Aminoborylene complexes of group 6 elements and iron: a synthetic, structural, and quantum chemical study

Transition-metal-borylene complexes of the type [(OC)(5)M=BR] {M=Cr, Mo, W; R=N(SiMe(3))(2), 1a-3a, Si(SiMe(3))(3), 4a} and [(OC)(4)Fe=B=N(SiMe(3))(2)] (8) were prepared by salt elimination reactions. Synthesis of the latter complex was accompanied by the formation of substantial amounts of an unusual dinuclear iron complex [Fe(2){mu-C(2)O(2)(BN(SiMe(3))(2))}(2)(CO)(6)] (9). The aminoborylene complexes of Group 6 metals were converted to trans-[(Cy(3)P)(CO)(4)M=B=N(SiMe(3))(2)] (5a-7a) by irradiation in the presence of PCy(3). Structural and spectroscopic parameters were discussed with respect to the trans-effect of the borylene ligand and the degree of M-B d(pi)-p(pi)-backbonding. Computational studies were performed on Group 6-borylene complexes. The population and topological analyses as well as the molecular orbital composition are consistent with the presence of both sigma-and pi-type interactions. There are, however, indications that the d(pi)-p(pi)-backbonding in the silylborylene complex is significantly more pronounced than in the aminoborylene complexes.     

Comparative quantum chemical study of cyclic ether-water complexes

The interaction of the cyclic ethers tetrahydrofuran (THF) and 1,4-dioxane (DO) with water molecules in 1: 1 and 1: 2 ratios and with water dimers has been investigated by the density functional theory method in the B3LYP/6–31+G** approximation. The most stable structures and geometric and energetic characteristics of the resulting complexes have been determined, and the atomic charges have been calculated. The inferences from this study have been corroborated by comparing the properties of aqueous solutions of THF and DO.     

Ambidentate and redox-properties of 4,7-phenanthroline-5,6-dione in cobalt complexes: a quantum chemical study

Mono- and dinuclear adducts of cobalt diketonates with tetradentate 4,7-phenanthroline- 5,6-dione were modeled within the framework of DFT UB3LYP*/6-311++G(d,p) approximation. A competitive coordination of the metal ion to different donor centers of the redox-active ligand was studied. Variation of substituents in the diketone moieties allowed one to reveal compounds than can undergo thermally initiated one- and two-step valence tautomeric rearrangements. The calculated energy and magnetic characteristics of the dinuclear complexes give reasons to consider them as potential basis of molecular electronics and spintronics devices.     

O-O bond cleavage in dinuclear peroxo complexes of iron porphyrins: a quantum chemical study

To gain insight into the mechanisms of O2 activation and cleavage in metalloenzymes, biomimetic metal complexes have been constructed and experimentally characterized. One such model complex is the dinuclear peroxo complex of iron porphyrins observed at low temperature in a non-coordinating solvent. The present theoretical study examines the O-O bond cleavage in these complexes, experimentally observed to occur either at increased temperature or when a strongly coordinating base is added. Using hybrid density functional theory, it is shown that the O-O bond cleavage always occurs in a state where two low-spin irons (S = +/-1/2) are antiferromagnetically coupled to a diamagnetic state. This state is the ground state when the strong base is present and forms an axial ligand to the free iron positions. In contrast, without the axial ligands, the ground state of the dinuclear peroxo complex has two high-spin irons (S = +/-5/2) coupled antiferromagnetically. Thus, the activation barrier for O-O bond cleavage is higher without the base because it includes also the promotion energy from the ground state to the reacting state. It is further found that this excitation energy, going from 10 unpaired electrons in the high-spin case to 2 in the low-spin case, is unusually difficult to determine accurately from density functional theory because it is extremely sensitive to the amount of exact exchange included in the functional.     

Environment effects on the CO vibrational shifts in erbium complexes: a quantum chemical study

The stability of lanthanide complexes and the efficiency of the energy transfer process, which makes these molecules interesting materials for technological applications, are correlated to the chemical environment surrounding the metal ion. In particular the efficiency depends on the relative position of the antenna (the ligand moiety that acts as photon absorption center) and the lanthanide ion (the emitting center), while the stability of the complex is correlated to the strength of the coordination between the rare earth and the ligands. For these reasons, knowledge of the structural properties of the complex is an interesting task to achieve. Since a large number of ligand structures hold the carboxylate group (COO(-)), which is used as an anchor for binding the antennae to the lanthanide ion, in this work we will show how the vibrational shifts of this group, induced by the interactions between the carboxylate moiety and the metal center of the lanthanide complex, can be used for obtaining in a simple way information on the structure of the chemical environment surrounding the lanthanide ion.     

