Ultrafast Dynamics Through Conical Intersections in 2,6-dimethylpyridine Studied with Time-resolved Photoelectron Imaging

The ultrafast dynamics through conical intersections in 2,6-dimethylpyridine has been stud-ied by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Upon absorption of 266 nm pump laser, 2,6-dimethylpyridine is excited to the S₂ state with a ππ^* character from S₀state. The time evolution of the parent ion sig-nals consists of two exponential decays. One is a fast component on a timescale of 635 fs and the other is a slow component with a timescale of 4.37 ps. Time-dependent photo-electron angular distributions and energy-resolved photoelectron spectroscopy are extracted from time-resolved photoelectron imaging and provide the evolutive information of S₂ state. In brief, the ultrafast component is a population transfer from S₂ to S₁ through the S₂/S₁ conical intersections, the slow component is attributed to simultaneous IC from the S₂ state and the higher vibrational levels of S₁ state to S₀ state, which involves the coupling of S₂/S₀ and S₁/S₀ conical intersections. Additionally, the observed ultrafast S₂→S₁ transition occurs only with an 18% branching ratio.     

Solvent Effect on the Ligand Exchange between Bis(diethyldithiocarbamato)copper(II) and Bis(diethyldiselenocarbamato)copper(II)

EPR and spectrophotometric study on the products of ligand‐exchange taking place on mixing bis(diethyldiselenocarbamato)copper(II), [Cu(Et₂dsc)₂], and bis(diethyldithiocarbamato)copper(II), [Cu(Et₂dtc)₂], solutions is reported. EPR spectra monitored at room temperature for one month period reveal a stable equilibrium among the parents (chromophores CuS₄ and CuSe₄) and the obtained mixed‐chelate [Cu(Et₂dtc)(Et₂dsc)] complex (chromophore CuS₂Se₂) in heptane, hexane, benzene, toluene, acetone, DMFA, DMSO and dichloromethane. In CCl₄ and CHCl₃ two new additional EPR spectra appear attributed to the mixed‐chelate complexes with the chromophores CuSSe₃ and CuS₃Se which are not observed with electronic spectroscopy. The intensities of all five EPR spectra decrease with the time. It is assumed that the new mixed‐chelates observed in CCl₄ and CHCl₃ are obtained in a reaction of [Cu(Et₂dtc)(Et₂dsc)] or [Cu(Et₂dtc)₂] with the ester of diselenocarbamic acid which is formed in a parallel reaction of [Cu(dsc)₂]with CCl₄ or CHCl₃.     

Synthesis, characterization and structural investigation of the first vanadocene(IV) carboxylic acid complexes prepared from the vanadocene dichloride

Two types of vanadocene complexes with carboxylic acids have been synthesized from the aqueous solution, Cp₂V(OOCR)₂ (R=H, CCl₃ and CF₃) and Cp₂V(OOC-A-COO) (A= - and CH₂), and characterized by EPR, IR, and Raman spectroscopy and X-ray diffraction analysis. Monocarboxylic and dicarboxylic acids form monodentate and chelate complexes, respectively. Both bonding types were evidenced by X-ray diffraction analysis. Structures and EPR HFC tensors were also calculated at the DFT level. Correlation between the complex structure and HFC tensor was established. HFC tensors are characteristic for the type of bond of carboxylic acid on vanadocene fragment. It is shown that the structure of complexes can be determined by the combination of theoretical method with experimental EPR spectra.     

Cation radicals of N-substituted phenothiazines

The reaction of N-substituted phenothiazines with certain acceptors (halogenated solvents CHCl₃, CH₂Br₂, CCl₄, o-chloranil, AlCl₃, SnCl₄, concentrated H₂SO₄, concentrated HNO₃ in HClO₄) has been studied by EPR spectroscopy. The hyperfine structure in the EPR spectra of the cation radicals is analyzed. To interpret the EPR spectra obtained in terms of the MNDO-PM3 method we carried out a calculation of the electronic structure of phenothiazine cation radicals containing N-CH₃ and N-CH₂R substituents. In these radical systems, there are significant steric hindrances to conformational rotation of the CH₂ substituent around the N-C bond leading to a conformation with magnetically non-equivalent protons in the methylene group.Mordovian State University, Saransk 430000. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 420–426, March, 1996. Original article submitted July 28, 1995.     

