Determination of some methylcarbamate insecticides by a.c. polarography and cyclic voltammetry

The study of adsorption-desorption phenomena represents an important extension to the range of organic compounds that can be determined by electrochemical methods. The present work has utilized this approach for the determination of a range of methylcarbamate insecticides. The tensammetric behaviour of this class of compound at the mercury electrode has been investigated by a.c. polarography and the optimum experimental conditions for their analytical determination have been derived. Cyclic voltammetry has also been used to study the electrode process and by using the peak obtained on the cathodic sweep it was possible to extend the limit of detection to the ppm level.     

Red edge excitation and proton association in the excited state of acridine

A comprehensive study of acridine spectra with variation of pH, wave length of excitation, deuteration of the solvent, etc., has been made. The excited state protonation of acridine is found extra-ordinarily excitation wavelength sensitive near the red edge of the first absorption band. The proton association takes place very fast (K _PT ~ 10¹⁰ sec^-1) on excitation at the red edge of the first absorption band (ree) and acridinium emission is observed while it is slow on short wavelength excitation (swe). The reaction rate slows down at lower temperature which is indicated by a delay in the initiation of the effect by ~ 8 nm on ree. The acridinium type emission with ree at 80 K shows that proton tunnelling is the chief mechanism of proton transfer. The quantum yields are also found wavelength dependent. Contrary to previous observations acridinium ion also shows a ree shift at 80 K.     

13C-NMR spectral studies on the distribution of substituents in some cellulose derivatives

¹³C-NMR spectra of trityl cellulose (Tr-Cell), tosyl cellulose (Ts-Cell), cellulose S-methyl xanthate (Cell-M-Xan), and cellulose formate (CF) in dimethylsulfoxide-d₆ were analyzed at 50.4 MHz. It was found that the distribution of substituents in the anhydroglucose units of these cellulose derivatives can be estimated from their ring carbon spectra. The results showed that (i) in Tr-Cell having degree of substitution (DS) lower than 1, the hydroxyl groups at C-6 carbon position are selectively tritylated, (ii) in the case of Ts-Cell, the difference in the relative DS value among three different types of hydroxyl groups is not large, although the relative reactivities of hydroxyl groups toward tosylation decrease in the order C-6 > C-2 > C-3, (iii) in Cell-M-Xan, the hydroxyl groups at C-3 carbon position are mainly substituted, and (iv) the ease of formylation is C-6 > C-2 > C-3. The 100.8 MHz ¹³C-NMR spectra of O-methyl cellulose (MC) revealed that the reactivity order in commercial MC prepared from alkali cellulose is C-6 ? C-2 > C-3. Concerning MC, its water solubility was also discussed in terms of the distribution of substituents along the cellulose chain.     

Solid state (13)C NMR and FT-IR spectroscopy of the cocoon silk of two common spiders

The structure of the silk from cocoons of two common spiders, Araneus diadematus (family Araneidae) and Achaearanea tepidariorum (family Theridiidae) was investigated by means of (13)C solid state NMR and FT-IR spectroscopies. The combined use of these two techniques allowed us to highlight differences in the two samples. The cocoon silk of Achaearanea tepidariorum is essentially constituted by helical and beta-sheet structures, whereas that of Araneus diadematus shows a more complex structure, containing also beta-strands and beta-turns. Moreover, the former silk is essentially crystalline while the latter contains more mobile domains. The structural differences of the two cocoon silks are ascribed to the different habitat of the two species.     

Electroactivity of a starburst hole-transport material in Langmuir-Blodgett films. Solid state effects and intervalence charge transfer

Here we report on the electroactivity properties of Langmuir-Blodgett (LB) films of the hole-transport molecule 4,4',4'-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA). Fairly stable Langmuir films at the air-water interface are accomplished, despite the non-amphiphilic character of the molecule. The reflection-absorption infrared spectroscopy (RAIRS) and Fourier transform infrared (FT-IR) analysis revealed that the molecules arrange with no neat preferential orientation, in agreement with the amorphous glassy nature of this starburst molecule. However, there is a tendency of the molecules to organize in a more planar conformation due to the intermolecular stacking induced by the LB technique. On the other hand, the fundamental electrochemistry (by cyclic voltammetry, CV) of the films is also analyzed. The CV studies of both solution and films reveal that both the solid state and the electrolyte's anions clearly affect the m-MTDATA's electroactivity, exhibiting a unique and broad redox process instead of the two reversible oxidations observed in solution. The oxidization mechanism is discussed. Finally, the spectroelectrochemistry studies evidence that the oxidization of the films leads to new absorption bands, among which the emerging bands in the NIR region ascribed to intervalence charge transfer (IVCT) between the generated aminyl radical cations should be pointed out.     

