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International audience; In this work, a series of Type I photoinitiators (PIs) based on the naphthoquinone scaffold were designed and synthesized for the first time, in order to induce photopolymerization under visible light. As a result, these PIs exhibit excellent photoinitiation abilities in the presence of acrylates monomers upon LED@405 or @455 nm irradiation. Interestingly, some compounds have better photoinitiation performance than the benchmark Type I phosphine-oxide i.e. diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (TPO) upon exposure to LED@455 nm. Chemical mechanisms supporting the photopolymerization process were investigated through different techniques as well as theoretical calculations. In addition, the new proposed structures were also investigated in two-component photoinitiating systems and exhibited a higher efficiency in free radical photopolymerization. Finally, direct laser write approach is successfully used to fabricate 3D objects.

A new organic material with three 4,4-difluoro-4-borata-3a-azonia-4a-aza-s-indacene dyes (BODIPYs) at the periphery of the central core is successfully synthesized (3BDP3T) and its corresponding aqueous nanoparticles are prepared via the encapsulation approach and characterized in detail both experimentally and theoretically with the aid of the Density Functional Theory (DFT). The linear and non-linear optical properties of the synthesized material are also studied. Until now, the development of organic materials with three BODIPYs as substituents is limited and their properties are not fully resolved. The obtained 3BDP3T-based nanoparticles exhibit far-red and near infrared (NIR) emission with photoluminescence quantum yields of 0.021, which is promising as a new fluorescent contrast agent in the far-red and NIR spectral regions.

NMR and DFT studies of phenol compounds as molecular sensors were carried out to investigate H2O/DMSO eutectic mixtures at a molecular level. The experimental 1H NMR chemical shifts of the OH groups, δexp(OH), of phenol, paracoumaric acid, and vanillic acid show maximum deshielding and, thus, hydrogen bond interactions in the range of mole fractions 0.20 < χ(DMSO) < 0.33. In the mole fractions χ(DMSO) < 0.2, a progressive decrease in δexp(OH) was observed which demonstrates a decrease in hydrogen bond interactions at infinite dilution in H2O, despite the increase in the number of available hydrogen bond acceptor and donor sites. DFT calculated δcalc(OH) of minimum energy solvation clusters were shown to be in reasonable agreement with the pattern in experimental δexp(OH) data. The chemical shift deshielding and, thus, increased hydrogen bond interactions in the natural product + DMSO + nH2O (n = 2, 3) solvation clusters, relative to complexes in DMSO or H2O solutions, cannot be attributed to a single structural parameter of the cooperative interactions between H2O and DMSO molecules with the phenol OH groups of the natural products. The minimum energy conformers of phenol compounds + 2H2O + DMSO complexes are in excellent agreement with a recent low temperature neutron diffraction experiment of 3D2O + DMSO and demonstrate a general structural motif of solvation complexes. The combined use of 1H NMR and DFT studies with emphasis on δ(OH) of phenol compounds, as molecular sensors, can provide an effective method for the study of solute–solvent interactions at the atomic level.

The development of new aqueous conjugated polymer nanoparticles with high photoluminescence quantum yields (PLQYs) at the far red and near infrared (NIR) spectral regions (>650 nm) as alternative polymer probes for fluorescence imaging is reported. This is achieved through an appropriate chemical design by the introduction of up to four fluorine atoms in the different positions of a donor–acceptor (D–A) polymer backbone consisting of thiophene as the electron donating unit and quinoxaline as the electron withdrawing building block. The resulting series of low synthetic complexity conjugated polymers enable us to perform a detailed structure–properties relationship study between the synthesized conjugated polymers and their corresponding aqueous nanoparticles as prepared by the nanoprecipitation and encapsulation methods. We manage to achieve PLQY for aqueous NIR conjugated polymer probes of 0.1 and a PLQY of 0.18 for the far-red emitting conjugated polymer probes. Furthermore, our work paves the way for the rational design of new conjugated polymers with enhanced PLQY, especially in the NIR spectral region, through better control of the intramolecular and intermolecular polymer chain interactions.

