# Stable conformer of protonated ethane-1,2-diamine

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?

• 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. May 5 at 19:23
• After @Geoff Hutchison 's answer I now asked for both cases without mentioning a specific pH, hope that's all right May 6 at 2:05
• @KimDokja Generally, it is discouraged to expand the scope of a question after having received an answer.
– Tyberius
May 6 at 13:33
• @Tyberius, Will take care of that in the future . Thank you for pointing it out. May 6 at 14:00

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:
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.
• The monoprotonated compound exhibits a strong hydrogen bond between the $\ce{NH3+}$ and the lone pair on the other $\ce{NH2}$ group. May 6 at 3:30