Spectra and structure of small ring compounds. XLIX—Raman and infrared spectra, conformational analysis and vibrational assignment of methylcyclobutane and methyl‐d3‐cyclobutane

Abstract

AbstractThe Raman (3500–20 cm−1) and infrared (3500–50 cm−1) spectra of gaseous and solid methylcyclobutane and methyl‐d3‐cyclobutane have been recorded. Additionally, the Raman spectra of the pure liquids have been recorded and qualitative depolarization values have been obtained. The spectra have been interpreted on the basis that both the axial and equatorial conformers are present in the fluid phases and that the equatorial form is thermodynamically preferred and the only form present in the annealed solid. All 39 of the normal vibrational modes have been assigned for both the d0 and d3 compounds. An abundance of spectral evidence is presented which indicates extensive coupling between the vibrations associated with the ring and those of the methyl group. The CH3 internal torsional mode has been observed in the Raman spectrum of the solid at 232 cm−1 and utilizing this frequency the periodic threefold barrier to internal rotation has been calculated to be 1190 cm−1 (3.40 kcal mol−1). The fundamental ring puckering mode has been assigned from the Raman spectra of the gases to weak features observed at 161 and 154 cm−1 for the d0 and d3 compounds, respectively. All of these results are compared with the corresponding quantities for some similar molecules.