Computational Study of the Rovibrational Spectrum of CO2–N2

The CO$_2$--N$_2$ complex is formed from two key components of Earth's atmosphere, and as such, has received some attention from both experimental and theoretical studies. On the theory side, a potential energy surface (PES) based on high level \textit{ab initio} data was reported [Nasri \textit{et al.}, J. Chem. Phys., 2015, \textbf{142}, 174301] and then used in more recently reported rovibrational calculations [Lara-Moreno \textit{et al.}, Phys. Chem. Chem. Phys., 2019, \textbf{21}, 3550]. Accuracy of about 1 percent was achieved for calculated rotational transitions of the ground vibrational state of the complex, compared with previously reported microwave spectra. However, a very recent measurement of the geared bending mode frequency [Barclay \textit{et al.}, J. Chem. Phys., 2020, \textbf{153}, 014303] recorded a value of 21.4~cm$^{-1}$, which is wildly different from the corresponding calculated value of 45.9~cm$^{-1}$. To provide some insight into this discrepancy, we have constructed a new more accurate PES, and used it to perform highly converged variational rovibrational calculations. %using a numerically exact Hamiltonian. Our new results yield a value of 21.1~cm$^{-1}$ for that bending frequency, in close agreement with the experiment. We also obtain significantly improved predicted rotational transitions. Finally, we note that a very shallow well, previously reported as a distinct second isomer, is not found on our new PES, but rather a transition structure is seen in that location.