Archive for January, 2016

Quintuple bonds: resurfaced.

Sunday, January 31st, 2016

Six years ago, I posted on the nature of a then recently reported[1] Cr-Cr quintuple bond. The topic resurfaced as part of the discussion on a more recent post on NSF3, and a sub-topic on the nature of the higher order bonding in C2. The comment made a connection between that discussion and the Cr-Cr bond alluded to above. I responded briefly to that comment, but because I want to include 3D rotatable surfaces, I expand the discussion here and not in the comment.

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References

  1. C. Hsu, J. Yu, C. Yen, G. Lee, Y. Wang, and Y. Tsai, "Quintuply‐Bonded Dichromium(I) Complexes Featuring Metal–Metal Bond Lengths of 1.74 Å", Angewandte Chemie International Edition, vol. 47, pp. 9933-9936, 2008. http://dx.doi.org/10.1002/anie.200803859

Kinetic isotope effect models as a function of ring substituent for indole-3-carboxylic acids and indolin-2-ones.

Wednesday, January 20th, 2016

The original strategic objective of my PhD researches in 1972-74 was to explore how primary kinetic hydrogen isotope effects might be influenced by the underlying structures of the transition states involved. Earlier posts dealt with how one can construct quantum-chemical models of these transition states that fit the known properties of the reactions. Now, one can reverse the strategy by computing the expected variation with structure to see if anything interesting might emerge, and then if it does, open up the prospect of further exploration by experiment. Here I will use the base-catalysed enolisation of 1,3-dimethylindolin-2-ones and the decarboxylation of 3-indole carboxylates to explore this aspect.

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VSEPR Theory: A closer look at trifluorothionitrile, NSF3.

Saturday, January 16th, 2016

The post on applying VSEPR ("valence shell electron pair repulsion") theory to the geometry of ClF3 has proved perennially popular. So here is a follow-up on another little molecue, F3SN. As the name implies, it is often represented with an S≡N bond. Here I take a look at the conventional analysis.

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I’ve started so I’ll finish. Kinetic isotope effect models for a general acid as a catalyst in the protiodecarboxylation of indoles.

Sunday, January 10th, 2016

Earlier I explored models for the heteroaromatic electrophilic protiodecarboxylation of an 3-substituted indole, focusing on the role of water as the proton transfer and delivery agent. Next, came models for both water and the general base catalysed ionization of indolinones. Here I explore general acid catalysis by evaluating the properties of two possible models for decarboxylation of 3-indole carboxylic acid, one involving proton transfer (PT) from neutral water in the presence of covalent un-ionized HCl (1) and one with PT from a protonated water resulting from ionised HCl (2).

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I’ve started so I’ll finish. The ionisation mechanism and kinetic isotope effects for 1,3-dimethylindolin-2 one

Thursday, January 7th, 2016

This is the third and final study deriving from my Ph.D.[1]. The first two topics dealt with the mechanism of heteroaromatic electrophilic attack using either a diazonium cation or a proton as electrophile, followed by either proton abstraction or carbon dioxide loss from the resulting Wheland intermediate. This final study inverts this sequence by starting with the proton abstraction from an indolinone by a base to create/aromatize to a indole-2-enolate intermediate, which only then is followed by electrophilic attack (by iodine).  Here I explore what light quantum chemical modelling might cast on the mechanism.

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References

  1. B.C. Challis, and H.S. Rzepa, "Heteroaromatic hydrogen exchange reactions. Part VIII. The ionisation of 1,3-dimethylindolin-2-one", Journal of the Chemical Society, Perkin Transactions 2, pp. 1822, 1975. http://dx.doi.org/10.1039/P29750001822

Some examples of open access publications citing managed research data (RDM).

Tuesday, January 5th, 2016

In May 2015, the EPSRC funding council in the UK required researchers to publish the outcomes of the funded work to include an OA (open access) version of the narrative and to cite the managed research data used to support the research with a DOI (digital object identifier). I was discussing these aspects with a senior manager (research outcomes) at the EPSRC and he asked me to provide some examples from my area of chemistry; here are some.

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I’ve started so I’ll finish. Mechanism and kinetic isotope effects for protiodecarboxylation of indoles.

Saturday, January 2nd, 2016

Another mechanistic study we started in 1972[1] is here 40+ years on subjected to quantum mechanical scrutiny.

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References

  1. B.C. Challis, and H.S. Rzepa, "Heteroaromatic hydrogen exchange reactions. Part 9. Acid catalysed decarboxylation of indole-3-carboxylic acids", Journal of the Chemical Society, Perkin Transactions 2, pp. 281, 1977. http://dx.doi.org/10.1039/P29770000281