Knowledge Bank

The abundant form of 1,2-difluoroethane is the gauche rotamer $(about85$ 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 $3002cm-1$ and a B-type band at $284cm-1,3,4$ At high resolution the C-type band had some assignable series, but gauche rotamer lines overwhelmed the center of the band and the P branch. In contrast, the B-type band due to $\nu 18(bu)$, 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 $0.0016cm-1$ resolution on a Bruker IFS 120 instrument. From the analysis of this band 1086 ground-state combination-differences gave, in $cm-1,A\prime \prime =1.0573856(11),B\prime \prime =0.12939034(26)andC\prime \prime =0.12065486(19)$. For $Ka\le 12$, 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 $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C2$ and $d4$ 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.