Comparative study of p‐amino benzhydrazide and m‐amino benzhydrazide by free radicals and free electron transfer

The bimolecular electron transfer from p‐amino benzhydrazide (PABH) and its meta‐derivative (m‐amino benzhydrazide (MABH)) to specific one‐electron oxidant results in the formation of anilino radical. In case of PABH, reaction with ^?OH radicals results in the synchronous formation of adduct as well as anilino radical. The interesting observation is that the yield of the anilino radical increases with increase in pH. The effect of substitution also has a significant effect on the formation of adduct. In case of MABH, significant yield of anilino radical gets form on reaction with ^?OH radical. The free radical electron transfer from PABH and MABH to parent radical cation of non‐polar solvent also results in the formation of anilino radical only suggesting that the radical cation of PABH and MABH has short life time. The above results were supported by quantum chemical calculations. Copyright © 2013 John Wiley & Sons, Ltd.     

Molecular structure and vibrational spectra of 3 (or 4 or 6)‐methyl‐5‐nitro‐2‐pyridinethiones: FT‐IR,FT‐Raman and DFT quantum chemical calculations

IR and Raman spectra (RS) of polycrystalline 3‐(or 4 or 6)‐methyl‐5‐nitro‐2‐pyridinethione have been measured and analyzed by means of density functional theory (DFT) quantum chemical calculations. The B3LYP/6‐311G(2d,2p) approach has been applied for both the thiol and thione tautomers due to the possibility of the formation of these two thiole forms. Molecular structures of these compounds have been optimized starting from different molecular geometries of the thiol group and thione group. Two conformations of the 2‐mercaptopyridine, trans and cis, have been taken into account. It was shown that the studied compounds appear in the solid state in the thione form. The effect of the hydrogen‐bond formation in the studied compounds has been considered. Copyright © 2008 John Wiley & Sons, Ltd.     

Time‐resolved resonance Raman and density functional theory study of the photochemistry of 4‐benzoylpyridine in acetonitrile and 2‐propanol

A nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic investigation of the intermolecular hydrogen‐abstraction reaction of the triplet state of 4‐benzoylpyridine (4‐BPy) in 2‐propanol solvent is reported. The TR³ results reveal a rapid hydrogen abstraction (<10 ns) by the 4‐BPy triplet state (nπ*) with the 2‐propanol solvent, leading to formation of a 4‐BPy ketyl radical and an associated dimethyl ketyl radical partner from the solvent. The recombination of these two radical species occurs with a time constant about 200 ns to produce a para‐N‐LAT (light absorbing transient). The structure, major spectral features, and identification of the ketyl radical and the para‐N‐LAT coupling complex have been determined and confirmed by comparison of the TR³ results with results from density functional theory (DFT) calculations. A reaction pathway for the photolysis of 4‐BPy in 2‐propanol deduced from the TR³ results is also presented. The electron‐withdrawing effect of the heterocyclic nitrogen for 4‐BPy on the triplet state makes it have a significantly higher chemical reactivity for the hydrogen abstraction with 2‐propanol compared to the previously reported corresponding benzophenone triplet reaction under similar reaction conditions. In addition, the 4‐BPy ketyl radical reacts with the dimethyl ketyl radical to attach at the para‐N atom position of the pyridine ring to form a cross‐coupling product such as 2‐[4‐(hydroxy‐phenyl‐methylene)‐4h‐pyridin‐1‐yl]‐propan‐2‐ol instead of attacking at the para‐C atom position as was observed for the corresponding benzophenone reaction reported in an earlier study. Copyright © 2008 John Wiley & Sons, Ltd.     

