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 13C2, d2, and d1 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 13C2 species, A = 1.2634959(27), B = 0.13403712(61), and C = 0.12110234(59) for the d2 species, and A = 1.5198818(34), B = 0.13429738(50), and C = 0.12331713(54) for the d1 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 rCH = 1.080 Å, rCC = 1.316 Å, rCF = 1.352 Å, αCCH = 126.3°, and αCCF = 119.2°. The most significant differences from the cis isomer are the 2.9° smaller CCF angle and the 2.4° 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 Å longer and both the CC and CH bonds are somewhat shorter in the trans isomer. The synthesis of trans-1,2-difluoroethylene-13C2 is described, and various intermediates are identified.


American Chemical Society

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Journal of Physical Chemistry A


Chemistry and Biochemistry

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