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Abstract
Advanced quantum mechanical computational techniques have been applied to probe two disparate systems. First, density functional theory (DFT) is used to obtain the first structural characterizations of the unsaturated dichromium carbonyl complexes Cr2(CO)x (x=10, 9, 8), which may contain metal-metal multiple bonds. Chromium is unique among the first row transition metal elements in its ability to form compounds with multiple bonds in the formal metal oxidation state chromium(0). The global minimum energy structure of Cr2(CO)10 for the lowest singlet state of C2h symmetry is consistent with a model of two interacting Cr(CO)5 fragments, with chromium-chromium distances of 2.93 (B3LYP) or 2.83 (BP86). The minimum energy Cr2(CO)9 structure of Cs symmetry is predicted to have a remarkably short metal-metal bond length of 2.31 (B3LYP) or 2.28 (BP86). This chromium-chromium distance is essentially identical to that reported experimentally for the established CrCr triple bond in (h5-Me5C5)2Cr2(CO)4. Cr2(CO)8 is predicted to have a short metal-metal bond length of 2.30 (B3LYP) or 2.28 (BP86) for the minimum energy structure of Cs symmetry. Second, extensive ab initio quantum mechanical methods have been employed to five lowest-lying electronic singlet states of astrophysically interesting AlOH, particularly for the two excited singlet states. To avoid the difficulty involved in applying multi-reference methods, we use the equation-of-motion coupled cluster method. The first singlet excited state ( 1A) is predicted to have a bond angle of 110 and to lie 114 kcal/mol (39 900 cm.1, 4.94 eV) above the ground state, whereas the second singlet excited state (B 1A) is predicted to have a bond angle of 116 and to be located 119 kcal/mol (41 700 cm.1, 5.17 eV) above the ground state. These theoretical energy separations are in excellent agreement with the experimental values T0 ( 1A) = 114.57 kcal/mol (40 073 cm.1, 4.968 eV) and T0 (B1A) = 119.36 kcal/mol (41 747 cm.1, 5.176 eV).