Conformational stabilities of dicyclopropyl ketone determined from variable temperature infrared spectra of xenon solutions and ab initio calculations

The infrared (3500–40 cm−1) spectra of gaseous and solid dicyclopropyl ketone, (c-C3H5)2CO, along with the Raman (3500–40 cm−1) spectra of the liquid and solid phases have been recorded. Only the major cis–cis (C2v) conformer (CO bond over the two three-membered rings) and minor cis–trans (Cs) form (CO bond eclipsing the hydrogen atom on one of the rings) have been identified in the fluid phases. Variable temperature (−55 to −100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. Utilizing six different combinations of pairs of bands from the C2v and Cs conformers, the average enthalpy difference between these two conformers has been determined to be 530 ± 27 cm−1 (6.34 ± 0.32 kJ/mol), with the C2v form the more stable rotamer. At ambient temperature it is estimated that there is only 13 ± 2% of the Cs conformer present. A complete vibrational assignment is given for the C2v form and several of the fundamentals of the Cs conformer have been identified by utilizing predictions of fundamental frequencies, infrared intensities, and Raman activities from MP2(full)/6-31G(d) ab initio calculations. The structural parameters, dipole moments and conformational stabilities have been obtained from ab initio calculations at the level of restricted Hartree–Fock (RHF), with full electron correlation by the perturbation method to second order (MP2(full)) and hybrid density functional theory (DFT) by the B3LYP method with a variety of basis sets. The predicted conformational stabilities are consistent with the experimental results. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.