Direct activation of aromatic C-H bonds in polyphenolic compounds in a single step, without the use of late transition metals, is demonstrated with the use of D2O and common phosphate buffer at neutral pD and near ambient temperatures. Detailed variable temperature and pD 1H NMR studies were carried out to investigate, for the first time, the Gibbs activation energy (ΔG‡), the activation enthalpy (ΔH‡), and activation entropy (TΔS‡) of H/D exchange reactions of the natural product catechin and the model compounds resorcinol and phloroglucinol. NMR and DFT calculations support a catalytic cycle comprising two water molecules in a keto-enol tautomeric process. The reduction of ΔG‡ values due to the catalytic role of two molecules of water by a factor of 20-30 kcal mol-1 and the resulting acceleration of the H/D exchange rate by a factor of 1020-1030 should be compared with a minor reduction in ΔG‡ of 0.4 to 4.5 kcal mol-1 due to the effect of an additional electron donating oxygen group and the deprotonation of OH groups. It can therefore be concluded that although the H/D exchange process can be accelerated by a small amount of an acid or a base to break a C-H bond, water as a catalyst plays the major role. This approach opens a new vistas for the combined use of NMR and DFT studies as tools to understand the molecular basis of the catalytic role of water.

A combination of selective 1D Total Correlation Spectroscopy (TOCSY) and 1H-13C Heteronuclear Multiple Bond Correlation (HMBC) NMR techniques has been employed for the identification of methyl linolenate primary oxidation products without the need for laborious isolation of the individual compounds. Complex hydroperoxides and diastereomeric endo-hydroperoxides were identified and quantified. Strongly deshielded C&ndash, O&ndash, H 1H-NMR resonances of diastereomeric endo-hydroperoxides in the region of 8.8 to 9.6 ppm were shown to be due to intramolecular hydrogen bonding interactions of the hydroperoxide proton with an oxygen atom of the five-member endo-peroxide ring. These strongly deshielded resonances were utilized as a new method to derive, for the first time, three-dimensional structures with an assignment of pairs of diastereomers in solution with the combined use of 1H-NMR chemical shifts, Density Functional Theory (DFT), and Our N-layered Integrated molecular Orbital and molecular Mechanics (ONIOM) calculations.

The 3D lanthanum-dicarboxylate MOF [La2(hex)3(H2O)4]3H2O, (1), (hex = trans,trans-muconic acid) has been obtained at 50 °C, in a silica gel column, either via in situ transformation of the 2D MOF [La2(hex)3(H2O)6]9H2O, (2), published previously, or directly. The structure is made up of metal–ligand layers pillared by a ligand dianion. The comparison of the crystal structures of the two MOFs allowed us to propose a possible mechanism of the 2D to 3D transformation. The presence of many short olefinic contacts allowed the formation of three photoproducts at different yields, the major one being the rtct isomer for the 3D MOF and the rttc one for the 2D MOF. In both cases, the three photoreactions proceed with the parallel reaction scheme. The comparison of the crystal structures of 1 and 2 allowed the photoreactivity differences observed to be explained.

DFT calculations of 1H NMR chemical shifts, using various functionals and basis sets, the conductor-like polarizable continuum model and discrete solute-solvent hydrogen bond interactions have been used to derive the solution structures of methyl salicylate and methyl 2,5-dihydroxybenzoate. We demonstrate that very good agreement between experimental and computed 1H NMR chemical shifts can be obtained for various basis sets. The DFT structures in solution were compared with the recently reported X-ray structure, solved by the crystalline-sponge method, of the methyl salicylate and the single-crystal X-ray structure of methyl 2,5-dihydroxybenzoate. It is demonstrated that the information provided by 1H NMR chemical shifts about the solution structure is significantly more precise than that obtained by the single-crystal X-ray and the crystalline-sponge methods.

An ab-initio CASSCF/CASMP2 study on the structures and energies of the classical and bridged forms of the vinyl cation is presented. Our calculations which are in agreement with the experimental results predict that the bridged form of the vinyl cation (C2H3+) is more stable than its classical counterpart and is the unique species on the potential energy surface. A quantum mechanical explanation based in the notion of electron correlation energy and in Boltzmann distribution shows that the classical form is considered to be a transient species having a fleeting existence.

