COMPLETE STRUCTURE OF THE ANTI ROTAMER OF 1,2-DIFLUOROETHANE FROM HIGH-RESOLUTION INFRARED-SPECTROSCOPY

B-type bands in the high-resolution, gas-phase infrared spectra of two isotopomers of 12-difluoroethane have been recorded, and analyzed in detail. These bands are due to the in-plane, antisymmetric CCF bending fundamental (v(18)) Of the low-abundance anti rotamer of the d(4) and C-13(2) species and are centered at 264.257 and 279.410 cm(-1), respe ctively. Watson-type rotational Hamiltonians have been fit to the grou nd state (GS) and exerted vibrational slate for each molecule, A compl ete structure for the anti rotamer has been derived from the GS rotati onal constants of these two isotopomers and the previously reported no rmal species. The r(0) values are r(CH) = 1.092 Angstrom, r(CC) = 1.50 6 Angstrom. r(CF) = 1.400 Angstrom, alpha(CCH) = 111.6 degrees, alpha( HCH) = 110.7 degrees, alpha(CCF) = 107.3 degrees, and alpha(FCH) = 107 .8 degrees. These geometric parameters are compared with those of the gauche rotamer and together with values for both rotamers from ab init io calculations. The changes in the parameters between the two rotamer s are discussed in relation to the gauche effect, the surprisingly low er electronic energy of the polar gauche rotamer.