Microwave spectrum, r0 structure, barriers to internal rotation and ab initio calculations of gauche-1,1-difluoropropane

The microwave spectra of four isotopomers, CH3CH212CHF2, CH3CH213CHF2, CD3CH212CHF2 and CD3CD212CHF2, of gauche-1,1-difluoropropane, 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 ΔJ, ΔJK, δJ and ΔK. The dipole moment components were evaluated from the measurements of the Stark effects of the ground state rotational transitions and the determined values are: |μa|=2.044(3), |μb|=0.851(5), |μc|=0.711(35) and |μtot|=2.326(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)/6-311+G(d,p) from which structural parameters, barriers to internal rotation, and distortion constants have been obtained. The C-H distance of 1.095±0.002 Å for the CHF2 moiety has been determined from the isolated stretching frequency from the CD3CD212CHF2 molecule. The r0 parameters have been obtained by combining the ab initio predicted values with the microwave rotational constants and these parameters for the heavy atoms are: rC1-F5=1.370(5), rC1-F6=1.372(5), rC1-C2=1.505(3), rC2-C3=1.522(3) Å, ∠C1C2C3=112.2(5), ∠C2C1F5=109.8(5), ∠C2C1F6=110.9(5)°. The adjusted r0 parameters have also been obtained for 2,2-difluoropropane, CH3CF2CH3. The results are compared to those of some similar molecules.