[khwcm]

Most internet result explain the low reactivity of oxygen diradical is due to its intrinsic triplet properties, where most other molecule exist as singlet.

So does it mean that there is a selection rule saying that a molecule with triplet state cannot react another molecule in singlet state? While for bromine radical, which is doublet, can react with methane, which is singlet state. why is this process allow but not the triplet oxygen one?

Thanks

[Corribus]

It's not total spin that matters, but that spin is conserved. Oxygen reacting with a hydrocarbon to produce water and carbon dioxide can be expressed as: triplet + singlet = singlet + singlet. Spin is not conserved, reaction is kinetically slow. In the case of a radical halogenation, a bromine radical (doublet, say) reacting with a hydrocarbon (singlet) produces a chain reaction where spin is ultimately conserved. (the radical halogen atom reacts with methane to produce a methyl radical plus H-X).

https://en.wikipedia.org/wiki/Free-radical_halogenation

Ultimately a violation of spin conservation doesn't prevent a reaction, just acts as a kinetic barrier. In the case of oxygen, formation of singlet oxygen makes the reaction very fast due to (at least one reason) removal of this kinetic barrier.



 

[khwcm]

Thanks !

Do you mean that if one of the product is high energy enough to be existed, it is possible to have something like:
A^singlet + O₂^triplet  C^singlet + D^triplet  ?

How about if a reaction have the No. of reactant different from the number of product? How to apply "selection rule" for these kind of reaction?
ie: O₂^singlet + B^singlet B-O₂^{Singlet or triplet?}

And how is the selection rule explain the thermally spontaneous recombination of radicals?
CH₃. + CH₃.  CH₃CH₃
Doublet + Doublet to give a singlet(or triplet?) product. Is this a spin allowed process?

thanks for your kind help again

[Corribus]

Do you mean that if one of the product is high energy enough to be existed, it is possible to have something like:
A^singlet + O₂^triplet  C^singlet + D^triplet  ?

Yes, this is a very well known type of process. You can read more about triplet sensitizers here:

http://www.photobiology.info/Oleinick.html

(See fig. 2)

How about if a reaction have the No. of reactant different from the number of product? How to apply "selection rule" for these kind of reaction?
ie: O₂^singlet + B^singlet B-O₂^{Singlet or triplet?}

I don't know that I'd call it a selection rule, per se. Typically if two singlets react, they most readily formed reaction will be another singlet. It may be more productive to consider specific cases rather than hypotheticals.

And how is the selection rule explain the thermally spontaneous recombination of radicals?
CH₃. + CH₃.  CH₃CH₃
Doublet + Doublet to give a singlet(or triplet?) product. Is this a spin allowed process?

Two triplets will also combine to form two singlets, in a process called triplet annihilation. 

Read more here:

http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Fundamentals/Dexter_Energy_Transfer

Here's a simplistic view:

A doublet (to make things easy) can either be a spin up or spin down. So while a  +      process maybe forbidden, the other option  +      isn't.

Triplet annihilation and other like processes are a fair bit more complicated than that, but maybe it gives you a simplistic picture of how the spins can balance in a radical recombination (termination) reaction. I am no expert on radical recombination chemistry, but I seem to recall that  disproportionation can result in formation of both singlet and triplet pairs. A (very) quick Google search seems to confirm this.

[khwcm]

Thanks! I am still abit confused, would you mind telling me whether I understand this correctly?

For a bimolecular reaction to give 2 products:
A^singlet + B^singlet  C^singlet + D^singlet or
A^singlet + B^triplet  C^singlet + D^triplet or
A^singlet + B^doublet  C^singlet + D^doublet and so on

and for a bimolecular reaction to give 1 or 3 products, there is no general selection rule?

[Corribus]

First, I would be careful with such linear thinking. Oxygen (triplet) reacts with hydrocarbons (singlets) all the time to form two singlet state products - combustion. The thing is that spin-balancing of a photophysical or photochemical process doesn't determine whether a reaction will happen; rather it determines the favorability (rate, say) of a process. An excited triplet chromophore will eventually decay back down to a singlet ground state even though it's a spin-forbidden process. The presence of a ground-state triplet (oxygen) can make this process occur near instantaneously via energy transfer. Some metal polypyridyl complexes have excited triplet lifetimes on the order of tens to hundreds of microseconds in deoxygenated solution. In the presence of oxygen, that lifetime goes down to about 1 microsecond - and this is only limited by the diffusion timescale of oxygen in solution. Note that the absolute rater of a process depends on a lot of factors. The rates of nominally spin-forbidden processes can differ by many orders of magnitude, from sub-microsecond to greater than second timescales. So, it is important not to oversimplify. Particularly in the case of chemical reactions (versus photophysical processes) that have additional complicating factors - diffusion rates being one of them. For a biomolecular reaction giving 1 or 3 products... this complicates things even further. I don't know if I'd feel comfortable trying to generalize expected results. Did you have a specific example in mind.

Bear in mind, also, that there are very few common molecules that have stable radicals or triplet states. Oxygen is very unique in this regard. Even in the case that triplet oxygen can sensitize the production of another triplet-state species, it is likely this product will relax eventually to a singlet (and singlet oxygen will relax back to a triplet by physical or chemical routes). So you have to consider what the downstream products are as well.

[khwcm]

Well, i was not intentionally simplify to the few equations, but as the saying "triplet oxygen have low reactivity due to the spin forbidden reaction with singlet molecule", and also knowing that "doublet radical react with singlet molecule to give singlet + doublet product, is a spin allow process", so i start wonder how the selection rule for a reaction instead of within a molecule.

for the combustion part, how about if i understand a combustion having high activation energy due to the conversion of triplet oxygen to singlet oxygen, which is reactive enough to carry on the spin allow process?

I could somehow understand the spin allow/forbidden process within a molecule(photochemical), but i just have no idea how would electron spin may affect a reaction theoretically.

so an examples in my mind would be carbene chemistry,
https://upload.wikimedia.org/wikipedia/commons/8/8f/Singletriplet.png
We can have
carbene^singlet + alkene^singlet  cyclopropane^singlet
carbene^triplet + alkene^singlet  diradical_intermediate^{unknown_state}  cyclopropane^singlet
so is it possible to know any of the process are spin forbidden theoretically?