Magnetic exchange coupling in transition metal complexes with bidentate bridging ligands: a quantum chemical study

The exchange coupling constants (J) were calculated and the spin density distributions were analyzed in the B3LYP/TZV approximation for the complex anions [L₂M(1)^IIILM(2)^IIL₂] ⁿ⁻, where L is ligand (L is oxalate, oxamide, dithiooxamide, hydroxamate) and M(1) and M(2) are atoms of the tri- and divalent 3d-transition metals, respectively, and n- is the charge of the anion. The largest J values were found for the complexes formed by the Cr^III-Ni^II and Cr^III-Co^II pairs with the dithiooxamide ligands. Differences between the calculated and experimental J values are at most a few cm⁻¹.     

A quantum chemical study of trimethoxyaluminum complexes with neutral molecules

The spatial and electronic structures of trimethoxyaluminum complexes with neutral molecules are calculated by the MP2/6-31(2d,p) method using the PC GAMESS-Firefly program package. The main characteristics of aluminum bonds in these molecules are determined by AIM and NBO methods. It is shown that these bonds can be characterized as the bonds between the atoms with closed shells.     

Uranium and thorium hydride complexes as multielectron reductants: a combined neutron diffraction and quantum chemical study

The unusual uranium reaction system in which uranium(4+) and uranium(3+) hydrides interconvert by formal bimetallic reductive elimination and oxidative addition reactions, [(C(5)Me(5))(2)UH(2)](2) (1) ? [(C(5)Me(5))(2)UH](2) (2) + H(2), was studied by employing multiconfigurational quantum chemical and density functional theory methods. 1 can act as a formal four-electron reductant, releasing H(2) gas as the byproduct of four H(2)/H(-) redox couples. The calculated structures for both reactants and products are in good agreement with the X-ray diffraction data on 2 and 1 and the neutron diffraction data on 1 obtained under H(2) pressure as part of this study. The interconversion of the uranium(4+) and uranium(3+) hydride species was calculated to be near thermoneutral (~-2 kcal/mol). Comparison with the unknown thorium analogue, [(C(5)Me(5))(2)ThH](2), shows that the thorium(4+) to thorium(3+) hydride interconversion reaction is endothermic by 26 kcal/mol.     

Conformational polymorphism in bicalutamide: a quantum chemical study

Bicalutamide is an anti-neoplastic drug widely used for the treatment of prostate cancer and it exhibits conformational polymorphism. Three crystal structures of bicalutamide are reported as racemic mixtures, two of which are polymorphs. In addition, three co-crystals are also reported—two with organic coformers and one with adrenoreceptor (the macromolecular target). All the reported structures show significant conformational differences. Quantum chemical B3LYP/6-31+G(d,p) analysis has been carried out to understand the interplay of intra- and intermolecular interactions leading to the conformational preferences in this molecule. The difference between the two polymorphic forms has been traced to the C5–S8–C11–C12 torsional angle. Inside the cavity of androgen receptor, a completely different conformation is found but it does not correspond to any local minima on the potential energy surface of the drug. A relatively rigid torsional angle C11–C12–C15–N17 is also expected due to a strong five-membered ring intramolecular hydrogen bond (H–O13–C12–C15–O16), which has been reported to be desirable; quantum chemical analysis revealed that this rigidity is of the order of 11 kcal/mol. Ab initio calculations demonstrate that polymorphs and polymorphic co-crystals differ in the extent of intra- and intermolecular hydrogen bonding interactions. The strength of the intermolecular interactions associated with these structures is analyzed in terms of energy release due to dimerization.     

Interaction of aluminum and cobalt chlorides and their complexes with propane: A quantum chemical study

Density functional calculations of binary and ternary propane complexes with aluminum and cobalt(II) chlorides were carried out. Interaction of metal chloride complexes with the methylene and methyl groups of propane is coupled with strong polarization of C-H bonds. The direction of polarization is opposite to that observed under interaction with the typical Lewis acids including aluminum halides. In the interaction of bimetallic (transition metal/aluminum) complexes with saturated C-H bonds, the key role is played by the transition metal ion rather than the stronger Lewis center (Al). The negative atomic charge on carbon increases, which may facilitate the formation of metal-alkyl compounds with a carbanion-type organic radical.     

