(BQ) Part 2 book "A guide to molecular mechanics and quantum chemical calculations" has contents: Obtaining and interpreting atomic charges; kinetically controlled reactions, applications of graphical models, obtaining and using transition state geometries. | Chapter 15 Obtaining and Using Transition-State Geometries This chapter addresses practical issues associated with establishing, verifying and using transition-state geometries. It outlines a number of practical strategies for finding transition states, and provides criteria for establishing whether or not a particular geometry actually corresponds to the transition state of interest. Most of the remainder of the chapter focuses on choice of transition-state geometry, and in particular, errors introduced by using transition-state (and reactant) geometries from one model for activation energy calculations with another (“better”) model. The chapter concludes with a discussion of “reactions without transition states”. Introduction The usual picture of a chemical reaction is in terms of a onedimensional potential energy (or reaction coordinate) diagram. transition state energy (E) reactants products reaction coordinate (R) The vertical axis corresponds to the energy of the system and the horizontal axis (the “reaction coordinate”) corresponds to the 409 Chapter 15 adf 409 3/25/03, 10:48 AM geometry of the system. The starting point on the diagram (“reactants”) is an energy minimum, as is the ending point (“products”). In this diagram, the energy of the reactants is higher than that of the products (an “exothermic reaction”) although this does not need to be the case. The energy of the reactants can be lower than that of the products (an “endothermic reaction”), or reactant and product energies may be the same (a “thermoneutral reaction”) either by coincidence or because the reactants and products are the same molecule (a “degenerate reaction”). Motion along the reaction coordinate is assumed to be continuous and pass through a single energy maximum (the “transition state”). According to transitionstate theory, the height of the transition state above the reactant relates to the overall rate of reaction (see Chapter 9). Reactants, products and transition state
