To quote from Wikipedia: in chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The most ubiquitous type of carbene of recent times is the one shown below as 1, often referred to as a resonance stabilised or persistent carbene. This type is of interest because of its ability to act as a ligand to an astonishingly wide variety of metals, with many of the resulting complexes being important catalysts. The Wiki page on persistent carbenes shows them throughout in form 1 below, thus reinforcing the belief that they have a valence of two and by implication six (2×2 shared + 2 unshared) electrons in the valence shell of carbon. Here I consider whether this name is really appropriate.
Posts Tagged ‘energy barrier’
The substitution of a nucleofuge (a good leaving group) by a nucleophile at a carbon centre occurs with inversion of configuration at the carbon, the mechanism being known by the term SN2 (a story I have also told in this post). Such displacement at silicon famously proceeds by a quite different mechanism, which I here quantify with some calculations.
Thalidomide is a chiral molecule, which was sold in the 1960s as a sedative in its (S,R)-racemic form. The tragedy was that the (S)-isomer was tetragenic, and only the (R) enantiomer acts as a sedative. What was not appreciated at the time is that interconversion of the (S)- and (R) forms takes place quite quickly in aqueous media. Nowadays, quantum modelling can provide good in-silico estimates of the (free) energy barriers for such processes, which in this case is a simple keto-enol tautomerism. In a recently published article, just such a simulation is reported. By involving two explicit water molecules in the transition state, an (~enthalpic) barrier of 27.7 kcal/mol was obtained. The simulation was conducted just with two water molecules acting as solvent, and without any additional continuum solvation applied. So I thought I would re-evaluate this result by computing it at the ωB97XD/6-311G(d,p)/SCRF=water level (a triple-ζ basis set rather than the double-ζ used before), and employing a dispersion-corrected DFT method rather than B3LYP.
- C. Tian, P. Xiu, Y. Meng, W. Zhao, Z. Wang, and R. Zhou, "Enantiomerization Mechanism of Thalidomide and the Role of Water and Hydroxide Ions", Chemistry - A European Journal, vol. 18, pp. 14305-14313, 2012. http://dx.doi.org/10.1002/chem.201202651