Conformational stability, r0 structural parameters, ab initio calculations, and vibrational assignment for fluorocyclopentane.

ABSTRACT

The infrared spectra (3200-50 cm(-1)) of the gas and solid and the Raman spectrum (3200-30 cm(-1)) of liquid and solid fluorocyclopentane, c-C5H9F, have been recorded. Additionally the infrared spectra (3200-400 cm(-1)) of liquid xenon solutions have been recorded at -65 and -95 degrees C. In all of the physical states, only the twisted C(1) conformer was detected. Ab initio calculations utilizing various basis sets up to MP2(full)/6-311+G(2df,2pd) with and without diffuse functions have been used to predict the conformational stabilities. These calculations predict only the twisted C1 conformer as the stable form. The two envelope (C(s) symmetry) forms with axial and equatorial structures were predicted to be first order saddle points with average higher energies of 75 +/- 33 and 683 +/- 44 cm(-1), respectively, from the C1 conformer but lower energies of 2442 and 1812 cm(-1), respectively, than the planar form by MP2 calculations. Similar values were obtained from the corresponding density functional theory calculations by the B3LYP method. A complete vibrational assignment is given for the twisted (C1) conformer which is supported by normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations. The adjusted r0 structural parameters have been obtained by systematically fitting the MP2(full)/6-311+G(d,p) predicted values with the rotational constants obtained from a microwave study. The determined heavy atom r0 distances in A are (C1C2) = 1.531(3), (C1C3) = 1.519(3), (C2C4) = 1.553(3), (C3C5) = 1.533(3), (C4C5) = 1.540(3), and (C1F6) = 1.411(3) and the angles in degrees are angle C3C1C2 = 105.5(5), angle C1C2C4 = 106.2(5), angle C1C3C5 = 102.9(5), angle F6C1C2 = 108.9(5), and angle F6C1C3 = 107.6(5) with a dihedral angle angle C2C4C5C3 = 25.3(3). These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.