Knowledge Bank

The microwave spectrum of trans-1-fluoro-2-butene has been recorded from 18.0 to 39.0 GHz. The R-branch assignments of the ground and first two excited states of the observed a-type spectrum have been made. The ground state rotational constants are: $A=18943\pm 317$,$B=2071.34\pm 0.04$ and $C=2022.11\pm 0.04$ MHz. It is shown that these rotational constants are consistent with a gauche orientation of the $CH2F$ moiety. Additionally, several lines observed in the microwave spectrum could not be assigned to the gauche rotamer but, on the basis of an investigation of the infrared and Raman spectra, they have been assigned to the cis conformer. An investigation of the far infrared spectrum of trans-1-fluoro-2-butene has allowed the assignment of the asymmetric torsional mode of the cis rotamer to a series of Q-branches beginning at $123.9cm-1$.These results will be compared to the corresponding quantities in similar molecules.