Topological properties of the electronic density of CO-MgF2 and OC-MgF2 adducts

By means of Bader's approach on topological properties of the electronic density, the major stability of the O-bonded adduct over the C-bonded one between MgF₂ and CO is reinforced.     

Anion substitution effects on structure and magnetism in the chromium chalcogenide Cr5Te8—Part I: Cluster glass behavior in trigonal Cr(1+x)Q2 with basic cell (Q=Te, Se; Te:Se=7:1)

The effect of substitution of the anion Te by Se in non-stoichiometric Cr₅Te₈ has been investigated with respect to its crystal structure, magnetic properties, and electronic structure. The compounds Cr_(1+x)Q₂ (Q=Te, Se; Te:Se=7:1; (1+x)=1.234(6), 1.264(6), 1.300(7)) were synthesized at elevated temperatures followed by quenching the samples to room temperature. The crystal structures have been refined with X-ray powder diffraction data with the Rietveld method in the trigonal space group with lattice parameters a=3.8651(1)-3.8831(1) Å and c=5.9917(2)-6.0528(2) Å. The structure is related to the NiAs structure with full and deficient metal layers stacking alternatively along the c-axis. The irreversibility in the field-cooled/zero-field-cooled magnetization suggests that the substitution effects of one Te by one Se is strong enough to cause cluster-glass behavior, from ferromagnetic Cr₅Te₈ to cluster-glass Cr_(1+x)Q₂. Non-saturation magnetizations at 5.5 T and the magnetic relaxation results further support the existence of cluster-glass behavior. Accompanying SPR-KKR (spin-polarized relativistic Korringa-Kohn-Rostoker) band structure calculations strongly support the observation that the Cr(1) sites are preferentially occupied by Cr atoms and predict that these compounds are metallic. Results for the spin-resolved DOS and magnetic moments on each crystallographic sites are presented.     

Absence of p,π-conjugation in the Cl2C=CHOMe molecule from data ofab initio calculations

ab initio Calculation of the Cl₂C=CHOMe molecule has been carried out with geometrical optimization in the split valence basis set in the RHF/6-31G^*//RHF/6-31G^* approximation. An analysis of the populations of the p-orbitals of the Cl atoms implies the absence of p,-conjugation between the unshared electron pairs of these atoms and the -bond. The asymmetry parameters of the electric field gradient at the³⁵Cl nuclei, calculated from the 3p-orbital populations of the Cl atoms, practically coincide with the experimental values.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2369–2372, December, 1995.The authors are grateful to G. S. Beloglazov for assistance in accomplishingab initio calculations.     

Structure of Charge Transfer Reaction Complexes Formed in Anionic Polymerization of Isoprene

A new mechanism is suggested for the anionic polymerization of isoprene. The key moment of this mechanism is thermal electron excitation of the complex of a living polymer with a monomer to the low lying S₁ (T₁) state involving a charge (electron) and (Li⁺) cation transfer from the terminal unit to the monomer molecule. It is stated that the probability of chemical bonding depends on the spin density on the radical centers of reactant molecules and on the geometry of the reaction complex. The semiempirical AM1 and ab initio 6-31G* quantum-chemical calculations revealed strong interaction for the ground electronic state of the complex (5-10 kcal/mole) and low energies of the excited triplet levels (<10 kcal/mole).     

Parameterization of peptide 13C carbonyl chemical shielding anisotropy in molecular dynamics simulations.

NMR chemical shielding anisotropy (CSA) relaxation is an important tool in the study of dynamical processes in proteins and nucleic acids in solution. Herein, we investigate how dynamical variations in local geometry affect the chemical shielding anisotropy relaxation of the carbonyl carbon nucleus, using the following protocol: 1) Using density functional theory, the carbonyl (13)C' CSA is computed for 103 conformations of the model peptide group N-methylacetamide (NMA). 2) The variations in computed (13)C' CSA parameters are fitted against quadratic hypersurfaces containing cross terms between the variables. 3) The predictive quality of the CSA hypersurfaces is validated by comparing the predicted and de novo calculated (13)C' CSAs for 20 molecular dynamics snapshots. 4) The CSA fluctuations and their autocorrelation and cross correlation functions due to bond-length and bond-angle distortions are predicted for a chemistry Harvard molecular mechanics (CHARMM) molecular dynamics trajectory of Ca(2+)-saturated calmodulin and GB3 from the hypersurfaces, as well as for a molecular dynamics (MD) simulation of an NMA trimer using a quantum mechanically correct forcefield. We find that the fluctuations can be represented by a 0.93 scaling factor of the CSA tensor for both R(1) and R(2) relaxations for residues in helix, coil, and sheet alike. This result is important, as it establishes that (13)C' relaxation is a valid tool for measurement of interesting dynamical events in proteins.     

