Knowledge Bank

A chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer based on the original Pate design., {\bf 79}, (2008), 053103.} has been constructed to allow analysis of any 480 MHz region in the 7 -- 18 GHz range. A 1 $\mu$s 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 $\mu$s 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 $\mu total=0.6-0.7$ D) and the more polar 3,3-difluoropentane (predicted $\mu total=2.5-2.8$ D). \vspace{1em} Two of the three possible conformers of divinyl silane were assigned (both having a C$1$=C$2$--Si--C$3$ dihedral angle of $-120irc$ and a {\hbox{C$2$--Si--C$3$=C$4$}} dihedral of either $0irc$ ($C1$ symmetry) or $-120irc$ ($C2$ symmetry)). For 3,3-difluoropentane, three of the four possible {\hbox{conformers}} were identified: anti-gauche ($C1$), gauche-gauche ($C2$) and anti-anti ($C2v$). While rotational spectra for only the silicon isotopologues were observed for divinyl silane, measurement of the $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}$C 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 {\hbox{spectrometer}}, and a Balle-Flygare FTMW spectrometer was used to compare frequencies of measured transitions and also to provide Stark effect data. Substitution ($rs$) and inertial fit ($r0$) structures will be compared with computational data and instrumental details will be presented.