Structural, electronic, and optical properties of 9-heterofluorenes: a quantum chemical study

Density-functional theory studies were applied to investigate the structural, electronic, and optical properties of 9-heterofluorenes achieved by substituting the carbon at 9 position of fluorene with silicon, germanium, nitrogen, phosphor, oxygen, sulfur, selenium, or boron. These heterofluorenes and their oligomers up to pentamers are highly aromatic and electrooptically active. The alkyl and aryl substituents of the heteroatom have limited influence, but the oxidation of the atom has significant influence on their molecular structures and properties. The highest occupied molecular orbital (HOMO)-lowest occupied molecular orbital (LUMO) interaction theory was successfully applied to analyze the energy levels and the frontier wave functions of these heterofluorenes. Most heterofluorenes belong to type B of interaction with low-lying LUMO and have the second kind of wave function. Carbazole and selenafluorene have type C of interaction with high-lying HOMO and the third kind of wave function. Types C and D of heterofluorenes, such as carbazole, oxygafluorene, sulfurafluorene, and selenafluorene also have high triplet state energies. The extrapolated HOMO and LUMO for polyheterofluorenes indicate that polyselenonafluorene has the lowest LUMO; polycarbazole has the highest HOMO; polyselenafluorene has the highest bandgap (E(g)); and polyborafluorene has the lowest E(g). Heterofluorenes and their oligomers and polymers are of great experimental interests, especially those having extraordinary properties revealed in this study.     

Tautomerism of peroxyacetic acid derivatives: a quantum chemical study

The electronic and molecular structures and the relative stabilities of organic peracids X=C(R)-COOH and their cyclic tautomers, dioxiranes

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, with R = Me, CF₃; X = O, NH, were studied using the ab initio Hartree-Fock method and the density functional theory (B3LYP version) as well as at the MP2-MP4 Møller-Plesset levels of perturbation theory. Geometry optimization was performed by the UHF and B3LYP methods with the 6-31G** basis set and at the MP2/cc-pvtz level of theory. The acyclic form of the peracid is more stable than the cyclic dioxirane form irrespective of the nature of the substituent. The energy difference between these tautomers increases as the CF₃ and NH groups are replaced by Me and O, respectively. Parameters of the activation barrier to tautomeric conversion increase in parallel with enhancement of the electron-accepting properties of both substituents. The transition state of tautomeric interconversion, which is topologically similar to the acyclic structure of the carbonyl oxide derivative R(HX)C=O⁺-O^?, was found and characterized taking peroxyacetic acid as an example. The characteristic features of the transition state are an intramolecular “multicenter” H-bond and the charge distribution that is inconsistent with the canonical structure mentioned above. An appropriate reaction coordinate for the transformation of the quasi-tetrahedral dioxirane structure into a planar peroxyacetic acid structure is provided by the dihedral angle. Deprotonated anionic systems are characterized by much smaller differences between the relative stabilities of the open and closed forms of isomers and by much lower activation barriers to isomeric conversions.

     

(67)Zn NMR chemical shifts and electric field gradients in zinc complexes: a quantum chemical investigation

We have used quantum chemical methods to predict 67Zn NMR chemical shifts as well as quadrupole coupling constants (CQ) in a series of biomimetic and inorganic zinc complexes. The 67Zn chemical shifts are predicted with an R2 = 0.975, corresponding to a 24.3 ppm or 6.7% error over the entire 365 ppm 67Zn chemical shift range. The 67Zn CQ values are predicted with an R2 = 0.991, corresponding to a 1.17 MHz or 3.0% error over the entire 38.75 MHz range. The 67Zn NMR shifts in a series of complexes containing N,O ligands are, in general, highly correlated with the number of oxygen ligands. The ability to compute 67Zn NMR shifts as well as CQ values opens up the possibility of using both of these properties in structure determination or refinement in proteins.     

Two-dimensional lattice oligomers of diboraspiropentadiene: a quantum chemical study

According to B3LYP/6-311G(d,p) density functional calculations, the most energetically favorable method of assembling diboraspiropentadiene molecules into dimers, trimers, tetramers, and higher two-dimensional (2D) oligomers is to join the diboraspiropentadiene units with the formation of C-B bonds and the diboraspiropentadiene units trans-arranged relative to these bonds. Using this procedure, stable 2D lattice structures comprising 4, 7, 9, 16, or 25 monomer units were constructed in such a manner that each unit retains a nonclassical planar configuration of four bonds formed by the central carbon atom. Enlargement of the planar oligomers results in narrowing of the HOMO-LUMO gap and a considerable increase in the dipole moment. High negative NICS values suggest that the three-membered rings retain their aromaticity, whereas the ten-membered rings formed involving different monomer units in constructing oligomeric structures are strongly antiaromatic.     

