Barrier Studies in the Halopropenes. III. Microwave Spectra, Molecular Structure, Quadrupole Coupling Constants, Orientation of the Field Gradient Tensor, and Barrier to Internal Rotation in cis-1-Chloro-2-fluoropropene

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Abstract
The microwave spectra of cis‐CH3FC:CH35Cl and cis‐CH3FC:CH37Cl have been observed in the ground torsional state. The ground‐state rotational constants for cis‐CH3FC:CH35Cl are A = 9958.22 ± 0.05 MHz , B = 2285.88 ± 0.02 MHz , and C = 1879.89 ± 0.02 MHz . The ground‐state rotational constants for cis‐CH3FC:CH37Cl are A = 9958.00 ± 0.08 MHz , B = 2225.36 ± 0.03 MHz , and C = 1838.75 ± 0.03 MHz . The 35Cl quadrupole coupling constants along the appropriate inertial axes in the ground state of cis‐CH3FC:CH35Cl are χ aa = − 53.09 ± 0.20 MHz , χ bb = + 19.45 ± 0.05 MHz , and χ cc = + 33.64 ± 0.20 MHz , and for the corresponding 37Cl isotope χ aa = − 41.70 ± 0.20 MHz , χ bb = + 15.17 ± 0.20 MHz , and χ cc = + 26.53 ± 0.20 MHz . Second‐order quadrupole coupling effects have been used to obtain a direct measurement of the nonzero off‐diagonal element of the χ tensor for the 35Cl species. The off‐diagonal element is χ ab = ± 43.5 ± 3 MHz . Analysis of the ground torsional state spectra for the 35Cl species yields a barrier to internal rotation of the methyl group of 2565 ± 30 cal/mole .