Configurational and conformational properties of cyclododeca-1,2,7,8-tetraene: An Ab inito study and NBO analysis

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-1,2,7,8-tetraene (1) has been under taken. Two axial symmetrical conformations are found for (E)4. The ( L)A-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/6-31G*HHI'/6-31G* levels of theory, respectively. Interconversion of (+/-)-1-TB and (+/-)-1-crown conformations call take place via the unsymmetrical twist form (+/-)-1-T, as a minimum geometry. Conformational interconversion barrier height between (+/-)-1-T-B and (+/-)-1-T forms is 11.48 kcal mol(-1), as calculated by B3LYP/6-31G*//H[F/6-31G* method. Also, based oil the B3LYP/6-31G*//HF/6-31G* results, the conformational inerconversion barrier height between ()I-T and (+/-:)-1-crown forms is 12.23 kcal mol(-1). Among the various conformations of meso-1 configuration, the unsymmetrical TBBC conformation is the most stable form. Conformational racemization of meso-1-TBBC form call 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/6-31G*//HF/6-31G* method. Furthermore, MP2/6-31G* and B3LYP/6-31 1+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-IJBBC form (most stable conformation of meso-1 configuration). Also, HF/6-31G*//HF/6-31G* and B3LYP/6-31G*//HF/6-31G* results showed that (+/-)-1-TB conformation is more stable than ineso-1-TTIBC 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 conforiviations are important because they are expected to be significantly populated at room temperature. Also, pi and pi* allenic bonding, antibonding orbital occupancies and the deviations of sigma and pi bonding orbitals of allenic moieties were investigated using NBO analysis. NBO results revealed that the sum of the pi* allenic antibonding orbital occupancies (Sigma pi*(occupancy)) in the most stable form of meso configuration is greater than the dl configuration. Also, NBO results indicate that the deviations of sigma and pi bonding orbitals of allenic moieties (Sigma sigma(dev)+ Sigma pi(dev)) 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.