Conformational behavior of ethyloxirane from infrared and Raman spectra, temperature dependent FT-IR spectra of xenon solutions, and ab initio calculations

Infrared spectra (3500-400 cm(-1)) of gaseous ethyloxirane, C-OC2H2C2H5, (1,2-epoxybutane) and xenon solutions of this molecule at temperatures from -60 to -100 degrees C have been recorded. In addition, Raman spectra of the liquid and polycrystalline phases have been obtained, and variable temperature Raman spectra (25 to -89 degrees C) of the liquid were also recorded. Spectroscopic evidence for the co-existence of the gauche-l and gauche-2 conformers of the title compound is found in the fluid phases, along with small amounts of the cis rotameric form. In xenon solutions, the enthalpy difference between the gauche-l and gauche-2 conformers has been determined to be 217 +/- 34 cm(-1) (620 +/- 97 cal mol(-1)), with the gauche-l rotamer the more stable form, whereas in the liquid phase this quantity is estimated to be 95 +/- 6 cm(-1) (272 +/- 17 cal mol(-1)). In the annealed solid phase, only the gauche-l conformer is found. Ab initio calculations, have been carried with different basis sets up to MP2/6-31G* from which structural parameters and conformation stabilities have been determined. These calculations support the experimental conformational conclusions. A force field from these calculations is used to support the vibrational assignments and simulate the observed infrared and Raman spectra. These results are compared to the corresponding quantities for some similar molecules.