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Updated: Dec 22, 2025

Spin Saturation Transfer Difference NMR SSTD NMR: A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
Published on: November 12, 2016
Validation of Enthalpy-Entropy Compensation Mechanism in Partial Amide Bond Rotation
Jacob Guerra1, Bhvandip Bajwa1, Prarthana Kumar1
1Department of Chemistry, California State University, Fresno, California 93740, United States.
Abstract:
The concept of enthalpy-entropy compensation (EEC) is one of the highly debated areas of thermodynamics. The conformational change due to restricted double-bond rotation shows a classic two-site chemical exchange phenomenon and has been extensively studied. Fifty-four analogs of N,N-diethyl-m-toluamide (DEET) as a model system were synthesized to study the thermodynamics of the partial amide bond character using nuclear magnetic resonance (NMR) spectroscopy. Line-shape analysis as a function of temperature is used to estimate the chemical exchange. Eyring analysis was then used to convert the chemical exchange rates to determine the transition state enthalpy and entropy of the molecules. The experimental design follows selective variations that perturb one aspect of the molecular system and its influence on the observed thermodynamic effect. The results of the study demonstrate that amide bond resonance in analogs of DEET follows an EEC mechanism. Simple modifications made to DEET's structural motif alter both the enthalpy and entropy of the system and were limited overall to a temperature compensation factor, T β = 292.20 K, 95% CI [290.66, 293.73]. We suggest EEC as a model to describe the kinetic compensation seen in chemical exchange phenomena in analogs of DEET.
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