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Henry Rzepa's Blog

Henry Rzepa's Blog
Chemistry with a twist
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The previous post demonstrated the simple iso-electronic progression from six-coordinate carbon to five coordinate nitrogen. Here, a further progression to oxygen is investigated computationally. The systems are formally constructed from a cyclobutadienyl di-anion and firstly the HO 5+ cation, giving a tri-cationic complex. There are no examples of the resulting motif in the Cambridge structure database.

Published

It is not only the non-classical norbornyl cation that has proved controversial in the past. A colleague mentioned at lunch (thanks Paul!) that tri-coordinate group 14 cations such as R 3 Si + have also had an interesting history.[cite]10.1021/ja990389u[/cite] Here I take a brief look at some of these systems. Their initial characterisations, as with the carbon analogues, was by 29 Si NMR.

Published

The example a few posts back of how methane might invert its configuration by transposing two hydrogen atoms illustrated the reaction mechanism by locating a transition state and following it down in energy using an intrinsic reaction coordinate (IRC). Here I explore an alternative method based instead on computing a molecular dynamics trajectory (MD). I have used ethane instead of methane, since it is now possible to

Published

In the previous post, I found intriguing the mechanism by which methane (CH 4 ) inverts by transposing two of its hydrogens. Here I take a look at silane, SiH 4 . It appears it is a three-stage process! Firstly, silane eliminates molecular hydrogen to form a molecular complex between H 2 and SiH 2 (DOI: 10.14469/hpc/2290). The barrier (~60 kcal/mol) is very much lower than with methane.

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This is a spin-off from the table I constructed here for further chemical examples of the classical/non-classical norbornyl cation conundrum. One possible entry would include the transition state for inversion of methane via a square planar geometry as compared with e.g. NiH 4 for which the square planar motif is its minimum.

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Cyclobutadiene is one of those small iconic molecules, the transience and instability of which was explained theoretically long before it was actually detected in 1965.[cite]10.1021/ja01092a049[/cite] Given that instability, I was intrigued as to how many crystal structures might have been reported for this ring system, along with the rather more stable congener cyclo-octatetraene. Here is what I found.

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The thread thus far. The post about Na 2 He introduced the electride anionic counter-ion to Na + as corresponding topologically to a rare feature known as a non-nuclear attractor. This prompted speculation about other systems with such a feature, and the focus shifted to a tetrahedral arrangement of four hydrogen atoms as a dication, sharing a total of two valence electrons. The story now continues here.

Published

I analysed the bonding in chlorine trifluoride a few years back in terms of VSEPR theory. I noticed that several searches on this topic which led people to this post also included a query about the differences between it and the bromine analogue. For those who posed this question, here is an equivalent analysis.