The Electronic Structure of ScAl + . Ground and Low-Lying Excited States Demeter Tzeli and Aristides Mavridis* Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrian UniVersity of Athens, P.O. Box 64 004, 157 10 Zografou, Athens, Greece ReceiVed: March 7, 2000; In Final Form: May 5, 2000 Using semiquantitative basis sets and ab initio multireference methods, we have investigated the electronic structure of scandium aluminide cation, ScAl + . In addition to the ground state (X 2 Δ), we have constructed potential energy curves for 20 more states spanning an energy range of no more than 1.5 eV. The first three states, X 2 Δ,1 2 Π, and 2 2 Σ + , are practically degenerate within the accuracy of our calculations. They have similar binding modes and a binding energy of about 30 kcal/mol with respect to their adiabatic fragments Sc( 2 D) + Al + ( 1 S). The rest of the states correlate to Sc + ( 3 D or 3 F) + Al( 2 P). For all states we report bond lengths, dissociation energies, harmonic frequencies, Mulliken charges, and energy gaps. 1. Introduction The 3d transition metal aluminides form an important class of high-temperature materials with some of them being candi- dates for permanent magnets. 1 A systematic study through electronic spectroscopy of 3d transition metal aluminides but with the exception of the ScAl species is given in ref 2, while for ScAl alloys there are some limited experimental data on enthalpies of formation. 3-5 Given the increasing importance of these materials, our interest in the isovalent metal borides, M-B + (M ) Sc, Ti, V, Cr), 6 and the complete lack of experimental and/or theoretical information on ScAl + , we have decided to probe the electronic structure of this diatomic cation by ab initio quantum methods. In the present work the electronic structure of ScAl + is investigated via complete active space SCF (CASSCF) and multireference configuration interaction (MRCI ) CASSCF + single + double replacements ) CASSCF +1 + 2) methods. In addition to the ground state (X 2 Δ, vide infra), 20 low-lying excited states have been also examined: 10 doublets {2 2 Σ + , 11 2 Σ + ,7 2 Σ - , 20 2 Σ - ,1 2 Π,9 2 Π, 13 2 Π,8 2 Δ, 14 2 Δ, 12 2 Φ} and 10 quartets {15 4 Σ + , 19 4 Σ + ,3 4 Σ - , 18 4 Σ - ,4 4 Π, 10 4 Π, 17 4 Π,6 4 Δ, 16 4 Δ,5 4 Φ}. One of the interesting aspects of the present report is the comparable ionization potentials of Sc and Al atoms, 6.56 and 5.98 eV, respectively; 7 as a result the ground (X 2 Δ) and the first two excited states (1 2 Π,2 2 Σ + ) of ScAl + correlate to Sc( 2 D) + Al + ( 1 S), while the rest of the states correlate to Sc + - ( 3 D or 3 F) + Al( 2 P). For all 21 states examined we report full potential energy curves (PEC), binding energies (D e ), bond distances (R e ), harmonic frequencies (ω e ), Mulliken populations, and energy gaps (T e ), while emphasis has been given in deciphering the bonding mechanism(s) with the help of simple valence bond Lewis (vbL) diagrams. 2. Methods For the Sc atom the ANO basis set of Bauschlicher 8 (21s16p9d6f4g) has been used but with the functions of g angular momentum removed. For the Al atom, the cc-pVTZ (15s9p2d1f) basis set of Dunning 9 was employed. Both sets were generally contracted to [(7s6p4d3f) Sc /(5s4p2d1f) Al ], numbering 100 spherical Gaussians. As already indicated, the complete active space SCF (CASS- CF) methodology was employed to describe the reference space, followed by single and double configuration interaction out of the CASSCF space (CASSCF + 1 + 2 ) MRCI). Five “valence” (active) electrons were distributed to 10 orbital functions for the quartets (one 4s and five 3d’s on Sc + one 3s and three 3p’s on Al), and to 11 orbitals (+ one 4p z ) for the doublets. The additional 4p z orbital function was deemed necessary for the X 2 Δ,1 2 Π, and 2 2 Σ + states correlating to Sc + Al + , but for reasons of uniformity the 11 orbital space was maintained in all doublets. Depending on the number of orbitals and the symmetry of the state, our reference spaces range from 432 to 1354 configuration functions (CF), with corresponding CI numbers ranging from 431 318 to 871 611 CFs at the MRCI level. By applying the internal contraction (icMRCI) technique, 10 the number of CFs is reduced by about half. All calculations were done under C 2V symmetry constraints; however care was exercised for the CASSCF wave functions to display correct axial angular momentum symmetry, i.e., |Λ| ) 0, 1, 2, and 3 or Σ ( , Π, Δ, and Φ, respectively. This means that Δ states are linear combinations of A 1 and A 2 symmetries, Π and Φ states are combinations of B 1 and B 2 symmetries, and Σ + and Σ - states correspond to the A 1 and A 2 symmetry species, respectively. MRCI wave functions do not display in general pure axial symmetry, being calculated as A 1 or A 2 and B 1 or B 2 . With the exception of the X 2 Δ,1 2 Π,2 2 Σ + , and 3 4 Σ - states, the state average (SA) approach 11,12 was used for the computation of all other states. Numerical experiments performed for some of the lowest states with and without the SA method and with and without internal contraction showed absolute energy losses of no more than 2 mhartrees collectively. Finally, the size nonextensivity errors of our SA-icMRCI wave functions are less than 1 mhartree. All calculations were performed with the MOLPRO suite of codes, 13 while some of the lower states were also checked with the COLUMBUS code. 14 3. Description of the Atoms Absolute energies of Sc( 2 D), Sc + ( 3 D, 1 D, 3 F), and Al( 2 P), Al + - ( 1 S), atomic energy separations, and ionization potentials are listed in Table 1. The SCF and CASSCF calculations were done 6861 J. Phys. Chem. A 2000, 104, 6861-6870 10.1021/jp000894+ CCC: $19.00 © 2000 American Chemical Society Published on Web 07/01/2000