Switching of hydrogen bonds of water in ionic liquid, 1‐butyl‐3‐methylimidazolium tetrafluoroborate

We have investigated temperature‐induced Raman spectral changes of deuterated water in an ionic liquid, 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([bmim][BF₄]), between room temperature and 77 K. The comparison of the OH and OD stretching vibrational spectra at 77 K shows that the strength of the hydrogen bonds in [bmim][BF₄]–water mixtures strongly depends on the type of water, i.e. H₂O and D₂O. In the [bmim][BF₄]–D₂O system, remarkably strong hydrogen bonds form at low temperatures, but they switch to nearly free hydrogen bonds on heating. Copyright © 2013 John Wiley & Sons, Ltd.     

Electrochemical reduction of 2,4‐dimethyl(diethyl)‐9‐oxo‐10‐(4‐heptoxyphenyl)‐9H‐thioxanthenium hexafluorophosphates and 2,4‐dimethyl(diethyl)‐9H‐thioxanthene‐9‐ones

Electrochemical reduction of 2,4‐dimethyl(diethyl)‐9‐oxo‐10‐(4‐heptoxyphenyl)‐9H‐thioxanthenium hexafluorophosphates in acetonitrile (MeCN) and N,N‐dimethylformamide is an irreversible 1‐electron process accompanied by the cleavage of the C(Ph)‐S bond in thioxanthenium cations with the formation of the corresponding 2,4‐dimethyl(diethyl)‐9H‐thioxanthene‐9‐ones. One‐electron reversible electrochemical reduction of the latter compounds occurs at more negative potentials and yields the corresponding radical anions, which have been characterized by electron paramagnetic resonance spectroscopy and density functional theory calculations at the (U)B3LYP/6‐31+G*/polarizable continuum model level of theory.     

Rotational disorder in 2‐piperidyl‐5‐nitro‐6 ‐methylpyridine: structural phase transition and its vibrational characteristics

X‐ray diffraction (XRD) studies have shown that 2‐piperidyl‐5‐nitro‐6‐methylpyridine, C₁₁H₁₅N₃O₂, undergoes a structural phase transition at T = 240 K. The room temperature structure is tetragonal, space group I4₁/a, with the unit‐cell dimensions a = 13.993(2) and c = 23.585(5) Å. The pyridine ring takes trans conformation with respect to the piperidine unit. While pyridine is well ordered, the piperidine moiety shows apparent disorder resulting from a libration about the linking N C bond. The low‐temperature phase is monoclinic, space group I2/a. Contraction of the unit‐cell volume by 2.3% at 170 K enables the C H···O linkage between the molecules of the neighbouring stacks. As result, the asymmetric unit becomes bi‐molecular. The thermal librations of the piperidine and methyl groups become considerably reduced at 170 K and nearly fully reduced at about 100 K. The IR spectra and polarised Raman spectra agree with the X‐ray structure and confirm the disorder effect on the piperidine ring. The assignment of the bands observed was made on the basis of DFT chemical quantum calculations. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and X‐ray structures of unexpected 2‐O‐(5‐deoxy‐1,2‐O‐isopropylidene‐α‐D‐glucofuranos‐5‐yloxy)quinoxalines

Reaction of 3‐methyl‐2(1H)‐quinoxalinone ( 4) and 2(1H)‐quinoxalinone ( 5) with 5,6‐anhydro‐1,2‐O‐isopropylidene‐ α‐D ‐glucofuranose 6 gives the unexpected O‐glucoquinoxalines derivatives by the intermediary novel intramolecular rearrangement of 5,6‐anhydro‐1,2‐O‐isopropylidene‐α‐D ‐glucofuranose to the corresponding 3,6‐anhydro form. The obtained O‐glucoquinoxalines 7,8 were identified by NMR spectroscopy. The X‐ray crystal structures have been determined at room temperature. Moreover, a solid–solid phase transition has been detected at 198.9 K for O‐glucoquinoxalines 7 and the structure of the low‐temperature phase has been solved at 188 K. Copyright © 2009 John Wiley & Sons, Ltd.     

