Configurational and conformational properties of 1,3,7,9-tetraphospha-cyclododeca-1,2,7,8-tetraene: An ab initio study and NBO analysis

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 1,3,7,9-tetraphospha-cyclododeca-1,2,7,8-tetraene (1) was undertaken. Two axial symmetrical conformations are found for (+/-)-1 configuration. (+/-)-1-TB axial symmetrical form is found to be about 0.35 and 0.99 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 unsymmetrical meso-1-TBCC form is found to be the most stable geometry, among the various conformations of meso-1 configuration. 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-TBCC by about 3.35 and 2.43 kcal mol(-1), respectively. In addition, MP2/6-31G* and B3LYP/6-311+G** results showed that the (+/-)-1-TB form is about 1.10 and 2.36 kcal mol(-1) more stable than the meso-1-TBCC form. Further, NBO results revealed that in the most stable form of meso configuration (meso-1-TBCC), the sum of the pi* allenic antibonding orbital occupancies (Sigma pi*(occupancy)) is greater than dl configuration ((+/-)-1-TB). Also, NBO results indicated that in the (+/-)-1-TB conformer, the sum of sigma and pi allenic moieties bonding orbital deviations (Sigma sigma(dev)+Sigma pi(dev)) from their normal values, is lower than in the meso-1-TBCC form.