Alkene Isomers

General Discussion
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Alkene Isomers Investigate the stability of alkene isomers due to steric interactions. Task Build and minimize isomeric alkenes and compare their relative energies to confirm empirical rules for evaluating alkene stability: Zaitsef’s rule, cis- vs trans-, isolate vs conjugated dienes. Examples: Compare: 1-hexene, 2-hexene (cis and trans), 2-methyl-2-pentene, 2,3-dimethyl-2-butene Compare: 1,4-pentadiene and 1,3-pentadiene See also Markovnikov’s rule Categories: Teaching Updated: November 26, 2009 Share on Twitter Facebook Google+ Reddit
This is a companion discussion topic for the original entry at https://avogadro.cc/teaching/alkene_isomers/
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Hi,

I compared 1,4-pentadiene and 1,3-pentadiene using UFF geometry optimization. 1,3 gives energy 2.8351307 and 1.4 gives lower energy of 2.7028027. I would expect 1,3-isomer to give lower energy due to double bonds conjugation… Any comments ?

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The UFF force field (or any current force field for that matter) does not have any interaction term for delocalization.

To get accurate relative energies for delocalization effects, you’ll need some level of QM method.

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Hello.
Did you fix your prooblem with delocalization&conjugation with some other software?

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As far as I know there is no force field method that exists that handles delocalization. I would say that if you need to work on specific interactions that occur from delocalization, then you need to use a more advanced QM package, something like ORCA.

There may come a time when Avogadro will have something like GFN2-XTB, which may be able to handle something like that with relative ease, but until then it won’t be accounted for here.

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