Spectroscopic study of magnesium dinitrogen and sodium dinitrogen cation

ABSTRACT

Despite its likely importance in astrochemistry, pure rotational spectra are not observable for gas-phase N2 since this molecule has no permanent dipole moment. Complexation of monomeric N2 with a cationic metal (MN2+) may be kinetically and thermodynamically favourable, and the detection of such MN2+ molecules could be useful tracers of N2 in order to probe its abundance and kinetics. Highly accurate quartic force field methods have been applied here to compute rotational and vibrational spectroscopic properties of the NaN2+ and MgN2+ molecules via a coupled cluster-based composite approach with additional corrections for post-CCSD(T) electron correlation and relativistic effects. The relative energies of various isomers have also been computed and show that both NaN2+ and MgN2+ have linear ground electronic states. At the highest level of theory, rotational constants (B0) of 4086.9 and 4106.0 MHz are predicted for NaN2+ and MgN2+, respectively, with dipole moments of 6.92 and 4.34 D, respectively, making them rotationally observable even at low concentrations. Post-CCSD(T) electron correlation corrections lower the N–N stretching frequency while relativistic corrections have a much smaller effect putting the fundamental frequencies at 2333.7 and 2313.6 cm−1, respective of NaN2+ and MgN2+ slightly above that in N2H+. Additive corrections do not significantly change the other two vibrational modes. An anharmonic, zero-point corrected N2 dissociation energy of 7.3 and 7.0 kcal mol−1 is, respectively, reported for NaN2+ and MgN2+ suggesting possible formation of these molecules in protoplanetary discs or planetary nebulae that are metal- and nitrogen-rich.