The AM1 SCF-MO method has been applied to simulate the transfer of a methoxycarbonyl group between isoquinoline and a range of substituted pyridines in the gas phase. The nucleophiles and leaving group were mimicked by ammonia molecules whose gas-phase basicities were modulated by suitably located dipoles of varying size. Linear rate-equilibrium correlations were obtained for reaction series involving activated complexes of variable structure. The meaning of the Brönsted exponents is discussed in regard to charge development, bond order, and a simple model of intersecting energy curves. It is concluded that linear rate-equilibrium relationships are not incompatible with the Leffler principle and Hammond postulate, and that Brönsted exponents do not measure activated-complex structure directly but do reflect the Morse curvature of the bond being made with the nucleophile.
|Number of pages||8|
|Journal||Journal of the Chemical Society, Perkin Transactions 2|
|Publication status||Published - 1 Dec 1989|
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