Ultrafast intramolecular charge transfer with N-(4-cyanophenyl)carbazole. Evidence for a LE precursor and dual LE + ICT fluorescence

The photophysics of N-(4-cyanophenyl)carbazole (NP4CN) was investigated by using absorption and fluorescence spectra, picosecond fluorescence decays, and femtosecond transient absorption. In the nonpolar n-hexane as well as in the polar solvent acetonitrile (MeCN), a locally excited (LE) state is detected, as a precursor for the intramolecular charge transfer (ICT) state. A LE → ICT reaction time τ(2) at 22 °C of 0.95 ps in ethyl cyanide (EtCN) and 0.32 ps in MeCN is determined from the decay of the LE excited state absorption (ESA) maximum around 620 nm. In the ESA spectrum of NP4CN in n-hexane at a pump-probe delay time of 100 ps, an important contribution of the LE band remains alongside the ICT band, in contrast to what is observed in EtCN and MeCN. This shows that a LE ? ICT equilibrium is established in this solvent and the ICT reaction time of 0.5 ps is equal to the reciprocal of the sum of the forward and backward ICT rate constants 1/(k(a) + k(d)). In the photostationary S(0) → S(n) absorption spectrum of NP4CN in n-hexane and MeCN, an additional CT absorption band appears, absent in the sum of the spectra of its electron donor (D) and acceptor (A) subgroups carbazole and benzonitrile. This CT band is located at an energy of ～4000 cm(-1) lower than for N-phenylcarbazole (NPC), due to the larger electron affinity of the benzonitrile moiety of NP4CN than the phenyl subunit of NPC. The fluorescence spectrum of NP4CN in n-hexane at 25 °C mainly consists of a structured LE emission, with a small ICT admixture, indicating that a LE → ICT reaction just starts to occur under these conditions. In di-n-pentyl ether (DPeE) and di-n-butyl ether (DBE), a LE emission is found upon cooling at the high-energy edge of the ICT fluorescence band, caused by the onset of dielectric solvent relaxation. This is not the case in more polar solvents, such as diethyl ether (DEE) and MeCN, in which a structureless ICT emission band fully overlaps the strongly quenched LE fluorescence. For the series of D/A molecules NPC, N-(4-fluorophenyl)carbazole (NP4F), N-[4-(trifluoromethyl)phenyl]carbazole (NP4CF), and NP4CN, with increasing electron affinity of their phenyl subgroup, an ICT emission in n-hexane 25 °C only is present for NP4CN, whereas in MeCN an ICT fluorescence is observed with NP4CF and NP4CN. The ICT fluorescence appears when for the energies E(ICT) of the ICT state and E(S(1)) of the lowest excited singlet state the condition E(ICT) ≤ E(S(1)) holds. E(ICT) is calculated from the difference E(D/D(+)) - E(A(-)/A) of the redox potentials of the D and A subgroups of the N-phenylcarbazoles. From solvatochromic measurements with NP4CN an ICT dipole moment μ(e)(ICT) = 19 D is obtained, somewhat larger than the literature values of 10-16 D, because of a different Onsager radius ρ. The carbazole/phenyl twist angle θ = 45° of NP4CN in the S(0) ground state, determined from X-ray crystal analysis, has become smaller for its ICT state, in analogy with similar conclusions for related N-phenylcarbazoles and other D/A molecules in the literature.     

Theoretical study of the strong intramolecular hydrogen bond and metal-ligand interactions in group 10 (Ni, Pd, Pt) bis(dimethylglyoximato) complexes

The structural and bonding characteristics of the bis(dimethylglyoximato) complexes of group 10 transition metals ([M(dmg)₂], where M = Ni, Pd and Pt) were investigated by means of quantum chemical computations. The equilibrium geometries, energetic and bonding properties were computed using the B3P86 exchange-correlation density functional in conjunction with a 6-311+(+)G^∗∗ basis set. The computations revealed that the strong O⁻?H-O hydrogen bond exists only in the presence of the metal cations. The free (dmg)₂²⁻ ligand has significantly different geometry in which the O⁻?H-O interaction is replaced by N?O-H bonds. The characteristics of the metal-ligand interactions were determined by natural bond orbital analysis.     

Homogeneous catalytic aminocarbonylation of nitrogen-containing iodo-heteroaromatics. Synthesis of N-substituted nicotinamide related compounds

Various primary and secondary amines, including amino acid methyl esters, were used as nucleophiles in palladium-catalysed aminocarbonylation of 2-iodopyridine, 3-iodopyridine and iodopyrazine. N-Substituted nicotinamides and 3-pyridyl-glyoxylamides (2-oxo-carboxamide type derivatives) of potential biological importance can be obtained from 3-iodopyridine as a result of simple and double carbon monoxide insertions, respectively. The latter examples can be obtained in synthetically acceptable yields by using elevated carbon monoxide pressure. On the contrary, N-alkyl/aryl-carboxamides were obtained exclusively in the whole pressure range by using 2-iodopyridine and iodopyrazine.     

