Complete structure of gauche 1,1,2,2-tetrafluoroethane determined by microwave spectroscopy

The complete molecular structure of the high-energy gauche rotamer of 1,1,2 ,2-tetrafluoroethane (TFEA), Freon F134, has been determined using pulsed-m olecular-beam Fourier transform microwave spectroscopy. Five different C-13 - and H-2-substituted isotopomers, in addition to the normal species, were studied. Unlike the nonpolar anti rotamer, the gauche form has a permanent electric dipole moment, which permits a direct rotational analysis. The ele ctric dipole moment was also determined using the Stark effect of two low-J transitions. The resulting dipole moment is mu (c) = 8.186(7) x 10(-30) C m [2.454(2) Debye] where the numbers in parentheses are the standard uncert ainties (i.e., 1 sigma). The experimental results for the geometric paramet ers of TFEA are compared with two recent ab initio calculations.