The geometries and stabilities of the FeFe cofactor at different oxidation states and its complexes with N2 have been determined by density functional calculations. These calculations support an EPR-inactive resting state of the FeFe cofactor with four Fe2+ and four Fe3+ sites (4Fe2+4Fe3+). FeFeco(μ6-N2) with a central dinitrogen ligand is predicted to be the most stable complex of the FeFe cofactor with N2. It is easily formed by penetration of N2 into the trigonal Fe6 prism of the FeFe cofactor with an approximate barrier of 4 kcal mol-1. The present DFT results suggest that an FeFeco-(μ6-N2) entity is a plausible intermediate in dinitrogen fixation by nitrogenase. CO is calculated to bind even more strongly than N2 to the FeFe cofactor so that CO may inhibit the reduction of nitrogen by Fe-only nitrogenase.

Densityfunct ional theory and CASSCF calculations have been used to optimize the geometries of binuclear gold(i) complexes [H3PAu(C C)nAuPH3] (n=1

Photoexcitations and photoisomerizations due to low-lying np* and pp* excited states of dimethylpyridines are investigated by density functional theory, CASSCF, CASPT2 and MRCI methodologies. Mechanistic details for the formation of Dewar dimethylpyridines and the interconversions of dimethylpyridines are rationalized through the characterization of minima and transition states on the singlet and triplet potential energy surfaces of relevant intermediates. Our present theoretical schemes suggest that Möbius dimethylpyridine intermediate 14 and azabenzvalene intermediate 10 can serve as possible precursors to Dewar dimethylpyridines and singlet phototransposition products, respectively. The calculations suggest that an S1(pp*)/S0 conical intersection in dimethylpyridines 2 is involved in the formation of 14. An azabenzvalene 10 might be formed through S2(ππ*)/S1(nπ*) interaction followed by an S1/S0 decay in dimethylpyridine 6. Calculated barriers of isomerizations from 14 to Dewar dimethylpyridine 7 and from 10 to 4 are 8.4 and 28.5 kcalmolÿ1 at the B3LYP/6-311G** level, respectively. In the suggested triplet multistage transposition mechanism, an out-of-plane distorted geometry 19 due to vibrational relaxation of the T1(3B1) excited state of 3,5-dimethylpyridine 6 is a precursor of the interconversion of 6 to 2,4-dimethylpyridine 4. The formation of a triplet azaprefulvene 21 with a barrier of 20.7 kcalmolÿ1 is a key step during the triplet migration process leading to another out-of-plane distorted structure 27. Subsequent rearomatization of 27 completes the interconversion of 6 with 4. Present calculations provide some insight into the photochemistry of dimethylpyridines at 254 nm.

Optimized structures for the redox species of the diiron active site in [Fe]-hydrogenase as observed by FTIR and for species in the catalytic cycle for the reversible H2 oxidation have been determined by densityfunctional calculations on the active site model, [(L)(CO)(CN)Fe(í-PDT)(í-CO)Fe(CO)(CN)(L')]q (L) H2O, CO, H2, H-; PDT) SCH2CH2CH2S, L') CH3S-, CH3SH; q) 0, 1-, 2-, 3-). Analytical DFT frequencies on model complexes (í-PDT)Fe2(CO)6 and [(í-PDT)Fe2(CO)4(CN)2]2- are used to calibrate the calculated CN-and CO frequencies against the measured FTIR bands in these model compounds. By comparing the predicted CN-and CO frequencies from DFT frequency calculations on the active site model with the observed bands of D. Vulgaris [Fe]-hydrogenase under various conditions, the oxidation states and structures for the diiron active site are proposed. The fully oxidized, EPR-silent form is an Fe(Ⅱ)-Fe(Ⅱ) species. Coordination of H2O to the empty site in the enzyme's diiron active center results in an oxidized inactive form (H2O)Fe-(Ⅱ)-Fe(Ⅱ). The calculations show that reduction of this inactive form releases the H2O to provide an open coordination site for H2. The partially oxidized active state, which has an S) 1/2 EPR signal, is an Fe(I)-Fe(Ⅱ) species. Fe(I)-Fe(I) species with and without bridging CO account for the fully reduced, EPR-silent state. For this fully reduced state, the species without the bridging CO is slightly more stable than the structure with the bridging CO. The correlation coefficient between the predicted CN-and CO frequencies for the proposed model species and the measured CN- and CO frequencies in the enzyme is 0.964. The proposed species are also consistent with the EPR, ENDOR, and Mossbauer spectroscopies for the enzyme states. Our results preclude the presence of Fe(Ⅲ)-Fe(Ⅱ) or Fe(Ⅲ)-Fe(Ⅲ) states among those observed by FTIR. A proposed reaction mechanism (catalytic cycle) based on the DFT calculations shows that heterolytic cleavage of H2 can occur from (è2-H2)Fe(Ⅱ)-Fe(Ⅱ) via a proton transfer to "spectator" ligands. Proton transfer to a CN- ligand is thermodynamically favored but kinetically unfavorable over proton transfer to the bridging S of the PDT. Proton migration from a metal hydride to a base (S, CN, or basic protein site) results in a two-electron reduction at the metals and explains in part the active site's dimetal requirement and ligand framework which supports low-oxidation-state metals. The calculations also suggest that species with a protonated Fe-Fe bond could be involved if the protein could accommodate such species.

Activation of C-C and C-H bonds by the Rh(I) and Ir(I) complexes (PCP) MCl (M) Rh, Ir; PCP=C6H3(CH3)(CH2PH2)2) has been studied by density functional methodology. C-H activation from either of the three-coordinate intermediates 1a and 1b has a high barrier (>25 kcal/mol). Direct C-C activation does not occur from either 1a or 1b because the C-C bond is sterically inaccessible. Plausible C-C and C-H activation mechanisms under mild conditions are related to four-coordinate