Microwave spectra, barrier heights to internal rotation of methyl group, and dipole moments for conjugated chain compounds Methacrolein

The microwave spectrum of methacrolein, CH ₂C (CH ₃)CHO, was measured in the frequency region from 7.6-25 GHz. The R- and Q-branches for the a-type have been assigned and the rotational constants for the A-level are determined to be A = 8612.39 MHz, B = 4403.08 MHz, and C = 2965.33 MHz. All the Q-branch transitions observed have been split into doublets by the hindering rotation of the methyl group, and the barrier height has been determined to be 1340 ± 20 cal/mole by using the frequency splittings of these lines. The value of the total dipole moment is 2.80 ± 0.03 D, somewhat different from the value previously obtained by dielectric constant measurements.