Stereoelectronic Effects on the Conformational Properties of 1,3-Dioxane, 1,3-Dithiane, and 1,3-Diselenane: An Ab Initio Study and NBO Analysis

Abstract

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 C─M 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 σ _{C2− M3} antibonding orbitals, decrease form 1 to 3. The LP(e)_M1→ σ _{C2− M3} resonance energies for compounds 1–3 are 12.66, 6.73 and 5.33 kcal mol⁻¹, respectively. The LP(e)_M1→ σ _{C2− M3} delocalizations could fairly explain the decrease of occupancies of LP(e)_M1 non-bonding orbitals and the increase of occupancies of σ _{C2− M3} anti-bonding orbitals, from compound 1 to 3. The electronic delocalization from LP(e)_M1 non-bonding orbitals to σ _{C2− M3} anti-bonding orbitals increase the ground state structure stability, therefore, the decrease of LP(e)_M1→ σ _{C2− M3} delocalizations could fairly explain the easiness of ring flipping processes from compound 1 to 3.

Keywords:

• Ab initio
• molecular modeling
• NBO
• 1,3-dioxane
• 1,3-dithiane
• 1,3-diselenane

Notes

^a Relative to the ground state

^b numbers in parenthesis are the corresponding Δ E values in kcal mol⁻¹

^c corrected by multiplying by a scaling factor (0.9804)

^d experimental value [see Refs. 19,33]