Vibrational Assignments and Potential Constants for cis- and trans-1,2-Difluoroethylenes and Their Deuterated Modifications

Abstract

Infrared and Raman spectra and a complete assignment of vibrational fundamentals are presented for cis- and trans-1,2-difluoroethylenes and their deuterated modifications. For cis-CFHCFH the fundamentals are: (a1) 3122, 1716, 1263, 1015, 237; (a2) 839, 495; (b1) 3136, 1374, 1130, 769; and (b2) 756 cm−1. For trans-CFHCFH the fundamentals are: (ag) 3111, 1694, 1286, 1123, 548; (au) 875, 329; (bg) 788; and (bu) 3114, 1274, 1159, 341 cm−1. The near degeneracy in ν7 and ν12 of the trans isomer is shown from low-temperature crystal and matrix studies and zero-order normal coordinate calculations. Analysis of the rotational structure in the region of the overlapped bands proves that these fundamentals are strongly perturbed by Coriolis coupling. From the vibrational fundamentals and a previous measurement of ΔH°650, the cis isomer is shown to have 1086 cal/mole less electronic energy than the trans isomer. For the in-plane vibrations refined Urey–Bradley potential constants are presented. These constants are obtained from an overlay calculation involving the full set of six molecules. For the out-of-plane vibrations, refined general valence constants, obtained for the separate isomers, are presented.

Theoretical prediction of nuclear magnetic shieldings and indirect spin-spin coupling constants in 1,1-, cis-, and trans-1,2-difluoroethylenes

“…36 Obviously, structural studies on difluoroethylenes were often based on their infrared spectra. 2,3 Moreover, vibrational analysis of their experimental data 3 was supported by theoretical predictions including anharmonic effects. [4][5][6][7][8][9][10] Among a large number of currently used density functionals 37 for practical reasons we selected B3LYP [38][39][40] and BLYP [39][40][41] for structural and vibrational calculations.…”

“…Experimental IR 103, 104 and semi-experimental 101 structural data for ethylene were compared to our parameters and the best reported theoretical results. 99,100 The observed harmonic (calculated benchmark data for cis-and trans-species) and fundamental wavenumbers for 1,1-difluoroethylene, 4, 5 cis-1,2-difluoroethylene, 3,29 and trans-1,2-difluoroethylene 3,8 were used for comparison with calculated frequencies.…”

A comparison of predicted and experimental vibrational spectra in some small fluorocarbons

“…The trans conformer has C 2h symmetry, and its 12 fundamentals are distributed in 5A g , 2A u , B g , and 4B u . After some preliminary assignments by early workers, Craig and coworkers 16,17 obtained a complete and consistent wave number assignment for the two isomers using liquid-phase Raman spectra, and infrared spectra in the gas phase and cold matrix. High-resolution IR studies of both the trans conformer 18 and the cis conformer 19 -21 provided some more precise band origins.…”

High-resolution infrared spectra of the two nonpolar isomers of 1,4-difluorobutadiene

“…The nearly perpendicular CF motions in these two modes are favorable for Coriolis coupling, and the direct product of these two modes has the correct symmetry species for Coriolis coupling. Such coupling could cause large perturbations in the rotational states and the intensities of the two bands, as was seen for trans-1,2-difluoroethylene, which is a similar case [14].…”

“…For the GS rotational constants, attempts to fit d J and d K gave uncertainties comparable to the values of the parameters. Consequently, GSCDs were fitted using fixed values for d J and d K predicted from a G03 calculation with a B3LYP/aug-cc-pVTZ model [12][13][14]. Supporting the use of the calculated values for d J and d K was the agreement within 3.2% between calculated D K , D JK , and D J and their observed values.…”