I am currently working on a lab, where I am trying to calculate whether the R group in penicillin type drugs has an affect on the ring strain of the beta lactam. I know how to find ring strain in cycloalkanes through the molar heat of combustion, however I can't find any information about the combustion of beta lactams as I'm sure it doesn't happen often, so is there another way I can calculate the ring strain of the beta lactams in different penicillin type drugs?


  • $\begingroup$ Are you looking for an experiment to confirm the ring strain or is a computer model also an option? $\endgroup$
    – Inselino
    Mar 24, 2021 at 12:29
  • $\begingroup$ I cannot currently do an experiment in a lab so I am looking for a way to calculate ring strain with data available online. Computer modelling is definitely an option. I have also tried using Webmo to calculate the ring strain, however the strain energy calculations were completely different each time even if I was using the exact same penicillin drug. $\endgroup$
    – Orion
    Mar 24, 2021 at 12:58
  • $\begingroup$ Would be the heat of formation instead / in addition to the heat of combustion you an alternative suitable for you? This then wider scope may increase the likelihood to identify literature. A first entry found was «AM1 study of the electronic structure of benzylpenicillin» by Venugopal et al. in IJOBC here, e.g. about the influence to protonate the lactam-N. $\endgroup$
    – Buttonwood
    Mar 24, 2021 at 13:23

1 Answer 1


For the calculation of ring strain for ring-opening metathesis polymerization, I used DFT calculation. In generell you open your molecule at the position you want to and saturate the bonds. Then you compare the enthalpy of products and educts.

As an example you can look at cyclopentene.If it is opened during Metathesis and you saturate the doublebonds at the end of the chain with two $\ce{CH2}$ or one ethene molecule.

enter image description here

This has also been reported in literature for example: https://onlinelibrary.wiley.com/doi/full/10.1002/pola.28695

I have also done a quick search and in this paper they have used the same technique (they just used ethane instead of ethene because they saturated single bonds instead of double bonds). They used post Hatree Fock B3LYP to calculate the enthalpy. I think you are fine to use either one.



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