The molecular structure of monofluoroacetic acid

Abstract

Monofluoroacetic acid was studied by microwave spectroscopy and electron diffraction. Two sets of rotational transitions were found, corresponding to two distinct conformations each having a plane of symmetry. From the dipole moments and from. results of isotopic substitution in the carboxyl group it was shown that in the “trans-conformation” the chain of atoms F-C-C-O-H is zig-zag, whereas in the “cis-conformation” it is cyclic.

The bond distances and bond angles could only be determined from a combination of electron diffraction and microwave spectroscopy data. The inertial moments were taken as observed quantities in the electron diffraction least squares procedure, with weights estimated from the expected differences between the results of the two techniques.

From microwave absorption coefficients the trans-conformation was found to be 604 ± 28 cal mole−1 lower in energy than the cis-conformation. A very rough estimate of about 6 kcal mole−1 is given for the height of the rotational barrier. These data are compared with the results of two semi-empirical quantum mechanical methods (EHT and CNDO/2).