Solid state fluorescence of Pigment Yellow 101 and derivatives: a conserved property of the individual molecules

The optical properties of the bisazomethine pigments Pigment Yellow 101 (P.Y.101) and three derivatives are investigated employing density functional theory (DFT) and time-dependent DFT (TDDFT). P.Y.101 and one of its derivatives exhibit unusual solid state fluorescence, although both possess OH groups and the latter pigment has particularly small intermolecular distances in its crystal, which are both properties that contradict common empirical rules for fluorescent pigments. Here it is shown that the OH groups are indeed essential for molecular fluorescence of the pigments due to the necessary formation of an intramolecular hydrogen bond with the lone pairs of the bisazomethine nitrogens. Furthermore, the quenching mechanism of molecular fluorescence in the non-fluorescent derivatives is analyzed in detail and a CNN bending motion of the central bisazomethine substructure is identified to be the relevant reaction coordinate along which efficient fluorescence quenching occurs in the individual molecule as well as in the crystal. Electron transfer quenching, which usually is expected to be an important quenching mechanism in aggregated media (here the crystals), is ruled out for the studied bisazomethine pigments. The solid state fluorescence properties of the pigments can finally be understood as conserved molecular properties of the individual pigment molecules.     

Solid state NMR spectroscopy as a precise tool for assigning the tautomeric form and proton position in the intramolecular bridges of o-hydroxy Schiff bases

Two analogous Schiff bases, (S,E)-2-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylimino)methyl)phenol (1) and (S,Z)-2-hydroxy-6-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylamino)methylene)cyclohexa-2,4-dienone (2), exist in the solid state as phenol-imine and keto-amine tautomers, respectively. Their crystal structures were solved using the X-ray diffraction method. Sample 1 forms orthorhombic crystals of space group P2(1)2(1)2(1), while 2 forms monoclinic crystals of space group P2(1). In each sample, one molecule is in the asymmetric unit of the crystal structure. One-dimensional and two-dimensional solid state NMR techniques were used for structure assignment and for inspection of the (13)C and (15)N δ(ii) of the chemical shift tensor (CST) values. NMR study indicates that the span (Ω = δ(11)-δ(33)) and the skew (κ = 3(δ(22)-δ(iso)/Ω) are extremely sensitive to change in the tautomeric form of the Schiff bases. Theoretical calculations of NMR shielding parameters for 1 and 2 and a model compound with reduced aliphatic residue were performed using the GIAO method with B3LYP functional and 6-311++g(d,p) basis sets. From comparative analysis of the experimental and theoretical parameters, it was concluded that the position of hydrogen in the intramolecular bridge has tremendous influence on (13)C and (15)N CST parameters. Inspection of Ω and κ parameters allowed for the establishment of the nature of the hydrogen bonding and the assignment of the equilibrium proton position in the intramolecular bridges in the solid state.     

Synthesis,spectral, thermal,solid state d.c. electrical conductivity and biological studies of lanthanum(III) and thorium(IV) complexes with thiocarbohydrazone

The La(III) and Th(IV) complexes have been synthesized by reacting La(III) and Th(IV) nitrate with the Schiff base derived from thiocarbohydrazide and thiophene-2-aldehyde. These complexes are soluble only to a larger extent in DMF and DMSO. The observed molar conductance values indicate that they are non-electrolytes. The elemental analyses of the complexes and confined to the stoichiometry of the type La.L.(NO₃)₃H₂O and Th.L.(NO₃)₄. Further, the complexes have been characterized by the spectral and thermogravimetric data. The solid state d.c. electrical conductivity of the La(III) and Th(IV) complexes has been investigated from room temperature to 205 °C; which indicates the electrical conductivity increases with increase in temperature. Hence, La(III) and Th(IV) complexes were considered as semiconductors. Fluorescence spectra of the Schiff base and its Th(IV) complex were investigated in various solvents; the Schiff base and its metal complexes were evaluated for their antimicrobial activity.     

Solid state photodiode system matched to high-gain low-noise d.c. and lock-in amplifiers for use in multichannel emission spectrochemical analysis

Various configurations of planar silicon photodiode arrays were tested as detection devices for multichannel emission spectrometry. The electronics comprised a laboratory-built preamplifier (open loop gain 25,000) that allowed measurements with commercial high-gain low-noise d.c. and lock-in amplifiers. An a.c. noise level of 2.5 × 10^?15 A (modulating frequency = 400 Hz, frequency band pass ≈ 1 Hz) and a low frequency noise level of 6 × 10^?15 A (3-db high-frequency roll-off = 3 Hz) at the output of the detector was reached at room temperature. This noise can be ascribed completely to the preamplifier. Cooling the photodiodes has no effect on the noise level and can be omitted. A perfectly linear response over at least three orders of magnitude above the noise level and a spectral resolution equivalent to that attained with photomultiplier and exit slit were established. Comparison of the signal-to-noise (S/N) ratio of the photodiodes with that of various types of photomultiplier showed that photomultipliers were superior by a factor of 100–500 in the S/N ratio at low light levels in the wavelength region between 2500 and 5500 Å. Prospects for further research on photodiode devices for dual-channel intensity measurements (simultaneous measurement of line-plus-background and background) are discussed.     

