Conformational stability of bicyclo[3.3.1]nonane-2,6-dione and bicyclo[3.3.1]nonane-2,9-dione: ab initio calculations and vibrational spectroscopy studies

Abstract

The conformational stability of newly synthesised bicyclo[3.3.1]nonane-2,6-dione C 9H 12O 2 (D26) and bicyclo[3.3.1]nonane-2,9-dione C 9H 12O 2 (D29) was studied. Ab initio calculations (HF level using 6-31G ∗∗ basis set) reveal that due to six-membered ring inversion four conformers exist in the case of D26, whereas five of them exist for D29. The double-chair (CC) conformer is the most stable for D26 and the chair–boat (CB) conformer is 3.4 kJ mol −1 higher in energy. On the contrary, the chair–boat (CBo) conformer of D29 is the most stable and the chair–chair conformer (CC) is 2.5 kJ mol −1 higher in energy. Raman and IR spectra of the title compounds were recorded in solid, liquid and gas phases for the first time. Assignment of vibrational spectral bands for both molecules was made by comparing theoretical and experimental spectra.