Ionizing yet more ultra-strong acids with water molecules.

March 20th, 2015
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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).

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A 5-high straight flush of water-ionised acids?

March 17th, 2015
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I do not play poker, and so I had to look up a 5-4-3-2-1(A), which Wikipedia informs me is a 5-high straight flush, also apparently known as a steel wheel. In previous posts  I have suggested acids which can be ionised by (probably) 5, 4, 3 or  1 discrete water molecules in the gas phase; now to try to track down  a candidate for ionisation by the required two water molecules to form that straight flush.

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Ionizing ultra-strong acids with water molecules.

March 15th, 2015
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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.

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How many water molecules does it take to ionise HCN/HNC? An NCI exploration.

March 2nd, 2015
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HCN is a weak acid (pKa +9.2, weaker than e.g. HF), although it does have an isomer, isocyanic acid or HNC (pka < +9.2 ?) which is simultaneously stronger and less stable. I conclude my halide acid series by investigating how many water molecules (in gas phase clusters) are required for ionisation of this “pseudo-halogen” acid.

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How many water molecules does it take to ionise HI?

February 28th, 2015
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Why is this post orphaned from the previous? In order to have the opportunity of noting that treating iodine computationally can be a little different from the procedures used for F, Cl and Br.

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How many water molecules does it take to ionise HF and HBr?

February 27th, 2015
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No doubt answers to the question posed in the previous post are already being obtained by experiment. Just in case that does not emerge in the next day or so, I offer a prediction here.

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How many water molecules does it take to ionise HCl?

February 14th, 2015
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According to Guggemos, Slavicek and Kresin, about 5-6![1]. This is one of those simple ideas, which is probably quite tough to do experimentally. It involved blasting water vapour through a pinhole, adding HCl and measuring the dipole-moment induced deflection by an electric field. They found “evidence for a noticeable rise in the dipole moment occurring at n56“.

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References

  1. N. Guggemos, P. Slavíček, and V.V. Kresin, "Electric Dipole Moments of Nanosolvated Acid Molecules in Water Clusters", Phys. Rev. Lett., vol. 114, 2015. http://dx.doi.org/10.1103/PhysRevLett.114.043401

How-open-is-it?

February 12th, 2015
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The title of this post refers to the site http://howopenisit.org/  which is in effect a license scraper for journal articles. In the past 2-3 years in the UK, we have been able to make use of grants to our university to pay publishers to convert our publications into Open Access (also called GOLD). I thought I might check out a few of my recent publications to see what http://howopenisit.org/ makes of them.

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Chiroptical spectroscopy of the natural product Steganone.

February 10th, 2015
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Steganone is an unusual natural product, known for about 40 years now. The assignment of its absolute configurations makes for an interesting, on occasion rather confusing, and perhaps not entirely atypical story. I will start with the modern accepted stereochemical structure of this molecule, which comes in the form of two separately isolable atropisomers.
steganone
The first reported synthesis of this system in 1977 was racemic, and no stereochemistry is shown in the article (structure 2).[1] Three years later an “Asymmetric total synthesis of (-)steganone and revision of its absolute configuration” shows how the then accepted configuration (structure 1 in this article) needs to be revised to the enantiomer shown as structure 12 in the article[2] and matching the above representation. The system has continued to attract interest ever since[3],[4],[5],[6], not least because of the presence of axial chirality in the form of atropisomerism. Thus early on it was shown that the alternative atropisomer, the (aS,R,R) configuration initially emerges out of several syntheses, and has to be converted to the (aR,R,R) configuration by heating[3]. One could easily be fooled by such isomerism!

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References

  1. D. Becker, L.R. Hughes, and R.A. Raphael, "Total synthesis of the antileukaemic lignan (�)-steganacin", J. Chem. Soc., Perkin Trans. 1, pp. 1674, 1977. http://dx.doi.org/10.1039/P19770001674
  2. J. Robin, O. Gringore, and E. Brown, "Asymmetric total synthesis of the antileukaemic lignan precursor (-)steganone and revision of its absolute configuration", Tetrahedron Letters, vol. 21, pp. 2709-2712, 1980. http://dx.doi.org/10.1016/S0040-4039(00)78586-8
  3. E.R. Larson, and R.A. Raphael, "Synthesis of (?)-steganone", J. Chem. Soc., Perkin Trans. 1, pp. 521, 1982. http://dx.doi.org/10.1039/P19820000521
  4. A. Bradley, W.B. Motherwell, and F. Ujjainwalla, "A concise approach towards the synthesis of steganone analogues", Chem. Commun., pp. 917-918, 1999. http://dx.doi.org/10.1039/A900743A
  5. M. Uemura, A. Daimon, and Y. Hayashi, "An asymmetric synthesis of an axially chiral biaryl via an (arene)chromium complex: formal synthesis of (?)-steganone", Journal of the Chemical Society, Chemical Communications, pp. 1943, 1995. http://dx.doi.org/10.1039/C39950001943
  6. B. Yalcouye, S. Choppin, A. Panossian, F.R. Leroux, and F. Colobert, "A Concise Atroposelective Formal Synthesis of (-)-Steganone", European Journal of Organic Chemistry, vol. 2014, pp. 6285-6294, 2014. http://dx.doi.org/10.1002/ejoc.201402761

Mechanism of the solvatochromic reaction of a spiropyran.

February 4th, 2015
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The journal of chemical education has many little gems providing inspiration for laboratory experiments. Jonathan Piard reports one based on the reaction below[1]; here I investigate the mechanism of this transformation.

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References

  1. J. Piard, "Influence of the Solvent on the Thermal Back Reaction of One Spiropyran", J. Chem. Educ., vol. 91, pp. 2105-2111, 2014. http://dx.doi.org/10.1021/ed4005003