Posts Tagged ‘free energy barrier’
Saturday, November 23rd, 2013
Mercury (IV) tetrafluoride attracted much interest when it was reported in 2007[1] as the first instance of the metal being induced to act as a proper transition element (utilising d-electrons for bonding) rather than a post-transition main group metal (utilising just s-electrons) for which the HgF2 dihalide would be more normal (“Is mercury now a transition element?”[2]). Perhaps this is the modern equivalent of transmutation! Well, now we have new speculation about how to induce the same sort of behaviour for caesium; might it form CsF3 (at high pressures) rather than the CsF we would be more familiar with.[3] Here I report some further calculations inspired by this report.
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References
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X. Wang, L. Andrews, S. Riedel, and M. Kaupp, "Mercury Is a Transition Metal: The First Experimental Evidence for HgF4", Angewandte Chemie International Edition, vol. 46, pp. 8371-8375, 2007. http://dx.doi.org/10.1002/anie.200703710
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W.B. Jensen, "Is Mercury Now a Transition Element?", Journal of Chemical Education, vol. 85, pp. 1182, 2008. http://dx.doi.org/10.1021/ed085p1182
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M. Miao, "Caesium in high oxidation states and as a p-block element", Nature Chemistry, vol. 5, pp. 846-852, 2013. http://dx.doi.org/10.1038/nchem.1754
Tags:animation, energy, free energy barrier, free energy change, metal, pence, post-transition main group metal, potential energy surface
Posted in Hypervalency, Interesting chemistry | 8 Comments »
Wednesday, June 26th, 2013
A reader asked me about the mechanism of the reaction of 2-picoline N-oxide with acetic anhydride to give 2-acetoxymethylpyridine (the Boekelheide Rearrangement[1]). He wrote ” I don’t understand why the system should prefer to go via fragmentation-recombination (… the evidence being that oxygen labelling shows scrambling) when there is an easy concerted pathway available (… a [3,3]sigmatropic shift). Furthermore, is it possible for two pathways to co-exist?” Here is how computation might enlighten us.
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References
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A. Massaro, A. Mordini, A. Mingardi, J. Klein, and D. Andreotti, "A New Sequential Intramolecular Cyclization Based on the Boekelheide Rearrangement", European Journal of Organic Chemistry, vol. 2011, pp. 271-279, 2010. http://dx.doi.org/10.1002/ejoc.201000936
Tags:ATM, CF 3 CO, CH 3 CO, extraneous product, free energy, free energy barrier, recombination
Posted in Interesting chemistry, pericyclic, reaction mechanism | 3 Comments »
Tuesday, January 8th, 2013
Eagle-eyed footnote readers might have spotted one at the bottom of the post on the benzidine rearrangement. I was comparing the N-N bond lengths in crystal structures of known diprotonated hydrazines (~1.45Å) with the computed N-N bond length at the start point of the intrinsic reaction coordinate for the [5,5] sigmatropic rearrangement of di-N-protonated diphenylhydrazine (the active species in the benzidine rearrangement itself), which was some 1Å longer. This post explores the implications of this oddity.
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Tags:free energy barrier, Reaction Mechanism
Posted in Uncategorized | 3 Comments »
Monday, December 3rd, 2012
I promised that the follow-up to on the topic of Birch reduction would focus on the proton transfer reaction between the radical anion of anisole and a proton source, as part of analysing whether the mechanistic pathway proceeds O or M.
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Tags:Birch reduction, dielectric, energy, free energy, free energy barrier, Hammonds postulate, proton transfer, Reaction Mechanism, Tutorial material
Posted in Uncategorized | 2 Comments »
Friday, November 4th, 2011
An attosecond is 10-18s. The chemistry that takes place on this timescale is called electron dynamics. For example, it is the time taken for an electron to traverse the 1s orbit in a hydrogen atom. And chemists are starting to manipulate electrons (and hence chemistry) on this timescale; for example a recent article (DOI: 10.1021/ja206193t) describes how to control the electrons in benzene using attosecond laser pulses.
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Tags:attosecond, chemical processes, exasecond, free energy barrier, G/RT, Inga Ulusoy, laser, Mathias Nest, Tutorial material
Posted in General, Interesting chemistry | 1 Comment »
Monday, May 9th, 2011
Introductory organic chemistry invariably features the mechanism of haloalkane solvolysis, and introduces both the Sn1 two-step mechanism, and the Sn2 one step mechanism to students. They are taught to balance electronic effects (the stabilization of carbocations) against steric effects in order to predict which mechanism prevails. It was whilst preparing a tutorial on this topic that I came across what was described as the special case of neopentyl bromide, the bimolecular solvolysis of which has been identified (DOI: 10.1021/ja01182a117) as being as much as 3 million times slower than methyl bromide. This is attributed to a very strong steric effect on the reaction, greater even than that which might be experienced by t-butyl bromide! Time I thought, to take a look at what might make neopentyl bromide so special, and what those supposed electronic and steric effects were really up to.
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Tags:free energy, free energy barrier, Historical, potential energy surface, Tutorial material
Posted in Uncategorized | 1 Comment »
Friday, January 7th, 2011
This story starts with a calixarene, a molecule (suitably adorned with substituents) frequently used as a host to entrap a guest and perchance make the guest do something interesting. Such a calixarene was at the heart of a recent story where an attempt was made to induce it to capture cyclobutadiene in its cavity.
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Tags:animation, calixarene, chiral, dielectric, free energy barrier, gas phase, gas phase model, pericyclic, proton transfer, watoc11, zwitterionic
Posted in Interesting chemistry | 2 Comments »