Detailed solvent and temperature effects on the experimental 1H-NMR chemical shifts of the natural products chrysophanol (1), emodin (2), and physcion (3) are reported for the investigation of hydrogen bonding, solvation and conformation effects in solution. Very small chemical shift of │&Delta, &delta, │ &lt, 0.3 ppm and temperature coefficients │&Delta, /&Delta, &Tau, │ &le, 2.1 ppb/K were observed in DMSO-d6, acetone-d6 and CDCl3 for the C(1)&ndash, OH and C(8)&ndash, OH groups which demonstrate that they are involved in a strong intramolecular hydrogen bond. On the contrary, large chemical shift differences of 5.23 ppm at 298 K and &Delta, values in the range of &minus, 5.3 to &minus, 19.1 ppb/K between DMSO-d6 and CDCl3 were observed for the C(3)&ndash, OH group which demonstrate that the solvation state of the hydroxyl proton is a key factor in determining the value of the chemical shift. DFT calculated 1H-NMR chemical shifts, using various functionals and basis sets, the conductor-like polarizable continuum model, and discrete solute-solvent hydrogen bond interactions, were found to be in very good agreement with the experimental 1H-NMR chemical shifts even with computationally less demanding level of theory. The 1H-NMR chemical shifts of the OH groups which participate in intramolecular hydrogen bond are dependent on the conformational state of substituents and, thus, can be used as molecular sensors in conformational analysis. When the X-ray structures of chrysophanol (1), emodin (2), and physcion (3) were used as input geometries, the DFT-calculated 1H-NMR chemical shifts were shown to strongly deviate from the experimental chemical shifts and no functional dependence could be obtained. Comparison of the most important intramolecular data of the DFT calculated and the X-ray structures demonstrate significant differences for distances involving hydrogen atoms, most notably the intramolecular hydrogen bond O&ndash, H and C&ndash, H bond lengths which deviate by 0.152 t&omicron, 0.132 &Aring, and 0.133 to 0.100 &Aring, respectively, in the two structural methods. Further differences were observed in the conformation of &ndash, OH, &ndash, CH3, and &ndash, OCH3 substituents.

Numerous gas phase electron diffraction, ultra-fast electron diffraction, X-ray and neutron diffraction experiments on β-dicarbonyl compounds exhibiting enol-enol tautomeric equilibrium, with emphasis on acetylacetone and dibenzoylmethane, have so far been reported with conflicting results on the structural details of the O-HO intramolecular hydrogen bond and resulted in alternative hypotheses on the intramolecular hydrogen bond potential function either a double minimum potential corresponding to two tautomeric forms in equilibrium or a single symmetrical one. We demonstrate herein, firstly, that the DFT calculated OH 1H NMR chemical shifts of acetylacetone and dibenzoylmethane exhibit a strong linear dependence on the computed OO hydrogen bond length of ∼-50 ppm A-1 and as a function of the O-HO bond angle of ∼1 ppm per degree, upon the transfer of the hydrogen atom from the ground state toward the transition state. Secondly, the refinement of labile hydrogen atomic positions in intramolecular hydrogen bonds based on the root-mean-square deviation between experimentally determined and DFT calculated 1H NMR chemical shifts in solution can provide high resolution structures of O-H and O(H)O bond lengths and O-HO bond angles with an accuracy of ∼10-2 A and ∼0.5°, respectively. Thirdly, the calculated 1H NMR chemical shifts in solution of the two ground state tautomers in equilibrium of acetylacetone and dibenzoylmethane are in excellent agreement with the experimental value, even for moderate basis sets for energy minimization. In contrast, the single symmetrical structure in a strongly delocalized system is a transition state with calculated 1H NMR chemical shifts which strongly deviate from the experimental value. Fourth, the DFT calculated ground state O-H bond lengths of acetylacetone and dibenzoylmethane are in quantitative agreement with the literature data which take into account the effect of quantum nuclear motion. The DFT structural results are critically discussed with respect to the state-of-the-art variable temperature X-ray and neutron diffraction methods.

An approach for investigating the impacts of hydrogen bonding and stereochemical interactions of enol-enol tautomeric equilibria of β-dicarbonyl compounds is presented. DFT quantum chemical calculations of O–H⋯O 1H NMR chemical shifts of enol pairs, weighted by their proportions.as predicted by the relative free energies computed for them at the DFT level, agree well with experimental NMR chemical shifts. The main parameters affecting equilibrium constants for a set of eleven compounds, each one of which exists in two tautomeric forms in CHCl3 solution, are ring strain and steric repulsion of substituents with differences in O–H⋯O hydrogen bond lengths playing a secondary role.

A 2D lanthanum-dicarboxylate MOF of the formula [La2(hex)3(H2O)6]·9H2O (H2hex = trans,trans-hexadiendioic acid or trans,trans-muconic acid) has been synthesized by the silica gel technique. The structure consists of metal–organic layers separated by a 2D network of water molecules. The metal–organic layers are made up of La3+ arrays linked by the muconate ligands. Within a layer there is an infinite chain of short contacts between olefinic bonds allowing formation, upon UV irradiation, of three photoproducts. One of them was completely unexpected and its formation is explained by complex molecular movements in the solid state. The thermal instability of the MOF allowed isolation of five photochemically active lower hydrates. The rate constants of all hydrates were determined and show significant variations between them, attributable to structural rearrangements within the metal–organic layer brought about upon dehydration.

