Torsional spectra of molecules with two C3v rotors—XXV. Rotational and vibrational spectra, r0 structure, barriers to internal rotation and ab initio calculations for 2,2-difluoropropane

Abstract

The microwave spectra of two isotopic species of 2,2-difluoropropane, (CH 3) 2 13CF 2 and (CD 3) 2 12CF 2, have been recorded from 26.5 to 40.0 GHz and the b-type R-branch transitions have been observed and assigned for the ground state. Utilizing the rotational transitions for these two isotopomers along with those for the normal species the following r o structural parameters have been determined: r(C-H) = 1.091±0.002Å, r(C-C) = 1.510 + 0.005 Å, r(C-F) = 1.378±0.005 Å, ∢CCH = 109.26±0.05°, ∢CCC = 116.44±0.54° and ∢FCF = 105.15±0.50°. The carbon-hydrogen distances have also been obtained from the frequency of the isolated carbon-hydrogen frequency in the IR spectrum. The far-IR spectra of 2,2-difluoropropane- d 0 and - d 6 in the gas were recorded at a resolution of 0.10 cm −1 and the torsional fundamentals, along with several hot band transitions, have been assigned. From these data the barrier to internal rotation of the coupled rotors is 1103±48cm −1 (3.15±0.14kcalmol −1). The IR spectra (3600–50cm −1) of gaseous and solid and the Raman spectra (3600–10 cm −1) of gaseous, liquid and solid (CD 3) 2CF 2 have been recorded. A complete vibrational assignment is proposed for both the normal and d 6 molecules based on depolarization values, relative intensities, isotopic shifts and normal coordinate calculations. The structural parameters, barriers to internal rotation and fundamental vibrational frequencies which have been determined experimentally are compared with those obtained from ab initio Hartree-Fock gradient calculations utilizing both the 3–21G and 6–31G* basis sets along with electron correlation at the MP2 level, and to the corresponding quantities obtained for some similar molecules.