The microwave spectra of four isotopomers, CH 3 CH 2 12 CHF 2 , CH 3 CH 2 13 CHF 2 , CD 3 CH 2 12 CHF 2 and CD 3 CD 2 12 CHF 2 , of gauche-1,1difluoropropane, in the ground vibrational state were recorded and assigned. From the measured frequencies for the R and Q branches of the a-, b-and c-type transitions, accurate values for the rotational constants were obtained along with values for D J , D JK , d J and D K . The dipole moment components were evaluated from the measurements of the Stark effects of the ground state rotational transitions and the determined values are: jm a jZ2.044(3), jm b jZ0.851(5), jm c jZ0.711(35) and jm tot jZ2.326(3) D. Ab initio molecular orbital calculations have been carried out with valence electron correlation by the perturbation method to second-order with various basis sets up to MP2(full)/6-311C G(d,p) from which structural parameters, barriers to internal rotation, and distortion constants have been obtained. The C-H distance of 1.095G0.002 A Λfor the CHF 2 moiety has been determined from the isolated stretching frequency from the CD 3 CD 2
12 CHF 2 molecule. The r 0 parameters have been obtained by combining the ab initio predicted values with the microwave rotational constants and these parameters for the heavy atoms are: rC 1 -F 5 Z1.370(5), rC 1 -F 6 Z1.372(5), rC 1 -C 2 Z1.505(3), rC 2 -C 3 Z1.522(3) A Λ, :C 1 C 2 C 3 Z112.2(5), :C 2 C 1 F 5 Z 109.8(5), :C 2 C 1 F 6 Z110.9(5)8. The adjusted r 0 parameters have also been obtained for 2,2-difluoropropane, CH 3 CF 2 CH 3 . The results are compared to those of some similar molecules.