Conformational stability of ethylcyclopropane from Raman spectra, temperature dependent FT-IR spectra of xenon solutions and ab initio calculations

Infrared spectra (3500 to 400 cm(-1)) of xenon solutions of ethylcyclopropane, C-C3H5C2H5, at temperatures from -60 to -100 degrees C have been recorded. In addition, Raman spectra of the liquid and polycrystalline phases have been obtained as well as infrared spectra of the gaseous phase. Spectroscopic evidence for the coexistence of the gauche and cis conformers of the title compound is found in the fluid phases. From the spectroscopic data, the enthalpy difference between the gauche and cis conformers has been estimated to be 385 cm(-1) (1.10 kcal mol(-1)), with the gauche rotamer the more stable form. In the annealed solid phase, only the gauche conformer is found. Ab initio calculations have been carried with different basis sets up to MP2/631G, from which structural parameters, conformational stabilities, force constants, and vibrational frequencies have been determined. These calculations support the experimental conclusions and aid in the vibrational assignment of the normal modes. These results are compared to the corresponding quantities for some similar molecules. (C) 1997 Elsevier Science B.V.