isokinetic relationship

When a series of structurally related substrates undergo the same general reaction or when the reaction conditions for a single substrate are changed in a systematic way, the enthalpies and entropies of activation sometimes satisfy the relation:
Δ ‡ H − β Δ ‡ S = constant
where the parameter β is independent of temperature. This equation (or some equivalent form) is said to represent an 'isokinetic relationship'. The temperature T = β (at which all members of a series obeying the isokinetic relationship react at the same rate) is termed the 'isokinetic temperature'. Supposed isokinetic relationships as established by direct correlation of Δ ‡ H with Δ ‡ S are often spurious and the calculated value of β is meaningless, because errors in Δ ‡ H lead to compensating errors in Δ ‡ S. Satisfactory methods of establishing such relationships have been devised.
See also: compensation effect, isoequilibrium relationship, isoselective relationship
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1129