The (X)over-tilde(1)A(1), (a)over-tilde(3)B(1) and (A)over-tilde(1)B(1) electronic states of the aluminum dihydride anion

The three lowest-lying electronic states of the aluminum dihydride anion (A lH2-) were systematically investigated using ab initio electronic structure theory. Self-consistent-field (SCF), two-configuration self-consistent-fie ld (TCSCF), complete active space self-consistent-field (CASSCF), configura tion 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-xi quality. All three electronic st ates were predicted to possess bent equilibrium geometries. Total electroni c energies as well as physical properties including dipole moments, harmoni c vibrational frequencies, and associated infrared (IR) intensities were de termined for each state. At the CISD level with the largest basis set emplo yed, triple-xi plus triple polarization augmented with two sets of higher a ngular momentum functions and two sets of diffuse functions [TZ3P-(2f.2d)2 diff)], the equilibrium geometries of the three states were predicted to be r(e) = 1.681 Angstrom and theta(e) = 95.6 degrees ((X) over tilde (1)A(1 )), r(e) = 1.617 Angstrom and theta(e) = 117.8 degrees ((a) over tilde B-3( 1)), and r(e) = 1.594 Angstrom and theta(e) = 118.7 degrees ((A) over tilde B-1(1)). At the same level of theory, the dipole moments with respect to t he center of mass were predicted to be 0.64 ((X) over tilde (1)A(1)), 0.03 ((a) over tilde B-3(1)), and 0.24 D ((A) over tilde B-1(1)). The energy sep arations (T-0) between the ground ((X) over tilde (1)A(1)) and first two ex cited states predicted at the CASSCF-SOCI level with the TZ3P(2f,2d)+2diff basis set were 14.1 ((a) over tilde B-3(1) <-- (X) over tilde (1)A(1)) and 29.0 kcal mol(-1) ((A) over tilde B-1(1) <-- (X) over tilde (1)A(1)).