Posts Tagged ‘professor’

The atom and the molecule: A one-day symposium on 23 March, 2016 celebrating Gilbert N. Lewis.

Friday, December 11th, 2015

You might have noticed the occasional reference here to the upcoming centenary of the publication of Gilbert N. Lewis’ famous article entitled “The atom and the molecule“.[1] A symposium exploring his scientific impact and legacy will be held in London on March 23, 2016, exactly 70 years to the day since his death. A list of the speakers and their titles is shown below; there is no attendance fee, but you must register as per the instructions below.

Royal Society of Chemistry Historical Group Meeting on 23th March 2016, Burlington House, Piccadilly, London: The atom and the molecule: A symposium celebrating Gilbert N. Lewis.

  • Dr Patrick Coffey (Berkeley, USA): Does Personality Influence Scientific Credit? Simultaneous Priority Disputes: Lewis vs. Langmuir and Langmuir vs. Harkins
  • Professor Robin Hendry (Durham, UK): Lewis on Structure and the Chemical Bond
  • Professor Alan Dronsfield (UK): An organic chemist reflects on the Lewis two-electron bond
  • Dr Julia Contreras-García (UPMC, France): Do bonds need a name?
  • Professor Nick Greeves (Liverpool, UK): The influence of Lewis on organic chemistry teaching, textbooks and beyond
  • Professor Clark Landis (UWM, USA): Lewis and Lewis-like Structures in the Quantum Era
  • Professor Michael Mingos (Oxford, UK): The Inorganic dimension to Lewis and Kossel’s landmark contributions
  • Dr Patrick Coffey (Berkeley, USA): Lewis’ Life, Death, and Missing Nobel Prize

Prior registration is essential. Please email your name and address to Professor John Nicholson,  jwnicholson01 @ gmail.com

References

  1. G.N. Lewis, "THE ATOM AND THE MOLECULE.", Journal of the American Chemical Society, vol. 38, pp. 762-785, 1916. https://doi.org/10.1021/ja02261a002

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Posted in Interesting chemistry | 1 Comment »

The demographics of a blog readership – updated

Thursday, January 8th, 2015

About two years ago, I posted on the distribution of readership of this blog. The passage of time has increased this from 144 to 176 countries. There are apparently between 189-196 such, so not quite yet complete coverage! 2015
Of course, it is the nature of the beast that whilst we can track countries, very little else is known about such readerships. Is the readership young or old, student or professor, chemist or not (although I fancy the latter is less likely). Another way of keeping tabs on some of the activity are aggregators such as Chemical Blogspace, which has been rather quiet recently. Perhaps we have become too obsessed by metrics, and with the Internet-of-things apparently the “next-big-thing”, the metrics are only likely to increase. This will only encourage “game playing“, and I urge you to see a prime example of this in the UK REF (research excellence framework), the measure which attempts to rank UK universities in terms of their “excellence”.

Ah well, I had better leave this blog and go off and check on my h-index just in case it has notched up another integer.

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Posted in General | 2 Comments »

Woodward’s symmetry considerations applied to electrocyclic reactions.

Monday, May 20th, 2013

Sometimes the originators of seminal theories in chemistry write a personal and anecdotal account of their work. Niels Bohr[1] was one such and four decades later Robert Woodward wrote “The conservation of orbital symmetry” (Chem. Soc. Special Publications (Aromaticity), 1967, 21, 217-249; it is not online and so no doi can be given). Much interesting chemistry is described there, but (like Bohr in his article), Woodward lists no citations at the end, merely giving attributions by name. Thus the following chemistry (p 236 of this article) is attributed to a Professor Fonken, and goes as follows (excluding the structure in red):

wood

A search of the literature reveals only one published article describing this reaction[2] by Dauben and Haubrich, published some 21 years after Woodward’s description (we might surmise that Gerhard Fonken never published his own results). In fact this more recent study was primarily concerned with 193-nm photochemical transforms (they conclude that “the Woodward-Hoffmann rules of orbital symmetry are not followed”) but you also find that the thermal outcome of heating 4 is a 3:2 mixture of compounds 5 and 6, and that only 6 goes on to give the final product 7. It does look like a classic and uncomplicated example of Woodward-Hoffmann rules.

