15min:

DANIEL A. OBENCHAIN, AMANDA L. STEBER, ASHLEY A. ELLIOTT, REBECCA A. PEEBLES AND SEAN A. PEEBLES, Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, IL 61920; CHARLES J. WURREY, Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110; GAMIL A. GUIRGIS, Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424.

A chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer based on the original Pate design has been constructed to allow analysis of any 480 MHz region in the 7 -- 18 GHz range. A 1 µs chirped-pulse (0 -- 240 MHz) from an arbitrary function generator is mixed with output from a microwave synthesizer and used to polarize a supersonic gas expansion; the resulting free induction decay is collected over 20 µs and Fourier-transformed on a 500 MHz oscilloscope to produce a rotational spectrum. A variety of molecules have now been studied with this instrument and results will be presented for numerous conformers of divinyl silane (predicted µ _total = 0.6 - 0.7 D) and the more polar 3,3-difluoropentane (predicted µ _total = 2.5 - 2.8 D).

Two of the three possible conformers of divinyl silane were assigned (both having a C₁=C₂--Si--C₃ dihedral angle of -120^o and a \hboxC₂--Si--C₃=C₄ dihedral of either 0^o (C₁ symmetry) or -120^o (C₂ symmetry)). For 3,3-difluoropentane, three of the four possible \hboxconformers were identified: anti-gauche (C₁), gauche-gauche (C₂) and anti-anti (C_2v). While rotational spectra for only the silicon isotopologues were observed for divinyl silane, measurement of the ¹³C spectra of 3,3-difluoropentane allowed heavy atom structure determinations for the anti-gauche and gauche-gauche conformers. Initial assignments of all spectra were made on the CP-FTMW \hboxspectrometer, and a Balle-Flygare FTMW spectrometer was used to compare frequencies of measured transitions and also to provide Stark effect data. Substitution (r_s) and inertial fit (r₀) structures will be compared with computational data and instrumental details will be presented.