Numerous gas phase electron diffraction, ultra-fast electron diffraction, X-ray and neutron diffraction experiments on β-dicarbonyl compounds exhibiting enol-enol tautomeric equilibrium, with emphasis on acetylacetone and dibenzoylmethane, have so far been reported with conflicting results on the structural details of the O-HO intramolecular hydrogen bond and resulted in alternative hypotheses on the intramolecular hydrogen bond potential function either a double minimum potential corresponding to two tautomeric forms in equilibrium or a single symmetrical one. We demonstrate herein, firstly, that the DFT calculated OH

Experimental parameters that influence the resolution of 1H-NMR phenol OH signals are critically evaluated with emphasis on the effects of pH, temperature and nature of the solvents. Extremely sharp peaks (Δν1/2 ≤ 2 Hz) can be obtained under optimized experimental conditions which allow the application of 1H-13C HMBC-NMR experiments to reveal long range coupling constants of hydroxyl protons and, thus, to provide unequivocal assignment of the OH signals even in cases of complex polyphenol natural products. Intramolecular and intermolecular hydrogen bonds have a very significant effect on 1H OH chemical shifts which cover a region from 4.5 up to 19 ppm. Solvent effects on –OH proton chemical shifts, temperature coefficients (Δδ/ΔT), OH diffusion coefficients, and nJ(13C, O1H) coupling constants are evaluated as indicators of hydrogen bonding and solvation state of phenol –OH groups. Accurate 1H chemical shifts of the OH groups can be calculated using a combination of DFT and discrete solute-solvent hydrogen bond interaction at relatively inexpensive levels of theory, namely, DFT/B3LYP/6-311++G (2d,p). Excellent correlations between experimental 1H chemical shifts and those calculated at the ab initio level can provide a method of primary interest in order to obtain structural and conformational description of solute-solvent interactions at a molecular level. The use of the high resolution phenol hydroxyl group 1H-NMR spectral region provides a general method for the analysis of complex plant extracts without the need for the isolation of the individual components.

The exact knowledge of hydrogen atomic positions of O–H···O hydrogen bonds in solution and in the solid state has been a major challenge in structural and physical organic chemistry. The objective of this review article is to summarize recent developments in the refinement of labile hydrogen positions with the use of: (i) density functional theory (DFT) calculations after a structure has been determined by X-ray from single crystals or from powders; (ii) 1H-NMR chemical shifts as constraints in DFT calculations, and (iii) use of root-mean-square deviation between experimentally determined and DFT calculated 1H-NMR chemical shifts considering the great sensitivity of 1H-NMR shielding to hydrogen bonding properties.

A series of co-crystals of trans-1,2-bis(4-pyridyl) ethylene (bpe) with aliphatic saturated or unsaturated dicarboxylic acids, of the type [(bpe)(L)] (L = dicarboxylic acid), were prepared and characterized by single crystal X-ray crystallography. In all cases chains of the type acid···bpe···acid were formed via strong O–H···N hydrogen bonds. The chains were further linked between them by weak C–H···O hydrogen bonds, π–π and herringbone interactions to form three-dimensional structures. Thermal analysis showed considerable elevation of the melting/decomposition point of the co-crystals with respect either to both components or to bpe, ascribed to the formation of multiple complementary C–H···O bonds as well as to π–π interactions between the two conformer molecules. When L = fumaric acid, the two components stack separately with their C═C bonds strictly parallel between them at 3.83 A from one another. However, photodimerization reaction takes place only between the olefin bonds of bpe. Photodimerization ...

