Abstract
AbstractA procedure is suggested to estimate the positions of a transition state (TS) along the reaction coordinate in reaction series involving several radical reactions. It is based on the dependence between experimental activation energies U± and heat effects Q. Taking into account the shift of a TS position, the following relation between these quantities is obtained: U± −; U = A (Q ‐ Q0)2 + B(Q ‐ Q0). The subscript 0 indicates the standard reference reaction. For three series, namely, the hydrogen abstraction from hydrocarbon substrates by radicals CH3˙, CF˙3, and Br·, the bond lengths characterizing the TS reaction center are evaluated. The TS positions appear to vary significantly in the reaction series, which accounts for significant changes in the experimentally observed activation volumes. The derivation of the Hammond rule and the range of its validity for the series with polar and steric substituent effects are discussed.
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