CROSS-CONJUGATED COMPOUNDS - MICROWAVE-SPECTRUM AND RING PLANARITY OF3-METHYLENE-1,4-CYCLOHEXADIENE

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. Rot ational transitions have been measured from 12 to 40 GHz. Sixty-three lines were assigned with J-quantum numbers up to 60. From the least-sq uares fit with a standard deviation of 23 kHz the three rotational con stants and all five quartic centrifugal distortion constants from Wats on's A-reduction could be determined as A = 5177.8216(37) MHz, B = 261 3.1518(10) MHz, C = 1755.8422(9) MHz, DELTA(J) = 0. 1384(65) kHz, DELT A(JK) = 0.135(16) kHz, DELTA(K) = 1.074(50) kHz, delta(J) = 0.0440(11) kHz and delta(K) = 0.333(24) kHz (representation I(R) used). The dipo le moment was found to be 0.8645(31) D from 32 frequency shifts in tot al of the M components of the transition 8(35) <-- 7(34). The 6-monode uterated species has also been prepared and its microwave spectrum mea sured. Using Kraitchman's equations, the coordinates of the ring methy lene hydrogens could be determined. The pseudoinertial defect was foun d to be -0.1435(56) angstrom2 which proves that the molecule possesses a planar ring. The results are compared with the data for 4,4-dimethy l-2,5-cyclohexadien-1-one.