ON THE CALCULATION OF TRANSITION STATE ACTIVITY COEFFICIENT AND SOLVENT EFFECTS ON CHEMICAL REACTIONS Alvaro DOMINGUEZ, Rafael JIMENEZ, Pilar LOPEZ-CORNEJO, Pilar PEREZ and Francisco SANCHEZ 1, * Department of Physical Chemistry, Seville University, c/Professor Garcia Gonzales s/n, 41012, Sevilla, Spain; e-mail: 1 amurillo@cica.es Received March 2, 1998 Accepted August 14, 1998 Solvent effects, when the classical transition state theory (TST) holds, can be interpreted following the Brønsted equation. However, when calculating the activity coefficient of the transition state, γ ≠ it is important to take into account that this coefficient is different from that of the precursor complex, γ PC . The activity coefficient of the latter is, in fact, that calculated in classical treatments of salt and solvent effects. In this paper it is shown how the quotients γ ≠ /γ PC change when the reaction medium changes. Therefore, the conclusions taken on the basis of classical treatments may be erroneous. Key words: Transition states; Precursor complex; Medium effect; Reaction kinetics; Activity coeffi- cients; Solvent effects. In order to analyze solvent effects on chemical reactivity it is important to realize that for processes in solution there are, at least, three steps: A + B PC (1) PC X ≠ SC (2) SC C + D . (3) The first step, described by Eq. (1), represents the formation of the precursor com- plex (PC) or encounter complex from reactants, the second one, expressed by Eq. ( 2), the activation of the precursor complex and the formation of the successor complex (SC) from the transition state (this step is globally characterized by a rate constant k, see Eq. ( 17), and the third one, described by Eq. (3), the separation of the successor complex to give the products. k –1 k 1 Calculation of Transition State Activity Coefficient 1969 Collect. Czech. Chem. Commun. (Vol. 63) (1998) * The author to whom correspondence should be addressed.