Km. Marstokk et H. Mollendal, STRUCTURAL AND CONFORMATIONAL PROPERTIES OF 1,3-DIFLUOROPROPANE AS STUDIED BY MICROWAVE SPECTROSCOPY AND AB-INITIO CALCULATIONS, Acta chemica Scandinavica, 51(11), 1997, pp. 1058-1065
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 t
otal 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 st
able 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 calculati
ons at the B3LYP/6-311 + + G* level. The structural parameters predic
ted in both these computational schemes are close to the accurate elec
tron-diffraction values apart from the C-F bond length that is systema
tically too long. The relative energies computed for the different con
formers were rather similar at both these levels of theory and in good
agreement with experiment.