Posts Tagged ‘chemical shift’

An unusual [1,6] shift in homotropylium cation exhibiting zones of aromaticity.

Tuesday, August 12th, 2014
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One thing leads to another. Thus in the previous post, I described a thermal pericyclic reaction that appears to exhibit two transition states resulting in two different stereochemical outcomes. I noted that another such reaction appeared to be a [1,6] carousel migration in homotropylium cation,[1] where transition states for both retention and inversion of the configuration of the migrating group (respectively formally allowed and forbidden) were reported (scheme below). Here I explore this system further. homotropylium Firstly, the pathway leading to inversion.[2] The reaction path (ωB97XD/6-311G(d,p)/SCRF=chloroform) has got a very odd (table-top mountain) shape, whereby the region of the transition state (IRC = 0.0) is very flat, and the region close to reactant and (identical) product is very steep. The gradient norm shows this best, with sharp spikes at IRC ± 4.2. Something clearly is happening here to cause this behaviour. Before moving on to analyze this, I want you first to observe the methyl groups below. Note how one of them rotates at the start of the process, and the other at the end. I have elsewhere called this behaviour the methyl flag, and it is due to stereoelectronic re-alignments of the C-H groups accompanying the changes in the conjugated array. htropa htrop htropG The homotropylium cation is said to be homoaromatic, indicating that cyclic conjugation can be maintained across a ring in which the σ framework is interrupted at one point. A NICS probe placed at the ring critical point of this molecule reveals a chemical shift of -11.3 ppm[3], very similar to eg that obtained for benzene itself. The three highest doubly occupied NBOs (below) show two normal π-type orbitals and one rather different one that spans the homo-bond (the MOs, before you ask, are a bit of a mess, with lots of mixed contributions from other parts of the σ framework).

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References

  1. A.M. Genaev, G.E. Sal’nikov, and V.G. Shubin, "Energy barriers to carousel rearrangements of carbocations: Quantum-chemical calculations vs. experiment", Russian Journal of Organic Chemistry, vol. 43, pp. 1134-1138, 2007. http://dx.doi.org/10.1134/S1070428007080076
  2. Henry S. Rzepa., "Gaussian Job Archive for C10H13(1+)", 2014. http://dx.doi.org/10.6084/m9.figshare.1134556
  3. Henry S. Rzepa., "Gaussian Job Archive for C10H13(1+)", 2014. http://dx.doi.org/10.6084/m9.figshare.1135694

Hexacoordinate hydrogen.

Monday, July 8th, 2013
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A feature of a blog which is quite different from a journal article is how rapidly a topic might evolve. Thus I started a few days ago with the theme of dicarbon (C2), identifying a metal carbide that showed C2 as a ligand, but which also entrapped a single carbon in hexa-coordinated mode. A comment was posted bringing attention to the origins of the discovery of hexacoordinated carbon, and we moved on to exploring the valency in one such species (CLi6). Here I ask if hydrogen itself might exhibit such coordination.

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Chemistry with a super-twist: A molecular trefoil knot, part 2.

Tuesday, June 1st, 2010
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A conjugated, (apparently) aromatic molecular trefoil might be expected to have some unusual, if not extreme properties. Here some of these are explored. (more…)