Barriers to Internal Rotation in Asymmetric Molecules: 3-Fluoropropene

Abstract

The potential function hindering internal rotation in 3-fluoropropene has been determined using microwave and far-infrared spectroscopic data. It is given by the six potential constants in cm−1 (calories/mole): V1 = − 247 ± 30(− 707), V2 = 185 ± 25(530), V3 = 857 ± 15(2449), V4 = 188 ± 25(538), V5 = 7 ± 5(20), V6 = − 93 ± 10(− 265). From these the cis–gauche and gauche–trans–gauche barrier heights were determined to be 1090 + 75 − 25 cm−1 and 520 ± 40 cm−1, respectively. The spectra were interpreted using a one-dimensional model in which the only internal motion is the rotation about the bond connecting the top (–CH2F) to the frame (H2C=CH–); the various parameters in the Hamiltonian which was derived using the theory of internal rotation in asymmetric molecules developed by Quade and Lin were determined numerically and expressed as Fourier expansions in the internal coordinate α. In addition, the general theory was rederived and extended to include the case where neither the top nor the frame has a plane of symmetry. The large torsion–rotation interaction terms were reduced numerically by means of a Van Vleck transformation. The angle dependence of the reduced moment of inertia for the torsion was removed by a nonlinear coordinate transformation which yields a new Hamiltonian giving the same energy spectrum as the old but having a constant kinetic energy and a modified potential function. Finally, our potential is compared with that which was calcuated from the electrostatic model of Lowe and Parr.