Knowledge Bank

Rovibrational transitions of the $\nu 3$ band (C-O anti-symmetric stretch) of the $Co(CO)2$ radical were observed in the frequency region of $1944-1964cm-1$. The $Co(CO)2$ radical was produced by the 248 nm excimer laser photolysis and the transient absorption signal was detected by time-resolved infrared diode laser spectroscopy. More than 160 absorption lines were assigned to the $\nu 3$ fundamental band. The $\nu 3$ band consists of two series of spectral lines spaced approximately by four times the rotational constant, due to the spin statistics of identical C and O nuclei, and the electronic ground state was confirmed to be $X2\Delta g$. Molecular constants including the band origin $\nu 0$, rotational constant, and centrifugal distortion constant were derived from the observed transitions. Hot bands originating from the $\nu 7$ and $2\nu 7$ vibrational excited states, where $\nu 7$ refers to the C-Co-C bending mode, were also observed. Because of the rovibronic interaction, each of the observed lines for the $\nu 3+\nu 7\leftarrow \nu 7$ and $\nu 3+2\nu 7\leftarrow 2\nu 7$ hot bands was split into two and three components, respectively. Ab initio calculation in $BP86/6-311+G\ast$ level suggests that the $Co(CO)2$ radical is slightly bent and has the $X2A1$ electronic ground state. The present experiment shows that the $Co(CO)2$ radical is linear in average, and it may probably be a quasi-linear molecule in which the ground state has energy higher than the top of the potential bump.