Glassy carbon electrode modified with hemin and new melamine compounds for H<Subscript>2</Subscript>O<Subscript>2</Subscript> amperometric detection

In an attempt to increase the stability and efficiency of hemin-modified electrodes, the present work reports the preparation of a new modified glassy carbon electrode obtained by immobilization of hemin (Hm) on the electrode surface together with a new N-substituted melamine (2,4,6-triamino-1,3,5-triazine) based G-2 dendrimer comprising p-aminophenol as peripheral unit (Den) or with one of its analogues, a melamine G-0 dimer (Dim). Basic structural features, able to determine intimate relationships between Hm and Dim (or Den) at room temperature in solid state, were evidenced with the use of vibrational analysis carried out by FT-IR. This method revealed contacts between Hm and Dim or Den respectively as H-bond interactions, proton-interchange, and π-π stacking interactions. The new modified electrodes were characterized by cyclic voltammetry and electrochemical impedance spectroscopy and tested for amperometric detection of H₂O₂. In this purpose, GC/Hm-Dim electrode exhibited better catalytic properties than GC/Hm-Den electrode, but lower stability.     

Preparation and structural studies on diorganotin(IV) complexes of N-nitroso-N-phenylhydroxylaminates

Diorganotin(IV)-complexes of the N-nitroso-N-phenylhydroxylaminates (hereinafter cupf), Et₂Sn(cupf)₂ (1), Bu₂Sn(cupf)₂ (2), {[Bu₂Sn(cupf)]₂O}₂ (3), t-Bu₂Sn(cupf)₂ (4) and Oc₂Sn(cupf)₂ (5, 6) were prepared and characterised by FT-IR and Mössbauer spectroscopic measurements. The binding modes of the ligand were identified by FT-IR spectroscopy, and it was found that the ligand is coordinated in chelating or bridging mode to the organotin(IV) center. The ¹¹⁹Sn Mössbauer and FT-IR studies support the formation of trans-O_h (1-6) structures. The X-ray diffraction analysis of 4 revealed that the tin centre is in a skew-trapezoidal geometry defined by four donors derived from the cupferronato ligands and two carbon atoms from the tin-bound ^tbutyl substituents. The ¹¹⁹Sn NMR investigations indicate that in solution 4 retains its hexacoordinated nature.     

Mössbauer study of the autoxidation of ethylenediaminetetraacetato-ferrate(II)

The autoxidation reaction of the Fe²⁺/ethylenediaminetetraacetic acid system has been studied in solution phase at neutral pH and in solid state using Mössbauer spectroscopy. It was found that despite the proposed reaction pathway consisting of several reaction steps under similar circumstances, no intermediate species could be seen in the Mössbauer spectra and instead of the formation of the well-known [Fe^III(EDTA)(H₂O)]^? species, the direct formation of its dimeric form was observed.     

Kinetics and mechanism of the oxidation of tetrathionate by iodine in a slightly acidic medium

The iodine-tetrathionate reaction has been reinvestigated spectrophotometrically at T = 25.0 +/- 0.1 degrees C and at an ionic strength of 0.5 M adjusted by sodium acetate as a buffer component in both the absence and presence of the iodide ion in the pH range of 4.25-5.55. The reaction was found to be independent of pH within the range studied, and it was clearly demonstrated that the reaction proceeds via an intermediate S4O6I- formed in a pre-equilibrium. Iodide dependence of the kinetic curves strongly suggests that the iodide ion has to be involved in this equilibrium. Further reactions of the intermediate, including its hydrolysis and reaction with iodide, leads to the strict stoichiometry characterized by S4O62- + 7I2 + 10H2O --> 4SO42- + 14I- + 20H+. A seven-step kinetic model with three fitted kinetic parameters is suggested and discussed. A rate equation is also derived from which a sound explanation of the iodide dependence of the apparent rate coefficient is presented. Furthermore, it has also been pointed out that formation of the triiodide ion alone is not sufficient to take the retardation effect of the iodide ion into account quantitatively.     

Surface and structural properties of pt/ceo2catalyst under preferential co oxidation in hydrogen (PROX)

Summary Preferential oxidation of CO in the presence of excess hydrogen was studied on Pt/CeO₂with 5% metal loading. Catalytic data were similar to those observed on 1% Pt/CeO₂earlier [16]. The optimum temperature region is T￡373 K; conversion and selectivity of CO oxidation strongly decreased at higher temperatures. High-pressure XPS indicated CO adsorbed on platinum particles and significant amount of water on the ceria surface. The top-most ceria surface re-oxidized as small amount of oxygen (3%) was introduced into the H₂/CO feed. Despite this surface re-oxidation, high-resolution TEM after reaction indicated oxygen deficient ceria bulk structure, in which the defects formed a super-cell, with CeO_1.695structure. The defective ceria is suggested to play an important role stabilizing the hydrogen bonded surface water, which (i) suppresses further hydrogen oxidation and (ii) reacts at the metal/support interface with linearly adsorbed CO in a low temperature water-gas-shift type reaction to produce CO₂.</o:p>     

Comparative laboratory steam-cracking of different raw materials

Summary Steam cracking of several gasoline and gas oil samples was carried out under different conditions and the results are reported. Some of sample feedstocks were previously cracked in a commercial plant so the laboratory and commercial results could be compared. It was found that the laboratory reactor was more sensitive to the changes of the composition of the raw materials.