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 occupancies17 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