Henry Rzepa's Blog Chemistry with a twist

March 15, 2015

Ionizing ultra-strong acids with water molecules.

Filed under: Interesting chemistry — Tags: , , — Henry Rzepa @ 6:53 pm

My previous posts have covered the ionization by a small number of discrete water molecules of the series of halogen acids, ranging from HI (the strongest, pKa -10) via HF (weaker, pKa 3.1) to the pseudo-halogen HCN (the weakest, pKa 9.2). Here I try out some even stronger acids to see what the least number of water molecule needed to ionize these might be.

Firstly what must surely be the ultimate acid H(CHB11Cl11), discovered by Christopher Reed[1] in 2006. This is so strong that it appears that it can even largely ionize itself; the form on the right (below) is the cationic acid, the form on the left is its anionic base. The proton itself is bridged[2] between the two in a manner similar to the structure of one form of HCl.4H2O reported in the earlier post on the topic.

Click  for  3D

Click for 3D

So it comes as no surprise to find[3] that just one water molecule can also ionize H(CHB11Cl11) to the anionic form (CHB11Cl11).

Click for  3D

Click for 3D

How about triflic acid, CF3SO2OH, pKa -16), which is also a fair bit more acidic than HI? Here, only three waters are needed (ωB97XD/6-311++G(2d,2p) prediction) to ionise to triflate anion.[4]

Click  for  3D

Click for 3D

So, if there a system which is ionised by precisely two water molecules I will record it here.

Perhaps also no surprise is that one H2S molecule can also perform this ionisation.[5] This leads us into another exploration, using molecules other than water to perform these ionisations.


  1. E.S. Stoyanov, S.P. Hoffmann, M. Juhasz, and C.A. Reed, "The Structure of the Strongest Brønsted Acid:  The Carborane Acid H(CHB11Cl11)", Journal of the American Chemical Society, vol. 128, pp. 3160-3161, 2006. http://dx.doi.org/10.1021/ja058581l
  2. Stoyanov, E.S.., Hoffmann, S.P.., Juhasz, M.., and Reed, C.A.., "CCDC 606170: Experimental Crystal Structure Determination", 2006. http://dx.doi.org/10.5517/CCNBRWL
  3. Henry S Rzepa., "C 1 H 4 B 11 Cl 11 O 1", 2015. http://dx.doi.org/10.14469/ch/191134
  4. Henry S Rzepa., "C 1 H 7 F 3 O 6 S 1", 2015. http://dx.doi.org/10.14469/ch/191129
  5. Henry S Rzepa., "C 1 H 4 B 11 Cl 11 S 1", 2015. http://dx.doi.org/10.14469/ch/191135


  1. Hi, for H(CHB11Cl11), what may happen if water molecule approaches CH? What if H3O+ approaches CH group?

    Comment by Cina — March 15, 2015 @ 7:11 pm

  2. There are a number of different isomeric triangular faces a water could approach, one of them being the face with the CH bond. I have not explored them all in this post, and it would be interesting to find out which face results in the lowest free energy.

    Comment by Henry Rzepa — March 16, 2015 @ 7:05 am

  3. Thank you for your reply. I guess one of possible reactions in this system is substitution of hydride ion, attached to acidic carbon with OH or H2O. Furthermore, an update 🙂 though not proven yet, I expect the strongest Bronsted acid to be [(H+)-(2aF-)] in the following article, since the strongest hypothetical Lewis acid is compound 2a in this article:
    This compound is few percent more acidic than a hypothetical carborane very similar to the molecule you studied but with fluorine substitutions instead of chlorine (see Müller, et. al. Angew. Chem. Int. Ed. 2008, 47, 7659).

    Comment by Cina — March 16, 2015 @ 9:44 am

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