Archive for the ‘General’ Category

Why is mercury a liquid at room temperatures?

Saturday, July 12th, 2014

Computational quantum chemistry has made fantastic strides in the last 30 years. Often deep insight into all sorts of questions regarding reactions and structures of molecules has become possible. But sometimes the simplest of questions can prove incredibly difficult to answer. One such is how accurately can the boiling point of water be predicted from first principles? Or its melting point? Another classic case is why mercury is a liquid at room temperatures? The answer to that question (along with another, why is gold the colour it is?) is often anecdotally attributed to Einstein. More accurately, to his special theory of relativity.[1] But finally in 2013 a computational proof of this was demonstrated for mercury.[2] The proof was built up in three stages.

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References

  1. A. Einstein, "Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?", Ann. Phys., vol. 323, pp. 639-641, 1905. http://dx.doi.org/10.1002/andp.19053231314
  2. F. Calvo, E. Pahl, M. Wormit, and P. Schwerdtfeger, "Evidence for Low-Temperature Melting of Mercury owing to Relativity", Angew. Chem. Int. Ed., vol. 52, pp. 7583-7585, 2013. http://dx.doi.org/10.1002/anie.201302742

The 5σ-confidence level: how much chemistry achieves this?

Saturday, July 5th, 2014

I was lucky enough to attend the announcement made in 2012 of the discovery of the Higgs Boson. It consisted of a hour-long talk mostly about statistics, and how the particle physics community can only claim a discovery when their data has achieved a 5σ confidence level. This represents a 1 in 3.5 million probability of the result occurring by chance. I started thinking: how much chemistry is asserted at that level of confidence? Today, I read Steve Bachrach’s post on the structure of Citrinalin B and how “use of Goodman’s DP4 method indicates a 100% probability that the structure of citrinalin B is (the structure below)”. Wow, that is even higher than the physicists. Of course, 100% has been obtained by rounding 99.7 (3σ is 99.73%) or whatever (this is one number that should never be so rounded!). pc But there was one aspect of this that I did want to have a confidence level for; the absolute configuration of citrinalin B. Reading the article Steve quotes[1], one sees this aspect is attributed to ref 5[2], dating from 2005. There the configuration was assigned on the basis of “comparison of the electronic circular dichroism (ECD) spectra for 1 and 2 with those of known spirooxiindole alkaloids“. However, this method can fail[3]. Also, one finds “comparison of the vibrational circular dichroism (VCD) spectra of 1 with those of model compounds[2]. Nowadays, one would say that there is no need for model compounds, why not measure and compute the VCD of the actual compound? Even a determination using the Flack crystallographic method can occasionally be wrong![4]. Which leads to asking what typical confidence levels might be for these three techniques, and indeed whether improving instrumentation means that the confidence level gets higher with time. OK, I am going to guess these.

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References

  1. E.V. Mercado-Marin, P. Garcia-Reynaga, S. Romminger, E.F. Pimenta, D.K. Romney, M.W. Lodewyk, D.E. Williams, R.J. Andersen, S.J. Miller, D.J. Tantillo, R.G.S. Berlinck, and R. Sarpong, "Total synthesis and isolation of citrinalin and cyclopiamine congeners", Nature, vol. 509, pp. 318-324, 2014. http://dx.doi.org/10.1038/nature13273
  2. T. Mugishima, M. Tsuda, Y. Kasai, H. Ishiyama, E. Fukushi, J. Kawabata, M. Watanabe, K. Akao, and J. Kobayashi, "Absolute Stereochemistry of Citrinadins A and B from Marine-Derived Fungus", J. Org. Chem., vol. 70, pp. 9430-9435, 2005. http://dx.doi.org/10.1021/jo051499o
  3. F. Cherblanc, Y. Lo, E. De Gussem, L. Alcazar-Fuoli, E. Bignell, Y. He, N. Chapman-Rothe, P. Bultinck, W.A. Herrebout, R. Brown, H.S. Rzepa, and M.J. Fuchter, "On the Determination of the Stereochemistry of Semisynthetic Natural Product Analogues using Chiroptical Spectroscopy: Desulfurization of Epidithiodioxopiperazine Fungal Metabolites", Chemistry - A European Journal, vol. 17, pp. 11868-11875, 2011. http://dx.doi.org/10.1002/chem.201101129
  4. F.L. Cherblanc, Y. Lo, W.A. Herrebout, P. Bultinck, H.S. Rzepa, and M.J. Fuchter, "Mechanistic and Chiroptical Studies on the Desulfurization of Epidithiodioxopiperazines Reveal Universal Retention of Configuration at the Bridgehead Carbon Atoms", J. Org. Chem., vol. 78, pp. 11646-11655, 2013. http://dx.doi.org/10.1021/jo401316a

The price of information: Evaluating big deal journal bundles

Thursday, July 3rd, 2014

Increasingly, our access to scientific information is becoming a research topic in itself. Thus an analysis of big deal journal bundles[1] has attracted much interesting commentary (including one from a large scientific publisher[2]). In the UK, our funding councils have been pro-active in promoting the so-called GOLD publishing model, where the authors (aided by grants from their own institution or others) pay the perpetual up-front publication costs (more precisely the costs demanded by the publishers, which is not necessarily the same thing) so that their article is removed from the normal subscription pay wall erected by the publisher and becomes accessible to anyone. As the proportion of GOLD content increases, it was anticipated (hoped?) that the costs of accessing the remaining non-GOLD articles via a pay-walled subscription would decrease.