Comparison of the99mTc-cysteine complex and131I-hippuran in biodistribution

This study compares the biological behavior of the^99mTc-cysteine complex with¹³¹I-hippuran (¹³¹I-OIH). The potential role of the^99mTc-cysteine complex as a renal imaging agent has also been evaluated. In addition the biological characteristics, i.e., biodistribution, blood clearance, excretory mechanism and the renal excretion pattern in the presence and absence of the tubular transport inhibitor (2,4-dinitrophenol, probenecid) has been examined. The experimental results reveal that kidneys are the target organ. The blood clearance of these radiopharmaceuticals is rapid, approximately 84% of the injected activity is excreted from the kidneys at 1 hour postinjection. Moreover, the renal inhibitor obviously depresses the excretory rate of the^99mTc-cysteine and¹³¹I-OIH, suggesting that such agents are excreted by renal tubular secretion. Results presented demonstrate that the^99mTc-cysteine is a remarkable renal dynamic imaging agent which is highly promising as a new renal imaging radiopharmacentical.     

Plasma-chemical reactions in weakly decomposed CCl4

For the case of weak feed gas decomposition, where the concentration of CCl₄ exceeds those of decomposition and built-up products, the emission of CCl* is shown to originate from dissociative excitation of CCl₄. With electron concentration measured independently, the kinetics of CCl₄ decomposition has been extracted from the time dependence of the CCl* intensity. Supported by EPR determinations of radical concentrations in rapidly flowing CCl₄ and CCl₄/O₂ afterglows, the primary decomposition reaction is shown to be the electron impact dissociation into CCl₃ and Cl. Its rate constant (k ₁=4×10^–8 cm³s^–1) indicates strongly that dissociative electron attachment is the main reaction channel at least at r.f. power densities just above the threshold of a self-maintaining discharge. At extremely low mean electron energies the emission of a continuum is observed, which is tentatively ascribed to the radiative CCl₃-Cl recombination.     

2H NMR spin relaxation study of the soft segment motion in alternating ethylene–propylene copolymers

We synthesized three partially deuterated polymer samples, namely a poly(ethylene‐alt‐propylene) (EP) alternating copolymer, a poly(styrene‐b‐EP) diblock copolymer (SEP) and a poly(styrene‐b‐EP‐b‐styrene) triblock copolymer (SEPS). The ²H spin–lattice relaxation time, T₁, of EP soft segments above their glass transition temperature was measured by solid‐state ²H NMR spectroscopy. It was found that the block copolymers had a fast and a slow T₁ component whereas EP copolymer had only a fast component. The fast T₁ components for SEP and SEPS are similar to the T₁ value of EP above ca 20°C. The slow T₁ component for SEP and SEPS exhibited a minimum at 60°C and approached the value of the fast component near the T_g of polystyrene. The motional behavior of the EP units for SEP is similar to that of SEPS over the entire range of temperature. Copyright © 2001 John Wiley & Sons, Ltd.     

Low temperature kinetics and energetics of the electron and hole traps in irradiated TiO2 nanoparticles as revealed by EPR spectroscopy

We have monitored exclusively the dynamics of photogenerated charge carriers trapping in deep traps and trapped electron-hole recombination in UV irradiated anatase TiO2 powders by electron paramagnetic resonance (EPR) spectroscopy at 10 K. The results reveal that the strategy of using low temperatures contributes to the stabilization of the charged pair states for hours by reducing the rate of electron-hole recombination processes. Since only the localized states such as holes trapped at oxygen anions and electrons trapped at coordinatively unsaturated cations are accessible to EPR spectroscopy, the time-dependent population and depopulation of these EPR signals reflect the kinetics and energetics of these trap states. The data support a model of sequential accumulation of deep trap site populations in which the initial fast direct trapping into a deep trap site is followed by slower carrier trap-to-trap hopping until a deep trap is encountered for both photogenerated electrons and holes. Effective modeling of the subsequent decay of trapped-holes is achieved by employing a first-order kinetics, whereas the decay of either surface- or inner-trapped electrons has both a fast and a slow component. The fast component is attributed to a trapped-electron and a free-hole recombination, and the slow component is attributed to trapped electron-hole recombination. The activation energies for the process of diffusion of trapped electrons from their Ti3+ trapping sites are estimated.     

