The X̃ 1A1, ã 3B1 and à 1B1 Electronic States of the Aluminum Dihydride Anion

Abstract

The three lowest-lying electronic states of the aluminum dihydride anion (AlH2-) were systematically investigated using ab initio electronic structure theory. Self-consistent-field (SCF), two-configuration self-consistent-field (TCSCF), complete active space self-consistent-field (CASSCF), configuration interaction including single and double excitations (CISD), and CASSCF-based second-order configuration interaction (SOCI) levels of theory were employed with five basis sets of triple-ζ quality. All three electronic states were predicted to possess bent equilibrium geometries. Total electronic energies as well as physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities were determined for each state. At the CISD level with the largest basis set employed, triple-ζ plus triple polarization augmented with two sets of higher angular momentum functions and two sets of diffuse functions [TZ3P(2f,2d)+2diff)], the equilibrium geometries of the three states were predicted to be re = 1.681 Å and θe = 95.6° (X̃ 1A1), re = 1.617 Å and θe = 117.8° (ã 3B1), and re = 1.594 Å and θe = 118.7° (Ã 1B1). At the same level of theory, the dipole moments with respect to the center of mass were predicted to be 0.64 (X̃ 1A1), 0.03 (ã 3B1), and 0.24 D (Ã 1B1). The energy separations (T0) between the ground (X̃ 1A1) and first two excited states predicted at the CASSCF-SOCI level with the TZ3P(2f,2d)+2diff basis set were 14.1 (ã 3B1 ← X̃ 1A1) and 29.0 kcal mol-1 (Ã 1B1 ← X̃ 1A1).