Role of solvent dielectric constant on the enthalpy-entropy compensation in the hindered amide bond rotation

The purpose of this investigation is to determine if enthalpy-entropy compensation (EEC) is a valid concept when the experiments are performed on a set of molecules in multiple solvents. Dynamic nuclear magnetic resonance (DNMR) experiments are performed on a series of analogs derived from N, N-diethyl-m-toluamide (DEET) in six different solvents of varying dielectric constant from toluene‑d₈ (ε = 2.38) to dimethyl sulfoxide-d₆ (ε = 46.7). The thermodynamic parameters of the restricted rotation to the solvent dielectric constant do not have a straightforward correlation relationship between them. However, the activation enthalpy and entropy of more than sixty different experimental measurements indicate that enthalpy-entropy compensation (EEC) is independent of the solvent condition with a compensation temperature of 287.81 ± 3.95 K. The results, therefore, support the broader role of EEC on the partial amide bond dynamics in the DEET analogs over the temperature range studied.