barriers above and beyond any thermodynamic energy differences. The enthalpies of formation of reactants and products contain the information about the reaction 's overall energy balance and need to be considered independently of the kind of orbital symmetry analysis just introduced As the above example illustrates, whether a chemical reaction occurs on the ground or an excited-state electronic surface is important to be aware of. This example shows that one might want to photo-excite the reactant molecules to cause the reaction to occur at an accelerated rate. with the electrons occupying the lowest-energy orbitals, the ring closure reaction can still occur, but it has to surmount a barrier to do so(it can employ thermal collisional energy to surmount this barrier), so its rate might be slow. If an electron is excited there is no symmetry barrier to surmount, so the rate can be greater. Reactions that take place on excited states also have a chance to produc products in excited electronic states, and such excited-state products may emit light. Such reactions are called chemiluminescent because they produce light (luminescence) by way of a chemical reaction Rates of change Rates of reactions play crucial roles in many aspects of our lives. Rates of various biological reactions determine how fast we metabolize food and rates at which fuels burn in air determine whether an explosion or a calm flame will result. Chemists view the rate of any reaction among molecules(and perhaps photons or electrons if they are used to induce excitation in reactant molecules)to be related to(1)the frequency with which the reacting species encounter one another and(2)the probability that a set of such species
16 barriers above and beyond any thermodynamic energy differences. The enthalpies of formation of reactants and products contain the information about the reaction's overall energy balance and need to be considered independently of the kind of orbital symmetry analysis just introduced. As the above example illustrates, whether a chemical reaction occurs on the ground or an excited-state electronic surface is important to be aware of. This example shows that one might want to photo-excite the reactant molecules to cause the reaction to occur at an accelerated rate. With the electrons occupying the lowest-energy orbitals, the ring closure reaction can still occur, but it has to surmount a barrier to do so (it can employ thermal collisional energy to surmount this barrier), so its rate might be slow. If an electron is excited, there is no symmetry barrier to surmount, so the rate can be greater. Reactions that take place on excited states also have a chance to produce products in excited electronic states, and such excited-state products may emit light. Such reactions are called chemiluminescent because they produce light (luminescence) by way of a chemical reaction. 4. Rates of change Rates of reactions play crucial roles in many aspects of our lives. Rates of various biological reactions determine how fast we metabolize food, and rates at which fuels burn in air determine whether an explosion or a calm flame will result. Chemists view the rate of any reaction among molecules (and perhaps photons or electrons if they are used to induce excitation in reactant molecules) to be related to (1) the frequency with which the reacting species encounter one another and (2) the probability that a set of such species
will react once they do encounter one another. The former aspects relate primarily to the concentrations of the reacting species and the speeds with which they are moving. The latter have more to do with whether the encountering species collide in a favorable orientation(e.g, do the enzyme and substrate"dock"properly, or does the Br ion collide with the H,c-end of H, c-Cl or with the Cl end in the sn2 reaction that yields CH, br+ Cr?)and with sufficient energy to surmount any barrier that must be passed to effect breaking bonds in reactants to form new bonds in products The rates of reactions can be altered by changing the concentrations of the reacting species, by changing the temperature, or by adding a catalyst Concentrations and temperature control the collision rates among molecules, and temperature al controls the energy available to surmount barriers. Catalysts are molecules that are not consumed during the reaction but which cause the rate of the reaction to be increased (species that slow the rate of a reaction are called inhibitors ). Most catalysts act by providing orbitals of their own that interact with the reacting molecules orbitals to cause the energies of the latter to be lowered as the reaction proceeds In the ring-closure reaction cited earlier, the catalysts orbitals would interact (i.e, overlap) with the 1, 3- butadiene's T orbitals in a manner that lowers their enrgies and thus reduces the energy barrier that must be overcome for reaction to proceed In addition to being capable of determining the geometries(bond lengths and angles) energies, vibrational frequencies of species such as the isomers of arginine discussed above, theory also addresses questions of how and how fast transitions among these isomers occur. The issue of how chemical reactions occur focuses on the mechanism of the reaction, meaning how the nuclei move and how the electronic orbital occupancies
