Posts Tagged ‘Mt. Everest’

Ionizing yet more ultra-strong acids with water molecules.

Friday, March 20th, 2015

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]

References

  1. H.S. Rzepa, "C 2 H 5 F 6 N 1 O 6 S 2", 2015. https://doi.org/10.14469/ch/191136
  2. H.S. Rzepa, "C 2 H 5 F 6 N 1 O 6 S 2", 2015. https://doi.org/10.14469/ch/191137
  3. H.S. Rzepa, "H 5 Cl 1 O 6", 2015. https://doi.org/10.14469/ch/191139
  4. H.S. Rzepa, "H 7 Cl 1 O 7", 2015. https://doi.org/10.14469/ch/191138
  5. H.S. Rzepa, "H 5 Br 1 O 6", 2015. https://doi.org/10.14469/ch/191140
  6. H.S. Rzepa, "H 5 F 1 O 5 S 1", 2015. https://doi.org/10.14469/ch/191143
  7. H.S. Rzepa, "H 5 F 1 O 5 Se 1", 2015. https://doi.org/10.14469/ch/191141
  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. https://doi.org/10.1021/jo5022647

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Quintuple bonds

Tuesday, February 16th, 2010

Climbers scale Mt. Everest, because its there, and chemists have their own version of this. Ever since G. N. Lewis introduced the concept of the electron-pair bond in 1916, the idea of a bond as having a formal bond-order has been seen as a useful way of thinking about molecules. The initial menagerie of single, double and triple formal bond orders (with a few half sizes) was extended in the 1960s to four, and in 2005 to five. Since then, something of a race has developed to produce the compound with the shortest quintuple bond. One of the candidates for this honour is shown below (2008, DOI: 10.1002/anie.200803859) which is a crystalline species (a few diatomics which exist in the gas phase are also candidates; for other reviews of the topic see 10.1038/nchem.359, 10.1021/ja905035f and 10.1246/cl.2009.1122).

A molecule with a Quintuple-bond

(OK, its shown as a quadruple bond, but Chemdraw cannot handle five!). The Cr…Cr length is 1.74Å (R=aryl). It was also reported that DFT calculations (BP86/triple-ζ) reproduce this length well. The five highest occupied molecular orbitals are all centred around the Cr-Cr region, and the bonding is formally described as five pairs of electrons filling 1σ, 2π, and 2δ type molecular orbitals.

So the electron pair bond, approaching its 100th birthday, is alive and well? But it does seem worth asking if those ten electrons really do cram together to occupy the region between the two Cr atoms. The stalwarts in these blog posts, AIM and ELF will be deployed to see if they too verify this simple concept. Firstly, AIM (calculated at the BP86/6-311G(d) level, DOI: 10042/to-4181 for a model system with R=H).

Quintuple bond complex, AIM analysis. Click for 3D

The Cr…Cr region has the requisite bond critical point, and the value of ρ(r) at this point has the large value (for Cr) of 0.313 au, indeed hinting at a large bond order. The Laplacian ∇2ρ(r) has the more extraordinary value of +1.45 at this point, which makes it the strongest charge-shift bond ever noted (typically, ∇2ρ(r) is ~+0.5 for other examples of homonuclear charge-shift bonds, see DOI: 10.1038/nchem.327).

This charge-shift character perhaps hints that this quintuple bond is no ordinary bond. Charge-shift bonds are characterized by valence bond structures where the covalent form may actually be repulsive, and the bond is stabilized instead by resonance with charge-shifted ionic valence bond forms. So given this, the ELF perhaps comes as no surprise.

Quintuple bond. ELF analysis

This diagram needs some explanation. The colour code is as follows: purple spheres represent the centroids of conventional disynaptic ELF basins. The only interesting ones are the four connecting the nitrogens to the Cr (21-24) which integrate to 3.35 electrons each. The cyan spheres (shown as 3,4 above) are the inner core-electrons of the Cr atoms (10.2 electrons of a neon core) and surrounding them are five further basins for each Cr integrating to 12 electrons per Cr. These include 8 of the outer-core (3s,3p) and four of the valence (3d, 4s) electrons, leaving ~2 valence Cr electrons not accounted for. Some of these final electrons are to be found in the basins represented by red spheres. The very diffuse (39, 40) basins far from the centre have a tiny electron integration (~0.003) and more missing Cr valence electrons are found in the bridging basins (32,36; 0.56 and 0.25 electrons each). Added to the 2*3.35 electrons found in the Cr-N bond, this suggests the 3d/4s shell of the Cr is occupied by ~11.5 electrons. The 3d-shell is thus full, and the system is indeed an 18-electron (8+10) system with some occupancy of the 4s shell as well. An alternative view of the ELF surface can be seen below, showing the unusual environment surrounding the Cr pair.