FT-IR and FT-raman spectroscopy study of the cyclic anhydride intermediates for esterification of cellulose: I. Formation of anhydrides without a catalyst

In recent years, extensive efforts have been made to find nonformaldehyde durable press finishes to replace the traditional formaldehyde-based reagents for producing wrinkle-free cotton fabrics. 1,2,3,4-butanetetracarboxylic acid (BTCA) has been the most effective nonformaldehyde crosslinking agent. Our previous research has indicated that a polycarboxylic acid esterifies cellulose in two steps: the formation of a 5-membered cyclic anhydride intermediate by the dehydration of two adjacent carboxyl groups, and the reaction between cellulose and the anhydride intermediate to form an ester linkage. In this research, we used Fourier transform infrared and Fourier transform Raman spectroscopy to study the formation of cyclic anhydride intermediates by BTCA and other polycarboxylic acids without the presence of a catalyst. We found that BTCA and other polycarboxylic acids in a crystalline state start to form 5-membered cyclic carboxylic anhydrides when the temperature reaches the vicinity of their melting points with the exception of bifunctional acids, which form cyclic anhydrides at temperatures much higher than their melting points. Intermolecular hydrogen bonding between carboxylic acid groups prevents the formation of the cyclic anhydride intermediates at lower temperatures. We also found that polycarboxylic acids in an amorphous state form cyclic anhydrides at much lower temperatures.     

Excited-state intramolecular proton transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of coupled proton and charge transfer in the excited state of some o-hydroxyarylbenzazoles

The influence of solvent, temperature, and viscosity on the phototautomerization processes of a series of o-hydroxyarylbenzazoles was studied by means of ultraviolet-visible (UV-vis) absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. The compounds studied were 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzothiazole (HBT), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (HPyBI), and the new derivative 2-(3'-hydroxy-2'-pyridyl)benzoxazole (HPyBO), this one studied in neutral and acid media. All of these compounds undergo an excited-state intramolecular proton transfer (ESIPT) from the hydroxyl group to the benzazole N3 to yield an excited tautomer in syn conformation. A temperature- and viscosity-dependent radiationless deactivation of the tautomer has been detected for all compounds except HBI and HPyBI. We show that this radiationless decay also takes place for 2-(3-methyl-1,3-benzothiazol-3-ium-2-yl)benzenolate (NMeOBT), the N-methylated analog of the tautomer, whose ground-state structure has anti conformation. In ethanol, the radiationless decay shows intrinsic activation energy for HPyBO and HBO; however, it is barrierless for HBT and NMeOBT and controlled instead by the solvent dynamics. The relative efficiency of the radiationless decay in the series of molecules studied supports the hypothesis that this transition is connected with a charge-transfer process taking place in the tautomer, its efficiency being related to the strength of the electron donor (dissociated phenol or pyridinol moiety) and electron acceptor (protonated benzazole). We propose that the charge transfer is associated with a large-amplitude conformational change of the tautomer, the process leading to a nonfluorescent charge-transfer intermediate. The previous ESIPT step generates the structure with the suitable redox pair to undergo the charge-transfer process; therefore, an excited-state intramolecular coupled proton and charge transfer takes place for these compounds.     

Exploring the coordination chemistry of isomerizable terpyridine derivatives for successful analyses of cis and trans isomers by travelling wave ion mobility mass spectrometry

The photochemical cis-trans isomerization of the 4-{4-[2-(pyridin-4-yl)ethenyl]phenyl}-2,2':6',2'-terpyridine ligand (vpytpy) was investigated by UV-vis, NMR and TWIM-MS. Ion mobility mass spectrometry was performed pursuing the quantification of the isomeric composition during photolysis, however an in-source trans-to-cis isomerization process was observed. In order to overcome this inherent phenomenon, the isomerization of the vpytpy species was suppressed by complexation, reacting with iron(ii) ions, and forming the [Fe(vpytpy)(2)](2+) complex. The strategy of "freezing" the cis-trans isomerizable ligand at a given geometric conformation was effective, preventing further isomerization, thus allowing the distinction of each one of the isomers in the photolysed mixture. In addition, the experimental drift times were related to the calculated surface areas of the three possible cis-cis, cis-trans and trans-trans iron(ii) complex isomers. The stabilization of the ligand in a given conformation also allows us to obtain the cis-cis and cis-trans complexes exhibiting the ligand in the metastable cis-conformation, as well as in the thermodynamically stable trans-conformation.     

