A partial structure of the anti rotamer of 1,2-difluoroethane from the analysis of a band in the high-resolution infrared spectrum

Two regions in the infrared spectrum of gaseous 1,2-difluoroethane at room temperature have been investigated at high resolution. Although bands due to the abundant gauche rotamer dominate the spectrum, a C-type band centered at 3001.89 cm(-1) and a largely B-type band centered at 284.260 cm(-1) have been shown to be due to the anti rotamer. From its rotational structure the C-type band is confirmed as being due to nu(7), the antisymmetric CH2 stretching mode of a(u) symmetry. The B-type band is due to nu(18), which is largely the antisymmetric CF bending mode of b(u) symmetry. The rotational structure of the B-type band, recorded with exceptional resolution in a difficult spectral region, has been analyzed in detail. From the assignment of over 2000 lines, rotational constants have been fitted to the ground state and the upper state. The ground state constants are 1.057 385 7 (11), 0.129 390 34 (26), and 0.120 654 86 (19) cm(-1) for this near-prolate symmetric top (kappa = -0.9813), These rotational constants imply an increase of 2.5 degrees in the CCF bond angle in going from the anti to the gauche rotamer. This adjustment is consistent with significantly larger CF bond dipole repulsion in the gauche rotamer than in the anti, despite the lower electronic energy of the gauche rotamer.