Posts Tagged ‘lowest energy form’

Azane oxide, a tautomer of hydroxylamine.

Friday, April 15th, 2016

In the previous post I described how hydronium hydroxide or H3O+…HO–, an intermolecular tautomer of water, has recently been observed captured inside an organic cage[1] and how the free-standing species in water can be captured computationally with the help of solvating water bridges. Here I explore azane oxide or H3N+-O–,‡ a tautomer of the better known hydroxylamine (H2N-OH).

The usual search[2] of the Cambridge structure database reveals only two (related) entries[3],[4] the second reported in 2015.[5].

NH3-8
NH3-8

Now, location of hydrogen atoms is always a bit tricky, but here we see two species H3N+-OH…–O-+NH3 connected by a strong hydrogen bond of 1.54Å (click on the above image to see this packing). However, it is noteworthy that the N-O bonds for each of these species are exactly the same length (1.412Å); one might have imagined that whether the oxygen carries a proton or not would affect its bond length to nitrogen. There is here a strong hint that energetically the azane oxide might be relatively low in energy relative to hydroxylamine and perhaps that the zwitterionic form might be favoured when captured with hydrogen bonds.

So certainly time for a computational exploration of this species. I am using the three water bridges as before, each comprised of three water molecules and the ωB97XD/6-311++G(d,p)/SCRF=water method. In fact the structure optimises[6] to a delightful propeller-like geometry in which bridges are formed from both two AND three waters across the ion-pair, with overall three-fold C3 symmetry (i.e. chiral! Indeed, this species has a predicted optical rotation of 40° at 589nm).

NH3-8

Hydroxylamine itself has a less symmetric arrangement of hydrogen bonds[7], with a free energy in fact very similar (within 1 kcal/mol) to the ion-pair isomer. Here, a stochastic (statistical) exploration of all the various arrangements of water would be needed to be confident that the lowest energy form had been located. I would note that the N-O bond lengths in the ion-pair and neutral forms are respectively 1.399 and 1.435Å.

NH3-8

Certainly, this very brief computational look at azane oxide suggests that concentrations of this species in aqueous solutions of hydroxylamine might be appreciable (detectable). Its "trapping" inside a suitably designed cavity must be a realistic possibility (the cavity used to trap hydronium hydroxide probably does not have the correct dimensions for this purpose), as indeed illustrated in the two crystal structures noted above.

‡ I have represented this species in ionic form, but you may find text books showing it in dative form, or H3N→O. My personal inclination is to always prefer the ionic form, if only because it enables connections/analogies such as the one here to hydronium hydroxide to be more easily made.

References

  1. M. Stapf, W. Seichter, and M. Mazik, "Unique Hydrogen‐Bonded Complex of Hydronium and Hydroxide Ions", Chemistry – A European Journal, vol. 21, pp. 6350-6354, 2015. https://doi.org/10.1002/chem.201406383
  2. H. Rzepa, "Search for Azane oxide", 2016. https://doi.org/10.14469/hpc/380
  3. Fischer, Dennis., Klapotke, Thomas M.., and Stierstorfer, Jorg., "CCDC 1054611: Experimental Crystal Structure Determination", 2015. https://doi.org/10.5517/cc14ddqn
  4. Fischer, D.., Klapotke, T.M.., and Stierstorfer, J.., "CCDC 827687: Experimental Crystal Structure Determination", 2012. https://doi.org/10.5517/ccws8lh
  5. D. Fischer, T.M. Klapötke, and J. Stierstorfer, "1,5‐Di(nitramino)tetrazole: High Sensitivity and Superior Explosive Performance", Angewandte Chemie International Edition, vol. 54, pp. 10299-10302, 2015. https://doi.org/10.1002/anie.201502919
  6. H.S. Rzepa, "H 21 N 1 O 10", 2016. https://doi.org/10.14469/ch/192000
  7. H.S. Rzepa, "H 21 N 1 O 10", 2016. https://doi.org/10.14469/ch/192001

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Interactions responsible for the lowest energy structure of the trimer of fluoroethanol.

Friday, October 23rd, 2015

Steve Bachrach on his own blog has commented on a recent article[1] discussing the structure of the trimer of fluoroethanol. Rather than the expected triangular form with three OH—O hydrogen bonds, the lowest energy form only had two such bonds, but it matched the microwave data much better. Here I explore this a bit more.

The stability of the lowest energy form, as is evident from the title of the article, was attributed to unusual H-Bond topology and bifurcated H-bonds as teased out from bond critical points in the QTAIM analysis of the topology of the electron density. Here I add to this analysis by displaying the computed NCI (non-covalent-interaction)[2] surfaces, as you might see in the comment I posted on Steve’s blog. In essence, the QTAIM had revealed bond paths connecting an oxygen to a H-C and also a bifurcation from an F to two H-C atoms, shown with orange lines in the diagram there. What might an NCI analysis reveal? The analysis[3] is shown below, where I have added orange arrows to indicate the location of these bond paths. The arrows point to an NCI feature which corresponds to a weak dispersion-like stabilisation.

Click for 3D

Click for 3D

However, as you can spot from the diagram above (and inspect in a 3D sense if you click on the diagram above to load a 3D Jmol model), there are many more regions where NCI features appear. The most obvious are the blue-coded ones, which in fact represent the conventional O…HO hydrogen bonds, but there are plenty of others as well, including a cyan one which is not part of the published attributions. I will recapitulate my comment on Steve’s blog; the point I make here is that apart from the two regions which have been picked out in the article as responsible for stabilisation of the low energy structure, there are around 4-5 OTHER regions that also may be stabilising but for which there is no corresponding critical point in the density. So whilst the above origins are not incorrect, they may well be very incomplete!.

There is a tendency to only highlight features which can be named, and perhaps to ignore or pay less attention to those which have no name. The latter may in fact be more common than we imagine, and cumulatively they can often have a big impact.

Postscript: A structure has recently been reported[4],[5] illustrating an exceptionally strong OH…F interaction of 1.52Å. This is noteworthy because such hydrogen bonds are rarely strong and indeed even their very existence is controversial. The cyan NCI region mentioned above is just such an interaction (of length ~2.0Å).

References

  1. J. Thomas, X. Liu, W. Jäger, and Y. Xu, "Unusual H‐Bond Topology and Bifurcated H‐bonds in the 2‐Fluoroethanol Trimer", Angewandte Chemie International Edition, vol. 54, pp. 11711-11715, 2015. https://doi.org/10.1002/anie.201505934
  2. J. Contreras-García, W. Yang, and E.R. Johnson, "Analysis of Hydrogen-Bond Interaction Potentials from the Electron Density: Integration of Noncovalent Interaction Regions", The Journal of Physical Chemistry A, vol. 115, pp. 12983-12990, 2011. https://doi.org/10.1021/jp204278k
  3. H.S. Rzepa, and H.S. Rzepa, "C 6 H 15 F 3 O 3", 2015. https://doi.org/10.14469/ch/191558
  4. M.D. Struble, C. Kelly, M.A. Siegler, and T. Lectka, "Search for a Strong, Virtually “No‐Shift” Hydrogen Bond: A Cage Molecule with an Exceptional OH⋅⋅⋅F Interaction", Angewandte Chemie International Edition, vol. 53, pp. 8924-8928, 2014. https://doi.org/10.1002/anie.201403599
  5. Struble, Mark D.., Kelly, Courtney., Siegler, Maxime A.., and Lectka, Thomas., "CCDC 991440: Experimental Crystal Structure Determination", 2014. https://doi.org/10.5517/cc128nyy

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