Ab initio molecular dynamics studies on the lowest triplet and singlet potential surfaces of the azide cation: Anharmonic effects on the vibrational spectra of linear and cyclic N3+

We study the finite temperature dynamical process followed by the ground state (linear triplet) and the first excited state (cyclic singlet) of the azide radical cation, N3+. Born-Oppenheimer ab initio molecular dynamics (AIMD) simulations are done at near-room temperature and at 700 K using a hybrid exchange-correlation (XC) functional. Basis set/XC functional calibrations were done and the vibrational spectra of both spin species are reported at the same level of theory for the first time. We have been able to identify the lowest vibrational modes for the triplet and the singlet species up to around 3000 cm−1, which correlate well with previous ab initio static predictions for the linear and cyclic species. The non-harmonic contributions on the spectra are found to be very large for the linear triplet modes involving ν1 and ν3 quanta. Our AIMD spectra reveal that the anharmonic effects are actually larger on the ν1 symmetric stretching (299 cm−1) than on the ν3 asymmetric stretching (165 cm−1) mode. The ν3 anharmonic effects are in good accordance with the results of Chambaud et al. [Chem. Phys. Lett. 231, 9 (1994)] obtained from static variational solutions to the nuclear Schrödinger equation. For the first excited state of the cation, the cyclic singlet, the vibrational peaks are in good agreement with the hyper-spherical coupled-channel spectrum based on the highly correlated CCSD/cc-pVTZ potential energy surface [J. Chem. Phys. 125, 084306 (2006)]; the anharmonic effects are considerably smaller for this cyclic closed-shell species. These AIMD spectra provide valuable information through the relative peak intensities which are absent in the previous theoretical studies for both species.