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

The (2)Delta and (4)Sigma (-) excited states of Ga .N-2, which were assigne d by Ellis et al. (Phys. Chem. Chem. Phys. 1999, 1, 2709) to the upper stat es of two LIF transitions observed from the Ga .N-2 (X) over tilde (2)Pi St ate with onsets of 33468 and 37633 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, us ing 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 (4)Pi and a number of T-shaped quartet states of Ga .N-2 were also studied. Fran ck-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 33468 cm(-1) LIF band is concluded to be the (2)Delta (3/2) <-- (2)Pi (1/2) transition of Ga .N-2. In addition, the measured T-0 position of this band is confirmed and the assignments of the observed vibrational progressions in this LIF band have been revised. As for the 37633 cm(-1) LIF band, ab in itio results and spectral simulations computed in this work do not support the assignment of the upper state as the (4)Sigma (-) state of Ga .N-2, whi ch was shown by ab initio calculations to be a charge-transfer state with a short computed Ga-N bond length (ca. 2.0 Angstrom) and large intermolecula r vibrational frequencies (> 200 cm(-1)). In addition, all low-lying Ga .N- 2 quartet states considered were found to be either very weakly bound van d er Waals states (Ga-N bond length ca. 5 Angstrom) or well-bound charge tran sfer states, and none of them can be assigned to the upper state of this LI F band. Doubts concerning the identity of the molecular carrier and the ele ctronic states involved in this LIF band remain. Finally, the stabilities o f the charge-transfer quartet states of Ga .N-2 investigated in this work h ave been rationalized in terms of bonding interaction between the HOMOs of Ga and the LUMOs of N-2 and electrostatic attraction resulting from charge transfer from Ga to N-2. Possible applications of this kind of bonding and charge-transfer interactions in an M .N-4 ring system have been discussed b riefly in relation to stabilizing an N-n system, where M .N-n represents a potential high energy density material.