Rotational Isomerism and Microwave Spectroscopy. III. The Microwave Spectrum of 3-Fluoropropene

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Abstract
The 3-fluoropropene molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, cis and gauche. Rotational constants in the ground vibrational state are A = 17236.638, B = 6002.915, C = 4579.827 Mc/sec for the cis and A = 27720.349, B = 4263.628, C = 4131.984 Mc/sec for the gauche form. Rotational transitions are also assigned for isotopic species, eight of the cis and nine of the gauche form. By using Costain's method the molecular structure is determined separately for the two isomers. The dipole moment and its components along the principal axes of inertia are μa=0.742±0.008, μb=1.601±0.011, μtotal=1.765±0.014 D for the cis and μa=1.595±0.006, μb=0.908±0.012, μc=0.623±0.014, μtotal=1.939±0.015 D for the gauche form. Through the temperature dependence of the intensity ratio the cis form is found more stable than the gauche form by 166±67 cal/mole. The energies of the first and the second excited states of the C–C torsion are determined by the relative intensity measurement to be 169.6±4.2 cm−1 and 322.2±9.9 cm−1 for the cis and 84.6±2.8 cm−1 and 185±25 cm−1 for the gauche form. Some of the spectra in the excited states of the gauche form are doublets due to the tunnel effect through the trans barrier. Using these data a potential function for the rotation around the C–C single bond is discussed. A correlation is pointed out between the potential function and the binding states of carbon atoms forming the rotation axis.

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