Solutions of lithium hexafluorophosphate, LiPF 6 , in propylene carbonate (4-methyl-1,3-dioxolan-2-one; denoted by PC hereafter) in the concentration range from 0.0 to 3.29 M (M ) mol dm -3 ) have been studied regarding their conductivities, viscosities, and self-diffusion coefficients of PC by the NMR field gradient technique, Raman spectra, and NMR spectra. Walden's products are almost constant in the range up to and over 3.0 M. Therefore, Li ions are considered to be quite free from the firm interaction with anions even in such concentrated solutions. The appearance of the maximum conductivity at about 0.8 M is explained by associating with the concentration dependence of the solution viscosity. A remarkable increase in the solution viscosity was observed in a concentration beyond 2.0 M, and it can be ascribed to the cluster formation of lithium ions with PC molecules of the solvent. Such an idea of clusters can reasonably interpret some of the characteristic changes of the viscosities, the diffusion coefficients, the Raman spectra, and the NMR spectra at concentrations over 2 M.

The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH 2 )(H 2 O)Fe(µ-O) 2 (η 2 -HCOO) 2 Fe(NH 2 )(H 2 O), I. The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two Fe IV atoms, L 4 Fe IV (µ-O) 2 Fe IV L 4 , the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L 4 Fe IV (µ-O)(µ-OH(‚‚‚CH 3 ))Fe III L 4 , which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an Fe III -Fe IV mixed valence state. This short-lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L 4 Fe III (OHCH 3 )-(µ-O)Fe III L 4 , which has an Fe III -Fe III core. The barrier of the rate-determining step, methane H-abstraction, was calculated to be 19 kcal/mol. The overall CH 4 oxidation reaction to form the methanol complex, I + CH 4 f VI, was found to be exothermic by 39 kcal/mol.

The hybrid density functional method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by the methane monooxygenase (MMO) enzyme. The key reactive compound Q of MMO was modeled by cis-(H 2 O)(NH 2 )Fe(µ-O) 2 (η 2 -HCOO) 2 Fe(NH 2 )(H 2 O), I, where the substrate molecule may coordinate to the bridging oxygen atoms, O 1 and O 2 , located on the H 2 O and NH 2 sides, leading to two different mechanisms, O-side and N-side pathways, respectively. Previously we have detailed the N-side pathway (Basch, H.; Mogi, K.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 1999, 121, 7249); here we discuss the O-side pathway, and compare the two. Calculations show that, like the N-side pathway, the O-side pathway of the reaction of I with CH 4 proceeds via a bound-radical mechanism. It starts from the bis(µ-oxo) compound I and goes over the rate-determining transition state III_O for H abstraction from methane to form a weak complex IV_O between the Fe(µ-O)(µ-OH)Fe moiety and a methyl radical. This bound-radical intermediate IV_O converts to the oxo-methanol complex VI_O via a low barrier at transition state V_O for the addition of the methyl radical to the µ-OH ligand. Complex VI_O easily (with about 7-8 kcal/mol barrier) eliminates the methanol molecule and produces the Fe(µ-O)Fe, VII_O, complex. During the entire process, the oxidation state of the Fe core changes from Fe IV-Fe IV in I to a mixed-valence Fe III-Fe IV in the short-lived intermediate IV_O, and finally to Fe III-Fe III in VI_O and VII_O. A comparison of the O-side and N-side pathways shows that both include similar intermediates, transition states, and products. The ratedetermining step of both pathways is the H-atom abstraction from the methane molecule, which occurs by 23.2 and 19.5 kcal/mol barrier for the O-side and N-side pathways, respectively, in the ground 9 A states of the systems. Thus, the N-side pathway is intrinsically more favorable kinetically than the O-side pathway by about 4 kcal/mol. However, experimentally in the enzyme the N side is blocked by unfavorable steric hindrance and the actual reaction has to take place on the O side.

The dissolution of PtO2 in concentrated H2SO4 under an atmosphere of CO results in the formation of hexacarbonyldiplatinum(I), [{Pt(CO)3}2]2+ (1), the first homoleptic, dinuclear, cationic platinum carbonyl complex, of which a prolonged evacuation leads to reversible disproportionation to give cis-[Pt(CO)2]2+ (solv) (2) and Pt(0). 1 has been completely characterized by NMR (13C and 195Pt), IR, Raman, and EXAFS spectroscopy. The structure of 1 is rigid on the NMR time scale at room temperature. NMR:  δ(13CA) 166.3, δ(13CB) 158.7, δ(195Pt) −211.0 ppm; 1 J(Pt−CA) = 1281.5 Hz, 1 J(Pt−CB) = 1595.7 Hz, 1 J(Pt−Pt‘) = 550.9 Hz. The strongly polarized, sharp Raman band at 165 cm-1 (ρ = ca. 0.25) indicates the presence of a direct Pt−Pt bond. The IR and Raman spectra in the CO stretching region are entirely consistent with the presence of only terminal CO's on a nonbridged Pt−Pt bond with D 2 d symmetry. ν(CO)IR:  2174 (E), 2187 (B 2), and 2218 cm-1 (B 2); ν(CO)Raman:  2173 (E), 2194 (B 2), 2219 (B 2), 2209 (A 1) and 2233 cm-1 (A 1). EXAFS measurements show that the Pt−Pt bond is 2.718 Å and the mean length of the Pt−C bonds is 1.960 Å. The geometric optimization for 1 by a density functional calculation at the B3LYP level of theory predicts that the dinuclear cation contains two essentially planar tricarbonyl platinum(I) units that are linked via a Pt−Pt bond about which they are twisted by exactly 90.0° with respect to each other.

