Microwave spectrum and rotational isomerism of 3,3-difluoropropene CHF2CHCH2

The analysis of the microwave spectrum of 3,3-difluoropropene has confirmed the existence of two rotational isomers, cis and gauche. The rotational constants in the ground vibrational state are A = 9126.08 MHz, B = 3722.120 MHz, and C = 2946.598 MHz for the cis form and A = 8901.64 MHz, B = 4192.759 MHz, and C = 3107.718 MHz for the gauche form. The dipole moment and its components along the principal axes of intertia are μ_a = 2.369 ± 0.015 D, μ_c = 0.70 ± 0.03 D, and μ_t = 2.47 ± 0.03 D for the cis form and μ_a = 1.535 ± 0.015 D, μ_b = 0.53 ± 0.04 D, μ_c = 1.36 ± 0.03 D, and μ_t = 2.12 ± 0.05 D for the gauche form. The relative intensity measurement indicates that the cis form is more stable than the gauche form by 260 ± 80 cm^?1. The energy of the first excited state with respect to the ground state was found to be 63 ± 8 cm^?1 for the cis form and 85 ± 10 cm^?1 for the gauche form. Two Fourier coefficients of the potential function restricting the torsion around the CC bond were determined to be V₁ = 266 ± 40 cm^?1 and V₃ = 508 ± 200 cm^?1, using the available data. The potential function thus obtained is compared to a prediction model which is derived assuming additivity of the potential as a function of substitution.