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Henry Rzepa's Blog

Henry Rzepa's Blog
Chemistry with a twist
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Crystal_structure_mining2-Norbornyl CationCarbocationsChemical BondingChemistryChimieAnglais
Publié

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.

Reaction MechanismAnimationChemical ReactionChemistryComputational ChemistryChimieAnglais
Publié

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

Crystal_structure_miningInteresting ChemistryMetal SaltsPeroxydisulfateChimieAnglais
Publié

The Wikipedia entry on peroxydisulfate is quite short (as of today). But I suspect this article may change things.[cite]10.1038/s41559-017-0083[/cite]. A search of the Cambridge structure database reveals around 18 high quality crystal structures containing this species are known, many as metal salts.

Crystal_structure_miningReaction MechanismAluminiumCalculated Free Energy BarrierCarbon MonoxideChimieAnglais
Publié

A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. Pyrophoric iron for example is prepared by heating anhydrous iron (II) oxalate in a sealed test tube ( i.e. to 600° or higher). When the tube is broken open and the contents released, a shower of sparks forms. Not all metals do this;

Reaction MechanismChemistryIndustrial GasesMethaneSiH2 ComplexChimieAnglais
Publié

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.

Reaction MechanismChemistryMethaneMolecular GeometryOrbital HybridisationChimieAnglais
Publié

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.

Crystal_structure_miningAmmoniaAmmoniumAromaticityCationsChimieAnglais
Publié

A few years back, I did a post about the Pirkle reagent[cite]10.1039/c39910000765[/cite] and the unusual π-facial hydrogen bonding structure[cite]10.1039/P29940000703[/cite] it exhibits. For the Pirkle reagent, this bonding manifests as a close contact between the acidic OH hydrogen and the edge of a phenyl ring; the hydrogen bond is off-centre from the middle of the aryl ring.

Chemical ITAPIChemical DatabasesCity: LondonCompany: TfLChimieAnglais
Publié

Living in London, travelling using public transport is often the best way to get around. Before setting out on a journey one checks the status of the network. Doing so today I came across this page: our open data from Transport for London.  I learnt that by making TFL travel data openly available, some 11,000 developers (sic!) have registered for access, out of which some 600 travel apps have emerged.

Crystal_structure_miningAntiaromaticityChemistryCyclobutadieneInstabilityChimieAnglais
Publié

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.