Alkane oxidation by the system ‘tert‐butyl hydroperoxide–[Mn2L2O3][PF6]2 (L = 1,4,7‐trimethyl‐1,4,7‐triazacyclononane)–carboxylic acid’

The kinetics of cyclohexane (CyH) oxygenation with tert‐butyl hydroperoxide (TBHP) in acetonitrile at 50 °C catalysed by a dinuclear manganese(IV) complex 1 containing 1,4,7‐trimethyl‐1,4,7‐triazacyclononane and co‐catalysed by oxalic acid have been studied. It has been shown that an active form of the catalyst (mixed‐valent dimeric species ‘Mn^IIIMn^IV’) is generated only in the interaction between complex 1 and TBHP and oxalic acid in the presence of water. The formation of this active form is assumed to be due to the hydrolysis of the Mn? O? Mn bonds in starting compound 1 and reduction of one Mn^IV to Mn^III. A species which induces the CyH oxidation is radical tert‐BuO ^. generated by the decomposition of a monoperoxo derivative of the active form. The constants of the equilibrium formation and the decomposition of the intermediate adduct between TBHP and 1 have been measured: K = 7.4 mol^?1 dm³ and k = 8.4 × 10^?2 s^?1, respectively, at [H₂O] = 1.5 mol dm^?3 and [oxalic acid] = 10^?2 mol dm^?3. The constant ratio for reactions of the monomolecular decomposition of tert‐butoxy radical (tert‐BuO ^. → CH₃COCH₃ + CH) and its interaction with the CyH (tert‐BuO ^. + CyH → tert‐BuOH + Cy ^. ) was calculated: 0.26 mol dm^?3. One of the reasons why oxalic acid accelerates the oxidation is due to the formation of an adduct between oxalic acid and 1 (K ≈ 10³ mol^?1 dm³). Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of the polymethine chain length, the polarity and temperature of the medium on the spectroscopic properties of merocyanine dyes

We have studied the luminescent spectral properties and calculated the quadratic polarizability of dimethine, tetramethine, and hexamethine merocyanines based on 1,3-dihydro-1,3,3-trimethyl-(2H)-indol-2-ylidene and malonodinitrile in solutions and polymer films at room temperature and 77 K. We have shown that increasing the polarity of the solvent and lengthening the polymethine chain in the studied merocyanines lead to an increase in fluorescence quantum yields and narrowing of the fluorescence bands. This is explained by the decrease in bond length alternation and weakening of vibronic interactions in the singlet-excited fluorescent state. The mirror symmetry in the structured absorption and fluorescence spectra of merocyanine dyes in ethanol at 77 K, observed here for the first time, is due to enhancement of electrostatic interactions of the merocyanine and solvent molecules. Increased stiffness of the medium considerably decreases the probability of nonradiative processes, and the major channel for deactivation of the electronic excitation energy at 77 K is fluorescence. The higher values of the quadratic polarizability (6.0·10⁻²⁸ esu in ethanol and 3.3·10⁻²⁸ esu in chloroform) for hexamethine merocyanine, estimated by the solvatochromic method, lets us consider this compound as a promising material for designing nonlinear optical converters for harmonics of laser radiation. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 26–35, January–February, 2006.     

Theoretical studies on the hydrogen abstraction reactions of methyl esters with HO2 radical and the following β‐scission reactions

Unsaturated fatty acid methyl esters are ubiquitous in biodiesel fuels. The C = C double bond greatly affects the combustion characteristics of biodiesel, especially its ignition behavior at low temperatures. In this work, we report detailed theoretical study on the mechanism and kinetics of the hydrogen abstraction reactions of linear unsaturated C6 methyl esters with hydroperoxy radical (HO₂), which play a critical role in the low‐temperature combustion of biodiesel. Reaction profiles are obtained via intrinsic reaction coordinate (IRC) analysis including the formation of reactant complexes and product complexes at the entrance and exit channels, respectively. The potential energy surfaces are explored at the CBS‐QB3 level. The following β‐scission reactions of the forming radicals are also investigated at the same level of theory. The high‐pressure limit rate constants for all the reactions in the temperature range from 500 to 2000 K are calculated via conventional transition‐state theory with quantum tunneling effect and fitted to the modified Arrhenius expression.     

