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.

Ab initio calculations are used to determine molecular properties of linear carbon clusters Cn (n=3-6) in ground and electronically excited states relative to photodissociation processes. MRD-CI calculations predict that in C3 and C5 the siglet-triiplet splitting between the 1пu and 3пu arising from the same configuration is about 1eV similar as in C2. The energy differences between 1∆g and 1Σ+g corresponding to the same 2g or 2u configuration in C4 and C6 are less than 0.2 eV. Calculations support experimental finding that the energetically most favorable fragmentation channel for linear carbon clusters Cn (n=4-6) corresponds to the loss of C3 to give its partner fragment Cn-3. Further fragmentation channels are discussed.

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

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.