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

Abstract

The microwave spectra of four isotopomers, CH 3CH 2 12CHF 2, CH 3CH 2 13CHF 2, CD 3CH 2 12CHF 2 and CD 3CD 2 12CHF 2, 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 Δ J , Δ JK , δ J and Δ 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: | μ a|=2.044(3), | μ b|=0.851(5), | μ c|=0.711(35) and | μ tot|=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 CHF 2 moiety has been determined from the isolated stretching frequency from the CD 3CD 2 12CHF 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=1.370(5), rC 1–F 6=1.372(5), rC 1–C 2=1.505(3), rC 2–C 3=1.522(3) Å, ∠C 1C 2C 3=112.2(5), ∠C 2C 1F 5=109.8(5), ∠C 2C 1F 6=110.9(5)°. The adjusted r 0 parameters have also been obtained for 2,2-difluoropropane, CH 3CF 2CH 3. The results are compared to those of some similar molecules.