Masahiko Hada

An ab initio molecular orbital study of the nuclear volume effects in uranium isotope fractionations

This paper discusses the nuclear volume dependence of uranium isotope fractionations in the U(3+)-U(4+) and U(4+)-UO(2) (2+) systems by reference to a series of ab initio molecular orbital calculations. Nuclear volume-dependent terms ( identical withln K(nv)) in isotope fractionation coefficients ( identical withepsilon) are calculated from the energetic balance of the isotopomers involved in the systems. We used the Dirac-Coulomb Hartree-Fock (DCHF) method with the Gaussian-type finite-nucleus model. We employed three types of generally contracted Gaussian basis sets to check the basis set dependences. In the U(3+)-U(4+) system, the present values of ln K(nv) for uranium, other than those with the smallest double-zeta basis set, are in good agreement with previous values of ln K(nv) obtained from a numerical atomic multiconfigurational DCHF method with the Fermi-type finite-nucleus model. The present calculations reasonably reproduce the experimental value of epsilon in the U(3+)-U(4+) system, and the value of ln K(nv) in the U(4+)-UO(2) (2+) system, obtained empirically by temperature-dependent fitting of the experimental epsilon values. For instance, in the U(4+)-UO(2) (2+) system, the present ab initio ln K(nv) value for a (235)U-(238)U isotope pair is 0.002 09 using the largest basis set, while the experimental value is 0.002 24. This paper also shows that nuclear volume effects are negligibly small on the U-O bond length and two force constants of UO(2) (2+). Hence, the molecular vibrational terms of the isotope fractionation coefficients mainly depend on the nuclear mass rather than the nuclear volume.

Dilithioplumbole: A Lead-Bearing Aromatic Cyclopentadienyl Analog

Aromatic Lead The bond stabilization, or aromaticity, observed in cyclic carbon molecules, such as benzene, relies on delocalization of electrons around the ring. Although electron distributions in heavier elements can complicate this arrangement, Saito et al. (p. 339) show that even lead, one of the heaviest metals, is able to participate in an otherwise carbon-based aromatic network. In an analog of the well-studied cyclopentadienyl anion, one carbon atom was replaced with lead, and the framework stabilized by appending phenyl groups to the other four carbons. Crystallography revealed a planar structure, which together with spectroscopic data and theoretical calculations confirmed the aromatic character of the product. Lead can participate in the delocalized electron network of an aromatic carbon ring. Although the concept of aromaticity has long played an important role in carbon chemistry, it has been unclear how applicable the stabilizing framework is to the heaviest elements. Here we report the synthesis of dilithiotetraphenylplumbole by reduction of hexaphenylplumbole. X-ray crystallography revealed a planar structure with no alternation of carbon–carbon bond lengths in the five-membered ring core. Nuclear magnetic resonance spectra and relativistic theoretical calculations show considerable aromatic character in the molecule, thus extending aromaticity to carbon’s heaviest congener.

Unique Properties and Reactivity of High-Valent Manganese―Oxo versus Manganese―Hydroxo in the Salen Platform

To gain an understanding of oxidation reactions by Mn(III)(salen), a reaction of Mn(III)(salen) with m-chloroperoxybenzoic acid in the absence of a substrate is investigated. UV-vis, perpendicular- and parallel-mode electron paramagnetic resonance, and X-ray absorption spectroscopy show that the resulting solution contains Mn(IV)(salen)(O) as a major product and Mn(IV)(salen)(OH) as a minor product. Mn(IV)(salen)(O) readily reacts with 4-H-2,6-tert-Bu(2)C(6)H(2)OH (homolytic bond dissociation energy of an OH bond, BDE(OH) = 82.8 kcal mol(-1)), 4-CH(3)CO-2,6-tert-Bu(2)C(6)H(2)OH (BDE(OH) = 83.1 kcal mol(-1)), and 4-NC-2,6-tert-Bu(2)C(6)H(2)OH (BDE(OH) = 84.2 kcal mol(-1)) at 203 K, following second-order rate kinetics. Mn(IV)(salen)(OH) reacts with 4-CH(3)CO-2,6-tert-Bu(2)C(6)H(2)OH (BDE(OH) = 83.1 kcal mol(-1)) much more slowly under identical conditions than Mn(IV)(salen)(O) and does not react with 4-NC-2,6-tert-Bu(2)C(6)H(2)OH (BDE(OH) = 84.2 kcal mol(-1)), suggesting that the thermodynamic hydrogen-atom-abstracting ability of Mn(IV)(salen)(OH) is about 83 kcal mol(-1). The rate constant for reactions of Mn(IV)(salen)(OH) with phenols is not dependent on the concentration of phenols, suggesting that Mn(IV)(salen)(OH) might bind phenols prior to the rate-limiting oxidation reactions. Quantum chemical calculations are carried out for Mn(IV)(salen)(O) and Mn(IV)(salen)(OH), both of which well reproduce the extended X-ray absorption fine structures as well as the electronic configurations. It is also indicated that protonation of Mn(IV)(salen)(OH) induces a drastic electronic structural change from manganese(IV) phenolate to a manganese(III) phenoxyl radical, which is also consistent with the experimental observation.