 So let us subject this system to the “reality check”. The transform of 4 → 5 rotates the two termini of the cleaving bond in a direction that produces the stereoisomer 5, with a trans alkene straddled by two cis-alkenes[3]. The two carbon atoms that define the termini of the newly formed hexatriene are ~ 4.7Å apart; too far to be able to close to form 7.

 4 → 5  4 → 6
8 8

But with any electrocyclic reaction, two directions of rotation are always possible, and it is a rotation in the other direction that gives 4 → 6[4], ending up with a hexatriene with the trans-alkene at one end and not the middle (for which the free energy of activation is 3.1 kcal/mol higher in energy). Now the two termini of the hexatriene end up ~3.0Å apart, much more amenable to forming a bond between them to form 7.

It is at this point that the apparently uncomplicated nature of this example starts to unravel. If one starts from the 3.0Å end-point of the above reaction coordinate and systematically contracts the bond between these two termini, a transition state is found leading not to 7 but to the (endothermic) isomer 8.[5]This form has a six-membered ring with a trans-alkene motif (which explains why it is so endothermic). 

wood1
6 ↠ 8
8 wood2

Before discussing the implications of this transition state, I illustrate another isomerism that 6 can undertake; a low-barrier atropisomerism[6] to form 9, followed by another reaction with a relatively low barrier, 9 ↠  7[7]to give the product that Woodward gives in his essay.

6 ↠ 9
6-atrop 6-atrop
9 ↠ 7
9to7a 9to7a

 We can now analyse the two transformations 6 ↠ 8 and 9 ↠  7. The first involves antarafacial bond formation (blue arrows) at the termini and an accompanying 180° twisting about the magenta bond which creates a second antarafacial component[8]. So this is a thermally allowed six-electron (4n+2) electrocyclisation with a double-Möbius twist[9]. The second reaction is a more conventional purely suprafacial version[10] (red arrows) of the type Woodward was certainly thinking of; it is 18.0 kcal/mol lower in free energy than the first (the transition state for 6 ↠ 9 is 10.8 kcal/mol lower than that for 9 ↠ 7).

I hope that this detailed exploration of what seems like a pretty simple example at first sight shows how applying a “reality-check” of computational quantum mechanics can cast (some unexpected?) new light on an old problem. We may of course speculate on how to inhibit the pathway 6 ↠ 9 ↠ 7 to allow only 6 ↠ 8 to proceed (the reverse barrier from 8 is quite low, so 8 would have to be trapped at very low temperatures). 

References

  1. N. Bohr, "Der Bau der Atome und die physikalischen und chemischen Eigenschaften der Elemente", Zeitschrift f�r Physik, vol. 9, pp. 1-67, 1922. https://doi.org/10.1007/bf01326955
  2. W.G. Dauben, and J.E. Haubrich, "The 193-nm photochemistry of some fused-ring cyclobutenes. Absence of orbital symmetry control", The Journal of Organic Chemistry, vol. 53, pp. 600-606, 1988. https://doi.org/10.1021/jo00238a023
  3. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704833
  4. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704834
  5. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704755
  6. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704754
  7. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704844
  8. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704841
  9. H.S. Rzepa, "Double-twist Möbius aromaticity in a 4n+ 2 electron electrocyclic reaction", Chemical Communications, pp. 5220, 2005. https://doi.org/10.1039/b510508k
  10. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704995

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Posted in pericyclic | 4 Comments »

Woodward's symmetry considerations applied to electrocyclic reactions.

Monday, May 20th, 2013

Sometimes the originators of seminal theories in chemistry write a personal and anecdotal account of their work. Niels Bohr[1] was one such and four decades later Robert Woodward wrote “The conservation of orbital symmetry” (Chem. Soc. Special Publications (Aromaticity), 1967, 21, 217-249; it is not online and so no doi can be given). Much interesting chemistry is described there, but (like Bohr in his article), Woodward lists no citations at the end, merely giving attributions by name. Thus the following chemistry (p 236 of this article) is attributed to a Professor Fonken, and goes as follows (excluding the structure in red):

wood

A search of the literature reveals only one published article describing this reaction[2] by Dauben and Haubrich, published some 21 years after Woodward’s description (we might surmise that Gerhard Fonken never published his own results). In fact this more recent study was primarily concerned with 193-nm photochemical transforms (they conclude that “the Woodward-Hoffmann rules of orbital symmetry are not followed”) but you also find that the thermal outcome of heating 4 is a 3:2 mixture of compounds 5 and 6, and that only 6 goes on to give the final product 7. It does look like a classic and uncomplicated example of Woodward-Hoffmann rules.

