Computational study of ring strain in 1,3,2-dioxathiolane, its 2-oxide andits 2,2-dioxide

The strain energies in the five-membered rings 1,3,2-dioxathiolane (1), its 2-oxide (2) and 2,2-dioxide (3) have been evaluated using B3LYP and QCISD( T)//MP2 calculations and the standard 6-31G* basis set. Two different model s have been used, one based on an isodesmic reaction involving methane and ethane, and one applying the chair-form of cyclohexane as a strain-free ref erence. Thermally corrected strain energies obtained by B3LYP and with cycl ohexane as a reference are 10.1, -1.3 and 5.8 kcal mol(-1) for 1, 2 and 3, respectively. The corresponding values obtained by QCI calculations are 8.1 , -2.1 and 4.4 kcal mol(-1), respectively. The negative value for 2 is inte rpreted in terms of absence of destabilizing interactions present in 1 and 3.