Editing Kinetic isotope effect (section)

=== Primary kinetic isotope effects ===
A '''primary kinetic isotope effect''' (PKIE) may be found when a bond to the isotopically labeled atom is being formed or broken.<ref name="Atkins" /><ref name="Laidler_1987" />{{rp|427}} Depending on the way a KIE is probed (parallel measurement of rates vs. intermolecular competition vs. intramolecular competition), the observation of a PKIE is indicative of breaking/forming a bond to the isotope at the rate-limiting step, or subsequent product-determining step(s). (The misconception that a PKIE must reflect bond cleavage/formation to the isotope at the rate-limiting step is often repeated in textbooks and the primary literature: ''see the section on [[#Experiments|experiments]] below.'')<ref name="Simmons_2012" />

For the aforementioned nucleophilic substitution reactions, PKIEs have been investigated for both the leaving groups, the nucleophiles, and the α-carbon at which the substitution occurs. Interpretation of the leaving group KIEs was difficult at first due to significant contributions from temperature independent factors. KIEs at the α-carbon can be used to develop some understanding into the symmetry of the transition state in S{{sub|N}}2 reactions, though this KIE is less sensitive than what would be ideal, also due to contribution from non-vibrational factors.<ref name="Westaway1" />