Knowledge Bank

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 inertia are $\mu a=2.369\pm 0.015D,\mu c=0.70 \pm 0.03D,$ and $\mu t=2.47 \pm 0.03$ D for the cis form and $\mu a=1.535\pm 0.015D,\mu b=0.53 \pm 0.04D,\mu c=1.36 \pm 0.03D,$ and $\mu t=2.12 \pm 0.05D$ for the gauche form. The relative intensity measurements indicate that the cis form is more stable than the gauche form by $260\pm 80cm-1$. The energy of the first excited state with respect to the ground state was found to be $63 \pm 8cm-1$ for the cis form and $85 \pm 10cm-1$ for the gauche form. Two Fourier coefficients of the potential function restricting torsion around the C-C bond were determined to be $V1=266 \pm 40cm-1$ and $V3=508 \pm 200cm-1$, using the available data. The potential function thus obtained is compared to the prediction of a model in which the additivity of the potential as a function of substitution is assumed.