Ionizing yet more ultra-strong acids with water molecules.

This might be seen as cranking a handle by producing yet more examples of acids ionised by a small number of water molecules. I justify it (probably only to myself) as an exercise in how a scientist might approach a problem, and how it linearly develops with time, not necessarily in the directions first envisaged. A conventional scientific narrative published in a conventional journal tells the story often with the benefit of hindsight, but rarely how the project actually unfolded chronologically. So by devoting 7 posts to this, you can judge for yourself how my thoughts might have developed (and I am prepared to acknowledge this may only serve to show my ignorance).

To pick up the story where it ended in the 6th post, I set off to hunt for a strong acid that might require precisely two water molecules to ionise it. So here are some more candidates:

Acid Acid…H length, Å OH length in 2H2O Data-DOI
bis-triflylamine NH=1.056 1.622 [1]
bis-triflylamine OH=1.575 1.007 [2]
Perchloric acid 1.024 1.540 [3]
Perchloric acid 1.514 (3H2O) 1.026 (3H2O) [4]
Perbromic acid 1.030 1.518 [5]
Fluorosulfonic acid 1.028 1.504 [6]
Fluoroselenic acid 1.025 1.522 [7]

Of these, perchloric acid is thought to be stronger than eg HBr, and indeed whereas the latter requires four water molecules for ionization, the former seems to require only three (I include this in the table above to show what happens to the bond lengths upon ionisation). But two is not quite enough, although it does appear to be on the edge. Nor does perbromic acid achieve this, or fluorosulfonic or fluoroselenic acids.

This search also illustrates another proclivity of humans, to set themselves targets, and on occasion fairly pointless targets. But one never knows whether even an apparently pointless target at the outset might not result in the discovery of something much more unexpected (even climbing Mt Everest might have brought some benefits to humanity, although I cannot name one here). I think a fair few discoveries have gone down that route. But, sadly, the hunt for acids ionized by precisely two water molecules in the gas-phase has not (yet?) borne such fruits.

We recently tried to write an article in such a chronological fashion. We had a hypothesis, initially thought we might be able to prove it, did more experiments and ultimately proved the hypothesis wrong (in solution!). The referees did not take to this perhaps slightly too honest account of our efforts. Since the hypothesis was wrong, why did we need to publish the story? Well, it did get published in the end, and you can make your own mind up.[8]


  1. Henry S Rzepa., "C 2 H 5 F 6 N 1 O 6 S 2", 2015.
  2. Henry S Rzepa., "C 2 H 5 F 6 N 1 O 6 S 2", 2015.
  3. Henry S Rzepa., "H 5 Cl 1 O 6", 2015.
  4. Henry S Rzepa., "H 7 Cl 1 O 7", 2015.
  5. Henry S Rzepa., "H 5 Br 1 O 6", 2015.
  6. Henry S Rzepa., "H 5 F 1 O 5 S 1", 2015.
  7. Henry S Rzepa., "H 5 F 1 O 5 Se 1", 2015.
  8. P. Bultinck, F.L. Cherblanc, M.J. Fuchter, W.A. Herrebout, Y. Lo, H.S. Rzepa, G. Siligardi, and M. Weimar, "Chiroptical Studies on Brevianamide B: Vibrational and Electronic Circular Dichroism Confronted", The Journal of Organic Chemistry, vol. 80, pp. 3359-3367, 2015.

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