Jumping off of that the only somewhat stretched bond I could find is within the following molecule, which I am not even attempting to name. The key features here are the alkynyl links to get the bulky diamantyl moieties far enough away from the double bond to simultaneously allow them to have positive dispersion interactions. Even with these rather extreme measures, I was only able to stretch the bond to 140.8 pm at the B97D3/def2-SVP139.7 pm at the B97D3/def2-TZVPP level of theory, calculated with Gaussian 09 Rev. E.01. (The image is from the previous B97D3/def2-SVP optimisation and hence shows 140.8 pm bond distance. The overall change is too small to justify re-uploading a new image.)
I hope that by providing one data-point I might get the ball rolling. Most of my attempts have been unfruitful, and I suspect that stretching a carbon-carbon double bond is difficult in general.
That is due to the fact that if introducing bulky substituents in most of the cases I looked at the planarity is lost to avoid stress, i.e. the methylene groups rotate and form an approximately 90 degree angle. Restricting this movement is more challenging than I previously thought.
In other cases the double bond is still typically around 134 pm, as the sum of their covalent radii would suggest.[1]
Jumping off of that the only somewhat stretched bond I could find is within the following molecule, which I am not even attempting to name. The key features here are the alkynyl links to get the bulky diamantyl moieties far enough away from the double bond to simultaneously allow them to have positive dispersion interactions. Even with these rather extreme measures, I was only able to stretch the bond to 140.8 pm at the B97D3/def2-SVP level of theory, calculated with Gaussian 09 Rev. E.01.
I have yet to increase the level of theory and obtain a frequency calculation, but since this is quite the sizeable molecule it'll take some more time.
- Pekka Pyykkö and Michiko Atsumi published covalent (single-, double-, triple-) bond radii for (almost) the entire periodic table in Chem. Eur. J. 2009, 15, 12770-12779. It is summarised in figure 3. These are obtained from experimental or theoretical data. The figure itself can be found on Pyykkö's homepage as a pdf-file.
xyz coordinates
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