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Given the chemical formula of a coumpound (and possibly the arrangement of the various atoms and connections inside), are there theoretical or numerical approaches to compute the enthalpy and heat capacity of the molecule without experimental data, or at least estimate them ?

Can any knowledge on a similar molecule (say with a few atoms less, and or a different arrangement) be used to improve the guess ?

Can latent heats of vaporisation, liquefaction... be estimated ? (Potentially introducing further assumptions)

Thank you for your answers !

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    $\begingroup$ Ever heard of quantum chemistry? $\endgroup$
    – Ivan Neretin
    Commented Dec 7 at 9:13
  • $\begingroup$ Well, the point is that I am not a chemist, so I am precisely asking for guidance. I'd be happy to see you elaborate on this ;) $\endgroup$
    – Laurent90
    Commented Dec 7 at 9:52
  • $\begingroup$ Then the answer is: yes there is an approach, but it is extremely (sometime prohibitively) costly in terms of computation time, and not terribly reliable anyway. $\endgroup$
    – Ivan Neretin
    Commented Dec 7 at 10:20

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Yes, there are methods to estimate the enthalpy and heat capacity of a molecule without experimental data. A good reference in this regard is The Properties of Gases and Liquids by Reid, Prausnitz, and Poling. Specifically, Chapter 2 (4th ed.) covers estimating critical properties and acentric factors given molecular formula/structure. These can be used to calculate enthalpy departure functions from an ideal gas using an EOS. The ideal gas enthalpy and Gibbs energy (and thus entropy) is covered in Chapter 6. Some of the methods demonstrated are named Joback, Yoneda, Thinh. Basically these methods sum up contributions by functional group to estimate the heat capacity or Gibbs energy. Many/most functional groups are included. Of course, actual experimental data will be more accurate than these methods.

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The best theory developed to calculate enthalpy, heat capacity and other molecular properties from atomic constants is the so-called Benson group increment theory. It has been developed by Sydney Benson. Look at these words on Wikipedia, where this theory is developed in detail, with plenty of examples.

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