[n]peristylanes and [n]oxa[n]peristylanes (n=3-6): a theoretical study

Abstract

Theoretical studies at the HF and Becke3LYP levels using 6-31G^∗ basis sets were carried out on a series of [n]peristylanes and [n]oxa[n]peristylanes (n = 3-6) to understand their structure and energetics. The structures of the [3]- and [4]peristylanes (1, 2) and their oxa-derivatives (5, 6) were calculated to have the anticipated high symmetry, C_nv. In contrast, a C_s structure (9) at HF/6-31G^∗ and another (25) at the Becke3LYP/6-31G^∗ level were calculated for the [5]oxa[5]peristylane. The energy difference between them is extremely small even though there are major differences in the structures indicating a very soft potential energy surface. On the other hand, the potential energy surface of [6]oxa[6]peristylane is not as soft. Similar structures were also calculated for the top rings. Calculations on the seco-compounds 11-14 and 15-19 (Table 4) indicate that there is no unusual strain involved in the formation of 27 from 19. The Li⁺ interaction energies of the [n]oxa[n]peristylanes are 61.7 (n = 3), 72.8 (n = 4), 84.2 (n = 5) and 91.7 (n = 6) kcal mol^-1 at the Becke3LYP/6-31G^∗ level. Dramatic differences between the C-C bond lengths obtained from the solid state X-ray diffraction studies and those from the calculations for the [n]oxa[n]peristylanes were also observed.