Ab initio calculations on the (1)₂Δ excited state and low-lying quartet states of Ga-N2: Simulation of its LIF spectrum

The2Δ and4∑ excited states of Ga-N2, which were assigned by Ellis et al. (Phys. Chem. Chem. Phys. 1999, 1, 2709) to the upper states of two LIF transitions observed from the Ga-N2X2II state with onsets of 33 468 and 37 633 cm-1, respectively, have been studied by high-level ab initio calculations. Minimum-energy geometrical parameters, harmonic vibrational frequencies, and relative energies were computed at the SERHF, CASSCF, B3LYP, MP2, QCISD, and CCSD(T) levels of calculation, using standard and specifically designed, all-electron and ECP (for Ga) basis sets of up to aug-cc-pVQZ quality. In addition, the low-lying linear 4n and a number of T-shaped quartet states of Ga·N2were also studied. Franck-Condon factors (FCFs) of selected electronic transitions were calculated. Absorption spectra were simulated by employing the computed FCFs. On the basis of ab initio results and spectral simulations, the assignment of the 33 468 cm-1LIF band is concluded to be the2Δ3/2←2Π1/2transition of Ga·N2. In addition, the measured T0position of this band is confirmed and the assignments of the observed vibrational progressions in this LIF band have been revised. As for the 37 633 cm-1LIF band, ab initio results and spectral simulations computed in this work do not support the assignment of the upper state as the4∑ state of Ga·N2, which was shown by ab initio calculations to be a charge-transfer state with a short computed Ga-N bond length (ca. 2.0 A) and large intermolecular vibrational frequencies (>200 cm-1). In addition, all low-lying Ga-N2quartet states considered were found to be either very weakly bound van der Waals states (Ga-N bond length ca. 5 A) or well-bound charge transfer states, and none of them can be assigned to the upper state of this LIF band. Doubts concerning the identity of the molecular carrier and the electronic states involved in this LIF band remain. Finally, the stabilities of the charge-transfer quartet states of Ga·N2investigated in this work have been rationalized in terms of bonding interaction between the HOMOs of Ga and the LUMOs of N2and electrostatic attraction resulting from charge transfer from Ga to N2. Possible applications of this kind of bonding and charge-transfer interactions in an M·N4ring system have been discussed briefly in relation to stabilizing an Nnsystem, where M·Nnrepresents a potential high energy density material.