Study of radical rearrangement during bromination of 3,3,3-trichloropropene

Conclusions It was shown by the EPR method that in the photochemical bromination of 3,3,3-trichloropropene at 77°K a bromine atom adds to the compound with the formation of the CCl₃CHCH₂Br radical, which is then converted to the CCl₂CHClCH₂Br radical.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 179–181, January, 1973.     

“Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study”—Response to a Comment

We have presented an EPR‐based approach for deducing the RAFT equilibrium constant, K_eq, of a dithiobenzoate‐mediated system [Meiser, W. and Buback M. Macromol. Rapid Commun. 2011 , 32, 1490]. Our value is by four orders of magnitude below K_eq from ab initio calculations for the identical monomer‐free system. Junkers et al. [Macromol. Rapid Commun. 2011 , 32, 1891] claim that our EPR approach would be model dependent and our data could be equally well fitted by assuming slow addition of radicals to the RAFT agent and slow fragmentation of the so‐obtained intermediate radical as well as high cross‐termination rate. By identification of all side products, our EPR‐based method is shown to be model independent and to provide reliable K_eq values, which demonstrate the validity of the intermediate radical termination model.     

A pH-Sensitive Zwitterionic Iron Complex Probe with High Biocompatibility for Tumor-Specific Magnetic Resonance Imaging

Accurate diagnosis of tumor characteristics, including its location and boundary, is of immense value to subsequent therapy. Activatable magnetic resonance imaging (MRI) contrast agents that respond to tumor-specific microenvironments, such as the redox state, pH, and enzyme activity, enable better mapping of tumor tissue. However, the practical application of most reported activatable agents is hampered by problems including potential toxicity, inefficient elimination, and slow activation. In this study, we developed a zwitterionic iron complex (Fe-ZDS) as a positive MRI contrast agent for tumor-specific imaging. Fe-ZDS could dissociate in weakly acidic solution rapidly, accompanied by clear longitudinal relaxivity (r₁) enhancement, which enabled the complex to act as a pH-sensitive contrast agent for tumor-specific MR imaging. In vivo experiments showed that Fe-ZDS rapidly enhanced the tumor-to-normal contrast ratio by >40 %, which assisted in distinguishing the tumor boundary. Furthermore, Fe-ZDS circulated freely in the bloodstream and was excreted relatively safely via kidneys owing to its zwitterionic nature. Therefore, Fe-ZDS is an ideal candidate for a tumor-specific MRI contrast agent and holds considerable potential for clinical translation.     

1,4-dithiane radical cations in the CF<Subscript>3</Subscript>CCl<Subscript>3</Subscript> matrix: Photoisomerization mechanism

It has been established that the photoinduced process observed in EPR and in optical absorption spectra for 1,4-dithiane radical cations in the CF₃CCl₃ matrix at 77 K can be ascribed to a “boat”—“twist boat” conformational transition.     

Catalytic Activity of Immobilized Transition Metal Complexes with Monoethanolamine in Carbon Tetrachloride Addition to Multiple Bonds

The rates of CCl₄ addition to octene-1 in the presence of monoethanolamine complexes with Mn, Cu, V, Co, and Ni ions immobilized on the surface of silica have been determined. The majority of catalysts exhibit an inverse dependence of relative catalytic activity on surface metal concentration. Changes in the total concentrations and ligand environments of paramagnetic ions in the course of reaction are studied by EPR spectroscopy for copper- and vanadyl-containing catalysts. It has been found that the activity of complexes correlates with the rate of reduction of a metal ion in the course of a catalytic reaction.     

Fullerene C60 Trichloromethylation Through CCl4 Plasmalysis or Sonolysis

The trichloromethylation of C₆₀ fullerene was achieved either under plasmalysis conditions, i.e. by arcing a solution of C₆₀ in CCl₄ between two graphite electrodes, or by sonolysis of C₆₀ in CCl₄. The resulting products were studied by UV–VIS and FT-IR spectroscopy as well as by thermogravimetric analysis in comparison to the trichloromethyl adducts formed by radiolysis or photolysis of C₆₀ in CCl₄. A reference study on the plasmalysis of pure CCl₄ has revealed the formation of carbon soot and hexachlorobenzene. The formation of the latter compound was suppressed when C₆₀ was present in CCl₄.     

