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), respectively. Watson-type rotational Hamiltonians have been fit to the ground state (GS) and exerted vibrational slate for each molecule, A complete structure for the anti rotamer has been derived from the GS rotational constants of these two isotopomers and the previously reported normal species. The r(0) values are r(CH) = 1.092 Angstrom, r(CC) = 1.506 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 initio calculations. The changes in the parameters between the two rotamers are discussed in relation to the gauche effect, the surprisingly lower electronic energy of the polar gauche rotamer.