Theoretical investigation of the cyclic GaO2 and GaS2 molecules at DFT andcorrelated wave function levels

The geometries and the bonding properties have been predicted for cyclic Ga O2 and GaS2 species at density functional theory (DFT), MPn (n = 2, 3, 4 wi th different substitutions), QCISD(T), and CCSD(T) all-electron correlation levels with 6-311+G* basis set. The geometrical optimizations and the harm onic vibrational frequency analysis are performed using DFT and second-orde r M phi ller-Plesset (MP2) methods. The relevant energy quantities are also calibrated at the high-order electron correlation levels [MP3, MP4, quadra tic configuration interaction (QCI), and coupled cluster (CC)]. Each specie s possesses a (2)A(2) ground state with a higher energy level (2)A(1) state . The corresponding state-state separations are about 32 kcal/mol for GaO2 species and about 20 kcal/mol for GaS2 species at the QCISD(T)/6-311+G* lev el. The QCISD(T) and CCSD(T) calculations yield dissociation energies of 42 .0 and 59.0 kcal/mol for two species, respectively, and other methods yield dissociation energies within similar to5 kcal/mol. Result analysis has ind icated that the cyclic GaO2 should be classified as superoxide and the GaS2 species should be classified as supersulfide in their ground state, and th ose in the excited state ((2)A(1)) Should not be. However, the cyclic GaS2 ((2)A(2)) is less ionic than the GaO2 ((2)A(2)) and they are far less ionic than NaO2. (C) 2001 John Wiley & Sons, Inc.