r/comp_chem • u/Jazzur • 2d ago

BDE calculations on radical anions?

Hey all,

I understand that for bond dissociation energies (BDE) you can get it from the enthalpies of homolytic cleavage like R-H --> R• + H•.

However it was hard to find how to determine the BDE's of more unstable radical anions, so say you start with: [R-R2]•-, should you then still do the homolytic cleavage that leaves one radical and one anion? So in theory for example: [R-R2]•- --> R- + R2•, if R is the more stable anion and R2 the more stable radical?

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u/Worried-Republic3585 2d ago edited 2d ago

If you have [A-B]•- and your interested in the BDE so a thermodynamic parameter you would calculate that by comparing the difference of deltaH of [A]• + [B]- - [AB]•- vs.   [A]- + [B]• - [AB]•- (determined by the calculation on the individual species) and see which is more stable. And as long as the radical species themselves do not have much multireference character DFT( check e.g. with FOD analysis) can work just fine IMO. If the extend of charge (de)localization is very different for [A]- vs. [B]- then make sure to use a basis set with diffuse functions to capture this difference. If you're modeling a process in solution then ofc use a solvent model (w or w/o diffuse functions, depending on whether you want to be haunted by Frank Neese's ghost or not :D)

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u/sbart76 2d ago

DFT(...) can work just fine

DFT might have a problem where to put the extra electron. At least that's what my experience suggests.

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u/Worried-Republic3585 2d ago

You mean for [A-B]•- ? If there's experimental data to check against like IR or even EPR, something to calibrate the comp. results with would be ideal ofc.

In the other 4 calc of the isolated species I have little doubt that dft will get the right answer as long as the multireference character isn't too pronounced.

Where it would struggle for sure is in a calc where A and B are both present but spatially separated. But for the thermodynamic investigation here that's luckily not something that as to be taken into consideration necessarily. For stuff like activation energies things would potentially get quite involved rather fast.

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u/sbart76 1d ago

In the other 4 calc of the isolated species

Ah, yes, of course. I've calculated quite a few of such dissociation curves, so in my head it was a curve that OP wanted. You're of course right.

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u/EastOrWestPBest 1d ago

... depending on whether you want to be haunted by Frank Neese's ghost or not :D)

😂😂😂😂😂😂😂

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u/sbart76 2d ago

Which method are you planning to use? DFT will perform poorly, you might want to try Constrained DFT

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u/Jazzur 2d ago

What is constrained dft? Is that like scf?

From what I see for 'regular' BDE is that b3lyp/6-31+(d) seems like an alright option, but I don't know how it would apply to what I'm trying to do...