Applicability of the hindered rotor scheme to the puckering mode in four-membered rings.

The hindered rotor scheme, originally developed for internal rotors in flexible chains (Van Speybroeck, V.; Van Neck, D.; Waroquier, M.; Wauters, S.; Saeys, M.; Marin, G. B. J. Phys. Chem. A 2000, 104, 10939), is extended to puckering motions in four-membered rings. The applicability of the approach is tested in a variety of heterocyclic compounds for which the partition function, entropy, and heat capacity are calculated. The entropy may be substantially altered by a correct description of the puckering mode. The equilibrium puckering angle ranges between 0 degrees and 30 degrees depending on the heterosubstitution X (CH2, O, S, NH, PH, C=O, C=S, C=NH, C=PH) in the four-membered ring.