17 will react once they do encounter one another. The former aspects relate primarily to the concentrations of the reacting species and the speeds with which they are moving. The latter have more to do with whether the encountering species collide in a favorable orientation (e.g., do the enzyme and substrate “dock” properly, or does the Br- ion collide with the H3C- end of H3C-Cl or with the Cl end in the SN2 reaction that yields CH3Br + Cl- ?) and with sufficient energy to surmount any barrier that must be passed to effect breaking bonds in reactants to form new bonds in products. The rates of reactions can be altered by changing the concentrations of the reacting species, by changing the temperature, or by adding a catalyst. Concentrations and temperature control the collision rates among molecules, and temperature also controls the energy available to surmount barriers. Catalysts are molecules that are not consumed during the reaction but which cause the rate of the reaction to be increased (species that slow the rate of a reaction are called inhibitors). Most catalysts act by providing orbitals of their own that interact with the reacting molecules’ orbitals to cause the energies of the latter to be lowered as the reaction proceeds. In the ring-closure reaction cited earlier, the catalyst’s orbitals would interact (i.e., overlap) with the 1,3- butadiene’s p orbitals in a manner that lowers their enrgies and thus reduces the energy barrier that must be overcome for reaction to proceed In addition to being capable of determining the geometries (bond lengths and angles), energies, vibrational frequencies of species such as the isomers of arginine discussed above, theory also addresses questions of how and how fast transitions among these isomers occur. The issue of how chemical reactions occur focuses on the mechanism of the reaction, meaning how the nuclei move and how the electronic orbital occupancies
change as the system evolves from reactants to products. In a sense, understanding the mechanism of a reaction in detail amounts to having a mental moving picture of how the atoms and electrons move as the reaction is occurring The issue of how fast reactions occur relates to the rates of chemical reactions In most cases, reaction rates are determined by the frequency with which the reacting molecules access a" critical geometry"(called the transition state or activated complex) near which bond breaking and bond forming takes place. The reacting molecules potential energy along the path connecting reactants through a transition state to produces is often represented as shown in Fig. 5.7 Region of activated complex Reactants Products igure 5.7 Energy vs reaction progress plot showing the transition state or activated complex and the activation energy
18 change as the system evolves from reactants to products. In a sense, understanding the mechanism of a reaction in detail amounts to having a mental moving picture of how the atoms and electrons move as the reaction is occurring. The issue of how fast reactions occur relates to the rates of chemical reactions. In most cases, reaction rates are determined by the frequency with which the reacting molecules access a “critical geometry” (called the transition state or activated complex) near which bond breaking and bond forming takes place. The reacting molecules’ potential energy along the path connecting reactants through a transition state to produces is often represented as shown in Fig. 5.7. Figure 5.7 Energy vs. reaction progress plot showing the transition state or activated complex and the activation energy
In this figure, the potential energy (i.e, the electronic energy without the nuclei's kinetic energy included)is plotted along a coordinate connecting reactants to products The geometries and energies of the reactants, products, and of the activated complex can be determined using the potential energy surface searching methods discussed briefl above and detailed earlier in the Background Material. Chapter 8 provides more information about the theory of reaction rates and how such rates depend upon geometrical, energetic, and vibrational properties of the reacting molecules The frequencies with which the transition state is accessed are determined by the amount of energy (termed the activation energy e*)needed to access this critic geometry. For systems at or near thermal equilbrium, the probability of the molecule gaining energy e* is shown for three temperatures in Fig. 5.8 Intermediate temperature High temperature
19 In this figure, the potential energy (i.e., the electronic energy without the nuclei’s kinetic energy included) is plotted along a coordinate connecting reactants to products. The geometries and energies of the reactants, products, and of the activated complex can be determined using the potential energy surface searching methods discussed briefly above and detailed earlier in the Background Material. Chapter 8 provides more information about the theory of reaction rates and how such rates depend upon geometrical, energetic, and vibrational properties of the reacting molecules. The frequencies with which the transition state is accessed are determined by the amount of energy (termed the activation energy E*) needed to access this critical geometry. For systems at or near thermal equilbrium, the probability of the molecule gaining energy E* is shown for three temperatures in Fig. 5.8
Figure 5.8 Distributions of energies at various temperatures For such cases, chemical reaction rates usually display a temperature depende characterized by linear plots of In(k)vs 1/T. Of course, not all reactions involve molecules that have been prepared at or near thermal equilibrium. For example, in supersonic molecular beam experiments, the kinetic energy distribution of the colliding molecules is more likely to be of the type shown in Fig. 5.9. -sonIc ozzie Maxwell- Boltzmann distribution Molecular speed igure 5.9 Molecular speed distributions in thermal and super-sonic beam cases In this figure, the probability is plotted as a function of the relative speed with which
20 Figure 5.8 Distributions of energies at various temperatures. For such cases, chemical reaction rates usually display a temperature dependence characterized by linear plots of ln(k) vs 1/T. Of course, not all reactions involve molecules that have been prepared at or near thermal equilibrium. For example, in supersonic molecular beam experiments, the kinetic energy distribution of the colliding molecules is more likely to be of the type shown in Fig. 5.9. Figure 5.9 Molecular speed distributions in thermal and super-sonic beam cases. In this figure, the probability is plotted as a function of the relative speed with which