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6960 J. Am. Chem. SOC.1984, 106, 6960-6962 Perpendicular Effects on Transition-State Structure

Summary: 6960 J. Am. Chem. SOC.1984, 106, 6960-6962
Perpendicular Effects on Transition-State Structure:
Model and Application to Cycloadditions and Sigmatropic
Noam Agmont
Contributionfrom the Department of Physical Chemistry, The Hebrew University of Jerusalem,
Jerusalem 91904, Israel. Received September 2, 1983
Abstract: Effects of structures perpendicular to the reaction coordinate can be described by a model of an upside-down free
energy profile whose minimum is constrainedto coincidewith the barrier height along the reaction coordinateand its "intrinsic
barrier" proportionalto that of the profile along the reaction coordinate. The model shows the same quality of semiquantitative
agreement with experimental results for sigmatropic shifts and cycloadditions as the models based on two-dimensional
"potential-energy surfaces". The model also predicts semiquantitatively the "tightness" of the transition state for hexadiene
derivatives, as measured by the secondary kinetic isotope effects.
"Perpendicular" substituent effects in organic reactivity have
been popularized in terms of two-dimensional potential energy
surfaces.'-I2 The profile along the reaction coordinate (RC) has
a maximum (the transition state, TS), which corresponds in a
Hammond-like fashionI3to variations in its end points (reagents
and products). Hence stabilizing the products moves the TS closer
to reactants (and lowers its energy). The profile perpendicular


Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem


Collections: Chemistry