Effect of a tridentate ligand on the structure, electronic structure, and reactivity of the copper(I) nitrite complex: role of the conserved three-histidine ligand environment of the type-2 copper site in copper-containing nitrite reductases.

It is postulated that the copper(I) nitrite complex is a key reaction intermediate of copper containing nitrite reductases (Cu-NiRs), which catalyze the reduction of nitrite to nitric oxide (NO) gas in bacterial denitrification. To investigate the structure-function relationship of Cu-NiR, we prepared five new copper(I) nitrite complexes with sterically hindered tris(4-imidazolyl)carbinols [Et-TIC = tris(1-methyl-2-ethyl-4-imidazolyl)carbinol and iPr-TIC = tris(1-methyl-2-isopropyl-4-imidazolyl)carbinol] or tris(1-pyrazolyl)methanes [Me-TPM = tris(3,5-dimethyl-1-pyrazolyl)methane; Et-TPM = tris(3,5-diethyl-1-pyrazolyl)methane; and iPr-TPM = tris(3,5-diisopropyl-1-pyrazolyl)methane]. The X-ray crystal structures of all of these copper(I) nitrite complexes were mononuclear eta(1)-N-bound nitrite complexes with a distorted tetrahedral geometry. The electronic structures of the complexes were investigated by absorption, magnetic circular dichroism (MCD), NMR, and vibrational spectroscopy. All of these complexes are good functional models of Cu-NiR that form NO and copper(II) acetate complexes well from reactions with acetic acid under anaerobic conditions. A comparison of the reactivity of these complexes, including previously reported (iPr-TACN)Cu(NO2) [iPr-TACN = 1,4,7-triisopropyl-1,4,7-triazacyclononane], clearly shows the drastic effects of the tridentate ligand on Cu-NiR activity. The copper(I) nitrite complex with the Et-TIC ligand, which is similar to the highly conserved three-histidine ((His)3) ligand environment in the catalytic site of Cu-NiR, had the highest Cu-NiR activity. This result suggests that the (His)3 ligand environment is essential for acceleration of the Cu-NiR reaction. The highest Cu-NiR activity for the Et-TIC complex can be explained by the structural and spectroscopic characterizations and the molecular orbital calculations presented in this paper. Based on these results, the functional role of the (His)3 ligand environment in Cu-NiR is discussed.

Effect of the axial ligand on the reactivity of the oxoiron(IV) porphyrin π-cation radical complex: higher stabilization of the product state relative to the reactant state.

The proximal heme axial ligand plays an important role in tuning the reactivity of oxoiron(IV) porphyrin π-cation radical species (compound I) in enzymatic and catalytic oxygenation reactions. To reveal the essence of the axial ligand effect on the reactivity, we investigated it from a thermodynamic viewpoint. Compound I model complexes, (TMP(+•))Fe(IV)O(L) (where TMP is 5,10,15,20-tetramesitylporphyrin and TMP(+•) is its π-cation radical), can be provided with altered reactivity by changing the identity of the axial ligand, but the reactivity is not correlated with spectroscopic data (ν(Fe═O), redox potential, and so on) of (TMP(+•))Fe(IV)O(L). Surprisingly, a clear correlation was found between the reactivity of (TMP(+•))Fe(IV)O(L) and the Fe(II)/Fe(III) redox potential of (TMP)Fe(III)L, the final reaction product. This suggests that the thermodynamic stability of (TMP)Fe(III)L is involved in the mechanism of the axial ligand effect. Axial ligand-exchange experiments and theoretical calculations demonstrate a linear free-energy relationship, in which the axial ligand modulates the reaction free energy by changing the thermodynamic stability of (TMP)Fe(III)(L) to a greater extent than (TMP(+•))Fe(IV)O(L). The linear free energy relationship could be found for a wide range of anionic axial ligands and for various types of reactions, such as epoxidation, demethylation, and hydrogen abstraction reactions. The essence of the axial ligand effect is neither the electron donor ability of the axial ligand nor the electron affinity of compound I, but the binding ability of the axial ligand (the stabilization by the axial ligand). An axial ligand that binds more strongly makes (TMP)Fe(III)(L) more stable and (TMP(+•))Fe(IV)O(L) more reactive. All results indicate that the axial ligand controls the reactivity of compound I (the stability of the transition state) by the stability of the ground state of the final reaction product and not by compound I itself.