Quintuple bond, showing ELF isosurface. Click for 3D

It seems that AIM (the topology of the electron density) and ELF (the topology of the electron localization function) are giving us quite different pictures of the quintuple bond. The latter does seem to indicate that the conventional covalent shared electron pair picture of this bond is not really what is going on, and that the idea of a quintuple bond as sharing five electron pairs in the bonding region between the two Cr atoms is not really realistic. It may be of course that the ELF concept also is not really applicable for such bonds (it is after all essentially an empirical function, the deeper significance of which is debatable).  Nonetheless, the quintuple bond clearly has some surprises for us, and it would itself be no surprise to find out that controversy about the meaning of such a bond continues apace.

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The chirality of Möbius annulenes

Wednesday, April 22nd, 2009

Much like climbing Mt. Everest because its there,  some hypothetical molecules are just too tantalizing for chemists to resist attempting a synthesis. Thus in 1964, Edgar Heilbronner  speculated on whether a conjugated annulene ring might be twistable into a  Möbius strip. It was essentially a fun thing to try to do, rather than the effort being based on some anticipated  (and useful) property it might have. If you read the original article (rumour has it the idea arose during a lunchtime conversation, and the manuscript was completed by the next day), you will notice one aspect of these molecules that is curious by its absence. There is no mention (10.1016/S0040-4039(01)89474-0) that such Möbius systems will be chiral. By their nature, they have only axes of symmetry, and no planes of symmetry, and such molecules therefore cannot be superimposed upon their mirror image; as is required of a chiral system (for a discussion of the origins and etymology of the term, see 10.1002/chir.20699).

The 16-annulene synthesized by Herges and his team.

The 16-annulene synthesized by Herges and his team. Click for 3D.

Heilbronner’s little vignette had little overt effect on the synthetic community until around 2003, when Rainer Herges announced that a crystalline annulene following this recipe had been rationally synthesized (10.1038/nature02224). This time, the chemical community really sat up and took notice. The synthesis was hailed as a major achievement, ranking (chemically) as similar to climbing Everest. But if you read Herges’ article carefully, yet again you will note the absence of any discussion of the chirality of their molecule. Their synthesis was of course racemic, in other words an equal proportion of both enantiomers was made. Indeed, it is not obvious how a non-racemic synthesis could be carried out, although resolution of the product might be an easier task. So in the absence of any pure enantiomer of this molecule, can one speculate on its chiral properties? One obvious such property is the optical rotation, and in particular the [α]D value in chloroform. Most optically pure molecules with molecular weights of < 500 Daltons  tend to have rotations also < 500°. Few molecules have values > 1000°. Now it should be said at the outset that a molecule with a large optical rotation is not more chiral than a molecule with a smaller value; indeed it seems generally agreed that the question “how chiral is this molecule” is either fairly, or even completely meaningless. But it seems a useful task of having a value to hand which is at least approximately accurate, so that some idea of whether any attempted resolution of the enantiomers has produced optically pure product or not. Fortunately, in the last decade or so, computing a value for [α]D has been entirely viable using the standard programs (see 10.1002/chir.20466 and 10.1021/jo070806i for a discussion). This is also useful for two reasons:

 

  1. If the magnitude of the rotation is > 100°, then the sign of this rotation can be very reliably matched to either enantiomer. This allows the absolute configuration to be assigned with a lot of confidence, and probably much more easily than trying to do it by other methods.
  2. The magnitude itself can be reliably predicted to within 10% of the true value if the molecule is conformationally rigid. However, if it has any rotatable groups (and that even includes e.g. OH groups), then the result can be enormously sensitive to that conformation (or Boltzmann mixture of conformations). Put the other way, calculating the optical rotation could be regarded as a very sensitive way of determining conformations!

So what of the 16-annulene synthesized by Herges and co-workers. Well at the B3LYP/6-311G(2df,2pd) and SCRF(CPCM,solvent=chloroform) level of theory (which is reasonably accurate, although one can do better of course), the enantiomer shown by clicking on the graphic above is predicted to have a rotation of -1355° (for the digital repository entry for the calculation, see 10042/to-2176). That is indeed a large value for such a relatively small molecule, and is probably more reliable because of the lack of conformational ambiguity. Well, you saw the prediction here! Anyone up for testing it experimentally?

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