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References

  1. T.C. Bergstrom, P.N. Courant, R.P. McAfee, and M.A. Williams, "Evaluating big deal journal bundles", Proceedings of the National Academy of Sciences, vol. 111, pp. 9425-9430, 2014. http://dx.doi.org/10.1073/pnas.1403006111
  2. C. Woolston, "Secret publishing deals exposed", Nature, vol. 510, pp. 447-447, 2014. http://dx.doi.org/10.1038/510447f

Amides and inverting the electronics of the Bürgi–Dunitz trajectory.

Thursday, June 26th, 2014

The Bürgi–Dunitz angle describes the trajectory of an approaching nucleophile towards the carbon atom of a carbonyl group. A colleague recently came to my office to ask about the inverse, that is what angle would an electrophile approach (an amide)? Thus it might approach either syn or anti with respect to the nitrogen, which is a feature not found with nucleophilic attack. amide My first thought was to calculate the wavefunction and identify the location and energy (= electrophilicity) of the lone pairs (the presumed attractor of an electrophile). But a better more direct approach soon dawned. A search of the crystal structure database. Here is the search definition, with the C=O-E angle, the O-E distance and the N-C=O-E torsion defined (also specified for R factor < 5%, no errors and no disorder). search   The first plot is of the torsion vs the distance, for E = H-X (X=O,F, Cl) amides

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Anchoring chemistry.

Wednesday, June 18th, 2014

I was reminded of this article by Michelle Francl[1], where she poses the question “What anchor values would most benefit students as they seek to hone their chemical intuition?” She gives as common examples: room temperature is 298.17K (actually 300K, but perhaps her climate is warmer than that of the UK!), the length of a carbon-carbon single bond, the atomic masses of the more common elements.

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References

  1. M. Francl, "Take a number", Nature Chem, vol. 5, pp. 725-726, 2013. http://dx.doi.org/10.1038/nchem.1733

Trigonal bipyramidal or square pyramidal: Another ten minute exploration.

Friday, May 2nd, 2014

This is rather cranking the handle, but taking my previous post and altering the search definition of the crystal structure database from 4- to 5-coordinate metals, one gets the following.

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Tetrahedral or square planar? A ten minute exploration.

Wednesday, April 30th, 2014

I love experiments where the insight-to-time-taken ratio is high. This one pertains to exploring the coordination chemistry of the transition metal region of the periodic table; specifically the tetra-coordination of the series headed by Mn-Ni. Is the geometry tetrahedral, square planar, or other? One can get a statistical answer in about ten minutes.
Tet-SP.jpgThe (CCDC database) search definition required is shown above. The central atom defines the column of the period table, it is specified to have precisely four other atoms bonded to it, which can be any other element. These four bonds are specified as acyclic (to avoid any bias introduced by rings). And two angles are defined subtending the central atom. And off we go, defining on the way that the hits must be refined to an R-factor of < 0.05, have no disorder, and no errors.

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What is the best way of folding a straight chain alkane?

Sunday, April 6th, 2014

In the previous post, I showed how modelling of unbranched alkenes depended on dispersion forces. When these are included, a bent (single-hairpin) form of C58H118 becomes lower in free energy than the fully extended linear form. Here I try to optimise these dispersion forces by adding further folds to see what happens.

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Modelling the geometry of unbranched alkanes.

Saturday, March 29th, 2014

By about C17H36, the geometry of “cold-isolated” unbranched saturated alkenes is supposed not to contain any fully anti-periplanar conformations. [1] Indeed, a (co-crystal) of C16H34 shows it to have two-gauche bends.[2]. Surprisingly, the longest linear alkane I was able to find a crystal structure for, C28H58 appears to be fully extended[3],[4] (an early report of a low quality structure for C36H74[5] also appears to show it as linear). Here I explore how standard DFT theories cope with these structures.

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References

  1. N.O.B. Lüttschwager, T.N. Wassermann, R.A. Mata, and M.A. Suhm, "The Last Globally Stable Extended Alkane", Angew. Chem. Int. Ed., vol. 52, pp. 463-466, 2012. http://dx.doi.org/10.1002/anie.201202894
  2. N. Cocherel, C. Poriel, J. Rault-Berthelot, F. Barrière, N. Audebrand, A.M.Z. Slawin, and L. Vignau, "New 3π-2Spiro Ladder-Type Phenylene Materials: Synthesis, Physicochemical Properties and Applications in OLEDs", Chemistry - A European Journal, vol. 14, pp. 11328-11342, 2008. http://dx.doi.org/10.1002/chem.200801428
  3. S.C. Nyburg, and A.R. Gerson, "Crystallography of the even n-alkanes: structure of C20H42", Acta Cryst Sect B, vol. 48, pp. 103-106, 1992. http://dx.doi.org/10.1107/S0108768191011059
  4. R. Boistelle, B. Simon, and G. Pèpe, "Polytypic structures of n-C28H58 (octacosane) and n-C36H74 (hexatriacontane)", Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, vol. 32, pp. 1240-1243, 1976. http://dx.doi.org/10.1107/S0567740876005025
  5. H.M.M. Shearer, and V. Vand, "The crystal structure of the monoclinic form of n-hexatriacontant", Acta Crystallographica, vol. 9, pp. 379-384, 1956. http://dx.doi.org/10.1107/S0365110X5600111X

Happy 25th birthday, WWW.

Wednesday, March 12th, 2014

A short post this, to remind that today is officially the 25th birthday of the World-Wide-Web, in March 1989. It took five years for a conference around the theme to be organised and below is a photo from that event.

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