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

Robert B. Hammond, Ian H. Williams

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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.

Original languageEnglish
Pages (from-to)59-66
Number of pages8
JournalJournal of the Chemical Society, Perkin Transactions 2
Volume1989
Issue number1
DOIs
Publication statusPublished - 1 Dec 1989

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

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title = "Br{\"o}nsted 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 Br{\"o}nsted 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 Br{\"o}nsted exponents do not measure activated-complex structure directly but do reflect the Morse curvature of the bond being made with the nucleophile.",
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T2 - An AM1 SCF-MO theoretical simulation of a rate-equilibrium correlation for transfer of the methoxycarbonyl group between isoquinoline and substituted pyridines

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AU - Williams, Ian H.

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N2 - 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.

AB - 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.

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