Knowledge Bank

The high-resolution infrared spectrum has been recorded for two C-type bands of {\em cis}-1,3,5-hexatriene. The resolution (0.0013 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) and the Doppler width (0.0012 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ at 900 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) are barely adequate for observing detailed rotational structure of this heavy molecule in a spectrum recorded at room temperature. An additional complication is the extensive hot band structure arising from excited states of the low frequency C-C torsional modes. A preliminary analysis of rotational structure yielded $R$ and $P$ series in each of the two bands, centered at 908 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ and 586 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. However, ground state combination differences (GSCDs) failed to fit a rotational Hamiltonian. Subsequent microwave spectroscopic measurements gave ground state rotational constants. Reliable GSCDs computed from the ground state rotational constants led to revisions in assignments of some of the sub-band series in the infrared spectrum and to a convincing assignment.