The photoinduced reaction of 2‐iodothiophene in solutions of n‐heptane,dichloromethane and methanol

The photoinduced reaction of 2‐iodothiophene in n‐heptane, dichloromethane and methanol was studied at room temperature from experiments carried out with degassed solutions. The photoproducts of the reaction were mainly thiophene and small amounts of iodine in all three solvents used. The concentration of 2‐iodothiophene decreases throughout photolysis, following a first‐order rate law and the pseudo‐first‐order rate constants were determined in the three solvents used. The photochemistry of the system was quantified determining the quantum yields of 2‐iodothiophene consumption and thiophene formation in n‐heptane solutions. The results show that under the experimental conditions of this research, products deriving only from the reaction of the thienyl radical were observed. To support the experimental results, calculations were performed of the ionization potential of the thienyl radical, electron affinity of the iodine atom and free energy of solvation of the corresponding iodide and carbocation in the different solvents used. Copyright © 2010 John Wiley & Sons, Ltd.     

On the structure of 3‐acetylamino‐5‐methyl‐1,2,4‐oxadiazole and on the fully degenerate rearrangements (FDR) of its anion: a stimulating comparison between the results of ‘in‐silicon chemistry’ and ‘laboratory chemistry’

An accurate crystal structure determination has provided evidence for a planar conformation for 3‐acetylamino‐5‐methyl‐1,2,4‐oxadiazole ( 5 ), in agreement with quantum‐mechanical calculations in the gas phase. In the crystal, a series of strong intermolecular N7H7^….O9 hydrogen bonds link the amido groups of different molecules, causing the formation of infinite parallel ordered chains. The effect of the DMSO solvent on the energy and charge distribution of compound 5 and on its relevant 5 ^? anion, involved in a fully degenerate rearrangement (FDR), has been deepened by quantum‐mechanical DFT calculations. The calculated energy barrier for the FDR increases in going from in vacuo to DMSO, in agreement with previsions based on the Hughes and Ingold rules concerning the nucleophilic substitution of an anionic reagent (the deprotonated amido group in the side chain) on a neutral substrate (the 1,2,4‐oxadiazole ring). Copyright © 2009 John Wiley & Sons, Ltd.     

On the photophysical model for polyenes: Experimental evidence of the 1,8‐diphenylocta‐1,3,5,7‐tetraene molecule dissolved in n‐octane,cyclooctane, and 2,2,4‐trimethylpentane

Spectroscopic evidence recorded in solutions of diphenyloctatetraene in n‐octane, cyclooctane, and 2,2,4‐trimethylpentane by sudden immersion of the sample at 77 K or by changing the solution temperature slowly between 293 and 77 K allow us to conclude that the distinct spectroscopic behavior of the polyene compounds, on extending its polyene chain, does not lie in the existence of a ghost state 2¹Ag, as first excited electronic state of the system, but resides in a structural problem that influences whether its first electronic transition for absorption can be monitored or not at its lowest vibrational levels.     

Synthesis,properties, and solvatochromism of 1,3‐dimethyl‐5‐{(thien‐2‐yl)‐[4‐(1‐piperidyl) phenyl]methylidene}‐(1H,3H)‐pyrimidine‐2,4,6‐trione

