Ionic photodissociation and ionic photosensitized dissociation of charge-transfer complexes

Flash spectroscopic and kinetic studies have been carried out on the charge-transfer complex of tetracyanoethylene with tetrahydrofuran in a liquid paraffin solution at room temperature. The rise and decay curves in transient electronic absorption intensity have been observed with a common rate constant, corresponding to the anion radical of tetracyanoethylene and the triplet state of the charge-transfer complex, respectively. From the kinetic analysis it has been concluded that the ionic photodissociation of this complex takes place in its lowest excited triplet state. This dissociation mechanism has also been confirmed by employing a triplet energy transfer technique with which “ionic photosensitized-dissociation” phenomenon is observed. Furthermore, a few other examples of ionic photosensitized-dissociation are demonstrated in rigid glasses with typical weak charge-transfer complexes whose photodissociation processes are well-known.     

Photodissociation of t-butyl nitrite by UV and blue photons

The photodissociation of t-butyl nitrite at 266, 355, 454, 474 and 494 nm has been studied by probing the NO(X²Π) fragment. Either two lasers at different wavelengths were used for dissociating and probing or a single laser was used for both. The results show strong dependence of the vibrational and rotational excitation of the NO on the photodissociation wavelength.     

A measurement of vibrational excitation in NO produced in the photodissociatton of NOCl and NOBr at 193nm

Infrared emission has been observed from the vibrational state v_n (n from 1 to ? 16) of the nascent NO(X²Π) photo-fragment produced in the photodissociation of NOCl and NOBr at 193 nm. The photodissociation was observed to be a single-photon process. Models and mechanisms of photodissociation are discussed in view of experimental observations.     

Origin of photoluminescence in SrTiO3: a combined experimental and theoretical study

A joint experimental and theoretical study has been carried out to rationalize the photoluminescence properties of SrTiO₃ perovskite thin films synthesized through a soft chemical processing. Only the amorphous samples present photoluminescence at room temperature. From the theoretical side, first principles quantum mechanical techniques, based on density functional theory at B3LYP level, have been employed to study the electronic structure of a crystalline (ST-c) and an asymmetric (ST-a) model. Electronic properties are analyzed in the light of the experimental results and their relevance in relation to the PL behavior of ST is discussed.     

Comparative study of H2S and SO2 adsorption on polycrystalline copper by surface potential measurements

Surface Potential Variations ΔΦ and XPS show the presence of S(a) on Cu after exposure to H₂S. On such a presulfided state, H₂S adsorbs molecularly and reversibly at room temperature. Exposure to SO₂ mainly leads to the formation of S(a) and SO_x(a) while molecular adsorption has not been evidenced at room temperature.     

Regioselective synthesis of pyridoquinolones and pyridocoumarins via molecular iodine-mediated 6-endo-dig electrophilic cylization

Angular-pyridoquinolone and pyridocoumarin derivatives have been efficiently synthesized in 60-95% yields by molecular iodine-mediated cyclization of easily available starting materials, 6-(N-propargyl)amino quinolone and coumarin derivatives, in the presence of NaHCO₃. The reaction was carried out at room temperature.     

Evolution des tenseurs de dilatation thermique en fonction de la temperature. II. Etude calorimetrique et cristallographique de la transition de phase ordre-desordre du thiophene chrome tricarbonyle C4H4SCr(CO)3

Benzenechromium tricarbonyl C₆H₆Cr(CO)₃ and thiophenechromium tricarbonyl C₄H₄SCr(CO)₃ are isomorphous at room temperature. We have measured, in the range 77–295°K, the variations of specific heat and of the tensor of thermal expansion for these two molecular compounds. The first one exhibits quite normal behavior; on the other hand, the second one exhibits a first order phase transition, monoclinic ? triclinic at T = 185°K, associated with an order-disorder phase transition. Structural evolution of thiophene-chromium tricarbonyl, during phase transition, has been explained from the variation of its tensor of thermal expansion.     

Charge ordering and dielectric properties in the near half-doped Pr0.79Na0.21MnO3 perovskite

The Pr_0.79Na_0.21MnO₃ perovskite has been prepared in polycrystalline form by a ceramic method. We have carried out its structural characterization by synchrotron X-ray powder diffraction (SXRPD) at room temperature and at 150 K (T<T_CO). In the charge ordering (CO) state, the SXRPD data can be refined both on the basis of the site-centred model and the bond-centred model proposed for half-doped manganites, without a clear advantage of one model over the other. From the dielectric point of view, at the CO temperature this manganite shows a maximum in the dielectric constant whose origin is intrinsic, and cannot be attributed to the presence of extrinsic factors. We relate this dielectric behaviour to the formation of polar entities at the temperature of charge condensation, due to an asymmetric charge distribution intermediate between site-centred and bond-centred type.     