 So let us subject this system to the “reality check”. The transform of 4 → 5 rotates the two termini of the cleaving bond in a direction that produces the stereoisomer 5, with a trans alkene straddled by two cis-alkenes[3]. The two carbon atoms that define the termini of the newly formed hexatriene are ~ 4.7Å apart; too far to be able to close to form 7.

 4 → 5  4 → 6
8 8

But with any electrocyclic reaction, two directions of rotation are always possible, and it is a rotation in the other direction that gives 4 → 6[4], ending up with a hexatriene with the trans-alkene at one end and not the middle (for which the free energy of activation is 3.1 kcal/mol higher in energy). Now the two termini of the hexatriene end up ~3.0Å apart, much more amenable to forming a bond between them to form 7.

It is at this point that the apparently uncomplicated nature of this example starts to unravel. If one starts from the 3.0Å end-point of the above reaction coordinate and systematically contracts the bond between these two termini, a transition state is found leading not to 7 but to the (endothermic) isomer 8.[5]This form has a six-membered ring with a trans-alkene motif (which explains why it is so endothermic). 

wood1
6 ↠ 8
8 wood2

Before discussing the implications of this transition state, I illustrate another isomerism that 6 can undertake; a low-barrier atropisomerism[6] to form 9, followed by another reaction with a relatively low barrier, 9 ↠  7[7]to give the product that Woodward gives in his essay.

6 ↠ 9
6-atrop 6-atrop
9 ↠ 7
9to7a 9to7a

 We can now analyse the two transformations 6 ↠ 8 and 9 ↠  7. The first involves antarafacial bond formation (blue arrows) at the termini and an accompanying 180° twisting about the magenta bond which creates a second antarafacial component[8]. So this is a thermally allowed six-electron (4n+2) electrocyclisation with a double-Möbius twist[9]. The second reaction is a more conventional purely suprafacial version[10] (red arrows) of the type Woodward was certainly thinking of; it is 18.0 kcal/mol lower in free energy than the first (the transition state for 6 ↠ 9 is 10.8 kcal/mol lower than that for 9 ↠ 7).

I hope that this detailed exploration of what seems like a pretty simple example at first sight shows how applying a “reality-check” of computational quantum mechanics can cast (some unexpected?) new light on an old problem. We may of course speculate on how to inhibit the pathway 6 ↠ 9 ↠ 7 to allow only 6 ↠ 8 to proceed (the reverse barrier from 8 is quite low, so 8 would have to be trapped at very low temperatures). 

References

  1. N. Bohr, "Der Bau der Atome und die physikalischen und chemischen Eigenschaften der Elemente", Zeitschrift f�r Physik, vol. 9, pp. 1-67, 1922. https://doi.org/10.1007/bf01326955
  2. W.G. Dauben, and J.E. Haubrich, "The 193-nm photochemistry of some fused-ring cyclobutenes. Absence of orbital symmetry control", The Journal of Organic Chemistry, vol. 53, pp. 600-606, 1988. https://doi.org/10.1021/jo00238a023
  3. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704833
  4. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704834
  5. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704755
  6. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704754
  7. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704844
  8. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704841
  9. H.S. Rzepa, "Double-twist Möbius aromaticity in a 4n+ 2 electron electrocyclic reaction", Chemical Communications, pp. 5220, 2005. https://doi.org/10.1039/b510508k
  10. H.S. Rzepa, "Gaussian Job Archive for C10H14", 2013. https://doi.org/10.6084/m9.figshare.704995

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Posted in Uncategorized | 3 Comments »

Henry Armstrong: almost an electronic theory of chemistry!

