COMPLETE STRUCTURE OF TRANS-1,2-DIFLUOROETHYLENE FROM THE ANALYSIS OFHIGH-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 spect ra 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 isot opomer, a Watson-type Hamiltonian has been fit to ground state combina tion 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 = 0.121102 34(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 an d those previously published for the normal species, substitution coor dinates have been found for the carbon and hydrogen atoms and effectiv e coordinates have been found for the fluorine atoms. The resulting pa rameters 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 degr ees. 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.