Spectra and structure of small ring compounds. XLVIII. Conformational stability of methylcyclobutane from low frequency Raman data of the gas

Abstract

The low frequency (500–80 cm−1) Raman spectra of gaseous methylcyclobutane, c-C4H7CH3, and methyl-d3-cyclobutane have been recorded. A series of Q branches beginning at 161 cm−1 for the light molecule and 154 cm−1 for the d3 compound with successive transitions falling to lower frequencies have been assigned to the ring puckering vibrations of both the low energy equatorial and high energy axial conformers. These data have been fitted to an asymmetric potential function of the form: V(cm−1)=(4.78±0.10)×105X4−(3.08±0.04)×104X2 +(2.18±0.1)×104X3 with an assumed reduced mass of 160 amu for the light compound. Utilizing this potential the difference between the puckering angles for the two conformers was calculated to be 4.3° with the equatorial conformer having the larger value of 20.7°. A similar potential was obtained for the d3 molecule. The energy difference between the equatorial and axial forms was found to be 247±20 cm−1 (706 cal/mol) and a barrier of 641±20 cm−1 (1.83 kcal/mol) was found for the interconversion. Experimental values for the ΔH of both the liquid (354±179 cm−1) and gas (295±75 cm−1) from relative intensities of different Raman band pairs over 85 and 37 °C temperature ranges, respectively, were found to be consistent with the value obtained from the potential function. These results are compared to the corresponding quantities for similar molecules.