Agostic interactions in d0 metal alkyl complexes

The phenomenon of agostic interactions is reviewed and the nature of the interaction is revisited. A historical perspective is followed by an overview of experimental techniques used to diagnose agostic behavior, and previous interpretations of agostic bonding are presented. A series of simple metal alkyl complexes is considered and a new model for the phenomenon in d(0) systems is developed which sets them apart from agostic late-transition-metal complexes. Factors such as the valence electron count and coordination number of the metal center are revealed to be unimportant in facilitating the interaction in most d(0) systems. The charge density distribution in several transition-metal alkyl complexes is explored by experimental and theoretical techniques, including the powerful "Atoms in Molecules" approach. Local charge concentrations are shown to play an important role in the agostic interaction. Finally, we demonstrate for the first time a way to manipulate and control the magnitude and disposition of such local charge concentrations, and hence the strength of agostic interactions in d(0) metal alkyl complexes.     

Synthesis,Redox Properties,and Electronic Spectra of Sterically Crowded Triarylphosphines Conjugated with Electron Acceptors

Abstract

Synthesis, redox properties, and electronic spectra of the sterically crowded triarylphosphines conjugated with π-electron systems, especially electron acceptors such as carbonyl group, are briefly reviewed. The sterically crowded triarylphosphines conjugated with various π-electron systems were synthesized from the common synthetic intermediate, (bromoaryl)phosphine, by conventional manner. The sterically crowded triarylphosphines conjugated with the electron acceptors exhibit visible absorption and fluorescence with large Stokes shift. Large solvent effect, redshift in polar solvent, and good correlation with the difference of the oxidation potential of the phosphine moiety and the reduction potential of the acceptor moiety suggest polar excited state resulting from HOMO-LUMO transition.     

Conformational and Electronic Interaction Studies of Some β-Ketosulfoxides

ABSTRACT.

The analysis of the carbonyl I.R. band and the U.V. phoelectron spectroscopic data along with the ab-initio calculations and X-ray diffraction of α-(methylsulfinyl)- and α-(phenylsulfinyl)-acetophenones indicated the predominance of the cis rotamers over the qauche ones.     

Acid-Catalyzed Hydrolysis of Some N,N-Dibenzylalkanesulfinamides in 50% Acetonitrile–Water

Abstract

The hydrolysis of some N,N-dibenzylalkanesulfinamides (RSONH(CH₂Ph)₂; 1, R = Me; 2, R = ⁱPr; 3, R = ^tBu; 4, R = 1-adamantyl) has been studied in 50% (v:v) acetonitrile–water solutions of hydrobromic and hydrochloric acids, mainly at 44.8 °C, using ultraviolet (UV) spectrophotometry to determine pseudo first-order rate constants. The compounds were found to hydrolyze by concurrent bimolecular neutral, acid-catalyzed, and acid-dependent nucleophilic (halide ion) catalysis pathways. The last-named is predominant in reactions in HBr solutions, but in HCl solutions, the acid-catalyzed pathway is predominant. The results indicate that both steric and electronic effects are important in these reactions. There appears to be no mechanistic switchover in the series 1→4.     

POLYCATION EFFECTS ON ELECTRONIC SPECTRA OF CONJUGATED POLYMERS IN PROGRAMMED ELECTROSTATIC ASSEMBLIES

A polysaccaride, chitosan, has been used to study polycation effects on electronic spectra of conjugated polymers in programmed electrostatic assemblies. Conjugated polyelectrolytes, poly[2-(3-thienyl)-ethanolhydroxycarbonylmethylurethane] (HPURET) and polydiacetylene from 5,7-dodecadiyn-1,12-bis-(hydroxycarbonylmethylurethane) (HP4BCMU), were assembled from aqueous solution alternately with the polycation, poly (diallydimethylammonium) chloride [PDADMAC], and with the polysaccharide cation, chitosan. Switching polycations did not significantly change the visible absorption spectra for HP4BCMU multilayers. The optical properties of the regiorandom polythiophene, HPURET, depend distinctly on the polycation and the pH of the solution, showing significant differences on visible absorption maxima of the assemblies ranging from 435 nm to 516 nm. After the assemblies were exposed to vapor of aqueous ammonia, they showed UV-vis maxima further red shifted and an enhanced emission intensity, compared to those of the original assemblies. A chitosan/HPURET complex was prepared as a precipitate from solution. Its visible absorption maximum is at 536 nm with a shoulder at 580 nm, reminiscent of regioregular poly(3-alkylthiophenes). This unusual sensitivity of conjugated polymers to polycations may have potential application in sensor devices.     

