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.

Source:

PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1129