2-Benzoylxanthone (BzX) was synthesized, characterized and used as type II photoinitiator (PI) in combination with triethylamine for the polymerization of methylmethacrylate (MMA). The photophysical/photochemical behaviour of the photoinitiator, the involved excited state and the reaction with the amine co-initiator was studied by means of absorption and nanosecond time-resolved absorption spectroscopy. Upon irradiation with 266 or 355 nm laser light, the triplet state 3BzX* (λmax = 355 nm and 530 nm) was generated as the only transient (lifetime of 22.7 μs) in nitrogen saturated MeCN solution. 3BzX* was confirmed through quenching experiments with oxygen, 2-methylbutadiene, perylene and MMA and spectral similarity to benzophenone triplet (3BP*). The quantum yield of its formation (ΦT = 0.8) and the molar absorption coefficient (ɛ = 7500 L mol−1 cm−1) was measured in MeCN. The triplet was photoreduced by triethylamine (TEA) via photoinduced electron/proton transfer giving the corresponding ketyl and α-amino ethyl radical ( CHMe–NEt2). From the reduction potential of BzX measured via cyclic voltametry (two cathodic peaks at −1.63 V and −1.91 V vs. Ag/AgCl), an exergonic electron transfer reaction results. The ketyl radical resembles the well-known benzophenone ketyl. In conclusion, the triplet 3BzX* corresponds to an n → π* transition localized on the benzoyl substituent and resembles the benzophenone triplet 3BP* but deviates from the triplet state of xanthone (3X*). This is supported through DFT/B3LYP calculations, viz., (i) fully ground and triplet state geometry optimizations show that charge and spin densities are localized on the benzoyl group, and (ii) calculation of the electronic transitions via TD-DFT at B3LYP/631G+(d) and PB1BPE/631G+(d) levels of theory shows the local character. Agreement with experiment is better by applying the conductor like polarized continuum model (CPCM) to consider the solvent effect (MeCN). The effectiveness of BzX as photoinitiator for the polymerization of MMA is found to be double that of the unsubstituted xanthone (X). The photopolymerization rates (Rp) were found to be 7.06 × 10−4 mol L−1 s−1 in the case of BzX and 3.04 × 10−4 mol L−1 s−1 in the case of BX. This is attributed to the fact that triplet 3BzX* behaves like the benzophenone triplet 3BP* and deviates from that of 3X*, i.e., it is the localization of the triplet excitation on the benzoyl subunit which renders BzX a good type-II photoinitiator.

The crystal structure and packing of a series of six nearly tetrahedral organic compounds 1– 6, of general formula PhNH-CAr 1Ar 2Ar 3 [ 1(Ph, Ph, Ph), 2(Ph, Ph, 2-Py), 3(Ph, Ph, 3-Py), 4(Ph, Ph, 4-Py), 5(Ph, 2-Py, 2-Py), and 6(2-Py, 2-Py, 2-Py)], were investigated by X-ray crystallography and supported, in some cases, by ab initio calculations, IR spectroscopy, and 1H NMR. All molecules have in common an aniline group, but they differ in the number of phenyl groups that are replaced by pyridyl ones ( 1, 2, 5, 6) or the position of the nitrogen heteroatom at the pyridyl ring ( 2– 4). Purely van der Waals solids are obtained when the molecule can form an intramolecular N−H···N hydrogen bond ( 2, 5, 6), and, in this case, a correlation between melting point and structure was attempted. However, when intermolecular N−H···N bonds were established, helices ( 4) or tetrameric (two organic molecules + two solvent molecules) molecular complexes ( 3) were obtained.

In this paper, the absorption and fluorescence spectra of xanthene, 9-trimethylsilyl xanthene (1) and 9-methyl-9-trimethylsilyl xanthene (2) are presented and discussed with the help of CNDO/S and ab initio calculations. Introduction of the trimethylsilyl group at the 9-position of the xanthene molecule considerably shifts the short-wavelength band (246 nm) to the red and reduces the fluorescence quantum yields (Φfl = 0.03 for xanthene, 0.008 for 1 and Si bond rupture. Using ps-laser flash photolysis (266 nm laser, MeCN) we observed a broad absorption spectrum peaking up at 960 nm and decaying monoexponentially with a lifetime of 130 ps, close to the measured fluorescence lifetime. We assigned therefore this transient to the singlet excited state of 1 (S1 → Sn absorption). We assume the S1 state as the origin of the photo-Fries rearrangement, giving via C Si bond dissociation a singlet geminal radical pair (9-xanthyl radical + SiMe3). In the next step, the radical pair undergoes predominantly in-cage recombination to the persistent photo-Fries intermediates with high quantum yields (Φ1CHT = 0.70 and Φ2CHT = 0.50), while to a lesser extent it escapes the solvent cage (Φ = 0.30 for 1) and undergoes typical free radical reactions (e.g., scavenging with O2). The estimation of the above quantum yields was possible only after determination of the absorption coefficients (ɛ) of the photo-Fries intermediates [ɛ1CHT(410 nm) = 16,800 M−1 cm−1 and ɛ2CHT(403 nm) = 15,900 M−1 cm−1] using three independent methods; this represents the first example in the literature.

