Knowledge Bank

The microwave spectrum of 3-fluoropropene $(CH2FCH=CH2)$ has been studied in the region from 8,000 to 42,000 Mc/sec. Rotational transitions have been assigned for two rotational isomers. The rotational constants obtained for the ground vibrational states are $$\def \a {\hphantom {0}}\def \b {\hphantom {.}}\begin{array}{clll}rotational isomer & \multicolumn{1}{c}{1 (cis)}& \multicolumn{1}{c}{H (gauche)}\ A &17,236.61 Mc/sec &27,720.31 Mc/sec\ B &\a6,003.05 &\a4,263.57\ C & \a4,579.72& \a4,132.01\\end{array}$$ The moments of inertia of the isomer I agree quite well with those predicted for a cis conformation, in which all heavy atoms lie in a plane. On the other hand the dihedral angle of the isomer II is about $124.4\circ$ measured from the cis position. A preliminary measurement of the relative intensity shows that the cis form is more stable than the gauche form by 170 cal/mole, and the C-C torsional frequency is $176cm-1$ for the cis form and $91cm-1$ for the gauche form. A remarkable feature of the vibrational satellites is the doublet structure observed for the first and the second excited state lines of the gauche form. The doublets have been explained by the tunneling through the barrier between two equivalent gauche forms. This barrier height has been estimated at 1.14 kcal/mole from the analysis of the doublet separation.

Author Institution: Mallinckrodt Chemical Laboratory, Harvard University