Stereoelectronic effects on the conformational properties of 1,3-dioxane, 1,3-dithiane, and 1,3-diselenane: An ab initio study and NBO analysis

NBO analysis, density functional theory (DFT: B3LYP/6-31G*//B3LYP/6-31G*) and ab initio molecular orbital (MO: MP2/6-31G*//B3LYP/6-31G*) based methods were used to study the stereoeletronic effects on the conformational properties of 1,3-dioxane (1), 1,3-dithiane (2) and 1,3-diselenane (3). The obtained B3LYP/6-31G*//B3LYP/6-31G* and MP2/6-31G*//B3LYP/6-31G* results show that the ring flipping barrier heights [via rotation about CM bond, (M O (1), M S (2) and M Se (3))] decrease from compound 1 to compound 3. Based on the optimized ground state geometries using B3LYP/6-31G** method, the NBO analysis of donor-acceptor (bond-antibond) interactions revealed that the stabilization energies associated with the electronic delocalization from the equatorial non-bonding Lone Pair orbitals [LP(e)(M1)] to sigma*(C2-M3) antibonding orbitals, decrease form 1 to 3. The LP(e)(M1) -> sigma*(C2-M3) resonance energies for compounds 1-3 are 12.66, 6.73 and 5.33 kcal mol(-1), respectively. The LP(e)(M1) -> sigma*(C2-M3) delocalizations could fairly explain the decrease of occupancies of LP(e)(M1) non-bonding orbitals and the increase of occupancies of sigma*(C2-M3) anti-bonding orbitals, from compound 1 to 3. The electronic delocalization from LP(e)(M1) non-bonding orbitals to sigma*(C2-M3) anti-bonding orbitals increase the ground state structure stability, therefore, the decrease of LP(e)(M1) -> sigma*(C2-M3) delocalizations could fairly explain the easiness of ring flipping processes from compound 1 to 3.