My understanding of exothermic reactions is that, in the aggregate, the sum of the bond energies of the products is lower than that of the reactants, and definitely lower than that of the sum of any excited state intermediates, and that the resulting difference in energy is accounted for by a decrease in potential energy for the bonding electrons involved, resulting in a release of photons of equivalent energy.

Seeing as how exothermic reactions generally raise the temperature of the environment they are in, it seems logical to me that those photons are rapidly absorbed (inelastically) by atoms/molecules immediately neighboring the reaction products and thus their kinetic energies are raised, although I honestly don't truly understand the mechanism for that, either.

Similarly, for reactions that are isothermal but which are chemiluminescent, photons of a wavelength not easily absorbed by neighboring atoms/molecules simply leave the environment.

However, does that mean that the inverse is true? Is IR radiation from the environment the direct mediator of energy transfer that raises bonding electrons in reactants to anti-bonding MOs so that new MOs can form among different configurations of the original reactant atoms, thus allowing electrons to occupy the bonding MOs of the products?

I ask because the basic collision theory proposed by Trautz and Lewis doesn't seem to account for quantum mechanical models of energy, but I admit my own understanding of the topic is about as limited as my question would demonstrate.

  • 1
    $\begingroup$ Within the BO approximation, collisions can transfer energy into vibrational modes (and vice versa). At thermal equilibrium a small percentage of molecules will have enough vibrational energy in a certain bond to break it. $\endgroup$
    – Jan Jensen
    Commented May 21, 2017 at 9:35

1 Answer 1


In your first paragraph you state at the end that photons are released. In all reactions, except the rather few chemiluminescent ones, this is not the case.

The energy difference is made up of vibrational and rotational energy of the products, which by collisions with solvent (or inert buffer gas) removes this energy. This energy transfer occurs as the product is initially 'hot' with respect to the surroundings and thus energy flows out of this into the surroundings, heating it a little, until equilibrium is reached. In femtosecond laser experiments it is possible to observe the flow of energy when in solution and it is very rapid, not much longer than a few picoseconds. Ir photons could be emitted but the radiative lifetime of levels is much, much longer than collision lifetime so that i.r. emission yield will be tiny.

In the low pressure gas phase (where collisions with buffer gas are absent) infra-red can be observed (called infra-red chemiluminescence as it comes from excited vibrational levels) in reactions such as $\ce{H + H2 \rightarrow HF(v'J') + H} $ where $\ce{v'J'}$ are excited vibrational and rotational levels. Many experiments have been reported by Polanyi and co workers for which Polanyi received the Chemistry Nobel prize. This reference https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1986/polanyi-lecture.pdf gives a clear account of this important work

In the reverse case it is the few molecules that by the Boltzmann distribution have enough energy to surmount the activation barrier that are important not promotion by ir photons.


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