Cross-conjugated compounds: microwave spectrum and ring planarity of 3-methylene-1,4-cyclohexadiene

Abstract The unstable 3-methylene-1,4-cyclohexadiene, an isomer of toluene, has been prepared from benzoic acid. The hydrocarbon can be handled quite easily in the solid state, and in the gas phase at low pressures. Rotational transitions have been measured from 12 to 40 GHz. Sixty-three lines were assigned with J -quantum numbers up to 60. From the least-squares fit with a standard deviation of 23 kHz the three rotational constants and all five quartic centrifugal distortion constants from Watson's A -reduction could be determined as A = 5177.8216(37) MHz, B = 2613.1518(10) MHz, C = 1755.8422(9) MHz, Δ j = 0.1384(65) kHz, Δ JK = 0.135(16) kHz, Δ K = 1.074(50) kHz, δ J = 0.0440(11) kHz and δ K = 0.333(24) kHz (representation I R used). The dipole moment was found to be 0.8645(31) D from 32 frequency shifts in total of the M components of the transition 8 35 ← 7 34 . The 6-monodeuterated species has also been prepared and its microwave spectrum measured. Using Kraitchman's equations, the coordinates of the ring methylene hydrogens could be determined. The pseudoinertial defect was found to be −0.1435(56) A 2 which proves that the molecule possesses a planar ring. The results are compared with the data for 4,4-dimethyl-2,5-cyclohexadien-1-one.