A series of CASSCF calculations were performed on the ground states of NiCO and FeCO. The contributions of the σπ interactions are checked by examining the validity of the CASSC calculation to describe the molecule with a particular choice of the active space. The calculation results substantiate that the stability of MCO is determined by a balance between π donation from the metal 3dπ to the CO2π and p repulsion between the metal σ electrons and the CO5σ lone pair and, at the same time, emphasizes the importance of the synergistic σπ interactions between the metal and the CO group. The relative importance of σπ interactions depends on the nature of the metal. In the case of NiCO, it is the π donation from Ni 3dπ to CO2π that makes the σ largest contribution to the formation of the Ni-CO bond, while in the case of FeCO, it is the correlation of σ electrons that holds the metal and CO together.

Periana et al. [Science 1998, 280, 560] previously reported two catalysts for lowtemperature methane activation to methanol: PtCl2(NH3)2 and PtCl2(bpym). It was shown that the ammine catalyst is much more active, but it decomposes rapidly in sulfuric acid to form a PtCl2 precipitate, while the bpym system is long-lived. To have a basis for developing new catalysts that would not decompose, we undertook a study of the structure, bonding, and stability of the PtCl2(NH3)2 and PtCl2(bpym) catalysts, using quantum mechanics (QM) [density functional theory (DFT) at the B3LYP/LACVP**(+) level] including solvation in sulfuric acid via the Poisson-Boltzmann continuum approximation. Critical results include the following: (1) The influence of a trans ligand Y on the Pt-X bond follows the order Cl->NH3(bpym)>OSO3H->0(empty site). Thus the Pt-N bond length is longer (up to 0.04 Å) and the Pt-N bond is weaker (up to 18 kcal/mol) when trans to a Cl-as compared to trans to OSO3H-. (2) The bpym ligand acts as both a ó-donor and a ð-acceptor. As bpym is protonated, the Pt-N bond strength decreases (by up to 51 kcal/mol). Thus, ¢H(soln, 453 K) for Pt(OSO3H)2(bpym) (69.6)> [Pt(OSO3H)2(bpymH)]+(49.4)>[Pt(OSO3H)2(bpymH2)]2+(18.7). (3) In sulfuric acid replacing the ammine ligands with bisulfate ligands is thermodynamically favorable [by△G(soln, 453 K)=-23 kcal/mol], whereas replacement of bpym with OSO3H- is unfavorable [by△G(soln, 453 K)=+16 kcal/mol]. (4) Replacement of chloride ligands with bisulfate ligands is thermodynamically unfavorable [by△G(soln, 453K)=～7kcal/mol for ammine and ～12 kcal/mol for bpym]. (5) Protonation of PtCl2(bpym) is thermodynamically favorable, leading to [PtCl2(bpymH)]+ as the stable species in sulfuric acid (by 8 kcal/mol). Thus we conclude that in hot concentrated sulfuric acid it is quite favorable for PtCl2(NH3)2 to lose its ammine ligands to form PtCl2, which in turn will dimerize and oligomerize, leading eventually to a (PtCl2)n precipitate and catalyst death. We find that PtCl2(bpym) is resistant to solvent attack, favoring retention of the bpym ligand in hot concentrated sulfuric acid. These results agree with experimental findings. The insights from these findings should help screen for stable new ligands in the design of new catalysts.

Based on density functional cluster model calculations, we present the first detailed mechanisms for the complete decomposition of NH3 to NHx(a) (x=0-2) on the Si(100)-(2

Stimulated by the recent surprising results from Wentworth et al. [Wentworth, A. D., Jones, L. H., Wentworth, P., Janda, K. D. & Lerner, R. A. (2000) Proc. Natl. Acad. Sci. USA 97, 10930–10935] that Abs efficiently catalyze the conversion of molecular singlet oxygen (1O2) plus water to hydrogen peroxide (HOOH), we used quantum chemical methods (B3LYP density functional theory) to delineate the most plausible mechanisms for the observed efficient conversion of water to HOOH. We find two reasonable pathways. In Pathway I, (i) H2O catalyzes the reaction of 1O2 with a second water to form HOOOH; (ii) two HOOOH form a dimer, which rearranges to form the HOO-HOOO＋H2O complex; (iii) HOO-HOOO rearranges to HOOH-OOO, which subsequently reacts with H2O to form H2O4＋HOOH; and (iv) H2O4 rearranges to the cyclic dimer (HO2)2, which in turn forms HOOH plus 1O2 or 3O2. Pathway II differs in that step ii is replaced with the reaction between HOOOH and 1O2, leading to the formation of HOO-HOOO. This then proceeds to similar products. For a system with 18O H2O, Pathway I leads to a 2.2:1 ratio of 16O:18O in the product HOOH, whereas Pathway II leads to 3:1. These ratios are in good agreement with the 2.2:1 ratio observed in isotope experiments by Wentworth et al. These mechanisms lead to two HOOH per initial 1O2 or one, depending on whether the product of step iv is 1O2 or 3O2, in good agreement with the experimental result of 2.0. In addition to the Ab-induced reactions, the hydrogen polyoxides (H2O3 and H2O4) formed in these mechanisms and their decomposition product polyoxide radicals (HO2, HO3) may play a role in combustion, explosions, atmospheric chemistry, and the radiation chemistry in aqueous systems.

Hydrogen adsorption at platinum electrodes was investigated by B3LYP quantum-chemical calculations. Electric-field effects were simulated either by charging the cluster models or by considering the cluster in the presence of a uniform external field. The results show that the large tuning rate of the Pt–H frequency should be attributed to the work function shift with the change of electrode potential, and the experimentally observed red-shift of the Pt-H frequency with decrease of electrode potential would originate from the lateral interaction of the Pt-H bonds.