Cross-conjugated compounds: microwave spectrum of 4,4-dimethyl-2,5-cyclohexadien-1-one

Abstract

4,4-Dimethyl-2,5-cyclohexadien-1-one has been prepared by a three-step reaction from 4,4-dimethyl-2-cyclohexen-1-one. Microwave transitions of the cross-conjugated compound have been measured over extended regions from 11 to 40 GHz. The spectrum is a classical a-type spectrum of a near-prolate top with many interspersed lines presumably originating from excited vibrational states. No internal rotation splitting could be observed. We were able to assign 108 rotational transitions with J-quantum numbers up to 40. The least-squares fit showed a standard deviation of 30 kHz and yielded the three rotational constants, A = 3332.158(13) MHz, B = 1193.07386(52) MHz, C = 1082.21280(45) MHz, as well as all five quartic centrifugal distortion constants from Watson's A-reduction: Δ J = 0.02149(73) kHz, Δ JK = 1.1008(16) kHz, Δ K = −68.42(32) kHz, δ J = 0.00332(16) kHz, δ K = −2.7513(99) kHz (representation I R used). The dipole moment was determined from the Stark shift of the M-components of two transitions and was found to be 4.4522(83) D from 93 measurements.