I want to calculate BDE for hydroxyl groups (-OH) present in my biological molecules. Could you please suggest me how to do these calculations in gaz and in solvent phase ? and if i need to optimised the radical obtained from the neutral molcule or do only a single point to calculate the Bond dissociation enthalpies ?????
Thanks,
That is a multi-step problem:
1) You need quantum chemically optimized structures (not single point) of all reactands on all sides of the reaction and if you investigate dissociation into radicals it will have to be radicals. Those may be difficult to calculate because unpaired electrons are often handled poorly by simple standard DFT. On the other hand, accurate wavefunction methods are often too expensive for large biomolecules. Probably it might be worth checking out so-called embedding methods (you calculate the radical surrounding with a sophisticated method, the rest with a simpler tool). In your case a WF/DFT embedding may be helpful. If you want solvent molecules around, you will have no other chance than using embedding since more than a few hundred atoms are impossible to handle, even for plain DFT.
2) If the binding energy should be accurate, a correction of basis set superposition errors might be necessary. The classical way to do that is the counterpoise correction, check out if your program offers that. Another option is just calculating the dissociation products at large distances, but in that case you have to check your orbitals manually that they are the same as in your isolated monomers [this method can be done single point].
3) Since you are asking for enthalpies instead of "0K-energies", you will have to apply statistical thermodynamics to your calculation result. Many QC packages nowadays include some sort of thermochemistry tool, so check out the manual.
If all that seems complicated, it's because it is. There are plenty of chances to ruin the result along the way so I highly recommend either taking a university course on the subject or at least cooperating with someone who has the expertise for that [not me, I've never actually used embedding].
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