Conformational stabilities of dicyclopropyl methane determined from variable temperature infrared spectra of rare gas solutions and ab initio calculations

Abstract

The infrared (3500–40cm−1) spectra of gaseous and solid dicyclopropyl methane, (c-C3H5)2CH2, along with the Raman (3500–30cm−1) spectra of the liquid and solid phases of this molecule have been recorded. Three of the four possible conformers have been identified in the fluid phases, and they are the gauche–gauche rotamer with C2 symmetry, gauche–gauche rotamer with Cs symmetry, and the gauche–cis rotamer with C1 symmetry. Variable temperature (−55 to −145°C) studies of the infrared spectra of the sample dissolved in liquid xenon and krypton have been carried out. Utilizing six different combinations of pairs of bands from the C2 and Cs conformers, the average value of ΔH, the enthalpy difference between these two conformers, is 239±13cm−1 (2.86±0.16kJ/mol) from the xenon solution and 211±12cm−1 (2.52±0.14kJ/mol) from the krypton solution, with the C2 form the more stable rotamer in both solvents. The enthalpy difference for the gauche–cis form was obtained from the temperature dependence of the relative intensities of three of the fundamentals of this conformer with three fundamentals of the C2 form and two fundamentals of the Cs form, from which a value of 479±84cm−1 (5.73±1.00kJ/mol) was obtained between the C2 rotamer and this less stable C1 form. A complete vibrational assignment is proposed for the C2 conformer and most of the fundamentals have been assigned for the Cs conformer, utilizing predictions of fundamental frequencies, infrared intensities, and Raman activities from MP2/6-31G(d) ab initio calculations. The structural parameters, dipole moments and conformational stabilities of the four possible conformers of dicyclopropyl methane have been obtained from RHF/6-31G(d), MP2/6-31G(d) and MP2/6-311+G(d,p) ab initio calculations, with the latter two incorporating full electron correlation. 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.