Structural and conformational properties of 1,3-difluoropropane as studied by microwave spectroscopy and ab initio calculations

The microwave spectrum of 1,3-difluoropropane has been investigated in the 10.0-36.0 GHz spectral region at dry-ice temperature (-79 degrees C). Four all-staggered rotameric forms are possible for this compound. The gas phase consists almost exclusively of an equilibrium mixture of two rotameric forms of the molecule denoted GG and AG for which a total of about 2600 transitions have been assigned. The GG rotamer was found to be the most stable conformer and is 4.0(2) kJ mol(-1) more stable than AG. The dipole moment is lying along the b-inertial axis in the GG conformer for symmetry reasons and is 6.946(29) x 10(-30) C m. The dipole moment of the AG conformer is (same units) mu(a) = 3.59(5), mu(b) = 5.32(9), mu(c) = 1.02(19), mu(tot) = 6.50(11). The microwave work has been assisted by ab initio computations at the MP2/6-311 + + G** (frozen core) level of theory, as well as density theory calculations at the B3LYP/6-311 + + G** level. The structural parameters predicted in both these computational schemes are close to the accurate electron-diffraction values apart from the C-F bond length that is systematically too long. The relative energies computed for the different conformers were rather similar at both these levels of theory and in good agreement with experiment.