Heme Fe?SO2− intermediates in sulfite reduction: Contrasts with Fe?OO2− species from oxygen–oxygen bond activating systems

Sulfite reductase (SiR) catalyzes a six electron and six proton reduction of sulfite to sulfide. Similarly to the cytochrome P450 (cytP450) family, the active site in SiR contains a (partially reduced) heme bound axially to a cysteinate ligand—though with an extra Fe₄S₄ cluster. Fe(III) SO²⁻, Fe(III) SOH⁻, and Fe(III) SO(H₂) intermediates have been proposed for the catalytic cycle of SiR, leading to a formally Fe(V)S species—akin to the widely accepted reaction mechanism in cytP450. Here, density functional theory (DFT) data is reported for of such FeSO(H₂) intermediates. The Fe(III) SO²⁻ models display relatively high energies for homolytic bond breaking compared to their isomeric oxygen‐bound Fe(III) OS²⁻ models, and thus offer a better alternative in terms of avoiding radical side products able to induce enzyme suicide. This could be due to the fact that the (iron‐bound) sulfur is more active from a redox standpoint compared to oxygen, thus permitting the departing oxygen to maintain a redox‐inert state. Di‐protonation of the oxygen is computed to lead to a compound I type Fe(IV)S coupled to a porphyrin radical anion—consistent with an intermediate previously observed by x‐ray crystallography.     

Evaluation of Electron Population Terms for 〈rSe−3〉4p, 〈rS−3〉3p,and 〈rO−3〉2p: How Do HOMO and LUMO Shrink or Expand Depending on Nuclear Charges?

Electron population terms are evaluated for N=Se, S, and O. Calculations are performed on HOMO and LUMO constructed by pure atomic 4p(Se), 3p(S), and 2p(O) orbitals, employing the 6-311+G(3d) and/or 6-311(++)G(3df,3pd) basis sets at the HF, MP2, and DFT (B3 LYP) levels. Se(4+), Se(2+), Se(0), and Se(2-) with the O(h) symmetry are called G(A: Se) and HSe(+), H(2)Se, and HSe(-) with the C(infinityh) or C(2v) symmetry are named G(B: Se), here [G(A+B: Se) in all]. HOMO and LUMO in G(A+B: N) (N=Se, S, and O) satisfy the conditions of the calculations for . The (4p), (3p), and (2p) values correlate well with the corresponding MO energies (epsilon(N)) for all calculation levels employed. Plots of (HOMO) and (LUMO) versus Q(N) (N=Se, S, and O) at the HF and MP2 levels are analyzed as two correlations. However, the plots at the DFT level can be analyzed as single correlation. A regression curve is assumed for the analysis. Behaviors of clarify how valence orbitals shrink or expand depending on Q(N). The applicability of is examined to establish a new method that enables us to analyze chemical shifts with the charge effect separately from others. A utility program derived from the Gaussian 03 (NMRANAL-NH03G) is applied to evaluate and examine the applicability to the NMR analysis.     

Mechanistic Insight into the NN Bond‐Cleavage of Azo‐Compounds that was Induced by an Al?Al‐bonded Compound [L2−AlII?AlIIL2−]

An α‐diimine‐stabilized Al? Al‐bonded compound [L^2?Al^II? Al^IIL^2?] (L=[{(2,6‐iPr₂C₆H₃)NC(Me)}₂]; 1 ) consists of dianionic α‐diimine ligands and sub‐valent Al²⁺ ions and thus could potentially behave as a multielectron reductant. The reactions of compound 1 with azo‐compounds afforded phenylimido‐bridged products [L^?Al^III(μ₂‐NPh)(μ₂‐NAr)Al^IIIL^?] ( 2 – 4 ). During the reaction, the dianionic ligands and Al²⁺ ions were oxidized into monoanions and Al³⁺, respectively, whilst the [NAr]^2? imides were produced by the four‐electron reductive cleavage of the N?N double bond. Upon further reduction by Na, the monoanionic ligands in compound 2 were reduced to the dianion to give [(L^2?)₂Al^III₂(μ₂‐NPh)₂Na₂(thf)₄] ( 5 ). Interestingly, when asymmetric azo‐compounds were used, the asymmetric adducts were isolated as the only products (compounds 3 and 4 ). DFT calculations indicated that the reaction was quite feasible in the singlet electronic state, but the final product with the triplet‐state monoanionic ligands could result from an exothermic singlet‐to‐triplet conversion during the reaction process.     

