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 ethylcyclo propane, C-C3H5C2H5, at temperatures from -60 to -100 degrees C have b een recorded. In addition, Raman spectra of the liquid and polycrystal line phases have been obtained as well as infrared spectra of the gase ous phase. Spectroscopic evidence for the coexistence of the gauche an d cis conformers of the title compound is found in the fluid phases. F rom 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 s olid phase, only the gauche conformer is found. Ab initio calculations have been carried with different basis sets up to MP2/631G, from whic h structural parameters, conformational stabilities, force constants, and vibrational frequencies have been determined. These calculations s upport the experimental conclusions and aid in the vibrational assignm ent of the normal modes. These results are compared to the correspondi ng quantities for some similar molecules. (C) 1997 Elsevier Science B. V.