 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 at 13:33
  • $\begingroup$ @Tyberius, Will take care of that in the future . Thank you for pointing it out. $\endgroup$ May 6 at 14:00

1 Answer 1

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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.

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  • $\begingroup$ 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 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 at 3:30

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