The Activation Entropy of Chemical Exchange Driven by Internal Rotation: A Computational Approach

ABSTRACT The activation entropy () of chemical exchange processes is usually determined using Nuclear Magnetic Resonance (NMR) bandshape analysis. However, the procedure is plagued with problems that render the results unreliable as demonstrated by the large uncertainties of the reported values. A computational approach to calculate using third‐law gas‐phase statistical entropies is presented here. The proposed approach is demonstrated by calculating the activation entropy of the chemical exchange driven by internal rotation in a series of amide compounds. The potential parameters were obtained using high level ab initio Molecular Orbital Theory, whereas the rotational eigenstates for the individual conformers were calculated with a suitable Hamiltonian for the internal rotation process. The gas‐phase values obtained for reproduce well previously reported experimental results for the compounds studied. The proposed method provides a complementary technique to the NMR bandshape analysis to determine the activation entropy of chemical exchange processes driven by internal rotation.