Monday, November 7th, 2011

Henry Armstrong studied at the Royal College of Chemistry from 1865-7 and spent his subsequent career as an organic chemist at the Central College of the Imperial college of Science and technology until he retired in 1912. He spent the rest of his long life railing against the state of modern chemistry, saving much of his vitriol against (inter alia) the absurdity of ions, electronic theory in chemistry, quantum mechanics and nuclear bombardment in physics. He snarled at Robinson’s and Ingold’s new invention (ca 1926-1930) of electronic arrow pushing with the put down “bent arrows never hit their marks“.1  He was dismissed as an “old fogy, stuck in a time warp about 1894.”1 So why on earth would I want to write about him? Read on…

He did worthy (nowadays this could mean dull) chemistry on e.g. naphthalenes, but I want to focus on two articles from the period 1887-1890 (10.1039/CT8875100258  and 10.1039/PL8900600095). Let me set the scene by reminding of an earlier post showing the structure of a bis(stilbyl)ketone, dated 1921. The two aromatic groups (yes, they really are such) are drawn in the manner we would nowadays draw cyclohexane. This practice in fact continued in texts and articles for perhaps 30 more years! Not much sign of electronic accounting there then! And by a professor at Imperial College no less, where Armstrong had been.

Aromatic molecule, circa 1921

So when would you date the diagrams below? So called Clar2 representations, originating from the 1950s? The one on the bottom below cites Clar and dates from 2010, DOI: 10.3390/sym2031653, but the one above it comes from Armstrong’s 1890 article!

Two representations of pyrene, 2010 and 1890.

Clar representations are used to count electrons (as coming in six packs). But there is little doubt that Armstrong’s use of a “C” (or inner circle, which is exactly what it is) means six as well. The evidence I present below, taken from his 1887 article.

Armstrongs six pack

  1. He counts the six carbons as having a total of 24 what he calls affinities (definition: An attraction or force between particles that causes them to combine), or four per carbon. Let us make life easy and equate affinity=electron (remember, the electron itself was not yet discovered or named!). He disposes of 12 affinities/electrons to form what we now call six carbon-carbon σ bonds, and a further six for the  six C-H bonds.
  2. He is left with exactly six affinities/electrons, which he presupposes to act upon each other, in the manner of resultants (the old term for vectors). In fact, he replaces these six vectors by a circle (the inner circle) in his second article of 1890.
  3. He invents delocalization in all but name when he states that any one atom has an influence on other atoms not contiguous to it in the ring (he really did have o/m/p directing influence in mind here).
  4. He compares the introduction of a substituent (R, which comes from the old name Radicle) perturbing the distribution of the affinity to how electric charges perturb each other. So, the affinity behaves as if it might have electrical (from which the name electron came of course) properties? And it might be described by a vector?
  5. Remember, this is a scientist who in later life did not believe in electronic theories of chemistry? Really? Well, again in 1890:

Is this an affinity (=electronic) theory of chemistry?

  1. Here, he is refining his vector representation of affinities, saying that these vectors in effect define a circle, an inner circle no less. One that can be disrupted  (Robinson some 30 years later wrote of how the cycle of six electrons are able to form a group that resists disruption) when an additive compound is formed (his examples are all electrophiles, what we now call electrophilic addition) such that the remaining carbons become merely unsaturated. There seems little doubt he is describing what we now call a Wheland Intermediate.
  2. Is this really a man who did not believe in electronic theories of chemistry? What about that concluding paragraph then? The laws of substitution require a knowledge of the inner structure of (what we now call the aromatic) hydrocarbons?
  3. And that such speculations may suggest fresh lines of experimental inquiry? This all sounds very much like the modern use of quantum mechanics and its electronic eigenvectors to describe the probability distribution of electrons (remember, Armstrong did not approve of this either) to probe the inner structure of molecules and to suggest new experiments.

We have a real mystery. Armstrong got so very close to a modern theory of chemistry. Was he asleep when Stoney named the electron around 1891 and Thomson discovered it in 1897? If only he had followed his own advice! Ah well, just as well he was ignored in the 20th century when he preached against it all.

  1. W. H. Brock, “The case of the Poisonous Socks”, chapter 20, RSC Publishing, 2011, 978-1-84973-324-3
  2. Clar, E. The Aromatic Sextet; Wiley: New York, NY, USA, 1972.

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Posted in Historical | 3 Comments »