ANALYSIS OF THE B-TYPE BAND OF THE ANTI ROTAMER OF 1,2-DIFLUOROETHANE AT $284.260 CM^{-1}$
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Date
1995
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Publisher
Ohio State University
Abstract
The abundant form of 1,2-difluoroethane is the gauche rotamer $(about 85% at room temperature),^{1}$ for which a complete microwave-based structure is $known.^{2}$ No structure exists, however, for the non-polar anti rotamer. Although the IR spectrum of 1,2-difluoroethane is dominated by bands of the gauche rotamer, low resolution IR spectra revealed two promising bands for high resolution investigation of the anti rotamer, a C-type band at $3002 cm^{-1}$ and a B-type band at $284 cm^{-1,3,4}$ At high resolution the C-type band had some assignable $^{R}R_{K}$ series, but gauche rotamer lines overwhelmed the center of the band and the P branch. In contrast, the B-type band due to $\nu_{18}(b_{u})$, the antisymmetric CF bending mode, was essentially free of gauche lines and largely unperturbed. The B-type band of this near-prolate top with $\kappa = -0.9814$ was recorded at $0.0016 cm^{-1}$ resolution on a Bruker IFS 120 instrument. From the analysis of this band 1086 ground-state combination-differences gave, in $cm^{-1}, A^{\prime\prime} = 1.0573856(11), B^{\prime\prime} = 0.12939034(26) and C^{\prime\prime} = 0.12065486(19)$. For $K_{a}\leq 12$, fitting the upper state gave $\nu_{0} = 284.260056(14), A^{\prime} = 1.0781157(11), B^{\prime} = 0.129524818(39) and C^{\prime} = 0.120581478(37). Comparison of the ground-state rotational constants for the anti rotamer with those predicted from the geometric parameters of the gauche rotamer shows that the principal adjustment is a decrease in the CCF bond angle in going to the anti rotamer. The B-type bands of the $^{13}C_{2}$ and $d_{4}$ isotopomers have been recorded and are being analyzed. From the full set of rotational constants we expect to derive a complete structure for the anti rotamer of 1,2-difluoroethane.
Description
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Author Institution: Oberlin College, Oberlin, OH 44074; Justus-Liebig-Universität, D-35392 Giessen, Germany
Author Institution: Oberlin College, Oberlin, OH 44074; Justus-Liebig-Universität, D-35392 Giessen, Germany