The influence of uncompensated solution resistance on the determination of standard electrochemical rate constants by cyclic voltammetry,and some comparisons with a.c. voltammetry

a digital simulation analysis is presented of the deleterious effects of uncompensated solution resistance, R_us, on the evaluation of standard rate constant, k^s_ob, by cyclic voltammetry. The results are expressed in terms of systematic deviations of “apparent measured” rate constants, k^s_ob(app), evaluated in the conventional manner without regard for R_us, from the corresponding actual values, k^s_ob(true), as a function of R_us and other experimental parameters. Attention is focused on the effects of altering the electrode area and the double-layer capacitance on the extent of the deviations between k^s_ob(app) and k^s_ob(true), and on comparisons with corresponding simulated results obtained from phase-selective a.c. impedance data. The extent to which k^s_ob(app) <k^s_ob(true) for small R_us values was found to be similar for the cyclic and a.c. voltammetric techniques. The latter method is, however, regarded as being preferable under most circumstances in view of the greater ease of minimising, as well as evaluating, R_us for a.c. impedance measurements. The influence of solution resistance on k^s_ob measurements with microelectrodes and without iR compensation is also considered.     

Open and cyclic multimers in alcohol solutions. A quantitative model for the association and the permittivity

When plotted versus concentration in an inert solvent, the dielectric Kirkwood g factor of most normal alcohols exhibits a minimum lower than unity followed by a rise up to values larger than unity in the neat alcohol. This behaviour is interpreted by the fact that H-bond association favours cyclic multimers at low concentrations and open multimers at high concentrations. A quantitative model using chain statistics and predicting this behaviour is summarized. A comparison is made between the results of this model and literature data on n-hexan-1-ol diluted in cyclohexane at 25 and 55°C. From the fitted values of the dissociation constants at these two temperatures it is deduced that the enhalpy for the destruction-formation of a hydrogen bond in this alcohol is about 20 kJ moL⁻¹.     

Synthesis,spectral, thermal,solid state d.c. electrical conductivity,fluorescence and biological studies of lanthanum(III) and thorium(IV) complexes of 24-membered macrocyclic triazoles

A series of La(III) and Th(IV) complexes have been synthesized by template condensation of 2,6-diformyl-4-methylphenol, bis-(4-amino-5-mercapto-1,2,4-triazol-3-yl)alkanes and La(NO₃)₃ ·?6H₂O/Th(NO₃)₄ ·?5H₂O in 2 : 2 : 1 molar ratio in ethanol. These complexes were characterized by elemental analyses, magnetic susceptibility, molar conductance, spectral (IR, UV–Vis, ¹H-NMR, FAB-mass), thermal, fluorescence and solid state d.c. electrical conductivity studies. The complexes are insoluble in water but soluble in DMF and DMSO. The observed molar conductance values indicate non electrolytes. Elemental analyses suggest 1 : 1 stoichiometry, [La(L^I–IV)(NO₃)(H₂O)₂] ·?3H₂O and [Th(L^I–IV)(NO₃)₂(H₂O)₂] ·?3H₂O. Spectroscopic studies indicate that coordination occurs through phenolic oxygen after deprotonation, nitrogen of azomethine group and bridging bidentate nitrates. The solid state d.c. electrical conductivity indicates semiconducting nature. All the Schiff bases and their La(III) and Th(IV) complexes were evaluated for biological properties; some compounds show promising results.     

Distribution of the electron density and proton chemical shifts in a series of nitromethane derivatives

Shifting of the PMR signal of the CH proton to the strong field with an increase in the positive charge on the proton was found in nitroalkanes and nitrocarboxylic acid esters. This shift is due to the presence of steric interactions between the substituents and the CH proton, which are manifested by a change in the paramagnetic contribution to the magnetic shielding constant.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1769–1773, August, 1989.     

The NMR spectra of cyclic nitrones. 3. Effect of protonation and a hydrogen bond on the chemical shifts in the13c NMR spectra of derivatives of 3-imidazoline 3-oxide

The effect of solvents on the¹³C NMR spectra of the cyclic nitrones of 3-imidazoline 3-oxide derivatives was studied. It was shown that in solvents capable of forming hydrogen bonds with the N-oxide group the signal for the nitrone carbon atom is shifted downfield by 1.5–2.5 ppm for solutions in chloroform and 5–9 ppm for solutions in methanol. The size of the shift depends on the substituent at position 1 of the imidazoline ring and decreases with increase in its accepting character. Analogous effects are observed during the formation of an intramolecular hydrogen bond with the oxygen atom of the nitrone group. During protonation of the nitrone group the downfield shift of the signal for the nitrone carbon atom amounts to 30–39 ppm.For Communication 2, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1065–1072, August, 1986.