Conformations of saturated five-membered heterocycles evaluated by MP2 calculations

The conformation of saturated heterocyclic rings is often a subject of computational investigations. In spite of their simple molecular structure, study of their conformation is often hindered by the high sensitivity for the type of the accepted potential of interactions and limitations inherent to introducing artificial constraints. Here we report the results of the MP2 calculations carried out within the aug-cc-pVTZ basis set and aimed to elucidate the basic conformational properties of a series of prototypic heterocyclic compounds, containing saturated five-membered rings. We have considered: oxolane, thiolane, pyrrolidine, 1,3-dioxolane, 1,2-oxathiolane, 1,3-oxathiolane pyrazolidine, imidazolidine, and cyclopentane (the latter included for comparative purposes). We have identified the local and global minima on the potential energy surface along the pseudorotation path. Moreover, the relative energies of structural and geometrical isomers were estimated. The results reveal a large degree of conformational heterogeneity of the studied compounds which always exhibit more than one type of conformation corresponding to the minimum of the energy. The information about the topology of the given compound is insufficient to predict its conformational properties, including the type of favorable ring shape. However, the magnitude of the out-of-plane deviations exhibit similar range of values for most of the studied compounds. The calculated puckering parameters will provide the essential data for the next stage of related studies exploiting the machine learning methodology.