X?X Through‐Cage Bonding in Cu,Ni, and Cr Complexes with M3X2 Cores (X=S,As)

Density functional calculations on trinuclear complexes bridged by two sulfur atoms, [(tmeda)₃Cu₃(μ‐S)₂]³⁺, [(tmeda)₃Ni₃(μ‐S)₂]²⁺, and [(tmeda)₃Ni₃(μ‐S₂)]⁴⁺, as well as on the formation of [(tmeda)₃Cu₃(μ‐S)₂]³⁺ from a dinuclear [(tmeda)₂Cu₂(μ‐S₂)]²⁺ complex and a mononuclear [(tmeda)Cu(η²‐S₂)]⁺ fragment, are reported. A qualitative orbital analysis of the M₃X₂ framework bonding is presented for the case in which each metal atom M has a square planar coordination sphere completed by one bidentate or two monodentate ligands (that is, [(L₂M)₃X₂] compounds). It is concluded that a framework electron count (FEC) of 12 corresponds to systems with six M? X bonds but no X? X bond through the cage, while an FEC of 10 favors the formation of an X? X bond. Framework electron counting rules are also presented for related M₃X₂ cores in [(L₅M)₃X₂] complexes, based on a qualitative molecular orbital (MO) analysis supported by DFT calculations on [(OC)₁₅Cr₃(μ‐As₂)].     

Structures and properties of π Br‐bond in complexes C2H4−nFn?BrF (n = 0–2)

Using the counterpoise‐corrected potential energy surface method, the stationary structures of the π Br‐bond complexes C₂H_4‐nF_n? BrF (n = 0–2) with all real frequencies have been obtained at MP2/aug‐cc‐pVDZ level. The order of the π Br‐bond length is 2.625 Å (C₂H₄? BrF) < 2.714 Å (C₂H₃F? BrF) < 2.751 Å (g‐C₂H₂F₂? BrF) < 2.771 Å (trans‐C₂H₂F₂? BrF) < 2.778 Å (cis‐C₂H₂F₂? BrF). The interaction energies (E_int) are, respectively,‐5.9 (C₂H₄? BrF),‐4.4 (C₂H₃F? BrF),‐3.7 (g‐C₂H₂F₂? BrF),‐3.1 (cis‐C₂H₂F₂? BrF),‐2.8 kcal/mol (trans‐C₂H₂F₂? BrF), at the CCSD (T)/aug‐cc‐pVDZ level, which include larger electron correlation contributions (E_corre). The order of E_corre is‐3.40 (C₂H₄? BrF),‐3.60 (C₂H₃F? BrF),‐3.85 (g‐C₂H₂F₂? BrF),‐3.86 (cis‐C₂H₂F₂? BrF),‐3.88 kcal/mol (trans‐C₂H₂F₂? BrF). The earlier results show above that the F substituent effect elongates the π Br‐bond, reduces the E_int, and increases the E_corre contribution of the interaction energy. Interestingly, the interaction energy of the cis‐C₂H₂F₂? BrF structure with longer interaction distance is larger than that of the corresponding trans‐C₂H₂F₂? BrF structure with shorter interaction distance. This reason comes from a special secondary interaction between lone pairs of Br atom with positive charge and some atoms (H, C) with positive charges of C₂H₂F₂ in the cis‐C₂H₂F₂? BrF structure. Comparing with corresponding C₂H_4‐nF_n? ClF and C₂H_4‐nF_n? HF, the C₂H_4‐nF_n? BrF system has the larger E_int in which main contribution comes from the larger E_corre, representing the larger dispersion interaction. The larger E_corre contribution of the E_int of π Br‐bond can be used to understand that the π Br‐bond is shorter and stronger than corresponding π Cl‐bond. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Ba2(Ni1−xLix)Ni2N2: Ein niedervalentes Nitridoniccolat mit gewellten Schichten [(NiN2/2)?(Ni1−xLix)?(NiN2/2)]

Ba₂(Ni_1?xLi_x)Ni₂N₂: A Low-Valency Nitridoniccolate with Puckered Layers [(NiN_2/2)? (Ni_1?xLi_x)? (NiN_2/2)] Ba₂(Ni_1?xLi_x)Ni₂N₂ is obtained by reaction of lithium-barium-melts (molar ratios Li : Ba between 1 : 1 and 3 : 1) with nitrogen (1 atm.) in nickel-crucibles within a period of 15 h. Single crystals with a dark-metallic lustre are formed by cooling the melt to room temperature with a rate of 10°C/h (orthorhombic, Cmca; a = 713.3(2)pm, b = 1027.4(7)pm, c = 752.2(4)pm; z = 4; D_xr = 5.50 g/cm³ with x = 0.43). The crystal structure contains nearly liner [NiN_2/2]-chains (N? Ni? N: 178.5(7)°, Ni? N? Ni 173.4(7)°; Ni? N: 178.6(1)pm), running parallel to the [100] direction, which are interconnected via (Ni_1?xLi_x)-sites (linear units (N? (Ni_1?xLi_x)? N); bond-lenths: 194.5(12)pm with x = 0.43) to form puckered layer [(Nin_2/2)? (Ni_1?xLi_x)? (NiN_2/2)]. Barium is in a distorted trigonal-planar coordination by nitrogen atoms (Ba? N: 281.1(11)pm ? 285.5(11)pm. The nitrogen-coordination corresponds to a distorted octahedron, NBa₃(Ni_1?xLi_x)Ni₂, with nickel in trans-position. The crystal structure of Ba₂(Ni_1?xLi_x)Ni₂N₂ is closely related to the Li₃N-type structure: Li₂[LiN] ? Ba{(Ni_1?xLi_x)_0.5 □ _0.5}[NiN]. Furthermore, this structure enlarges the scope of barium-nitrido-niccolates which up to now were found to contain merely [NiN_2/2]-chains(BaNiN: Planar zigzag-chains; Ba₈Ni₆N₇ helical zigzag-chains).     

