Knowledge Bank

High-resolution rovibrational absorption spectra of the weakly bonded CO-$Cl2$ complex have been recorded in the $2143cm-1$ region by existing the CO chromophore with a tunable diode laser. The spectra indicate that CO-$Cl2$ is linear and semi-rigid. By fitting the data to a linear-molecule Hamiltonian, the following constants (in $cm-1$) were obtained; $v0=2149.5424(4),B\prime =0.031582339),B\prime =0.0314867(52),DJ\prime =4.37(25)\times 10-8$, and $DJ\prime =4.58(35)\times 10-8$. The distance between the CO and $Cl2$ centers of mass is approximately 4.78 \AA. The orientation of CO is not determined experimentally. However, $Cl2$ appears to act as a classical $\sigma$-electron acceptor, while CO behaves like a weak Lewis base, donating charge from the carbon side via the weakly antibonding 5$\sigma$ orbital, thereby raising the CO vibrational frequency.""