A new merocyanine dye, 1,3‐Dimethyl‐5‐{(thien‐2‐yl)‐[4‐(1‐piperidyl)phenyl]methylidene}‐ (1H, 3H)‐pyrimidine‐2,4,6‐trione 3 , has been synthesized by condensation of 2‐[4‐(piperidyl)benzoyl]thiophene 1 with N,N′‐dimethyl barbituric acid 2 . The solvatochromic response of 3 dissolved in 26 solvents of different polarity has been measured. The solvent‐dependent long‐wavelength UV/Vis spectroscopic absorption maxima, v_max, are analyzed using the empirical Kamlet–Taft solvent parameters π* (dipolarity/polarizability), α (hydrogen‐bond donating capacity), and β (hydrogen‐bond accepting ability) in terms of the well‐established linear solvation energy relationship (LSER): (1) The solvent independent coefficients s , a , and b and (v_max)₀ have been determined. The McRae equation and the empirical solvent polarity index, E_T(30) have been also used to study the solvatochromism of 3 . Copyright © 2007 John Wiley & Sons, Ltd.     

Chemical and photochemical reactivity of 6‐hydroxymethyl‐7,8‐dihydropterin in aqueous solutions

6‐Hydroxymethyl‐7,8‐dihydropterin (H₂Hmp) is an intermediate in the biosynthesis of folate, a precursor of coenzymes involved in the metabolism of nucleotides and amino acids. In this work, we have investigated the reactions undergone by H₂Hmp in aqueous solutions at physiological pH, in the absence and in the presence of UV‐A radiation (320–400 nm). In air‐equilibrated solutions, H₂Hmp undergoes slow thermal oxidation (half‐life 37 h) to yield 7,8‐dihydroxanthopterin (H₂Xap) as the main product. The reaction of H₂Hmp with hydrogen peroxide also yields H₂Xap as a main product. In contrast, UV‐A excitation of H₂Hmp leads to the formation of a dimer identified by electrospray ionization mass spectrometry. The corresponding quantum yield of H₂Hmp consumption (Φ_?R) was independent of O₂ and reactant concentration and has a value of 0.10 (±0.02), more than twice higher than that measured for other 6‐subtituted 7,8‐dihydropterins. Copyright © 2012 John Wiley & Sons, Ltd.     

1,1,1‐Trichloro‐3‐(1‐phenethylamino‐ethylidene)‐ pentane‐2,4‐dione—synthesis,spectroscopic, theoretical and structural elucidation

1,1,1‐Trichloro‐3‐(1‐phenethylamino‐ethylidene)‐pentane‐2,4‐dione is spectroscopically and structurally elucidated by means of linear‐polarized IR spectroscopy (IR‐LD) of oriented solids as a colloidal suspension in nematic liquid crystal. Structural information and IR‐spectroscopic assignment are supported by quantum chemical calculations at MP2 and B3LYP level of theory and 6‐311++G** basis set. The geometry is characterized with an inramolecular hydrogen bond of NH^…O?C with length of 2.526 Å and a NHO angle of 140.5(1)°. The NH? C(CH₃)C?C? C?O(CH₃) fragment is nearly flat with a maximal deviation of total planarity of 10.4°. Copyright © 2007 John Wiley & Sons, Ltd.     

Energetic and structural characterization of 2‐R‐3‐methylquinoxaline‐1,4‐dioxides (R = benzoyl or tert‐butoxycarbonyl): experimental and computational studies

The gaseous standard molar enthalpies of formation of two 2‐R‐3‐methylquinoxaline‐1,4‐dioxides (R = benzoyl or tert‐butoxycarbonyl), at T = 298.15 K, were derived using the values for the enthalpies of formation of the compounds in the condensed phase, measured by static bomb combustion calorimetry, and for the enthalpies of sublimation, measured by Knudsen effusion, using a quartz crystal oscillator. The three dimensional structure of 2‐tert‐butoxycarbonyl‐3‐methylquinoxaline‐1,4‐dioxide has been obtained by X‐ray crystallography showing that the two N? O bond lengths in this compound are identical. The experimentally determined geometry in the crystal is similar to that obtained in the gas‐phase after computations performed at the B3LYP/6‐311 + G(2d,2p) level of theory. The experimental and computational results reported allow to extend the discussion about the influence of the molecular structure on the dissociation enthalpy of the N? O bonds for quinoxaline 1,4‐dioxide derivatives. As found previously, similar N? O bond lengths in quinoxaline‐1,4‐dioxide compounds are not linked with N? O bonds having the same strength. Copyright © 2007 John Wiley & Sons, Ltd.     

