Ab initio molecular orbital calculations of 3,4-dihydro-1,2-dioxin, 3,6-dihydro-1,2-dioxin, 4H-1,3-dioxin (1,3-diox-4-ene), and 2,3-dihydro-1,4-dioxin (1,4-dioxene)

Optimized geometries and total energies for 3,4-dihydro-1,2-dioxin ( 1 ), 3,6-dihydro-1,2-dioxin ( 2 ), 4H-1,3-dioxin (1,3-diox-4-ene, 3 ), and 2,3-dihydro-1,4-dioxin (1,4-dioxene, 4 ) were calculated using ab initio 3-21G, 6-31G*, and MP2/6-31G*//6-31G* methods. The half-chair conformers of 1 (C₁), 2 (C₂), 3 (C₁), and 4 (C₂) are more stable than their respective planar structures 1 (C_s), 2 (C_2v), 3 (C_s), and 4 (C_2v)]. Among the four isomers 1 – 4 , the half-chair conformer of 3 is the most stable. It is 53.1, 54.6, and 3.4 kcal mol⁻¹ more stable than 1 , 2 , and 4 , respectively. The largest energy difference (19.0 kcal mol⁻¹) is observed between the half-chair and planar conformers of 2 . The boat conformers of 2 and 4 are less stable than their respective half-chair conformers, but are more stable than their planar structures. Hyperconjugative orbital interactions (anomeric effects) contribute to the greater stability of 3 (n_O(3) →σ*_C(2)—O(1), n_O(3)→σ*,n _O(3)→σ*) and of 4 (n_O(1)→ σ*). The ab initio calculated structural features of the half-chair conformations of the dihydrodioxins 1 – 4 are compared with the half-chair conformations of cyclohexene and the chair conformations of cyclohexane, oxacyclohexane (tetrahydropyran), 1,2-dioxacyclohexane (1,2-dioxane), 1,3-dioxacyclohexane (1,3-dioxane), and 1,4-dioxacyclohexane (1,4-dioxane) © 1997 by John Wiley & Sons, Inc. J Comput Chem 18 : 1392–1406, 1997