Complete structure of trans-1,2-difluoroethylene from the analysis of high-resolution infrared spectra

For comparison to the structure of the cis isomer with its puzzlingly lower energy, the complete structure of trans-1,2-difluoroethylene, a nonpolar molecule, has been determined. High-resolution infrared spectra have been recorded for three isotopomers, the C-13(2), d(2), and d(1) species. From the analysis of at least two band types for each isotopomer, a Watson-type Hamiltonian has been fit to ground state combination differences. Ground state rotational constants (in cm(-1)) are A = 1.8247501(32), B = 0.13396608(66), and C = 0.12473595(65) for the C-13(2) species, A = 1.2634959(27), B = 0.13403712(61), and C = 0.12110234(59) for the d(2) species, and A = 1.5198818(34), B = 0.13429738(50), and C = 0.12331713(54) for the d(1) species. From these constants and those previously published for the normal species, substitution coordinates have been found for the carbon and hydrogen atoms and effective coordinates have been found for the fluorine atoms. The resulting parameters are r(CH) = 1.080 Angstrom, r(CC) = 1.316 Angstrom, r(CF) = 1.352 Angstrom, alpha(CCH) = 126.3 degrees, and alpha(CCF) = 119.2 degrees. The most significant differences from the cis isomer are the 2.9 degrees smaller CCF angle and the 2.4 degrees larger CCH angle for the trans isomer. These differences correlate with a larger repulsion of CF bond dipoles in the cis isomer than in the trans isomer despite the lower energy of the cis isomer. In addition, the CF bond length is 0.015 Angstrom longer and both the CC and CH bonds are somewhat shorter in the trans isomer. The synthesis of trans-1,2-difluoroethylene-C-13(2) is described, and various intermediates are identified.