Synthesis,Crystal Structure,and IR Investigations of [(NH3)5V?O?V(NH3)5]I4 · NH3 and [(NH3)5Ti?O?Ti(NH3)5]I4 · NH3

The reaction of VI₂ or TiI₃, respectively, with ammonia in the presence of traces of water or oxygen, respectively, leads to [(NH₃)₅M? O? M(NH₃)₅]I₄ · NH₃ with M = V, Ti. Their structures were solved by X-ray single crystal data: Pbca (No. 61), Z = 4, M = V: a = 12.482(4) Å, b = 14.819(6) Å, c = 13.286(5) Å, N(F ? 3σF) = 983, N(variables) = 88, R/R_w = 0.053/0.063, M = Ti: a = 12.628(4) Å, b = 14.970(4) Å, c = 13.359(3) Å, N(F ? 3σF) = 1188, N(variables) = 88, R/R_w = 0.043/0.047. The structures consist of corner sharing octahedra double units [(NH₃)₅M? O? M(NH₃)₅]⁴⁺ with eclipsed conformation which are stacked together according to the motif of a distorted cubic face centered arrangement for the bridging oxygen atoms. IR spectroscopic investigations of the undeuterated vanadium compound and of 5% deuterated samples hint to N? H … I hydrogen bridge bonds and to remarkable π-bonding between the transition metal and the bridging oxygen atoms.     

Concerted Interaction between Pnicogen and Halogen Bonds in XCl?FH2P?NH3 (X=F,OH, CN,NC, and FCC)

We analyze the interplay between pnicogen‐bonding and halogen‐bonding interactions in the XCl? FH₂P? NH₃ (X=F, OH, CN, NC, and FCC) complex at the MP2/aug‐cc‐pVTZ level. Synergetic effects are observed when pnicogen and halogen bonds coexist in the same complex. These effects are studied in terms of geometric and energetic features of the complexes. Natural bond orbital theory and Bader’s theory of “atoms in molecules” are used to characterize the interactions and analyze their enhancement with varying electron density at critical points and orbital interactions. The physical nature of the interactions and the mechanism of the synergetic effects are studied using symmetry‐adapted perturbation theory. By taking advantage of all the aforementioned computational methods, the present study examines how both interactions mutually influence each other.     

Dihydrogen Bonding: Donor–Acceptor Bonding (AH⋅⋅⋅HX) versus the H2 Molecule (A?H2?X)

Dihydrogen bond or H ₂ molecule? The central H? H bond in linear H₄ can exist in two qualitatively different bonding modes corresponding to two different electronic states, namely a donor–acceptor dihydrogen bond (DHB) and a central H₂ molecule with an electron‐pair bond (see figure). This insight evolves from Kohn–Sham density functional analysis and it is further applied here to understand the bonding in more realistic model systems.

     

Changing Rules of Bonding Electron Pair Correlation Energies of CH3X （X = F, OH, NH2） Systems

1 INTRODUCTION It has been known that the electron correlation energy of molecular systems was, and still is, one of the most serious bottleneck problems to the chemis- try accuracy of computational quantum chemistry. Since L鰓din[1] gave the definition …     

N?H···S and S?H···N intramolecular hydrogen bond in β‐thioaminoacrolein: A quantum chemical study

The conformational study of β‐thioaminoacrolein was performed at various theoretical levels, HF, B3LYP, and MP2 with 6‐311++G(d,p) basis set, and the equilibrium conformations were determined. To have more reliable energies, the total energies of all conformers were recomputed at high‐level ab initio methods, G2MP2, G3, and CBS‐QB3. According to these calculations, the intramolecular hydrogen bond is accepted as the origin of conformational preference in thialamine (TAA) and thiolimine groups. The hydrogen bond strength in various resonance‐assisted hydrogen bond systems was evaluated by HB energy, geometrical parameters, topological parameters, and charge transfers corresponding to orbital interactions. Furthermore, our results reveal that the TAA tautomer has extra stability with respect to the other tautomers. The population analyses of the possible conformations by NBO predict that the origin of this preference is mainly due to the π‐electron delocalization in framework of TAA forms, especially usual π_C?C → π*_C?S and Lp (N) → π*_C?C charge transfers. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011