Parent radical cations of nonpolar solvents (alkanes and alkyl chlorides) ionize 9-(trimethylsilyl)xanthenes and 9-(trimethylsilyl)fluorenes in a diffusion-controlled electron transfer. The actual electron jump as the deciding part of the process does not require a defined encounter complex, and therefore the reactants are not subjected to any geometry optimization. Considering the molecule dynamics of the donors, bending motions of the silyl group are concerted with fluctuations of the highest occupied molecular orbital electrons. Ionizing such a standing conformer mixture creates metastable (microsecond) as well as dissociative donor radical cations. A mobility restriction of the benzylic silane group in positions vertical to the phenyl plane stabilizes the radical cations and accounts for a declining amount of dissociative radical cations, which undergo C-Si bond fragmentation in the order benzylsilane > xanthenylsilane > fluorenylsilane.

The direct photolysis of the title aniline compoundPh2(Me3C)C-NHPh in acetonitrile with 248 nm laser light results in the dissociation of the C-N bond giving the corresponding (tert-Bu)diphenylmethyl radical1a•and the anilino radicalPhNH•detectable byns-laser flash photolysis (LFP) and ESR spectroscopy. The same radical1a•is also produced from the corresponding (tert-Bu)diphenylmethyl chloridePh2(Me3C)C-Cl either photolytically in MeCN through C-Cl bond homolysis or pulse radiolytically in THF; the (tert-Bu)diphenylmethyl cation is also detected, however, at sorter time scales because of its high reactivity in the solvent MeCN. At higher laser pulse intensities the radical1a•is reexcited by a second photon leading through electrocyclization to the 4a,4b-dihydro-9-tert-butylfluorenyl radical (DHBF•) and finally to fluorenyl products. Product analyses of steady state photolyses using GC and GC-MS gave as principal products (tert-Bu)diphenylmethane, aniline and various 9-substituted fluorenyl derivatives, corroborating further the primary C-N bond cleavage and the formation of the radicals1a•andPhNH•. Other conceivable dissociation pathways leading to theC-CMe3or N-H bond rupture were not observed.

Molecular oscillations are accompanied with electron density fluctuations mainly of the higher occupied orbitals. This is particularly marked for rotational motions of large substituents of aromatic molecules. Hence, the extremely rapid and non-adiabatic free electron transfer from benzyltrimethylsilanes to n-butyl chloride parent radical cations affords the existence of different short-living molecular conformers. This is indicated by a characteristic pattern of transient products including metastable radical cations and fragment radicals derived from a dissociative and very unstable ionisation product.

In this work photodegradation of four organophosphorus insecticides (ethyl-parathion, methyl-parathion, fenitrothion, fenthion) in different natural waters and soils was studied under sunlight. The origin of the waters was from the region of Ioannina (underground, lake, and river water) and from Preveza (sea water) in Northern-West Greece. The soils used had different percentages of organic matter (0.9–3.5%) and their characterization were SCL, CL, and SL respectively. The photodegradation kinetics of these insecticides were followed by GC-FTD. The identification of the photodegradation by-products was made by using GC-MS. The half-lives of the organophosphorus insecti cides vary from 0.4 to 35.4 days in natural waters and from 3.4 to 21.3 days in soils. The humic substances and the other components of these environmental matrices seem to influence the degradation kinetics. The use of GC-MS allowed the identification of some important photodegradation by-products such as: fenthion sulfone, fenthion sulfoxide, fenoxon, 4-methylthio-3,5-dimethyl phenol, O,O,O-triethyl phosphoro thioate, paraoxon, 4-nitrophenol, aminoparathion. International Journal of Environmental and Analytical Chemistry

Contrary to dialkylaminoethyl halides, 2-picolyl chloride reacts with alkaline arsenite to give nearly quantitative yields 2-picolylarsonic acid. This acid is decomposed by ascorbic acid in the presence of catalytic amounts of iodine to 2-picoline and arsenious acid, most likely by hydride transfer from the ascorbic acid. Thiophenol decomposes this arsonic acid very quickly to 2-picoline, diphenyl disulfide and triphenyl trithioarsenite. In this case a proton from the thiophenol is transferred to the incipient 2-picolyl carbanion.

The synthesis of p-acetyl and p-benzoyl derivatives of various benzylsilanes (p-R1CO–C6H4–CR2R3–SiMe3: R1, R2, R3=Me, H, H; Me, H, Ph; Me, H, Me; Me, Me, Me; Ph, H, H; Ph, H, Ph; Ph, H, Me; Ph, Me, Me), xanthenes (3-benzoyl and 3,6-dibenzoyl), and 9-Me3Si-xanthenes (3-benzoyl, 3,6-dibenzoyl, 3-acetyl and 3,6-diacetyl) using Friedel–Crafts reactions and typical Lewis acid catalysts (AlCl3, FeCl3, ZnCl2) is described. Desilylation side reactions caused by AlCl3 become significant in substrates with weak carbon–silicon bonds (e.g. Ph2CH–SiMe3), and in the case of Ph3C–SiMe3 only the desilylation product Ph3CH was isolated.