Critical Role of External Axial Ligands in Chirality Amplification of trans-Cyclohexane-1,2-diamine in Salen Complexes

A series of Mn(IV)(salen)(L)(2) complexes bearing different external axial ligands (L = Cl, NO(3), N(3), and OCH(2)CF(3)) from chiral salen ligands with trans-cyclohexane-1,2-diamine as a chiral scaffold are synthesized, to gain insight into conformational properties of metal salen complexes. X-ray crystal structures show that Mn(IV)(salen)(OCH(2)CF(3))(2) and Mn(IV)(salen)(N(3))(2) adopt a stepped conformation with one of two salicylidene rings pointing upward and the other pointing downward due to the bias from the trans-cyclohexane-1,2-diamine moiety, which is in clear contrast to a relatively planar solid-state conformation for Mn(IV)(salen)(Cl)(2). The CH(2)Cl(2) solution of Mn(IV)(salen)(L)(2) shows circular dichroism of increasing intensity in the order L = Cl < NO(3) << N(3) < OCH(2)CF(3), which indicates Mn(IV)(salen)(L)(2) adopts a solution conformation of an increasing chiral distortion in this order. Quantum-chemical calculations with a symmetry adapted cluster-configuration interaction method indicate that a stepped conformation exhibits more intense circular dichroism than a planar conformation. The present study clarifies an unexpected new finding that the external axial ligands (L) play a critical role in amplifying the chirality in trans-cyclohexane-1,2-diamine in Mn(IV)(salen)(L)(2) to facilitate the formation of a chirally distorted conformation, possibly a stepped conformation.

Ligand effect on uranium isotope fractionations caused by nuclear volume effects: An ab initio relativistic molecular orbital study

We have calculated the nuclear volume term (ln K(nv)) of the isotope fractionation coefficient (epsilon) between (235)U-(238)U isotope pairs by considering the effect of ligand coordination in a U(IV)-U(VI) reaction system. The reactants were modeled as [UO(2)Cl(3)](-) and [UO(2)Cl(4)](2-) for U(VI), and UCl(4) for U(IV). We adopted the Dirac-Coulomb Hartree-Fock method with the Gaussian-type finite nucleus model. The result obtained was ln K(nv)=0.001 90 at 308 K, while the experimentally estimated value of ln K(nv) is 0.002 24. We also discuss how the ligand affects the value of ln K(nv), especially for the various structures of different compounds, and different ligands within the halogen ion series (F, Cl, and Br).

Ab initio study on vibrational dipole moments of XH+ molecular ions: X = 24Mg, 40Ca, 64Zn, 88Sr, 114Cd, 138Ba, 174Yb and 202Hg

The vibrational matrix elements of electric dipole moments were theoretically estimated for the electronic ground state of XH+ molecular ions (X = 24Mg, 40Ca, 64Zn, 88Sr, 114Cd, 138Ba, 174Yb and 202Hg) using the complete active space second-order perturbation theory method. Because of the large rotational constant and zero X-nuclear spin, these molecules are advantageous to be localized to a single (v, J, F) state, where v, J, F are quantum numbers of the vibrational, rotational and hyperfine states, respectively. The information of the dipole moments is very useful to discuss the period to localize the molecular ion to the (v, J, F) = (0, 0, 1/2) state and also the period to remain in this state, which is limited by the interaction with the black body radiation. The agreement of experimental and our theoretical spectroscopic constants ensures the accuracy of our results. Vibrational permanent and transition dipole moments were obtained with special care of accuracy in numerical integration. Spontaneous emission rates were calculated from the vibrational dipole moments and transition energies.

Magnetic shielding constants calculated by the infinite-order Douglas-Kroll-Hess method with electron-electron relativistic corrections

We presented a two-component relativistic quantum-chemical theory for magnetic shielding constants, which is based on the infinite-order Douglas-Kroll (IODK) transformation. Two-electron relativistic corrections were also generated using the IODK transformation, although negligibly small terms were discarded. The use of small-component basis functions was completely excluded from the present theory. We examined the combination of the levels of relativistic one- and two-electron terms and magnetic interaction terms using the first-order Foldy-Wouthuysen (FW1), the second-order Douglas-Kroll (DK2), and the infinite-order Douglas-Kroll (IODK) transformations, as well as the lowest-order (c(-2)) Breit-Pauli approximation. We calculated the magnetic shielding constants of several closed-shell atoms using the FW1, DK2, IODK, and Breit-Pauli Hamiltonians. The IODK Hamiltonian reproduced well the results calculated by the four-component Dirac-Fock-Coulomb theory: The maximum deviation is only about 2.2%. We found that the accuracy of the magnetic shielding constants is strongly affected by the relativistic treatments of one-electron magnetic interaction, while the effect of the two-component two-electron relativistic corrections is relatively small. We also discussed the picture change effect on magnetic operators.

An ab initio study based on a finite nucleus model for isotope fractionation in the U(III)-U(IV) exchange reaction system.

Isotope fractionation in the U(III)-U(IV) reaction system was investigated by a series of atomic relativistic ab initio calculations using the multiconfigurational Dirac-Coulomb Hartree-Fock method. To evaluate the nuclear volume effect on the fractionation, the Fermi statistical distribution function was adopted for nuclear charge density. The isotope fractionation coefficient epsilon resulting from the nuclear volume difference was evaluated from the total electronic energies of U3+ and U4+, based on the theoretical equation proposed by Bigeleisen [J. Am. Chem. Soc. 118, 3676 (1996)]. The calculated fractionation coefficient epsilon in the present work for the isotopic pair 235U and 238U at 293 K is 0.0031, which is quite close to the experimentally observed value of 0.0027. Discussion is extended to the nuclear volume effects on isotopic fractionations in the Pu(III)-Pu(IV) and Eu(II)-Eu(III) exchange systems.