The ultra-fast process of picosecond time-resolved energy transfer in liquid cyclohexane by picosecond single-pulse radiolysis

Two formation processes of singlet states of solute molecules in irradiated liquid cyclohexane have been observed by using a single picosecond electron pulse-radiolysis technique. The slower process corresponds to the very fast formation process reported by Beck et al. The faster process is an ultra-fast process which finishes immediately after a picosecond electron pulse, and is hardly quenched by CCl₄ on triethylamine. The ratio of the faster component to the slower component in liquid cyclohexane is very different from that in liquid toluene.     

Alkali‐Metal Trichloroacetates for Dichloromethylenation of Fullerenes: Nucleophilic Addition‐Substitution Route

The first experimental evidence that fullerenes react with alkali‐metal trichloroacetates through a nucleophilic addition‐substitution route, yielding dichloromethylenefullerenes as the final products, is reported. The intermediates, C₆₀(CCl₃)^? and C₇₀(CCl₃)^? anions, have been isolated in their protonated forms as ortho‐C₆₀(CCl₃)H, as well as three ortho and one para isomer of C₇₀(CCl₃)H. The structures were unambiguously determined by means of ¹H, ¹³C, and ¹H–¹³C HMBC NMR spectroscopy along with UV/Vis spectroscopy. The observed regiochemistry was analyzed with the aid of quantum chemical calculations. Conversion of the protonated compounds into the [6,6]‐closed C_60/70(CCl₂) cycloadducts under basic conditions can be effected only for the ortho isomers, whereas para‐C₇₀(CCl₃)H decomposes back into pristine C₇₀.     

Die Reaktion von Molybdänpentachlorid mit (SCN)2, (SeCN)2 und ICN

Reaction of Molybdenum Pentachloride with (SCN)₂, (SeCN)₂, and ICN By reaction of MoCl₅ with (SCN)₂, (SeCN)₂, and ICN in CCl₄ or H₂CCl₂ the compounds MoCl₅(NCS)₂, MoCl₅(NCSe)₂, and MoCl₅NCI were obtained. They are very sensitive towards hydrolysis and decompose on heating. The compounds are characterized by their vibrational and EPR spectra which indicate that the pseudohalogen is bonded via a nitrogen atom.     

The radiation chemistry of acyclic hydrofluoro and perhalogenated ether and hydrocarbon compounds

The radiolytic stability of some hydrofluoroethers and hydrofluorocarbons was investigated and compared with those of perfluoropolyethers (PFPEs) and the CCl₂FCClF₂ (CFC 113). The experimental results indicate that stability depends mainly on the relative abundance of hydrogen atoms in the molecule; however, a significant role is played also by the chemical structure (i.e. the relative positions of the hydrogen atoms in the molecule). As a result, molecules containing hydrogen atoms as OCF₂H chain ends show a higher stability compared with the other hydrofluoro compounds. Based on the analysis of the end products and on the nature of radicals detected by EPR, radiolysis mechanisms are proposed and discussed. Due to their high dipole moments the hydrofluoro compounds and CCl₂FCClF₂ degrade mainly through an ionic mechanism.     

<Superscript>99m</Superscript>Tc-labelled biguanide derivatives: chemical speciation modelling thereof and evaluation in vervets

^99mTc-DMSA (dimercaptosuccinic acid) is known to be a safe and effective agent for static renal imaging. However, it has a long uptake time which is a limiting factor in diagnostic procedures and also leads to a relatively high radiation dose being administered to patients. There is a constant search for possible new renal imaging agents with a good resolution, kidney/liver contrast and low radiation dose to all organs. A series of biguanide derivatives (potential as non-insulin-dependent diabetes mellitus agents) labelled with ^99mTc were investigated as potential alternative kidney-imaging agents on theoretical grounds (in silico) and their biodistribution (in vivo) verified in a limited number of animal experiments. Such a dual approach has the benefit that it reduces the number of animal experiments needed to evaluate a potential radiopharmaceutical. The blood plasma model shows little or no complexation of the biguanide type ligands by the metal ions in blood plasma. It was therefore expected that these ligands will clear rapidly through the kidneys and liver (increased lipophilicity). These predictions were verified by studies on single vervets comparing them with ^99mTc-DMSA as gold standard. All the biguanide derivatives labelled with ^99mTc show liver, kidney and gallbladder uptake in vervets. It was shown that the agent ^99mTc-CBIG (carboxylbiguanide) has a very fast kidney clearance, which will reduce the dose to organs (as experienced for ^99mTc-DMSA), although it’s potential as a kidney agent is limited by its gallbladder uptake.