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

The infrared (3500-40 cm(-1)) spectra of gaseous and solid dicyclopropyl me thane, (c-C3H5)(2)CH2, along with the Raman (3500-30 cm(-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 C-2 symmetry, gauche-gauche rotamer wit h C-s symmetry, and the gauche-cis rotamer with C-1 symmetry. Variable temp erature (-55 to -145 degreesC) studies of the infrared spectra of the sampl e dissolved in liquid xenon and krypton have been carried out. Utilizing si x different combinations of pairs of bands from the C-2 and C-3 conformers, the average value of DeltaH, the enthalpy difference between these two con formers, is 239 +/- 13 cm(-1) (2.86 +/- 0.16 KJ/mol) from the xenon solutio n and 211 +/- 12 cm(-1) (2.52 +/- 0.14 kJ/mol) from the krypton solution, w ith the C-2 form the more stable rotamer in both solvents. The enthalpy dif ference for the gauche-cis form was obtained from the temperature dependenc e of the relative intensities of three of the fundamentals of this conforme r with three fundamentals of the C-2 form and two fundamentals of the C-s f orm, from which a value of 479 +/- 84 cm(-1) (5.73 +/- 1.00 KJ/mol) was obt ained between the C-2 rotamer and this less stable C-1 form. A complete vib rational assignment is proposed for the C-2 conformer and most of the funda mentals have been assigned for the C-s conformer, utilizing predictions of fundamental frequencies, infrared intensities, and Raman activities from MP 2/6-31G(d) ab initio calculations. The structural parameters, dipole moment s and conformational stabilities of the four possible conformers of dicyclo propyl 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 ful l electron correlation. The predicted conformational stabilities are consis tent with the experimental results. These experimental and theoretical resu lts are compared to the corresponding quantities of some similar molecules. (C) 2001 Elsevier Science B.V. All rights reserved.