Ab initio study and NBO analysis of configurational and conformational properties of cyclododeca-1,2,7,8-tetraene

Abstract

The results concerning an investigation employing the ab initio molecular orbital (MO) and density functional theory (DFT) methods to calculate structural optimization and conformational interconversion pathways for the two diastereoisomeric forms, (±) and meso configurations of cyclododeca-1,2,7,8-tetraene ( 1) are reported in this work. Two axial symmetrical conformations are found for (±)- 1 configuration. (±)- 1-TB axial symmetrical form is found to be about 4.97 and 4.48 kcal mol −1 more stable than (±)- 1-crown axial symmetrical conformation, as calculated by HF/6-31G ∗//HF/6-31G ∗ and B3LYP/6-31G ∗//HF/6-31G ∗ levels of theory, respectively. The results show that the interconversion of (±)- 1-TB and (±)- 1-crown conformations can take place via (±)- 1-T unsymmetrical twist minimum geometry. Conformational interconversion barrier height between (±)- 1-TB and (±)- 1-T forms is found to be 11.48 kcal mol −1, as calculated by B3LYP/6-31G ∗//HF/6-31G ∗ method. Also, based on the B3LYP/6-31G ∗//HF/6-31G ∗ results, the conformational interconversion barrier height between (±)- 1-T and (±)- 1-crown forms is found to be 12.23 kcal mol −1. The unsymmetrical meso- 1-TBBC form is found to be the most stable geometry, among the various conformations of meso- 1 configuration. Conformational racemization of meso- 1-TBBC form can take place via another energy minimum geometry, namely meso- 1-TBCC. Conformational interconversion barrier height between meso- 1-TBBC and meso- 1-TBCC forms is 10.56 kcal mol −1, as calculated by B3LYP/6-31G ∗//HF/6-31G ∗ method. The results show also that conformational racemization of meso- 1 -TBCC can take place via the plane symmetrical meso- 1 -BCC geometry, and requires an energy about 5.35 kcal mol −1, as calculated by B3LYP/6-31G ∗//HF/6-31G ∗ method. In addition, HF/6-31G ∗//HF/6-31G ∗ and B3LYP/6-31G ∗//HF/6-31G ∗ results showed that between the two most stable conformations of (±) and meso configurations, (±)- 1-TB is more stable than meso- 1-TBBC by about 4.55 and 4.04 kcal mol −1, respectively. Also, MP2/6-31G ∗ and B3LYP/6-311+G ∗∗ methods were used to evaluate the HF/6-31G ∗//HF/6-31G ∗ and B3LYP/6-31G ∗//HF/6-31G ∗ results for the more stable conformations of (±) and meso configurations of compound 1 (namely: (±)- 1-TB, meso- 1-TBBC, meso- 1-TCCC and meso- 1-TBCC). Accordingly, MP2/6-31G ∗ and B3LYP/6-311+G ∗ ∗ results showed that the (±)- 1-TB form is about 3.55 and 3.54 kcal mol −1 more stable than the meso- 1-TBBC form. MP2/6-31G ∗ and B3LYP/6-311+G ∗ ∗ calculated energy differences between the most stable conformations of (±) and meso-configurations of compound 1, are found to be in good agreement with HF/6-31G ∗//HF/6-31G ∗ and B3LYP/6-31G ∗//HF/6-31G data. Further, using NBO (Natural Bond Orbital) analysis, π and π ∗ allenic bonding and antibonding orbital occupancies and also the deviations of σ and π bonding orbitals of allenic moieties were investigated. NBO results revealed that in the most stable form of meso configuration ( meso- 1-TBBC), the sum of the π ∗ allenic antibonding orbital occupancies ( Σ π occupancy ∗ ) is greater than dl configuration ((±)- 1-TB). In addition, NBO results indicated that in the (±)- 1-TB conformer, the sum of σ and π allenic moieties bonding orbital deviations ( Σ σ dev+Σπ dev), from their normal values, is lower than in the meso- 1-TBBC form. All these facts could explain the relative more stability of (±)- 1-TB conformer, as compared to the meso- 1-TBBC form.