Hypercoordinated carbon in C-doped boron fullerenes: a quantum chemical study

The structures and stability of C-doped boron fullerenes with the three-dimensional arrangement of non-classical pentacoordinated quasi-flat carbon centers were studied using the density functional theory (DFT) B3LYP/6-311+G(d,p) method. The doping with carbon atoms in apical positions above the five-membered rings stabilizes the spherical boron fullerene forms due to multicenter interactions of p_z-orbitals of the carbons and adjacent boron atoms. Increasing in the size of the fullerene cluster is accompanied by change in the bonding pattern and by flattening of the hypercoordinated carbon centers. Endohedral metal atoms significantly affect on the structure and stability of the fullerene systems with hypercoordinated carbon centers.     

Orientation of amide-nitrogen-15 chemical shift tensors in peptides: a quantum chemical study.

Knowledge of the orientation of the nitrogen-15 chemical shift anisotropy (CSA) tensor is critical for a variety of experiments that provide information on protein structure and dynamics in the solid and solution states. Unfortunately, the methods available for determining the orientation of the CSA tensor experimentally have inherent limitations. Rotation studies of a single crystal provide complete information but are tedious and limited in applicability. Solid-state NMR studies on powder samples can be applied to a greater range of samples but suffer from ambiguities in the results obtained. Density functional gauge-including-atomic-orbitals (GIAO) calculations of the orientations of (15)N CSA tensors in peptides are presented here as an independent source of confirmation for these studies. A comparison of the calculated (15)N CSA orientations with the available experimental values from single-crystal and powder studies shows excellent agreement after a partial, constrained optimization of some of the crystal structures used in the calculation. The results from this study suggest that the orientation as well as the magnitudes of (15)N CSA tensors may vary from molecule to molecule. The calculated alpha(N) angle varies from 0 degrees to 24 degrees with the majority in the 10 degrees to 20 degrees range and the beta(N) angle varies from 17 degrees to 24 degrees in good agreement with most of the solid-state NMR experimental results. Hydrogen bonding is shown to have negligible effect on the orientation of (15)N CSA tensor in accordance with recent theoretical predictions. Furthermore, it is demonstrated that the orientation of the (15)N CSA can be calculated accurately with much smaller basis sets than is needed to calculate the chemical shift, suggesting that the routine application of ab initio calculations to the determination of (15)N CSA tensor orientations in large biomolecules might be possible.     

A quantum chemical study of red-shift and blue-shift hydrogen bonds in bimolecular and trimolecular methylhydrazine-hydrate complexes

This study presents a theoretical discussion of the geometry and molecular parameters of the methylhydrazine-hydrate complex. Using B3LYP/6-311++G(d,p) calculations, two geometries for the methylhydrazine-hydrate complex were analyzed by considering the interaction between water in: (i) a lone nitrogen pair assisted by a methyl (I); and (ii) the nitrogen of the NH₂ group (II). These geometries were examined by examining the formation of (N···H) hydrogen bonds, which were characterized using ChelpG charge transfer amounts, as well as by means of topological parameters derived from Quantum Theory of Atoms in Molecules (QTAIM) calculations. In a qualitative evaluation, both complexes (a) and (b) were compared with the corresponding trimolecular system (c) formed by methylhydrazine and two water molecules. A conclusion was then obtained by means of vibrational analysis, in which, in addition to δυ(H–O) red-shifts in water molecules, the stretch frequency of the H–C bond of methyl group shifted upwards, indicating the formation of a blue-shifting hydrogen bond in the methylhydrazine-bihydrate complex.     

X-ray and quantum chemical study of the electronic structure of glycine and its metal complexes

OK_α spectra of glycine and some transition metal complexes with glycine ligands were obtained. The electronic structure of the glycine zwitterion is calculated by a quantum chemical method, and a theoretical X-ray spectrum of the glycine molecule is constructed. The nature of the metal-ligand bond in the compounds is discussed on the basis of experimental spectra and theoretical calculations. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Moscow State Academy of Light Industry, Novosibirsk Branch. Moscow State Academy of Light Industry. Novosibirsk State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 112–116, July–August, 1994. Translated by L. Smolina