Knowledge Bank

We report a comprehensive new study of the electronic spectroscopy of the A$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B1$-X$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1$ system of CCl$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ in the region 400-600 nm. The spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotationally analyzed to yield precise values for the band origins and $A$ rotational constants. As described by Kable and co-workers,} the spectrum can be separated into three distinct regions. Region 1, lying below 20300 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, displays regular vibrational structure, and a Dunham expansion fit of the band origins for the C$\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}$Cl$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ and C$\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}$Cl$\mathrm{<mrow\; data-mjx-texclass="ORD">37</mrow>}$Cl isotopomers reproduces the experimental term energies to within a standard deviation of 1 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. In region 2, lying between 20300 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ and roughly 21500 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, the rotational structure of the bands is largely unperturbed; however, vibrational mixing is extensive due to near resonances among the states 1$^{n}$2$^{m}$ having the same polyad number $p$=$2n$+$m$. Above 21500 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ (Region 3), the rotational structure of the bands changes markedly, such that above 22500 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ only subbands terminating in $Ka\prime$ = 0 appear strongly in the spectra, indicating that the Renner-Teller (RT) intersection has been exceeded. In this talk, we will focus on the correlation of spectra and lifetime measurements; as found for other simple carbenes, the approach to the RT intersection is evidenced in a pronounced lifetime lengthening for states with $Ka\prime$ $>$ 0. The measured $A$ rotational constants exhibit a sudden and significant increase in the vicinity of the barrier. We will also compare our lifetime measurements with previous experimental results. Good agreement is found with a previous matrix isolation study when accounting for matrix effects; however, our lifetimes are not in good agreement with most previous gas-phase studies.