Archive for December, 2009
Wednesday, December 30th, 2009
A previous post posed the question; during the transformation of one molecule to another, what is the maximum number of electron pairs that can simultaneously move either to or from any one atom-pair bond as part of the reaction? A rather artificial example (atom-swapping between three nitrosonium cations) was used to illustrate the concept, in which three electron pairs would all move from a triple bond to a region not previously containing any electrons to form new triple bonds and destroy the old. Here is a slightly more realistic example of the phenomenon, illustrated by the (narcisistic) reaction below of a bis(sulfur trifluoride) carbene. Close relatives of this molecule are actually known, with either one SF3 of the units replaced by a CF3 group or a SF5 replacing the SF3[1]
(more…)
References
- B. Poetter, K. Seppelt, A. Simon, E.M. Peters, and B. Hettich, "Trifluoroethylidynesulfur trifluoride, CF3C.tplbond.SF3, and its dimer", Journal of the American Chemical Society, vol. 107, pp. 980-985, 1985. https://doi.org/10.1021/ja00290a038
Tags:animation, calculated free energy barrier, Hypervalency, inorganic chemist, Interesting chemistry
Posted in Hypervalency, Interesting chemistry | 3 Comments »
Wednesday, December 9th, 2009
Clar islands are found not so much in an ocean, but in a type of molecule known as polycyclic aromatic hydrocarbons (PAH). One member of this class, graphene, is attracting a lot of attention recently as a potential material for use in computer chips. Clar coined the term in 1972 to explain the properties of PAHs, and the background is covered in a recent article by Fowler and co-workers[1]. The concept is illustrated by the following hydrocarbon:
(more…)
References
Tags:aromaticity, Clar, Clar islands, Cloud Clar islands, computer chips, ELF, Henry Armstrong, Interesting chemistry, non-planar systems, pence, Santos
Posted in Interesting chemistry | 1 Comment »
Sunday, December 6th, 2009
In the previous two posts, a strategy for tuning the nature of the CS bond in the molecule HO-S≡C-H was developed, based largely on the lone pair of electrons identified on the carbon atom. By replacing the HO group by one with greater σ-electron withdrawing propensity, the stereo-electronic effect between the O-S bond and the carbon lone pair was enhanced, and in the process, the SC bond was strengthened. It is time to do a control experiment in the other direction. Now, the HO-S group is replaced by a H2B-S group. The B-S bond, boron being very much less electronegative than oxygen, should be a very poor σ-acceptor. In addition, whereas oxygen was a π-electron donor (acting to strengthen the S=C region), boron is a π-acceptor, and will also act in the opposite direction. So now, this group should serve to weaken the S-C bond.
(more…)
Tags:Hypervalency, Interesting chemistry
Posted in Hypervalency, Interesting chemistry | No Comments »
Saturday, December 5th, 2009
In my first post on this theme, an ELF (Electron localization function) analysis of the bonding in the molecule HO-S≡C-H [1] was presented. This analysis identified a lone pair of electrons localized on the carbon (integrating in fact to almost exactly 2.0) in addition to electrons in the CC region. This picture seems to indicate that the triple bond splits into two well defined regions of electron density (synaptic basins). In a comment to this post, I also pointed out that an NBO analysis showed a large interaction energy between the carbon lone pair and the S-O σ* orbital, characteristic of anomeric effects (in eg sugars). This latter observation gives us a handle on possibly tweaking the effect. Thus if the S-O σ* orbital can be made a better electron acceptor, then its interaction with the lone pair could be enhanced.
(more…)
References
- P. Schreiner, H. Reisenauer, J. Romanski, and G. Mloston, "A Formal Carbon–Sulfur Triple Bond: HCSOH", Angewandte Chemie International Edition, vol. 48, pp. 8133-8136, 2009. https://doi.org/10.1002/anie.200903969
Tags:Hypervalency, Interesting chemistry, large interaction energy
Posted in Hypervalency, Interesting chemistry | 3 Comments »
Tuesday, December 1st, 2009
The Grignard reaction is encountered early on in most chemistry courses, and most labs include the preparation of this reagent, typically by the following reaction:
(more…)
Tags:Hypervalency, Interesting chemistry, metal, metal atom, metal centres, Mg atom
Posted in Hypervalency, Interesting chemistry | 1 Comment »