Structural and conformational properties of 1,2-difluoropropane as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectrum of 1,2-difluoropropane has been investigated in the 10.0-40.0 GHz spectral region at dry-ice temperature (-78 degrees C). Three all-staggered rotameric forms are possible for this compound. Two of these rotamers, denoted Conformer I and Conformer II, respectively. were assigned in this work, These two forms both have a F-C-C-F gauche atomic arrangement. The methyl group is anti to the Cl-F bond in Conformer I and gauche to this bond in Conformer II. The third form, Conformer III, which has a F-C-C-F anti arrangement, is likely to be present but could not be assigned, presumably because of its small dipole moment. Conformer II is 1.2(4) kJ mol(-1) more stable than I. The dipole moments (in units of 10(-30) C m) are mu(a) = 5.12(4), mu(b) = 8.64(8), mu(c) = 0.11(2), and mu(tot) = 10.05(8) for Conformer I, and mu(a) = 1.108(3), mu(b) = 4.46(3). mu(c) = 8.30(6), and mu(tot) = 9.49(6) for Conformer II, respectively. Three vibrationally excited states of I belonging to three different normal modes were assigned, while two excited states of two different normal modes were assigned for II. The barrier to internal rotation of the methyl group in Conformer I was determined from the splittings of the first excited states of the methyl group torsional vibration and is 11.88(20) kJ mol(-1). 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 functional theory calculations at the B3LYP/6-311++G** level. Internal energy differences between the three conformers of less than 0.5 kJ mol(-1) were computed at both these levels of theory. The best predictions of the rotational constants were found in the MP2/6-311++G** computations which are therefore assumed to predict the most accurate geometries for the conformers. Best predictions of the dipole moment are found in the B3LYP/6-311++G** calculations.