2-Arsonohexanoic acid is decomposed during its attempted reduction to 2-arsenosohexanoic acid by triphenylphosphine in the presence of a catalytic amount of iodine. When ascorbic acid is substituted for the triphenylphosphine, ­hexanoic acid is obtained, implying that the arscorbic acid acts as a hydride donor. Copyright © 2001 John Wiley & Sons, Ltd.

On irradiation in hexane (248- and 308-nm laser light) 4-diphenyl(trimethylsilyl)methyl-N,N-dimethylaniline, 2, undergoes photodissociation of the C-Si bond giving 4-N,N-dimethylamino-triphenylmethyl radical, 3(*) (lambda(max) at 343 and 403 nm), in very high quantum yield (Phi = 0.92). The intervention of the triplet state of 2 (lambda(max) at 515 nm) is clearly demonstrated through quenching experiments with 2,3-dimethylbuta-1,3-diene, styrene, and methyl methacrylate using nanosecond laser flash photolysis (LFP). The formation of 3(*) is further demonstrated using EPR spectroscopy. The detection of the S(1) state of 2 was achieved using 266-nm picosecond LFP, and its lifetime was found to be 1400 ps, in agreement with the fluorescence lifetime (tau(f) = 1500 ps, Phi(f) = 0.085). The S(1) state is converted almost exclusively to the T(1) state (Phi(T) = 0.92). In polar solvents such as MeCN, 2 undergoes (1) photoionization to its radical cation 2(*)(+), and (2) photodissociation of the C-Si bond, giving radical 3(*) as before in hexane. The formation of 2(*)(+) occurs through a two-photon process. Radical cation 2(*)(+) does not fragment further, as would be expected, to 3(*) via a nucleophile(MeCN)-assisted C-Si bond cleavage but regenerates the parent compound 2. Obviously, the bulkiness of the triphenylmethyl group prevents interaction of 2(*)(+) with the solvent (MeCN) and transfer to it of the electrofugal group Me(3)Si(+). The above results of the laser flash photolysis are supported by pulse radiolysis, fluorescence measurements, and product analysis.

Increased phenylation in the molecular series PhCH(2)NHPh (1), Ph(2)CHNHPh (2), and Ph(3)CNHPh(3) has two important consequences on the photophysical/photochemical behavior: (i) decrease in the fluorescence quantum yields (cyclohexane), Phi(f) = 0.115, 0.063, 0.001 (lambda(exc) = 254 nm) and 0.164, 0.089, 0.019 (lambda(exc) = 290 nm), respectively, and (ii) increase in the quantum yield (MeCN) of the photodissociation products PhCH(2)(*), Ph(2)CH(*), and Ph(3)C(*), Phi(radical) = 0.16, 0.25, 0.65 (lambda(exc) = 248 nm) and (not measured), 0.18, 0.29 (lambda(exc) = 308 nm), respectively. As the C-N bond progressively weakens in the series 1, 2, 3 (bond dissociation enthalpy: 52, 48, 39 kcal/mol, respectively), the C-N fission channel becomes obviously more favorable and competes effectively with fluorescence. The involved intermediates PhCH(2)(*), Ph(2)CH(*), Ph(3)C(*), and PhNH(*) were identified using laser flash photolysis (248 and 308 nm). Product analysis (lamp irradiation) gives as main products aniline and (i) 1,1-diphenylethane and o- and p-benzylaniline for 1, (ii) 1,1,2,2-tetraphenylethane for 2, (iii) Ph(3)CH and 9-Ph-fluorene for 3; all these compounds are formed from the above radicals through coupling or H-abstraction reactions.

The polymerization of methyl methacrylate (MMA) and styrene (St) has been studied using 4-[diphenyl(trimethylsilyl)methyl]benzophenon 1 as photoinitiator. The polymerization follows a free radical mechanism; the polymerization rate increases linearly with the monomer concentration and was found to be proportional to the 0.33 and 1.40 power of the photoinitiator and the monomer (MMA) concentration, respectively. The overall activation energy in the case of MMA photopolymerization was calculated to be 25.0 kJ/ mol. From 'H NMR studies it is concluded that the obtained polymers contain two different trimethylsilyl moieties, one at the head and the other at the tail of the polymer chain, showing primary termination reactions even at low initiator concentrations. The p-benzoyltrityl radical 1 . is incorporated into the polymer chain to a very small extent, acting as a scavenger. This is also concluded by laser flash photolysis (LFP) and ESR spectroscopy measurements. A living character of the polymerization was observed only at very low initiator concentrations. The triplet state ( 3 1 * ) of the initiator was quenched by styrene, reducing its efficiency. The rate constant kq of the quenching process of 3 1* was measured by LFP (k q = 3.1 10 9 M -1 . s -1 ). The triplet state and the photodissociation efficiency of the initiator is unaffected by MMA at various concentrations.

