Parametric method in the theory of the vibronic spectra of complex molecules. Absorption spectrum of decapentaene

The absorption spectra of decapentaene are analyzed and calculated using the available efficient methods of calculating the matric elements of the vibronic problem, a new parametric method for determining the potential surfaces of excited molecules, and a fragmentation approach to the design of molecular models. Good agreement for the fine vibrational structure of the calculated and experimental spectra (differences between the main band frequencies and intensities less than 20 cm⁻¹ and 10%, respectively) is obtained without correcting the parameters of the method. The mechanisms of excitation-induced structural changes in linear polyenes and the effect of bond angle changes on the vibrational structure of the spectrum are revealed. A new interpretation of the absorption spectrum of decapentaene in an argon matrix is proposed [J. Mol. Spectrosc.,114, 54–59 (1985)]. It is shown that the parametric method allows quantitative prediction of fine structure for the vibronic spectra of complex molecules. K. A. Timiryazev Moscow Agricultural Academy. Translated fromZhurnal Strukturmoi Khimii, Vol. 37, No. 6, pp. 1031–1039, November–December, 1996. Translated by I. Izvekova     

Validation of a high-performance liquid chromatographic method for determination of isoflavonoids in soybeans. Study of the extraction procedure by experimental design

Summary An improved analytical method, reversed-phase high-performance liquid chromatography on a narrow-bore C₁₈ column, has been developed for the simultaneous determination of genistein, daidzein, formononetin, and biochanin A. The method was validated in terms of detection limits, quantitation limits (LOQ), linearity, and precision.LOQ in the 0.04–0.1 μg mL⁻¹ range were calculated, enabling determination of these compounds of nutritional concern at trace levels. Good linearity was demonstrated over three orders of magnitude of concentration for each analyte (r ²=0.998–1.000). The intra-day repeatability was evaluated in terms ofRSD (%) at two concentration levels for each analyte (RSD (%) <1.8%). Good inter-day reproducibility of data was proved by performing homoscedasticity and ANOVA tests (P>0.05 at the 95% confidence level). The method was applied to the determination of genistein and daidzein in yellow soybeans, after optimization of the method for extraction of isoflavonoid aglycones from soybeans by experimental design, i.e. central composite design. Extraction recoveries up to 87±4% were obtained when the corresponding glycosidic forms (genistin and daidzin) were added to soybean samples.     

Spectrophotometric determination of methimazole,D-penicillamine,captopril, and disulfiram in pure form and drug formulations

A spectrophotometric method for the determination of methimazole (MT), D-penicillamine (PA), captopril (CA), and disulfiram (DM) was proposed. The method is based on the reaction of sulfur compound with N,N-dimethyl-p-phenylenediamine (DMPD) in acidic solution, in the presence of Fe³⁺ ions as oxidizing agent. The Beer’s law was obeyed in the range 16–110 mg of MT, 19–260 mg of PA, 29–160 mg of CA, and 36–110 mg of DM. The method was successfully applied to the quantification of these compounds in pharmaceuticals.     

Spectra-structure relationships in the near ultraviolet optical activity of chiral barbituric acid derivatives

The chiroptical properties associated with then-π^* (singlet-singlet) transitions in dissymmetric barbituric acid derivatives are examined on the basis of two theoretical models. The lower singlet excited states of unsubstituted and alkyl substituted barbituric acids are calculated on the semi-empirical CNDO/S-CI molecular orbital model, and the spectroscopic properties associated with transitions to these states are computed. In the structures we examined, threen-π^* transitions are found at λ>220 nm, two of which are nearly degenerate. Each of these transitions is computed to be strongly magnetic dipole allowed and to be forbidden or very weak (depending upon the exact symmetry and geometry of the trioxopyrimidine moiety) in electric dipole radiation. Contributions from chiral distortions within the trioxopyrimidine chromophoric system to the rotatory strengths of the three lowest energyn-π^* transitions are calculated directly from wave functions obtained by the CNDO/S-CI method. Contributions to then-π^* rotatory strengths arising from “vicinal” interactions between the trioxopyrimidine chromophore and asymmetric substituent groups are calculated by a perturbation method based on an independent systems representation of the optically active compounds. Various spectra-structure relationships are considered and correlations between experimental data and theoretically calculated results are examined.     

Determination of carbofuran, carbaryl and their main metabolites in plasma samples of agricultural populations using gas chromatography–tandem mass spectrometry

