Fourier transform microwave spectroscopy and molecular structure of the trans-1,2-difluoroethylene-hydrogen fluoride complex.

Guided by ab initio calculations, Fourier transform microwave rotation spectra in the 6.5-22 GHz region are obtained for the complex formed between trans-1,2-difluoroethylene and hydrogen fluoride, including the normal isotopomer and two singly substituted (13)C species in natural abundance. Spectra are also obtained for the analogous three species formed using deuterium fluoride. Analysis of the spectra provides rotational and hyperfine constants that are used to determine a structure for trans-CHFCHF-HF. This structure is similar to that obtained for 1,1-difluoroethylene-HF [H. O. Leung et al., J. Chem. Phys. 131, 204301 (2009)] in that a primary, hydrogen bonding interaction exists between the HF donor and a F atom acceptor on the 1,2-difluoroethylene moiety, while a secondary interaction occurs between the F atom on the HF molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene and causes the hydrogen bond to deviate from linearity. Because the two F atoms and the two H atoms in trans-1,2-difluoroethylene form electrostatically equivalent pairs, the structure of the complex with HF provides insight into the contribution of steric effects to the observed geometries of fluoroethylene-protic acid complexes. A comparison of the observed hydrogen bond lengths and deviations from linearity in 1,1-difluoroethylene-HF and trans-1,2-difluoroethylene-HF suggests that the F atoms in trans-1,2-difluoroethylene are more nucleophilic than those in 1,1-difluoroethylene and that the H atoms are similarly more acidic. Ab initio calculations of electrostatic potentials mapped onto total electron density surfaces for these two molecules support these conclusions.