Acetylacetonate (acac) anion in the gas phase: predicted structures, vibrational spectra, and photodetachment energies

The geometry and vibrational spectrum of the gas-phase acetylacetonate anion [acac(-); (CH(3)CO)(2)CH(-)] are predicted using ab initio molecular orbital theory. At the MP2/6-31+G**//HF/6-31+G* level there are three stable conformers. In order of increasing energy, they are (1) a C(s) structure with the carbonyl groups in an anti conformation, (2) a C(2) structure with the carbonyls pointed exo (about 21 kJ mol(-1) higher than 1), and (3) a C(2v) structure with the carbonyls endo (about 27 kJ mol(-1) higher than 1). The third, least stable conformation corresponds to the geometry found in moat beta-diketonate coordination complexes. These three conformations are separated by barriers of about 60 kJ mol(-1). The infrared spectra of 1 and 2 are quite similar, but that of the endo structure 3 features two strong peaks instead of only one in the 1400-1700 cm(-1) region. The enol (4) and di-enol (5) isomers are 60 kJ mol(-1) and 230 kJ mol(-1) respectively, less stable than structure 1. Vertical photodetachment of acac- can produce neutral acac in a few low-lying electronic states. At the spin-restricted B3LYP/6-31+G*//HF/6-31+G* level, the lowest vertical detachment energies are predicted to be 2.9, 2.9, 2.9, 2.1, and 1.2 eV for 1-5, respectively, with an estimated uncertainty 2 sigma = 0.2 eV. The corresponding adiabatic detachment energies (EA(o)) are predicted to be 2.8, 2.8, 2.8, 2.1, and 1.2 eV. The vibrational spectra of the neutral radicals are also predicted and the vibrational structure on the photodetachment spectra is discussed.