Cyclic ozone (O3) has not been isolated so far, despite its computed kinetic persistence. Possibilities of "trapping" this molecule (or the valence-isoelectronic cyclic thiozone, S3) in transition metal complexes are investigated in this paper. Candidates were constructed, first using the 18-electron rule as a guide and then optimizing the structures with the DFT-B3LYP method. A variety of structures result: oxo-peroxo species, di-sigma- and pi-bonded open ozone complexes, some eta1 and eta2 cyclic ozone complexes, and a few bona fide eta3 cyclic O3 and S3 complexes. MLn fragments suitable for complex formation would need to contain very strong pi-acceptor ligands. Nitrosyl ligands were chosen to minimize an energy mismatch between the O3 donor orbitals and the MLn acceptor orbitals. On this basis, the existence of the complexes [S3W(NO)3]3+, [O3M(NO)3]3+ (M = Cr, Mo, W, Fe, Ru, Os), and [S3W(NO)2(CO)]2+ containing cyclic O3 and S3 is suggested. In another approach, facing up to the oxidizing power of O3, potential systems were built from late transition metals in high oxidation states, and also d0 early transition metal centers.
The structure of the adsorbate systems formed by mercaptobenzothiazole (MBT) and analogue molecules on the CdS(101 h0) surface is studied quantum-chemically using density functional theory. Preliminary calculations of the free adsorptive molecules indicate an energetic preference of their thione form compared to the thiol form. For the anions of the adsorptive molecules, the role of the endocyclic nitrogen and the exocyclic sulfur as possible donor atoms is examined by means of known chelate complexes. Clusters with 24 and 28 atoms that are saturated by point charges have been developed as surface models. Geometry optimizations show that the structure of the adsorbate systems is dominated by the formation of two coordinative bonds from the donor atoms of the adsorptive anions to two adjacent cadmium atoms of the surface. It results that the molecular plane of the adsorptives is tilted with respect to the normal of the crystal face. The calculated tilt angle for the MBT adsorbate agrees with angle-dependent XANES measurements, the only structural information presently available from experiment. It is found that the tilt angle changes with the variation of the heteroatom in the five-membered ring of the adsorptives. The molecule-surface interactions leading to these structural differences are analyzed. Further, the relaxation of the surface is included in the investigation. It becomes obvious that the direction of the relaxation of the free surface is reversed by the formation of the adsorbate bonds.
The mechanism for the transformation of adsorbed cyclopropylmethyloxide into its ring-opened form, 3-butenyloxide, on the Mo(110) surface is explored theoretically. An alternative emerges to the radical clock mechanism that involves the cleavage of the C-O bond in the adsorbate as the critical reaction step. The alternative pathway involves the cleavage of a C-C bond in the three-membered ring leading to a diradical, which could transform via a 1,2-H shift to the same reaction product. Density functional theory (DFT) calculations for relevant reaction intermediates in molecular model systems show an energetic preference for the C-C cleavage in the initial step. The barrier of the subsequent 1,2-H shift of the singlet diradical is slightly lower than the barrier for the radical clock rearrangement, rendering the diradical pathway a possible alternative. Adsorbate structures for the reactant and the product were obtained by DFT slab calculations. We carry out a MO analysis of the bonding in the adsorbate, comparing also C-C and C-O bonding.
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