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Molecules 2013, 18(2), 2281-2296; doi:10.3390/molecules18022281

Enthalpy/Entropy Contributions to Conformational KIEs: Theoretical Predictions and Comparison with Experiment

1
Department of Chemistry, Pomona College, 645 North College Avenue, Claremont, CA 91711, USA
2
Department of Chemistry, University of California, Merced, Atwater, CA 95301, USA
*
Authors to whom correspondence should be addressed.
Received: 4 December 2012 / Revised: 22 January 2013 / Accepted: 1 February 2013 / Published: 18 February 2013
(This article belongs to the Special Issue Isotope Effects)
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Abstract

Previous theoretical studies of Mislow’s doubly-bridged biphenyl ketone 1 and dihydrodimethylphenanthrene 2 have determined significant entropic contributions to their normal (1) and inverse (2) conformational kinetic isotope effects (CKIEs). To broaden our investigation, we have used density functional methods to characterize the potential energy surfaces and vibrational frequencies for ground and transition structures of additional systems with measured CKIEs, including [2.2]-metaparacyclophane-d (3), 1,1'-binaphthyl (4), 2,2'-dibromo-[1,1'-biphenyl]-4,4'-dicarboxylic acid (5), and the 2-(N,N,N-trimethyl)-2'-(N,N-dimethyl)-diaminobiphenyl cation (6). We have also computed CKIEs in a number of systems whose experimental CKIEs are unknown. These include analogs of 1 in which the C=O groups have been replaced with CH2 (7), O (8), and S (9) atoms and ring-expanded variants of 2 containing CH2 (10), O (11), S (12), or C=O (13) groups. Vibrational entropy contributes to the CKIEs in all of these systems with the exception of cyclophane 3, whose isotope effect is predicted to be purely enthalpic in origin and whose Bigeleisen-Mayer ZPE term is equivalent to ΔΔ H. There is variable correspondence between these terms in the other molecules studied, thus identifying additional examples of systems in which the Bigeleisen-Mayer formalism does not correlate with ΔHS dissections.
Keywords: conformational kinetic isotope effect; steric isotope effect; Bigeleisen-Mayer formalism; enthalpy/entropy contributions to isotope effects conformational kinetic isotope effect; steric isotope effect; Bigeleisen-Mayer formalism; enthalpy/entropy contributions to isotope effects
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Fong, A.; Meyer, M.P.; O'Leary, D.J. Enthalpy/Entropy Contributions to Conformational KIEs: Theoretical Predictions and Comparison with Experiment. Molecules 2013, 18, 2281-2296.

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