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

Mid-infrared spectra (3500-400 cm(-1)) of gaseous and polycrystalline solid ethylcyclobutane have been recorded, as well as the far-infrared (400-50 cm(-1)) spectrum of ethylcyclobutane in the annealed solid phase. Additionally, the Raman spectra (4000-20 cm(-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 intensifies, 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 161 cm(-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. (C) 2000 Elsevier Science B.V. All rights reserved.