Conformational stability, structural parameters, and vibrational frequency assignments of ethylcyclobutane using infrared and Raman spectroscopy and ab initio calculations

Abstract

Mid-infrared spectra (3500–400cm−1) of gaseous and polycrystalline solid ethylcyclobutane have been recorded, as well as the far-infrared (400–50cm−1) spectrum of ethylcyclobutane in the annealed solid phase. Additionally, the Raman spectra (4000–20cm−1) of liquid and annealed solid ethylcyclobutane have also been obtained. Spectroscopic evidence for the coexistence of the equatorial–gauche, axial–gauche and equatorial–cis conformers of the title compound is found in the fluid phase, with the equatorial–gauche rotamer the most stable form, and the only rotamer present in the annealed solid. Ab initio calculations have been carried out with different basis sets up to MP2/6-31G∗, from which the structural parameters, conformational stabilities, force constants, infrared and Raman intensities, and vibrational frequencies have been determined. By incorporating the results from the ab initio calculations, a normal coordinate analysis of the title compound has also been performed and this aids in the vibrational assignments of the normal modes. The weak features at 172 and 161cm−1 in the far-IR and Raman spectra of the solid have been assigned to the ring puckering vibrations, and a theoretically predicted ring-puckering potential function has been determined.