Aerobic oxidation of benzylic aldehydes to acids catalyzed by iron （Ⅲ） meso-tetraphenylporphyrin chloride under ambient conditions

Highly efficient aerobic oxidation of benzylic aldehydes to the corresponding acids catalyzed by iron (Ⅲ) meso-tetraphe- nylporphyrin chloride (Fe(TPP)Cl) under ambient conditions was developed.The catalyst has been proved to be an excellent catalyst for the system in the presence of molecular oxygen and isobutryaldehyde at room temperature.     

An expedient stereoselective synthesis of (Z)- and (E)-allyl iodides from Baylis-Hillman adducts along with selective iodination of benzylic alcohols using the polymethylhydrosiloxane-iodine system

A stereoselective method has been developed for the synthesis of (Z)- and (E)-allyl iodides from Baylis-Hillman adducts using polymethylhydrosiloxane (PMHS) and iodine in chloroform at room temperature. In addition, the reagent system has been utilized for the iodination of benzylic alcohols selectively.     

Photodissociation and spectroscopic study of cold protonated dipeptides

A photodissociation spectrometer, containing a spray ionization source and a temperature-variable multipole ion trap, has been constructed to examine the structure and reactivity of gas phase biological molecular ions at various temperatures. Ultraviolet (UV) and infrared (IR) photodissociation spectra of protonated alanyltryptophan (Ala-TrpH+) and tryptophanylglycine (Trp-GlyH+) have been measured. In UV spectra, the S1-S0 band origin of Ala-TrpH+ exhibits a significant red shift with respect to those of protonated tryptophan (TrpH+) and Trp-GlyH+. This red shift is ascribed to the stabilization of the excited state due to the strong interaction between the NH3+ group and indole ring. We also discuss the temperature effect on the structure and reactivity for these peptides. In addition to the UV photodissociation spectra of the dipeptides, IR spectra of the complex of Ala-TrpH+ with methanol are measured. IR photodissociation spectra of solvated ions show that Ala-TrpH+-methanol has the closed structure, which is consistent with the large spectral shift in UV spectrum of bare dipeptide.     

Effect of doping and temperature on the crystal structure of (V1−xMox)2O3 above and below the metal/insulator transition

The crystal structure of new molybdenum-doped vanadium sesquioxides (V_1−xMo_x)₂O₃ (0?x?0.20) has been studied at low temperature (10 K) and up to room temperature, through neutron and X-ray powder diffraction. The transition from insulating I- to metallic M-type phases, either by doping or thermally driven, is accompanied by an abrupt decrease of all interatomic distances. Within each structural type however, at 10 K, the effect of doping is essentially the same as at room temperature: it increases cation-oxygen distances, and decreases cation-cation distances, making the cationic coordination octahedra more regular. Thermal effects differ for each phase type: all interatomic distances normally increase in the M-type phase (but with different octahedral modifications depending on doping), but they decrease or remain constant in the I-type phase. This produces an unusual negative thermal expansion coefficient up to 5% at low temperature for the doped compounds.     

Room temperature photoluminescence of amorphous BaxSr1−xTiO3 doped with chromium

ABO₃ amorphous materials, such as BaTiO₃ (BT), SrTiO₃ (ST), PbTiO₃ (PT), and Ba_xSr_1−xTiO₃ (BST) have recently attracted a good deal of attention due to their ferroelectric and electro-optical properties. Intense photoluminescence at room temperature was observed in amorphous titanate doped with chromium (Ba_xSr_1−xTi_1−yCr_yO₃) prepared by the polymeric precursor method. Results indicated that substantial luminescence at room temperature was achieved with the addition of small Cr contents to amorphous Ba_xSr_1−xTi_1−yCr_yO₃. Further addition of Cr or crystallization were deleterious to the intensity of the luminescent peak obtained for excitation using λ=488.0 nm.     

