kinetic isotope effect

Here is a clever detective trick chemists use to see inside a mechanism. If you replace an ordinary hydrogen atom in a molecule with deuterium (heavy hydrogen, which is chemically almost identical but twice as heavy) and the reaction suddenly slows down, that tells you the bond to that hydrogen is being broken in the slow step. This change in rate upon swapping isotopes is the kinetic isotope effect.

The reason lies in subtle physics. A C-D bond sits at slightly lower energy than a C-H bond (because the heavier deuterium vibrates more sluggishly), so it takes a bit more energy to break, giving a higher activation barrier and a slower reaction. The effect is large, the C-H reaction running up to about six or seven times faster than the C-D one, only when that bond is actually breaking in the rate-determining step. This is called a primary kinetic isotope effect. If the labeled bond merely sits nearby but is not broken, the effect is small (a secondary KIE) or absent.

The kinetic isotope effect is one of the sharpest experimental probes of mechanism. A large primary effect is strong evidence that C-H bond cleavage happens in the slow step, which is exactly how chemists distinguish, say, an E2 elimination (which breaks a C-H bond in its single slow step, large KIE) from an E1 (which does not, small KIE). It turns an invisible question about timing into a measurable number, letting chemists peer at which bond breaks when.