The Ligand‐Based Quintuple Bond‐Shortening Concept and Some of Its Limitations

Herein, the ligand‐based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium–diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6‐dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride–ate complex gave a quintuply bonded bimetallic complex with a Cr? Cr distance of 1.7056 (12) Å. Its metal–metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C? C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr^I dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by‐product. The formation of this by‐product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over‐reduction takes place, and a chromium–arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.     

Efficient implementation of restricted active space configuration interaction with the hole and particle approximation

The restricted active space configuration interaction (RASCI) formalism with the hole and particle truncation of the wavefunction, that is, RASCI(h,p), holds very nice properties such as balanced treatment of ground and low‐lying excited states, spin‐completeness, large flexibility of the wavefunction, and moderate computational cost. In this article, I present a new implementation of the RASCI(h,p) method using a general algorithm based on the integral‐driven approach. The new implementation allows to choose any electronic configuration as the single reference in combination with an excitation operator with any number of ionization, electron attachment, or spin‐flip (SF) excitations. The applicability and good performance of the new computational code is tested in the ground state calculation of water molecule with increasingly large active spaces and up to the full‐CI limit, the calculation of all‐trans linear polyenes with variable number of SF excitations, and the low‐lying states of fluorine molecule with a double‐ionization potential operator. © 2012 Wiley Periodicals, Inc.     

A comparison of the effect of nanotube chirality and electronic properties on the π–π interaction of single‐wall carbon nanotubes with pyrazinamide antitubercular drug

Theoretical investigation on local electronic structure and stability of the π–π stacking interaction of pyrazinamide (PZA) with armchair (5,5) and zigzag (9,0) single‐walled carbon nanotubes (SWCNTs) is performed using density functional theory (DFT). PZA is physisorbed onto nanotube sidewall through interaction of π orbitals of PZA and SWCNT and the enhanced structural stability of PZA/SWCNT systems is due to weak side‐on rather than the head‐on π‐interactions. The physisorption of PZA onto SWCNT sidewall is thermodynamically favored; as a consequence, it modulates the electronic properties of pristine nanotube in the vicinity of Fermi region and π–π stacked interactions is stronger in (9,0) SWCNT compared to (5,5) SWCNT. The density of states (DOS) analysis show that PZA contributes toward the enhancement of electronic states. Projected DOS and frontier orbital analysis in the vicinity of Fermi level region suggest the electronic states to be contributed from SWCNT rather than PZA. In addition, hybrid DFT calculation which includes the dispersion correction is employed to explain the non‐covalent π–π stacking interaction between PZA and SWCNT. The local density approximation and GGA results are compared with DFT‐D to explain near about accurately the weak nonbonded van der Waals interactions between PZA and SWCNTs. © 2012 Wiley Periodicals, Inc.     

Investigation on the dissolution of Mn ions from LiMn2O4 cathode in the application of lithium ion batteries: First principle molecular orbital method

The dissolution phenomenon of Mn ions in LiMn₂O₄ (LMO) cathode material for lithium ion batteries (LIBs) was investigated by a first principle calculation using the discrete variational Xα molecular orbital method. It was found that the oxidation number of Mn ions easily increases at high temperatures due to the empty levels of Mn 3d orbitals located in the vicinity of the Fermi energy level of LMO crystal. The changes of density of states (DOS) and Mn‐O bonding properties with doping were examined. Analysis of DOS showed that the substitution of elements with a smaller oxidation number than Mn was found effective in keeping Mn ions at higher oxidation states. From the calculation of bonding properties, the dissolution of Mn was found to be strongly correlated with the covalent nature of Mn‐O bond. Based on the results, we concluded that increasing the covalent character of Mn‐O bond is effective to minimize the dissolution of Mn ions, along with suppressing the formation of Jahn‐Teller‐active Mn³⁺ by inducing Mn ions at high oxidation state with proper selection of doping elements. © 2012 Wiley Periodicals, Inc.