ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 9, pp. 1505–1515. © Pleiades Publishing, Ltd., 2011. Published in Russian in Zhurnal Fizicheskoi Khimii, 2011, Vol. 85, No. 9, pp. 1621–1632. 1505 INTRODUCTION Kinetic and activation parameters (Е а , k(Т), ΔH # , ΔS # , and ΔG # ) are one set of tools for the mechanistic study of chemical reactions. If we perform a detailed analysis of the experimental data for a series of related reactions with variation of a certain reaction parame- ter (e.g., the polarity or solvating ability of the solvent, the electronic or steric effects of the substituents, the pressure), the result could be the detection of phe- nomena known as the isokinetic relationship (IKR) and the enthalpy–entropy compensation effect (EEC). There are many works in which various IKR/EEC concepts and interpretations are com- pared, detailed theoretical proofs and algorithms for the verification of these effects are produced (with examples of their pseudo-existence when mandatory conditions seem to be met), and the mathematical and statistical aspects needed to interpret IKRs and the EEC are considered [1–5]. Vlasov’s work, in which the much of the literature data on the activation parameters of bimolecular nucleophilic reactions are generalized and systematized, can be very useful in understanding both the correlation between a reaction mechanism and its activation parameters, and the suitability of the EEC in performing comparative anal- ysis of various mechanisms [6]. Linear correlation ln k 0, i –E a, i , for a series of related reactions is the first criterion of an IKR for which there is an isokinetic temperature (T iso ) when all stud- ied reactions proceed at equal rates. The evaluated T iso Activation Parameters of Supercritical and Gas-Phase β-Pinene Thermal Isomerization 1 A. M. Chibiryaev a, b , A. Yermakova c , and I. V. Kozhevnikov b, c a Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia b Department of Natural Sciences, Novosibirsk State University, Novosibirsk, 630090 Russia c Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia e-mail: chibirv@nioch.nsc.ru Received September 6, 2010 Abstract—New data on enthalpy and entropy contributions to the energy barrier of β-pinene thermal isomerization were obtained. The rate of β-pinene conversion is higher in supercritical EtOH (P = 120 atm) than in the gas phase (P ≤ 1 atm, without solvent, or for inert carrier gas N 2 ) at equal temperatures. The high- est activation energy E Σ of total β-pinene conversion is also observed in reactions in the supercritical (sc) con- dition. Activation parameters , , and depend strongly on the reaction pressure. Thus, at P ≤ 1 atm (gas-phase reaction) the values of are negative, while at sc conditions at P = 120 atm is positive. The linear dependences ln k Σ0 – E Σ and – indicate an isokinetic relation (IKR) and enthalpy–entropy compensation effect (EEC). The isokinetic temperature was calculated (T iso = 605.5 ± 22.7 K). It was shown that elevation of temperature reduces the value of (T) upon sc thermolysis only, whereas in all gas-phase reactions (T) increases. At equal reaction temperatures, the greatest values of (T) proved to be typical for thermolysis in sc-EtOH. We hypothesize that the rate of total β-pinene conversion increases dramatically due to a considerable shift in equilibrium toward higher concentrations of activated complex . A detailed analysis of activation parameters shows that the IKR and EEC coincide, evidence of a common mechanism of β-pinene conversion observed under different reaction conditions, including thermolysis in sc-EtOH. Keywords: β-Pinene thermal isomerization, activation parameters, pressure dependence, isokinetic relation- ship, enthalpy-entropy compensation effect. DOI: 10.1134/S0036024411090068 ΔH Σ # ΔS Σ # ΔG Σ # ΔS Σ # Δ H Σ # ΔS Σ # ΔG Σ # ΔG Σ # K eq # y TS # CHEMICAL THERMODYNAMICS AND THERMOCHEMISTRY 1 This article was translated by the authors.