Preparation of PEGMA-functionalized latex particles. 2. System styrene/<Emphasis Type="Italic">N</Emphasis>-vinylcaprolactam

In a previous paper [Pich A, Lu Y, Adler H-J (2003) Colloid Polym Sci (submitted)], the synthesis of polystyrene-poly(ethylene glycol) methacrylate (PST/PEGMA) particles has been described. In the present paper polymeric particles have been prepared by emulsion co-polymerization of styrene/N-vinylcaprolactam (ST/VCL) or styrene/n-butylacrylate (ST/BA) mixtures in presence of poly(ethylene glycol) methacrylate (PEGMA). The influence of the monomer composition and PEGMA concentration on the particle size and particle size distribution was studied. Increase of VCL content in reaction mixture leads to dramatic increase of the final particle size. Particle size distribution becomes broader at higher VCL contents. Poly(ST/VCL) particles show dramatic change of the size with the temperature.     

Branching ratio for S(33Pj) AND S(31D2) atom production in the photodissociation of CS2 at 193 nm

Resonance-enhanced photoionization has been used to follow S(³P₂) in the photodissociation of CS₂ at 193 nm. The contributions from initial photodissociation and from S(¹D) relaxation have been resolved and give a (15±5)% yield of S(¹D). The possibility of secondary production of S(³P_j) by CS photodissociation with a second 193 nm photon is discussed. Although this might raise the S(¹D) yield to (26±8)%, production of S(³P_J) is still the dominant photodissociation channel.     

Radiolysis of acid water (0.4 M H2SO4) at elevated temperatures with fast neutrons and proton beam

To elucidate the G-values of water decomposition products at elevated temperatures up to 270°C, radiolysis of acid water, 0.4 M H₂SO₄, has been carried out with fast neutrons in a reactor by a combination of the Fricke, cerium and bichromate dosimeters. At room temperature, the radical yield is smaller and the molecular yield is larger than those established in γ-radiolysis and net water decomposition is smaller. With increasing temperature, the ratio of the radical to molecular yields becomes similar to that in γ-radiolysis at room temperature. In order to check the above evaluation, proton beam radiolysis was also conducted at elevated temperatures as a model experiment. Although the temperature range was limited between room temperature and 80°C, the proton beam experiment confirms the evaluation obtained in the fast neutron radiolysis.     

Four hydroxyls are better than two. The use of a chiral lithium salt of 3,3′-bis-methanol-2,2′-binaphthol as a multifunctional catalyst of enantioselective Michael addition reactions

The catalytic performance of the Li salt of (S)- or (R)-3,3′-bis[bis-(phenyl)hydroxymethyl]-2,2′-dihydroxy-dinaphthalene-1,1′ (BIMBOL) in asymmetric Michael additions of malonic acid derivatives and toluedine has been studied. Nitrostyrene and cyclohex-2-enone were chosen as Michael acceptors. Efficient asymmetric C-C and C-N bond formations with ee’s of up to 95% at room temperature were observed. A transition state model of the malonic ester addition to cyclohex-2-enone has been proposed based on the molecular structure of the acetone solvate of BIMBOL. The impact of the catalyst self-association on its performance is also discussed.     

A highly efficient stereoselective synthesis of (Z)- and (E)-allyl iodides from Baylis-Hillman adducts

A highly efficient synthesis of (Z)- and (E)-allyl iodides has been accomplished by treatment of Baylis-Hillman adducts with iodine and triphenylphosphine in methylene chloride at room temperature. The method is associated with mild reaction conditions, high yields and excellent stereoselectivity.     

Mild and practical stereoselective synthesis of (Z)- and (E)-allyl bromides from Baylis-Hillman adducts using Appel agents (PPh3/CBr4): a facile synthesis of semiplenamides C and E

Appel agents (PPh₃/CBr₄) have been utilized for high-yielding stereoselective synthesis of (Z)- and (E)-allyl bromides from Baylis-Hillman adducts at room temperature. The method has been applied for the synthesis of naturally occurring bioactive fatty acid amides, semiplenamides C and E.     

Polarization of thallium atoms produced in molecular photodissociation: experiment and theory

A study has been made of the oriented ground state Tl(6² P _1/2) atoms produced in the photodissociation of TlBr molecules by circularly polarized 266-nm laser light. A significant degree of atomic orientation (15%) has been measured in the experiment which corresponds to the initial degree of orientation of 37%. A high value of depolarization cross section (210 Ų) for the oriented Tl atoms colliding with TlBr molecules has been also observed. The obtained experimental results have been treated theoretically. We present a general quantum mechanical theory of the orientation phenomenon in which all possible nonadiabatic interactions as well as molecular rotation are properly treated. The application of the theory to the case of TlBr photodissociation allowed to understand the obtained experimental results and to evaluate the probability of the earlier unknown radial nonadiabatic transition in the decaying molecule.