ABSTRACT

An investigation employing ab initio molecular orbital (MO) and density functional theory (DFT) methods to calculate structure optimization and conformatinal interconverstion pathways for the two diastereoisomeric forms, (±) and meso configurations, of cyclododeca-l,2,7,8-tetraene (1) has been under taken. Two axial symmetrical conformations are found for (±)-1. The (±)-1-TB axial symmetrical form is about 4.97 and 4.48 kcal mol-1 more stable than the axial symmetrical crown conformation, (±)-1-crown, as calculated at HF/6-31G*//HF/6-31G* and B3LYP/6-31GV/HF/6-31G* levels of theory, respectively. Interconversion of (±)-1-TB and (±)-1-crown conformations can take place via the unsymmetrical twist form, (±)-1-T, as a minimum geometry. Conformational interconversion barrier height between (±)-1-TB and (±)-1-T forms is 11.48 kcal mol1, as calculated by B3LYP/6-31GV/HF/6-31G* method. Also, based on the B3LYP/6-31G*//HF/6-31G* results, the conformational inerconversion barrier height between (±)-1-T and (±)-1-crown forms is 12.23 kcal mol”1. Among the various conformations of mesoa configuration, the unsymmetrical TBBC conformation is the most stable form. 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. Also, conformational racemization of the meso-1- TBCC can take place via the plane symmetrical BCC geometry, and requires energy about 5.35 kcal mol”1, as calculated by B3LYP/6-31GV/HF/6-31G* method. Furthermore, MP2/6-31G* and B3LYP/6-311+G** results showed that the (±)-1-TB form, as the most stable conformation of the (±)-1 configuration, is 3.55 and 3.54 kcal mol-1 more stable than the meso-1-tbbc form (most stable conformation of meso-1 configuration). Also, HF/6-31GV/HF/6-31G* and B3LYP/6-31GV/HF/6-31G* results showed that (±)-1-TB conformation is more stable than meso-1-TBBC form, about 4.55 and 4.04 kcal mol-1, respectively. Among the various conformations of meso and (±) configurations of compound 1, the meso-1-TBBC, meso-1-TCCC and (±)-1-TB conformations are important because they are expected to be significantly populated at room temperature.

Also, π and π* allenic bonding, antibonding orbital occupancies and the deviations of σ and π bonding orbitals of allenic moieties were investigated using NBO analysis. NBO results revealed that the sum of the π* allenic antibonding orbital occupancies (σπ*occupancy ) in the most stable form of meso configuration is greater than the dl configuration. Also, NBO results indicate that the deviations of σ and π bonding orbitals of allenic moieties (Σσdev +Σπdev ) in the (±)-1-TB conformer is lower than in the meso-1-TBBC form. These facts would explain the stability of (±)-1-TB conformer, compared to the meso-1-TBBC form.