Where does light go?

[5614]

The reaction:

[math]CH_4 + Cl_2 \to CH_3 Cl + HCl[/math]

requires UV light to overcome the activation energy barrier (ie. take place).

[VendingMenace]

The reaction:

[math]CH_4 + Cl_2 \to CH_3 Cl + HCl[/math]

requires UV light to overcome the activation energy barrier (ie. take place).

 

You are correct.

 

However' date=' UV light excites an [i']electronic[/i] transition which is accompanied by a vibronic and rotational transition. I had ask for an example of a purely vibrational or rotational transition providing the requisite activation energy.

 

The only example that I can think of that even comes close might be in some isomerizations (ie. switching between chair conformations in a hexane molecule) however, this is much more a physical process than a chemical process, which is what I think we were discussing.

[5614]

However, UV light excites an electronic transition which is accompanied by a vibronic and rotational transition. I had ask for an example of a purely vibrational or rotational transition providing the requisite activation energy.

I was referring to where silkworm said "it can do useful work overcomming an activation barrier or decomposing a molecule".

 

reading "the vibrations it causes can do useful work overcomming an activation barrier"

 

So' date=' i would love to hear some examples where purely vibrational events provide the activation energy or lead to decomposition.[/quote']

 

Silkworm said "vibrations it causes", whereas you then added in the "purely" which changed the meaning. Silkworm failed to mention about rotation which could be misleading, but he didn't specifically state there was no rotational movement.

 

I don't know of an example with solely a vibrational or rotational transition, but I would certainly not make a conclusion about whether or not it is possible based on what I currently know.

[VendingMenace]

Silkworm said "vibrations it causes"' date=' whereas you then added in the "purely" which changed the meaning. Silkworm failed to mention about rotation which could be misleading, but he didn't specifically state there was no rotational movement.

[/quote']

 

At the risk of getting involved in an argument online, i will add this...

 

There are three basic types of transitions that light can excite in a molecule (at least as far as this thread is concnered).

 

1) Electronic

2) Vibrational

3) Rotational

 

These have been listed in order of descending energy. This means that the energy required to effect an electronic transition is less than that for a vibrational one, which is less than that for a rotational one. It also happens that electronic energy levels (the stationary states) have less energy than vibrations which have less energy than rotations.

 

The point being this; if vibrations do not have enough energy to overcome an activation energy barrier, then rotations certainly do not. Thus, my consideration of a purely vibrational transition is a simplification of a vibrational/rotational transition.

 

So silkworm's assertion that, "the vibrations it causes can do useful work overcomming an activation barrier" can be adressed by considering purely vibrational transitions. Since he is asserting that it is the vibrations that provide the activation energy, it is acceptable to isolate the vibrational motions and transitions and ask if they are able to provide the requisite energy for a chemical process. WHich is what I did.

 

I hope that makes some sense?

 

I don't know of an example with solely a vibrational or rotational transition, but I would certainly not make a conclusion about whether or not it is possible based on what I currently know.

 

Fair enough. I am not sure myself. I think it is highly unlikely, but I do not really know, which is why i ask silkworm what he was thinking of, since he made the above claim. Just curious is all, really.

[Norman Albers]

Yo, did you make a mistake? You say descending order but then say 'less'...

[5614]

I think you meant ascending order of energy (ie. energy getting bigger as you do down the list). So for energy: 1<2<3

 

And you've contradicted yourself, I think. You said that vibrational energy is less than rotational, but that "if vibrations do not have enough energy to overcome an activation energy barrier, then rotations certainly do not" which is illogical because rotational contains more energy, so could over the activation energy barrier.

 

So what are you trying to argue (without having an argument!) here? If for energy: electronic < vibrational < rotational then you could vibrational without roational (if there wasn't sufficient energy), but if you had vibrational then would you have to still have electronic?

[swansont]

I think you meant ascending order of energy (ie. energy getting bigger as you do down the list). So for energy: 1<2<3

 

And you've contradicted yourself' date=' I think. You said that vibrational energy is less than rotational, but that [i']"if vibrations do not have enough energy to overcome an activation energy barrier, then rotations certainly do not"[/i] which is illogical because rotational contains more energy, so could over the activation energy barrier.

 

So what are you trying to argue (without having an argument!) here? If for energy: electronic < vibrational < rotational then you could vibrational without roational (if there wasn't sufficient energy), but if you had vibrational then would you have to still have electronic?

 

Electronic transitions are highest energy of the three.

[swansont]

So silkworm's assertion that, "the vibrations it causes can do useful work overcomming an activation barrier" can be adressed by considering purely vibrational transitions.

 

 

It depends on what "it" is referring to in the claim. I took it to mean the photon, not the vibrational energy.

[5614]

Electronic transitions are highest energy of the three.

Ah, ok, thanks.

 

So is there still a dependancy of one on the other? Like can electronic happen without vibrational and rotational?

 

Also as electronic is the highest then surely rotational could happen without vibrational if there is limited energy?

 

But could vibrational occur without the lower energy rotational?

[Norman Albers]

I am enjoying the same confusions over in: http://www.scienceforums.net/forums/showthread.php?t=13250

[VendingMenace]

lol...stupid me. I did mean to say "more" when i wrote "less" instead. I think swansont has cleared up this confusion. Wow, that was dumb (on my part).

 

It depends on what "it" is referring to in the claim. I took it to mean the photon, not the vibrational energy.

 

I guess...though post#11 makes it pretty clear that he was talking about the vibrations of the molecule.

 

Whatever. This discussion has way outlived its usefullness, i think it is clear that vibrations and rotations are usually not of the energy required for the breaking of a chemical bond or the overcomming of a activation energy...at least not anything that anyone has thought of yet.

 

THough, i have been thinking about this and there seems to be a few cases where this might not be the case...

1) If you count the temperature being raised in solution by vibrational and rotational transitions (ie. in a microwave).

2) Perhaps in a very unstable molecule like tetrazine vibrational excitment *might* lead to decomposition. But i really don't know.

3) In redox systems that are undergoing self-exchange of electrons, if the system is strongly coupled enough, then there can be a phenomenon called vibrational coupling, where the electron transfer event can depend strongly on the vibrational modes of the molecule.

 

I think that is it for now. At least that is what i have come up with so far...

 

 

To answer 5614;

 

Vibrational transitions can be accompanied by rotational transitions.

 

and

 

Electronic transitions can be accompanied by vibronic transitions.

[5614]

Vibrational transitions can be accompanied by rotational transitions.

Yes I know they can... but do they *have* to? ie. could a vibrational transitions occur without being accompanied by rotational transitions?

 

Going back to the original question about solely vibrational or rotational transitions... surely it would be theoretically possible to have rotational transition only if there was not enough energy to induce a vibrational transition as well.

[Norman Albers]

Vending Menace, your last statement may be what I have been trying to get at.. My question was, how does visible light (alone, say) leave us warm???

[swansont]

Vending Menace, your last statement may be what I have been trying to get at.. My question was, how does visible light (alone, say) leave us warm???

 

Electronic excitation, followed by (some) nonradiative relaxation. Here is more. And here is lots more.

[Norman Albers]

Very nice, thank you!