Complete structure of anti-1,1,2,2-tetrafluoroethane by high-resolution infrared spectroscopy

High-resolution (0.0018 cm(-1)) infrared spectra have been recorded in the CF stretching region for 1,1,2,2-tetrafluoroethane and its d(2) and C-13(2) isotopomers. These bands are sufficiently intense to give useful gasphase spectra at -100 degreesC in a 3 m cell. For each spectrum the rotational structure of the A/C-type and B-type bands due to the anti rotamer, which is a highly asymmetric rotor with kappa = -0.295, have been analyzed. Rotational constants for a Watson-type Hamiltonian have been fitted for the common ground state of each isotopomer and the separate upper states. A complete structure for the nonpolar anti rotamer has been derived and compared with the structure of the polar gauche rotamer obtained from microwave spectroscopy, The geometric parameters for the anti rotamer are r(CH) 1.087(5) Angstrom, r(CC) = 1.511(4) Angstrom, r(CF) = 1.359(7) Angstrom, alpha (CCH) = 112.9(3)degrees, alpha (CCF) = 108.5(6)degrees, alpha (FCF) = 107.4(6)degrees, and tau (CCF) = 63.6(12)degrees. The structural parameters for the two isomers of 1,1,2,2-tetrafluoroethane are compared with recent calculations, and trends in these parameters and those of the rotamers of 1,2-difluoroethane are discussed.