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 compoun d. Two of these rotamers, denoted Conformer I and Conformer II, respec tively. 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 for m, Conformer III, which has a F-C-C-F anti arrangement, is likely to b e present but could not be assigned, presumably because of its small d ipole moment. Conformer II is 1.2(4) kJ mol(-1) more stable than I. Th e 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, an d 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 stat es 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 micr owave work has been assisted by ab initio computations at the MP2/6-31 1++G* (frozen core) level of theory, as well as density functional th eory calculations at the B3LYP/6-311++G* level. Internal energy diffe rences between the three conformers of less than 0.5 kJ mol(-1) were c omputed at both these levels of theory. The best predictions of the ro tational 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 B3 LYP/6-311++G* calculations.