Methylenecyclobutane: Ring-Puckering Potential Function from Mid-Infrared Combination Bands

The mid-infrared spectra (4000–250 cm−1) of methylenecyclobutane, methylene-d2-cyclobutane-2,2-d2, and methylene-d2-cyclobutane-d6 have been examined closely for the occurrence of combination bands involving the ring-puckering vibration. A progression of sum bands for methylenecyclobutane and a progression of difference bands for methylene-d2-cyclobutane-2,2-d2 have been simultaneously fit by the one-dimensional potential function in the ring-puckering coordinate x: V(x) = 4.229 × 105x4 − 1.694 × 104x2. This potential function yields a barrier of 168 ± 10 cm−1 at the planar conformation, in good agreement with the barrier of 160 ± 40 cm−1 determined by other workers from the vibrational dependence of the rotational constants in the microwave spectrum. The detailed shape of the function does not agree as well and particularly the failure of the infrared function to account for the observed 0–1 splitting is noted. This had been noted earlier in the case of silacyclobutane and to a lesser degree, in the case of cyclopentene.