Theory and simulation of anisotropic pair correlations in ferrofluids in magnetic fields

Anisotropic pair correlations in ferrofluids exposed to magnetic fields are studied using a combination of statistical-mechanical theory and computer simulations. A simple dipolar hard-sphere model of the magnetic colloidal particles is studied in detail. A virial-expansion theory is constructed for the pair distribution function (PDF) which depends not only on the length of the pair separation vector, but also on its orientation with respect to the field. A detailed comparison is made between the theoretical predictions and accurate simulation data, and it is found that the theory works well for realistic values of the dipolar coupling constant (λ = 1), volume fraction (φ ≤ 0.1), and magnetic field strength. The structure factor is computed for wavevectors either parallel or perpendicular to the field. The comparison between theory and simulation is generally very good with realistic ferrofluid parameters. For both the PDF and the structure factor, there are some deviations between theory and simulation at uncommonly high dipolar coupling constants, and with very strong magnetic fields. In particular, the theory is less successful at predicting the behavior of the structure factors at very low wavevectors, and perpendicular Gaussian density fluctuations arising from strongly correlated pairs of magnetic particles. Overall, though, the theory provides reliable predictions for the nature and degree of pair correlations in ferrofluids in magnetic fields, and hence should be of use in the design of functional magnetic materials.     

The extended view on the empty C2(3)-C82 fullerene: isolation, spectroscopic, electrochemical, and spectroelectrochemical characterization and DFT calculations

An extended study of the spectroscopic and redox properties of the C(82) fullerene is presented. Among the nine isolated-pentagon-rule (IPR) isomers of the C(82) fullerene the C(82)(3) isomer with C(2) symmetry is the only stable, empty fullerene structure formed in the arc burning process that can be isolated in an isomerically pure form. Here, its formation and isolation are described and its structure is confirmed by experimental spectroscopic studies as well as time-dependent DFT calculations. The electrochemistry of the C(82)(3) isomer is studied in detail by cyclic voltammetry and spectroelectrochemistry. The anionic species of C(82) with the charge ranging from C(82) (-) to C(82) (4-) were successively generated in o-dichlorobenzene solution at room temperature and characterized by in situ ESR and visible/near-infrared (Vis/NIR) spectroscopy. The data give new insights into the charged states of the C(82)(3) fullerene.     

Synthesis of heterocyclic compounds possessing the 4H-thieno[3,2-b]pyrrole moiety

A series of novel heterocyclic combinatorial libraries containing 4H-thieno[3,2-b]pyrrole, thieno[2',3':4,5]-pyrrol[1,2-d][1,2,4]triazine and thieno[2',3':4,5]pyrrolo[1,2-a]pyrazine heterocyclic moieties were obtained by parallel solution-phase synthesis. Key steps include different reactions of initial alkyl 4H-thieno[3,2-b]-pyrrole-5-carboxylates, such as alkylation with alkylating agents; transformation of the carboxylate group into different reactive functionalities, followed by reactions with electrophilic species; intramolecular cyclizations; and amide bond formation. Simple manual techniques for parallel reactions were coupled with easy purification procedures to give high-purity final products.     

"Direct" detection and separation of DNA using nanoporous alumina filters

The concept of using alumina nanoporous filters (AAO) modified with DNA for "label-free" detection and separation/purification of the target ss-DNA is demonstrated. The high surface density of DNA (4 x 10(12) cm(-2)) and high efficiency of hybridization (ca. 70%) in combination with increased effective surface area make this system very attractive for development of various ss-DNA (or RNA) detection methods. Moderate transparency of AAO in the UV and IR regions allows direct detection of DNA hybridization by optical and IR absorption. Close to the quantitative efficiency of binding the target ss-DNA from solution using a single pass through the modified filter is observed.     

