Bond dissociation energy

[Morgan]

Hi,

One often say that there is no stupid question, but I have maybe one...

I would like to determine the Bond Dissociation Energy of some molecules I 'm working with. I already carried out PM3 and PM6 via MOPAC, and the strategy is simple, optimize the geometry, calculate the enthalpy of formation of species and calculate the energy of the reaction corresponding to the homolitic cleavage.

I would like to confirm my results via DFT using Turbomole. I optimize the geometry and then carried out a signle point calculation with frequency analysis. Tehn by using the keyword freeh I can generate thermodynamical properties, but I cannot find the "enthlpy of formation". Where can I get this data ?

Thanks in advance for your help

Morgan

[andreas]

Hi Morgan,

freeh computes the (in a sense absolute) inner energy U and the entropy S of the molecule
at given T and p (where p should be not too large as everything refers to a gas-phase molecule
without intermolecular interactions). Using these you can do your thermodynamics for the reaction.

The enthalpy of formation is not available, as this is relative to the reference states of the elements,
which is often difficult, if not impossible to calculate (think of C --> graphite). But you do not need them
for reaction energies.

Best,

 Andreas