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Enthalpy/Entropy Contributions to Conformational KIEs: Theoretical Predictions and Comparison with Experiment
Department of Chemistry, Pomona College, 645 North College Avenue, Claremont, CA 91711, USA
Department of Chemistry, University of California, Merced, Atwater, CA 95301, USA
* Authors to whom correspondence should be addressed.
Received: 4 December 2012; in revised form: 22 January 2013 / Accepted: 1 February 2013 / Published: 18 February 2013
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 ΔH/ΔS dissections.
Keywords: conformational kinetic isotope effect; steric isotope effect; Bigeleisen-Mayer formalism; enthalpy/entropy contributions to isotope effects
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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.
Fong A, Meyer MP, O'Leary DJ. Enthalpy/Entropy Contributions to Conformational KIEs: Theoretical Predictions and Comparison with Experiment. Molecules. 2013; 18(2):2281-2296.
Fong, Aaron; Meyer, Matthew P.; O'Leary, Daniel J. 2013. "Enthalpy/Entropy Contributions to Conformational KIEs: Theoretical Predictions and Comparison with Experiment." Molecules 18, no. 2: 2281-2296.