Determination of Transition Structures and Reaction Pathways

Chemists are interested not only in the thermodynamics of a process (the relative stability of the various species) but also in its kinetics (the rate of conversion from one structure to another). JCnowledge of the minimum points on an energy surface enables thermodynamic data to be interpreted, but for the kinetics it is necessary to investigate the nature of the 

Fig 5 21. The energy profile for the gas-phase Cl + MeCl reaction (Adapted in part from Chandrasekhar, S F Smith and W L Jorgensen 1985 Theoretical Examination of the Sf 2 Reaction Involving Chloride Ion and Methyl Chloride in the Gas Phase and Aqueous Solution Journal of the American Qienucal Society 107154-163.) 

Fig 5 22 Schematic representation of some of the lower frequencies in the ion-dipole complex for the Cl + MeCl reaction and the imaginary frequency of the transition structure, calculated using a 6-31G basis set. 

Fig 5 23 The transition structure fin- theH - - CH3CH2 — H2 + CH2=CH2 reaction. At low temperature the transition structure corresponds to the transition state (maximum cffree energy) At high temperature the transition state moves closer to the products, as can be seen from the graph. (Redrawn from Doubleday C, Mclver, M Page and T Zielinski 1985. Temperature Dependence of the Transition-State Structure for the Disproportionation of Hydrogen Atom with Ethyl Radical. Journal of the American Chemical Society 107 5800-5801) 

The eigenvalues of this matrix are obtained by setting the secular determinant to zero  

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