The microwave spectrum of 1-chloromethyl-1-fluorosilacyclopentane has been recorded for the first time using the chirped pulse, Fourier transform microwave technique. Quantum chemical calculations show the two lowest energy conformers as being a twist-trans and a gauche form with the gauche form previously being shown as having two separate conformations, a gauche+ (lower in energy) and gauche- (higher in energy) form. Analysis of the spectrum provided the observation of the twist-trans conformer only, with 253 and 85 transitions being assigned to the 35Cl and 37Cl isotopologues, respectively. R-branch, a- and b-type transitions were observed. The spectrum was fit to a Watson S-reduced Hamiltonian and consisted of rotational constants, quartic centrifugal distortion constants, and nuclear quadrupole coupling constants, including the determination of the off-diagonal nuclear quadrupole coupling constant, χab. Interpretation of the structure was provided using second moments and is found to have a similar ring structure to other known silacyclopentanes. Analysis of the χzz has been carried out and compared to other similar molecules. An investigation of the known quantum chemical energies of the gauche conformer reveals that the reported B3LYP energies do not align with the observed microwave results.

FRANK E MARSHALL, Department of Chemistry, Missouri University of Science and Technology, Rolla, MO, USA; DANIEL V. HICKMAN, GAMIL A GUIRGIS, Chemistry, College of Charleston, Charleston, SC, USA; MICHAEL H. PALMER, School of Chemistry, University of Edinburgh, Edinburgh, United Kingdom; CHARLES J. WURREY, Department of Chemistry, University of Missouri Kansas City, Kansas City, MO, USA; NICOLE MOON, THOMAS D. PERSINGER, G. S. GRUBBS II, Department of Chemistry, Missouri University of Science and Technology, Rolla, MO, USA.

The FT-microwave spectrum (6.5-26 GHz) of (chloromethyl)fluorosilane (ClCH2-SiH2F) has been recorded and 250 transitions for the parent species along with (13)C, (37)Cl, (29)Si, and (30)Si isotopologues have been assigned for trans conformer. Infrared spectra (3100 to 400 cm(-1)) of gas, solid, and the variable temperature (-100 to -60 °C) studies of the infrared spectra of the sample dissolved in xenon have been recorded. Additionally, the variable temperature (-153 to -133 °C) studies of the Raman spectra of the sample dissolved in krypton have been recorded. The enthalpy difference between the trans and gauche conformers in xenon solutions has been determined to be 109 ± 15 cm(-1) (1.47 ± 0.16 kJ mol(-1)), and in krypton solution, the enthalpy difference has been determined to be 97 ± 16 cm(-1) (1.16 ± 0.19 kJ mol(-1)) with the trans conformer as the more stable form. Approximately 46 ± 2% of the trans form is present at ambient temperature. By utilizing the microwave rotational constants of five isotopologues for trans and the structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r0 parameters have been obtained for trans conformer. The r0 structural parameter values for the trans form are for the heavy atom distances (Å): Si-F = 1.608 (3); C-Cl = 1.771 (3); Si-C = 1.884 (3); and angles (deg): ∠FSiC = 108.9 (5); ∠ClCSi = 104.9 (5). The results are discussed and compared to some related molecules.

The infrared and Raman spectra of 1,1,3,3,5,5-hexafluoro-1,3,5-trisilacyclohexane (c-C3H6Si3F6) as a solid powder have been recorded. The vibrational spectra reveal that the crystalline compound exists as a chair conformer with C3v symmetry at ambient temperature. Additional conformers such as twist or boat were not detected although quantum chemical calculations indicated a negligible energy difference between chair, twist and boat forms. The wavenumbers of the IR and Raman bands were measured and the assignments were initially supported by quantum chemical B3LYP/cc-pVTZ calculations in the harmonic approximation. These vibrational modes were scaled with a common factor of 0.98, and the 11 totally symmetric A1 and 16 doubly degenerate E fundamentals were assigned on the basis of the infrared and Raman intensities. An average relative deviation of ca. 2.9% between the observed and the scaled harmonic calculations was found. Additional calculations were made in the anharmonic approximation, in which the symmetry was reduced to Cs symmetry by a slight distortion of the hexagonal ring. Employing these calculations without scaling, a better agreement between the observed and calculated wavenumbers was obtained (1.5%). Further calculations, involving a twist conformer was not consistent with the spectral results, neither in terms of the number of modes nor their wavenumbers.