Brönsted exponents and activated-complex structure: An AM1 SCF-MO theoretical simulation of a rate-equilibrium correlation for transfer of the methoxycarbonyl group between isoquinoline and substituted pyridines

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.