Mechanistic aspects of the biological activation of O2 catalyzed by methane monooxygenase (MMO) were investigated by using a hybrid density functional method. The reduced form of the metalloenzyme was modeled by cis-(H2O)(NH2)Fe(η2-HCOO)2Fe(NH2)(H2O), where the O2 molecule may coordinate the Fe centers from two different sides, the H2O-side and the NH2-side, leading to two different mechanisms, O-side and N-side pathways, respectively. Calculations show that both pathways proceed via similar intermediates. The energy profile for the reaction of O2 coming from the O-side, however, is more consistent with available experimental data than for the N-side. On the other hand, the N-side mechanism is thermodynamically more favorable. This study suggests that, if the protein backbone did not block the N-side, the O2 molecule would most likely approach the dinuclear iron center from this side rather than from the O-side. Several mixed-valence intermediates have been found during the reaction, including an FeII−FeIII mixed-valence species, P*, prior to formation of intermediate P, and a species similar to intermediate X in the analogous mechanism of Ribonucleotide Reductase, as well as an FeIII−FeIV mixed-valence species prior to formation of intermediate Q. Our theoretical findings give support to the idea that electrons do not need to be transferred by pairs in the studied diiron system. This is the first time that a structure for intermediate P* has been proposed in the literature.

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B3LYP level optimizations were performed on the structures of the octasilsesquioxane (Si 8 O 12 H 8 , HT 8 ) double four-ring (D4R) cage and single hydrogen atom-trapped HT 8 (H@HT 8 ). Moreover, the transition state in the detrapping process of the hydrogen atom from the D4R cage was examined. The basis sets used were 6-31G** for HT 8 and (3 1 1/1*1*/1*) for the trapped hydrogen atom. Both HT 8 and H@HT 8 were structure-optimized with O h molecular symmetry and the resulting cage conformations were similar. The trapped H atom was located at the center of the D4R cage. The weak interaction between the D4R cage and the trapped H atom in H@HT 8 was determined by examining the singly occupied molecular orbital (SOMO) [8a 1g ] of H@HT 8 . The SOMO was constructed from an antibonding interaction between the lowest unoccupied molecular orbital (LUMO) [8a 1g ] of HT 8 and the 1s orbital of the trapped H atom. For the transition state, the structure was optimized with C 4V molecular symmetry. As a result, the position of the Si 8 cube framework was unchanged, and four O atoms in a silicon single four-ring were displaced, thereby opening one of the oxygen windows of the D4R cage. The detrapping H atom was located near the center of the oxygen window and the MO illustrations showed a change in shape from spherical to ellipsoid. Consequently, it is clear that the detrapping process is not due to the formation of chemical bonding. The calculated activation and reaction energies of this detrapping process were +98.6 and -26.1 kJ/mol, respectively. In addition, single-point calculations at the MP2 level were done for each optimized structure, and the obtained activation and reaction energies were +128.7 and -9.3 kJ/mol, respectively. Both calculated activation energies were comparable to Sto ¨sser's experimental data (+109.6 ( 3.1 kJ/mol) for H•:Si 8 O 12 (OSi(CH 3 ) 3 ) 8 (Q 8 M 8 ). Furthermore, additional explanations are given on the IR vibrational frequencies of HT 8 and H@HT 8 and the hyperfine coupling constant for caged atomic hydrogen by ESR.

Low-lying electronic states of TiCl and ZrCl were investigated by the complete active space SCF ͑CASSCF͒, multi-reference singly and doubly excited configuration interaction ͑MRSDCI͒, and multi-reference coupled pair approximation ͑MRCPA͒ calculations using the model core potential ͑MCP͒ method. Relativistic effects were incorporated in the MCP and basis sets for Zr at the level of Cowan and Griffin's quasi-relativistic Hartree-Fock method. The 4 ⌽ state was found to be the ground state of TiCl, whereas the 2 ⌬ state was the ground state of ZrCl at all levels of calculation. Two low-lying excited states were very close in energy to the ground state. The excited 4 ⌺ Ϫ and 2 ⌬ states of TiCl were higher than the ground state by 0.102 eV and 0.458 eV, respectively, and the excited 4 ⌽ and 4 ⌺ Ϫ states of ZrCl were higher by 0.094 eV and 0.110 eV, respectively, at the MRCPA level. The calculated values of r e (2.319 Å) and e (382 cm Ϫ1 ) for the ground 4 ⌽ state of TiCl are quite close to the values of r e (2.351 Å) and e (383 cm Ϫ1 ) for the ground 2 ⌬ state of ZrCl. The values of r e , e , B e , and ␣ e are reasonably comparable to the observed values for both TiCl and ZrCl.