Transient kinetic isotope fractionation

Okay, so first we need to understand what "isotope" means. Do you remember learning about atoms and how they have a nucleus with protons and neutrons? Well, different elements can have different numbers of neutrons in their nuclei, and this is what we call isotopes. For example, carbon can have 6 or 7 neutrons, so we call these carbon-12 and carbon-13, respectively.

Now, when certain chemical reactions happen really fast, we call them "transient." And sometimes, when an isotope is involved in one of these reactions, it can behave differently than the other isotopes of that element. This is what we call "kinetic isotope fractionation."

For example, imagine you have a bunch of carbon dioxide (CO2) molecules, and you want to see how fast they get broken down into carbon monoxide (CO) and oxygen (O2). If you label some of the CO2 molecules with carbon-13 instead of carbon-12, you might find that they get broken down at a slightly different rate than the normal CO2 molecules. This is because the carbon-13 isotope is slightly heavier than the carbon-12, and this can affect how quickly it moves and reacts in the reaction.

So "transient kinetic isotope fractionation" is just a fancy way of saying that sometimes, when certain chemical reactions happen really fast, different isotopes of the same element can behave differently and get separated from each other. This can tell us a lot about how the reaction is happening and what's going on at the molecular level.