Hydrogen‐bonded nucleophile effects in ANS: the reactions of 1‐chloro and 1‐fluoro‐2,4‐dinitrobenzene with 2‐guanidinobenzimidazole, 1‐(2‐aminoethyl)piperidine and N‐(3‐aminopropyl)morpholine in aprotic solvents

The kinetics of the reactions of 2,4‐dinitrofluorobenzene (DNFB) and 2,4‐dinitrochlorobenzene (DNClB) with 2‐guanidinobenzimidazole (2‐GB) at 40 ± 0.2 °C in dimethylsulphoxide (DMSO), toluene, and in toluene–DMSO mixtures, and with 1‐(2‐aminoethyl)piperidine (2‐AEPip) and N‐(3‐aminopropyl)morpholine (3‐APMo) in toluene at 25 ± 0.2 °C were studied under pseudo first‐order conditions. For the reactions of 2‐GB carried out in pure DMSO, the second‐order rate coefficients were independent of the amine concentration. In contrast, the reactions of 2‐GB with DNFB in toluene, showed a kinetic behaviour consistent with a base‐catalysed decomposition of the zwitterionic intermediate. These results suggest an intramolecular H‐bonding of 2‐GB in toluene, which is not present in DMSO. To confirm this interpretation the reactions were studied in DMSO–toluene mixtures. Small amounts of DMSO produce significant increase in rate that is not expected on the basis of the classical effect of a dipolar aprotic medium; the effect is consistent with the formation of a nucleophile/co‐solvent mixed aggregate. For the reactions of 3‐APMo with both substrates in toluene, the second‐order rate coefficients, k_A, show a linear dependence on the [amine]. 3‐APMo is able to form a six‐membered ring by an intramolecular H‐bond which prevents the formation of self‐aggregates. In contrast, a third order was observed in the reactions with 2‐AEPip: these results can be interpreted as a H‐bonded homo‐aggregate of the amine acting as a better nucleophile than the monomer. Most of these results can be well explained within the frame of the ‘dimer nucleophile’ mechanism. Copyright © 2010 John Wiley & Sons, Ltd.     

Spectral tuning of InGaAs/GaAs quantum dot infrared photodetectors with bandpass guided‐mode resonance filters

Quantum dot infrared photodetectors can be coupled with micro‐structured filters to create narrowband sensors. Guided‐mode resonance filters based on a high‐index dielectric slab can exhibit bandpass characteristics that are suitable for monolithic integration with focal‐plane arrays. Here, patterned Ge filters were integrated with InGaAs/GaAs quantum dot detectors to linearly tune their 77 K photoresponse peaks from 5.6 µm to 6.2 µm. The dark current was not influenced by these filters but the ability to narrow the photoresponse linewidth was limited by substrate scattering, which is often encountered with front‐side illumination architectures. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Radical anions containing the dioxidated 1,2,5‐thiadiazole heterocycle

Radical anions of 3,4‐aryl disubstituted 1,2,5‐thiadiazole 1,1‐dioxide were obtained by chemical and electrochemical reduction of their substrates, and characterized by ESR spectroscopy and cyclic voltammetry. The radical anion of the phenanthro[9,10‐c]‐1,2,5‐thiadiazole 1,1‐dioxide was found to be very stable in an aprotic solvent solution and did not react readily when water was added to the aprotic solvent, or the solution was saturated with oxygen gas. The radical formation chemical reaction competed with nucleophilic addition to the C?N bond of the thiadiazoles. A possible reaction mechanism, and a common reaction intermediate, supported by density functional theory calculations, is presented for the most stable radical. Copyright © 2009 John Wiley & Sons, Ltd.