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

Abstract

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.