Knowledge Bank

Samples of {\em cis,cis}- and {\em trans,trans}-1,4-difluorobutadiene-1-d$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ (DFBD) and {\em trans,trans}-DFBD-1,4-d$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ have been synthesized and investigated with high-resolution (0.0015 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) infrared spectroscopy. For the first two species the rotational structure in more than one band has been analyzed. For the 1,4-d$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ species the spectrum of only one C-type band was available in an isotopic mixture. Ground state rotational constants are reported for all three molecules. It is proposed that quartic centrifugal distortion constants computed with a B3LYP/cc-pVTZ model can be used to assess the quality of observed rotational constants. The favorable comparison of predicted and observed ground state rotational constants for all four $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}$C species of {\em cis,trans}-DFBD, which is MW active, demonstrates that the ground state rotational constants for the $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}$C species of the {\em cis,cis} and {\em trans,trans} isomers can be successfully predicted with high accuracy. Rotational constants for a full set of isotopologues will be used to determine accurate semiexperimental equilibrium structures of the {\em cis,cis} and {\em trans,trans} species of DFBD.