# Is there a way to efficiently estimate the enthalpy of formation for an organic molecule in computational chemistry?

I'm exploring the platonic hydrocarbons and one of the methods I'd like to use is to compare their enthalpies of formation.

The issue is that carbon's standard form is a covalent lattice and I can't think of a way to efficiently obtain its electronic and thermal enthalpies without resorting to optimising a giant PAH.

FWIW I'm using GaussView/Gaussian.

I would say yes, you could estimate standard enthalpies of formation for molecules more or less accurately by application of quantum and statistical mechanics, although there is a 'but': the usual procedure works for gas-phase heats of formation only.

And thus, if the standard state of the material of interest corresponds to the condensed phase, you have to resort to some procedure which allows you to calculate a condensed phase heat of formation given the corresponding gas-phase one. Such procedures are based on a trivial application of the infamous Hess's law, which for the case of solids can be written as follows $$\Delta H_{\mathrm{solid}} = \Delta H_{\mathrm{gas}} - \Delta H_{\mathrm{subl}} \, ,$$ where $\Delta H_{\mathrm{subl}}$, the heat of sublimation, is estimated usually from experiementally available data on simillar compounds by a correlation scheme. However, as with any empirically fitted scheme the reliability and the accuracy of any such approach are doubtful.

Alternatively, in many cases, it is enough to compare the gas-phase heat of formations, even if the standard state of material is solid or liquid. To calculate the gas-phase heat of formation for a substance in Gaussian all you need is to run frequencies calculations on the previously optimised geometry of a single molecule.

If you would like to give the gas-phase heats of formations a try, I suggest running G4 calculations first. In the route section just write

# G4


and that would do it. Alternatively, in GaussView's Gaussian Calculation Setup window first choose "Energy" at the "Job Type" tab and then at the "Method" tab choose "Compound..." in the second combobox and "G4" in the third one as shown below.

• I did do calculations using atomic carbon but the enthalpies made no sense; the value for tetrahedrane was -2920 kJ/mol. – gsurfer04 Apr 3 '15 at 18:55
• @gsurfer04, what do you mean by "using atomic carbon"? Also, the level of theory is important. For thermodynamical quantities you could try G3/G4 composite methods first. – Wildcat Apr 4 '15 at 12:27
• @gsurfer04, by G3/G4 I mean use G4 if you have Gaussian 09 and G3 only if you still use the previous Gaussian 03 in which G4 is not available. – Wildcat Apr 4 '15 at 12:39
• G4 is much more expensive, maybe G3 is good enough – user1420303 Apr 4 '15 at 12:47
• @user1420303, may be. I do not remember now, since once I switched to G4, I've never looked back. :D It works surprisingly well. – Wildcat Apr 4 '15 at 13:04