Synthesis, characterization, and ethylene polymerization behavior of [(RR′-admpzp)2Ti(OPr)2] complexes (RR′-admpzp = 1-dialkylamino-3-(3,5-dimethyl-pyrazol-1-yl)-propan-2-olate)

[(RR′-admpzp)₂Ti(OPrⁱ)₂] complexes (2a-c), synthesized from reaction of Ti(OPrⁱ)₃Cl (0.5 equiv) with 1-dialkylamino-3-(3,5-dimethyl-pyrazol-1-yl)-propan-2-ol compounds in the presence of triethylamine (0.5 equiv), are pseudo-octahedral with each RR′-admpzp ligand κ²-O,N(pyrazolyl) coordinated to the titanium center. In solution, 2a-c adopt isomeric structures that are in dynamic equilibrium. At 23 °C, 2a-c/1000 MAO catalyst systems furnished high molecular weight polymers with narrow molecular weight distributions (M_w/M_n = 2.7-2.8). At 100 °C, 2a-c/MAO catalyst systems exhibited increased polymerization activity and 2c/1000 MAO system furnished high molecular weight polyethylene with a molecular weight distribution (M_w/M_n = 2.1) that is close to that found for single-site catalysts.     

Remarkable temperature effect on intramolecular [3+2] cyclization

A preference of intramolecular cyclizations over oligomerization can often be achieved by conducting the reaction at higher temperatures. This effect which has obviously been underestimated so far may be useful in synthetic organic chemistry. Herein, we report a novel example supporting this observation—an intramolecular nitrile oxide 1,3-dipolar cycloaddition. The yield of the desired product rose from 53% at 0 °C to 79% at 80 °C. A plausible kinetic background of this phenomenon is proposed in terms of activation enthalpy.     

Electronic structure and spectroscopic studies of D3d-C60Cl30, a chlorofullerene with a [18]trannulene ring, and its relation to other [18]trannulenes

Detailed spectroscopic characterization of D3d-C60Cl30, including IR, Raman, UV-vis absorption, and fluorescence spectra, is presented for the first time. Assignment of the vibrational spectra is proposed on the basis of density functional theory computations. Electronic structure and excitations of C60Cl30 and other [18]trannulenes are studied theoretically with the use of time-dependent density functional theory and time-dependent Hartree-Fock approximation. Assignment of the low-energy part of electronic spectra of C60-based [18]trannulenes is given and importance of the interactions between trannulene moiety and remaining pi-subsystems in these molecules is established.     

Electron transfer between guanosine radicals and amino acids in aqueous solution. II. Reduction of guanosine radicals by tryptophan

The efficiency of the chemical pathway of DNA repair is studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP) using the model system containing guanosyl base radicals, and tryptophan as the electron donor. Radicals were generated photochemically by pulsed laser irradiation of a solution containing the photosensitizer 2,2'-dipyridyl, guanosine-5'-monophosphate, and N-acetyl tryptophan. Depending on the pH of the aqueous solution, four protonation states of the guanosyl radical are formed via electron or hydrogen atom transfer to the triplet excited dye. The rate constants of electron transfer from the amino acid to the guanosyl radical were determined by quantitative analysis of the CIDNP kinetics, which is very sensitive to the efficiency of radical reactions in the bulk, and rate constants vary from (1.0 +/- 0.3) x 10(9) M(-1) s(-1) for the cation and dication radicals of the nucleotide to (1.2 +/- 0.3) x 10(7) M(-1) s(-1) for the radical in its anionic form. They were found to be higher than the corresponding values for electron transfer in the case of N-acetyl tyrosine as the reducing agent.     

Formation of a C–C double bond from two aliphatic carbons. Multiple C–H activations in an iridium pincer complex

The search for novel, atom-economic methods for the formation of C–C bonds is of crucial importance in synthetic chemistry. Especially attractive are reactions where C–C bonds are formed through C–H activation, but the coupling of unactivated, alkane-type C_sp³–H bonds remains an unsolved challenge. Here, we report iridium-mediated intramolecular coupling reactions involving up to four unactivated C_sp³–H bonds to give carbon–carbon double bonds under the extrusion of dihydrogen. The reaction described herein is completely reversible and the direction can be controlled by altering the reaction conditions. With a hydrogen acceptor present a C–C double bond is formed, while reacting under dihydrogen pressure leads to the reverse process, with some of the steps representing net C_sp³–C_sp³ bond cleavage. Mechanistic investigations revealed a conceptually-novel overall reactivity pattern where insertion or deinsertion of an Ir carbene moiety, formed via double C–H activation, into an Ir–C bond is responsible for the key C–C bond formation and cleavage steps.