The new facial tripod ligand 3,3-bis(1-methylimidazol-2-yl)propionic acid and carbonyl complexes thereof containing manganese and rhenium

The new facially coordinating tripod ligand 3,3-bis(1-methylimidazol-2-yl)propionate (bmip) has been studied. A synthetic route to sodium 3,3-bis(1-methylimidazol-2-yl)propionate (Na[bmip], 2a) and its hydrochloride (Hbmip · 2HCl, 2b) is reported. The electronic properties of Hbmip were calculated by DFT methods and are compared to those of structurally similar bis(pyrazol-1-yl)acetic acids. The ligand was applied in the synthesis of the two tricarbonyl complexes [Re(bmip)(CO)₃] (3) and [Mn(bmip)(CO)₃] (4). Methyl 3,3-bis(1-methylimidazol-2-yl)propionate (bmipme) (1), which is the precursor of Hbmip, and the complexes [Re(bmip)(CO)₃] (3) and [Mn(bmip)(CO)₃] (4) were characterised by single crystal X-ray analysis.     

On Mercury(I) Oxo Compounds — Quasi‐Relativistic Computational and Experimental Studies

About 60 molecular species composed of up to 10 mercury atoms and of oxygen atoms and/or of some other elements or groups (such as halogen, OH₂, OH, H, alkali, NO₃) have been investigated quantum chemically. Different density functional approaches and the ab initio SCF‐MP2 method were applied, comparing different basis sets and different atomic core sizes. It is important not to treat the Hg 5s, p, d as inactive core shells, and to use sufficiently many polarization functions. The shape of the 〉O‐Hg‐Hg‐O〈 units is not favorable concerning the formation of lattices composed of Hg^I, O and OH only. Despite its bulkiness, the OHgHgO units can easily come into contact with each other and then disproportionate. This is prevented in the so‐called ternary M‐Hg^I oxides by the embedded oxometallate (oxoacidic) anions. Furthermore, the Hg^I and Hg^II oxide bond energies are less favorable towards the stability of Hg^I oxo compounds, as compared to Hg halidic or oxoacidic compounds. Both points are not promising concerning the search for Hg^I oxides/hydroxides, although the preparation of such compounds, including spacer groups, by topochemical reactions can still not be excluded. So far, experimental efforts towards the synthesis of such a new class of compounds have only demonstrated that Hg^II is strictly preferred over Hg^I in the formation of solids of binary Hg‐O or ternary A‐Hg‐O composition (A = electropositive metal such as alkali, in contrast to M = transition or semi‐metal). This is so even if compounds containing ‘electron rich Hg^δ— atoms’ (i.e. A‐Hg amalgams) are oxidized under mild conditions.     

Ab Initio High Pressure Investigations of InI

The high pressure behaviour of InI is studied by DFT‐calculations and compared with experimental data. The existence of a 5s² electron pair in In⁺ represents an unfavourable bonding situation for high symmetry structures because of effective closed shell repulsion. Since cations with a ns² electron pair are highly polarizable and the electronic situation is more favourable in the low symmetry structure InI prefers a TlI‐type structure at ambient pressure. A pressure induced transition to the more densely packed high symmetry CsCl‐type structure takes place at about 19 GPa according to our calculations. At ambient pressure the interactions are predominantly ionic. However with increasing pressure the distances between In⁺ cations in the TlI‐type structure diminish drastically, mainly due to the changing space requirement of the lone electron pair. Apart from ionic interactions further bonding interactions between the In⁺ cations occur. At elevated pressure the electron localization function (ELF) as well as the band structure diagrams suggest metallic bonding between the In⁺ within the zigzag chain, i. e. increasing bonding interactions between the In⁺ cations due to the electron pair and its s‐p‐mixing. At ambient pressure In‐In interactions are rather weak and the space requirement of the lone electron pair mainly determines the characteristic arrangement of the ions. At elevated pressure the In‐In interactions become stronger and stabilise themselves additionally the specific structural arrangement.     

Computer modelling of the conformational equilibrium of 2-and 2,5,5-substituted 1,3,2-dioxaborinanes

Empirical, semiempirical, and nonempirical quantum-chemical methods were used to study the conformational equilibrium of 2,5,5-substituted 1,3,2-dioxaborinanes. The sofa invertomers were found to correspond to the local and global minima on the potential energy surface. The position of the equilibrium between these forms is a function of the substituents at C₍₅₎ of the heterocycle. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1860–1865, December, 2007.     

Intramolecular charge transfer in pyrromethene laser dyes: photophysical behaviour of PM650.

Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media.