CHED 299 |
| The synthesis and characterization of the tri-trans (CTCTCT) isomer of [12]annulene was reported by Oth in 1970. Tri-trans [12]annulene is the first [4n]annulene for which there is no experimental evidence that planar, degenerate bond shifting occurs. Based on NMR data, Oth reported that the barrier for planar bond shifting must be greater than 17.4kcal/mol. This barrier is significantly higher than the barriers for both cyclooctatetraene and [16]annulene. The transition state for planar bond shifting should be a singlet diradical with anti-aromatic properties. To investigate this phenomenon, various calculations were run on tri-trans [12]annulene using (unrestricted) density functional theory (U)DFT and multi-configurational methods such as CASPT2. Initial (U)DFT results gave remarkably low barriers, between 9.0 kcal/mol (UB3LYP/6-311+G**) to 5.60 kcal/mol (UBH&HLYP/6-311+G**). CASPT2(12,12)/6-31G*//UDFT/6-311+G** results also provided surprisingly low energy barriers of 11.2 kcal/mol (UB3LYP) to 10.9 kcal/mol(UBH&H). This barrier was further reduced to 10.4 kcal/mol using CASPT2(12,12)/6-31G*//CASSCF(12,12)/6-31G*. However, CASSCF(12,12)/6-31G* results revealed a barrier of 25.5 kcal/mol, in agreement with the observations of Oth. Calculations on planar bond shifting in 1,7-methano[12]annulene and [16]annulene will also be presented.
|
|
Undergraduate Research Poster Session
2:30 PM-4:30 PM, Monday, August 20, 2007 BCEC -- Exhibit Hall - B2, Poster
Division of Chemical Education |
