Structure of cis,trans-1,4-difluorobutadiene from microwave spectroscopy

Microwave spectra in the 5.5 - 17.5-GHz region have been observed for cis,trans-1,4-difluorobutadiene, for its C-13 isotopomers in natural abundance, for the four singly substituted deuterium isotopomers, and for the 1,4-d(2) species. For the parent species fitting 37 lines to a Watson-type Hamiltonian with the five quartic centrifugal distortion constants gave A = 12 988.333(1) B = 1467.8791(3), and C = 1318.5845(3) MHz. Stark effect measurements on three transitions of the parent species gave a dipole moment of 2.309(5) D with components mu(a) = 0.660(4) D and mu(b) = 2.213(5) D. Surprisingly, the B rotational constant for the 2-d(1) isotopomer, as indexed from the trans end, is larger than that for the normal species, and the rotational constants for the 3-d(1) isotopomer are also anomalous. Nonetheless, a complete, 17-parameter structure is proposed for this planar molecule and compared with density functional theory predictions made with the adiabatic connection method. Structural parameters are also compared with those of butadiene itself.