Knowledge Bank

The sub-millimeter wave spectrum of the CoCl radical has been observed in the frequency range $340-510$ GHz using direct absorption techniques. This work is the first pure rotational study of this molecule in the laboratory. Rotational transitions from all three spin-orbit components ($\Omega =4,3$, and 2) have been recorded, along with magnetic hyperfine splittings due to the $\mathrm{<mrow\; data-mjx-texclass="ORD">59</mrow>}Co$ nucleus $(I=7/2)$. Transitions from the $Co37Cl$ isotopomer were also measured, as well as several excited vibrational modes. Interestingly, the $\Omega =3$ spin component was found to be shifted to lower frequency relative to the lowest spin state $(\Omega =4)$, while the $\Omega =2$ lies to higher frequency. Thus, this molecule is undergoing large perturbations, likely caused by excited electronic states. The data were fit with a case (a) Hamiltonian, and spectroscopic constants have been determined, improving upon those derived from previous optical data. These include the first assignment of the spinorbit constant A. The hyperfine parameters for CoCl, while similar to those found for other cobalt molecules, indicate a higher degree of covalency than in other such species. Comparison of periodic trends for transition metal chlorides and their fluoride analogues indicates that the two series are quite similar.