TAILIEUCHUNG - Ebook A guide to molecular mechanics and quantum chemical calculations (3e): Part 2

(BQ) Part 2 book "A guide to molecular mechanics and quantum chemical calculations" has contents: Dealing with flexible molecules; obtaining and using transition state geometries; obtaining and interpreting atomic charges; obtaining and interpreting atomic charges,.and other contents. | Chapter 14 Dealing with Flexible Molecules This chapter addresses practical issues which arise in dealing with flexible molecules. These include identification of the “important” conformer (or set of conformers) and location of this conformer. The chapter concludes with guidelines for fitting potential energy functions for bond rotation to simple Fourier series. Introduction Conformation dictates overall molecular size and shape, and influences molecular properties as well as chemical reactivity. Experimental information about conformation is often scarce, and computational methods may need to stand on their own. There are actually two different problems associated with treatment of conformationally-flexible molecules. The first is to identify the appropriate conformer (or conformers), and the second is to locate it (them). Both of these will be touched on in turn. Identifying the “Important” Conformer The equilibrium (“thermodynamic”) abundance of conformational forms depends on their relative energies. According to the Boltzmann equation, the lowest-energy conformer (global minimum) will be present in the greatest amount, the second lowest-energy conformer in the next greatest amount, and so forth.* This implies that reactions under thermodynamic control and involving conformationally-flexible reagents need to be described in terms of the properties of global * This is not strictly true where certain conformers possess elements of symmetry. Here, the number of occurences of each “unique” conformer also needs to be taken into account. 393 Chapter 14 asfd 393 3/25/03, 10:46 AM minima, or more precisely in terms of the properties of all minima weighted by their relative Boltzmann populations. The situation may be markedly different for reactions under kinetic control. Here, the lowest-energy conformer(s) of the reagent(s) may not be the one(s) involved in the reaction. A simple but obvious example of this is provided by the Diels-Alder cycloaddition .

• Toán học
• Dealing with flexible molecules
• Molecular mechanics
• Quantum chemical calculations
• Using transition state geometries
• Interpreting atomic charges
• Kinetically controlled reactions