Energy decomposition analysis of hindered alkenes: Tetra t-butylethene and others.

In the previous post,[1] I introduced the N=N double bond in nitrosobenzene dimer, arguing that even though it was a formal double bond, its bond dissociation energy made it nonetheless a very weak double bond! This was backed up by a technique known as energy decomposition analysis or EDA. Here I use a variant of this method  known as  NEDA to look at some other strained alkenes, including the famously non-existent tetra t-Butyl ethene.

The NEDA procedure gives a fragment interaction energy (decomposing it into fundamental quantum mechanically derived energies if required) with respect to a reference state for the fragments. In this case, the fragments are obtained by cutting the double bond, resulting in triplet state carbenes as the reference state. The calculations (B3LYP+GD3+BJ/Def2-TZVPP) are available here.[2]

1. Compound 1, a relatively unstrained alkene, ΔE = -177.0 kcal/mol, R_CC 1.341Å
2. Compound 2 (PUVQUE, [3], [4]), ΔE = -164.3 kcal/mol, R_CC 1.362Å, CC torsion 16.5°
3. Compound 3 (CUBVOK, [5]) ΔE = -167.9 kcal/mol, R_CC 1.351Å, CC torsion 9.2°
4. Compound 4 (currently unknown) ΔE = -135.8 kcal/mol, R_CC 1.380Å, CC torsion 54.5°

The NEDA interaction energy is directly proportional to both the CC bond length and the C-C=C-C torsion angle. What is interesting however is the large interaction energy gap in ΔE between the two known hindered alkenes (2 and 3) and the unknown tetra-t-butyl ethene 4. It seems moving from say compound 2 by converting the two iso-propyl substituents to full t-butyl ones is just too large a change to bridge. Unless one day isolated as a very very unstable species, compound 4 seems destined not to exist!

This post has DOI: 10.59350/rzepa.29410

References

1. H. Rzepa, “The mysterious N=N double bond in nitrosobenzene dimer.”, 2025.
2. H. Rzepa, “Energy decomposition analysis of hindered alkenes: Tetra-tert-butyl ethene and others.”, 2025.
3. R. Boese, W.R. Roth, D. Bläser, R. Latz, and A. Bäumen, “(E)-3,4-Diisopropyl-2,5-dimethylhex-3-ene at 125K”, Acta Crystallographica Section C Crystal Structure Communications, vol. 54, pp. IUC9800055, 1998.
4. R. Boese, W. Roth, D. Blaser, R. Latz, and A. Baumen, “CCDC 130610: Experimental Crystal Structure Determination”, 1999.
5. J. Deuter, H. Rodewald, H. Irngartinger, T. Loerzer, and W. Lüttke, “Kristall- und molekularstruktur von tetrakis(1-methylcyclopropyl)ethylen”, Tetrahedron Letters, vol. 26, pp. 1031-1034, 1985.