In the previous post,[cite]10.59350/rzepa.29383[/cite] 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.[cite]10.14469/hpc/15463[/cite]
- Compound 1, a relatively unstrained alkene, ΔE = -177.0 kcal/mol, RCC 1.341Å
- Compound 2 (PUVQUE, [cite]10.1107/S0108270198099247[/cite], [cite]10.5517/cc4cx7m[/cite]), ΔE = -164.3 kcal/mol, RCC 1.362Å, CC torsion 16.5°
- Compound 3 (CUBVOK, [cite]10.1016/S0040-4039(00)98504-6[/cite]) ΔE = -167.9 kcal/mol, RCC 1.351Å, CC torsion 9.2°
- Compound 4 (currently unknown) ΔE = -135.8 kcal/mol, RCC 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
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