N−(Triphenylmethyl)anilines (Ph3C-NHPh-R, R = H, o-Me, m-Me, p-Me, p-CPh3), upon photolysis with 248- and 308-nm laser light, as well as by lamp irradiation with 254-nm light in acetonitrile and hexane solutions, undergo exclusively C−N bond homolysis to give the triphenylmethyl radical in a monophotonic process. Product analysis gives as main products Ph3CH, 9-Ph-fluorene, and PhNH2. The quantum yields for the formation of Ph3C• are high (0.6−0.8, 248-nm excitation) and independent of the solvent. This effective homolytic dissociation results from the low electronegativity difference between the carbon and nitrogen atoms constituting the bond to be broken, the low bond dissociation enthalpy of the C−N bond, the high excitation energy of the local chromophore (aniline), and probably from a favorable alignment of the C−N bond in a plane perpendicular to the anilino chromophore (due to the large steric requirements of the trityl group), thus enabling an effective hyperconjugative interaction with it. The ab...

Accurate (1)H chemical shifts of the -OH groups of polyphenol compounds can be calculated, compared to experimental values, using a combination of DFT, polarizable continuum model (PCM) and discrete solute-solvent hydrogen bond interactions. The study focuses on three molecular solutes: phenol, 4-methylcatechol and the natural product genkwanin in DMSO, acetone, acetonitrile, and chloroform. Excellent linear correlation between experimental and computed chemical shifts (with the GIAO method at the DFT/B3LYP/6-311++G(2d,p) level) was obtained with minimization of the solvation complexes at the DFT/B3LYP/6-31+G(d) and DFT/B3LYP/6-311++G(d,p) level of theory with a correlation coefficient of 0.991. The use of the DFT/B3LYP/6-31+G(d) level of theory for minimization could provide an excellent means for the accurate prediction of the experimental OH chemical shift range of over 8 ppm due to: (i) strong intramolecular and solute-solvent intermolecular hydrogen bonds, (ii) flip-flop intramolecular hydrogen bonds, and (iii) conformational effects of substituents of genkwanin. The combined use of ab initio calculations and experimental (1)H chemical shifts of phenol -OH groups provides a method of primary interest in order to obtain detailed structural, conformation and electronic description of solute-solvent interactions at a molecular level.

Three 2D and 3D photoreactive MOFs of trans,trans-muconic acid with Er(3+) and their corresponding isomorphous Y(3+) phases were synthesized and their photoreactivity was studied as a function of the crystal environment.

Propargylchloride (PCl) and propargylbromide (PBr) were polymerized in good yields using M(CO)5PPh3/RxAlCl3-x as catalysts (M=W, Mo). Mo was found to be more effective than W. The presence of chlorine atoms in co-catalyst was important for the activation of the catalyst. The obtained polymers were coloured, insoluble in all organic solvents and stable until 150°C. From their IR and ESR spectra is concluded that the polymers have highly conjugated structures. The oligomeric products were constituted mainly from cyclotrimers. In the case of Mo-catalyst only 1,2,4-cyclotrimer was obtained; a mixure of 1,2,4 and 1,3,5-derivatives were formed using W-catalysts.

The assembly of Er(3+) and Y(3+) cations with trans,trans-muconic acid affords a photoreactive 3D microporous MOF that, upon UV irradiation, undergoes a cycloaddition reaction (SCSC up to 55%), with in situ formation of a strained ladderane. Chemical Communications

The reaction of N-substituted anilines 2 with diphenylmethyl halides 1 at room temperature in the presence of AlCl3 affords p-substituted derivatives 3 in good yields according to an electrophilic aromatic substitution. In contrast, aniline itself is only converted to the N-substituted compounds 4. A novel rearrangement from Ph3C-NHPh (4c) to p-(triphenylmethyl)-aniline (5) is described. Unexpected photofragmentations of 4a, b are studied by using ESR and ENDOR spectroscopy; e.g., irradiation of 4b with quartz- or pyrex-filtered light leads to the formation of the radicals Ph2C-SiMe3 (7b) and Ph2C – NHPh (6) respectively, following selective cleavage of the C – N and C – Si bond.