A gas chromatography–tandem mass spectrometric (GC-MS/MS) method has been developed for the determination of carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate), carbaryl (1-naphthyl-N-methylcarbamate) and their main metabolites in human blood plasma. Optimization of the isolation of the compounds from plasma matrix included the precipitation, denaturation and digestion of plasma proteins. Derivatization was achieved by the use of trifluoroacetic acid anhydride and was optimized for temperature, time and volume of derivatization agent. In the proposed method, a mild precipitation technique was applied using β-mercaptoethanol and ascorbic acid in combination with solid-phase extraction technique using Oasis HLB (Hydrophobic Lipophilic Balance) cartridges for further clean up of samples. Carbamate linkage was not hydrolyzed to its phenol product, but both carbamate phenol and ketones were transformed into trifluoroacetyl derivatives in order to become volatile compounds and were determined using tandem mass spectrometry. The linearity of the method was shown for nine concentrations in the range of 0.50–250 ng mL⁻¹ in fortified plasma aliquots. Limits of detection (LODs) for all compounds ranged from 0.015–0.151 ng mL⁻¹. Inter-day and intra-day assays (RSD) for all compounds, at three concentration levels of 2.5, 25 and 100 ng mL⁻¹ (n=3) in fortified plasma samples were less than 18%. Accuracy (%E _r) was calculated at three concentration levels, 8, 80 and 160 ng mL⁻¹ (n=3), and ranged from −12.0 to 15.0%. Matrix effect was evaluated so mean recoveries were calculated for all compounds and ranged from 81–107%. Specificity for the use of this method to biological monitoring studies was achieved including four main metabolites of CF, 1-naphthol and 2-naphthol from the naphthalene metabolism pathways, and both the parent compound of carbofuran and carbaryl. The proposed method was applied to plasma samples of pesticide users.     

Synthesis and crystal structure of new double mercury silver phosphide iodide Hg<Subscript>12</Subscript>Ag<Subscript>41</Subscript>P<Subscript>88</Subscript>I<Subscript>41</Subscript>

New double mercury silver phosphide iodide Hg₁₂Ag₄₁P₈₈I₄₁ (1) was synthesized and its crystal structure was established. Compound 1 crystallizes in the cubic system. The characteristic feature of the crystal structure 1 is the presence of the anionic cage clusters P₁₁ ³⁻, which have been previously found in alkali metal compounds only. The well-ordered P₁₁ ³⁻ clusters form a system of polyhedra, which encapsulate various disordered α-AgI-type fragments. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1882–1886, October, 2007.     

Comparison of structure,log<Emphasis Type="Italic">P</Emphasis> and P388 cytotoxicity of some phenyl and ferrocenyl cyclic chalcone analogues. Application of RP-TLC for log<Emphasis Type="Italic">P</Emphasis> determination of the ferrocenyl analogues

Cyclic chalcone analogues (2–5) and their ferrocenyl counterparts (6–10) were synthesized and their logP and P388 cyctotoxity were investigated. The structures of the newly synthesized compounds were confirmed by IR ¹H and ¹³NMR spectroscopy. Comparison of conjugation and stereochemistry of the respective derivatives showed similar characteristics compared to ones with some higher degree of conjugation in the ferrocenyl series. Comparison of logP of the ferrocenyl derivatives determined by a validated RP-TLC method showed the ferrocenyl derivatives to have higher logP _TLC. The results demonstrate that the differences in three dimensional shape, conjugation and lipophilicity do not have strong influence on the P388 cytotoxicity of the investigated phenyl (1−5) and ferrocenyl (6−10) enones.      

Rapid and Simple Method for Simultaneous Determination of Escin and Diethylamine Salicylate in Pharmaceutical Preparations by Partial Least-Squares Multivariate Calibration

Summary. A partial least-squares calibration (PLS) method has been developed for simultaneous quantitative determination of escin (ES) and diethylamine salicylate (DAS) in pharmaceutical preparations. The resolution of these mixtures has been accomplished without prior separation or derivatisation, by using partial least-squares (PLS-2) regression analysis of electronic absorption spectral data. The experimental calibration matrix was constructed with 9 samples. The concentration ranges considered were 10, 20, 30 (ES) and 40, 50, 60 (DAS) μg cm⁻³. The absorbances were recorded between 200 and 325 nm every 5 nm. Proposed method was compared with conventional spectrophotometric method. The results show that PLS-2 is a simple, rapid, and accurate method applied to the determination of these compounds in pharmaceuticals.     

Rapid and Simple Method for Simultaneous Determination of Escin and Diethylamine Salicylate in Pharmaceutical Preparations by Partial Least-Squares Multivariate Calibration

A partial least-squares calibration (PLS) method has been developed for simultaneous quantitative determination of escin (ES) and diethylamine salicylate (DAS) in pharmaceutical preparations. The resolution of these mixtures has been accomplished without prior separation or derivatisation, by using partial least-squares (PLS-2) regression analysis of electronic absorption spectral data. The experimental calibration matrix was constructed with 9 samples. The concentration ranges considered were 10, 20, 30 (ES) and 40, 50, 60 (DAS) μg cm⁻³. The absorbances were recorded between 200 and 325 nm every 5 nm. Proposed method was compared with conventional spectrophotometric method. The results show that PLS-2 is a simple, rapid, and accurate method applied to the determination of these compounds in pharmaceuticals.     

Complete basis set,hybrid-DFT study,and NBO interpretations of the conformational behavior of 1,2-dihaloethanes

The conformational behavior of 1,2-difluoroethane (1), 1,2-dichloroethane (2), 1,2-dibromoethane (3), and 1,2-diiodoethane (4) have been analyzed by means of complete basis set CBS-QB3, hybrid-density functional theory (B3LYP/Def2-TZVPP) based methods and natural bond orbital (NBO) interpretation. Both methods showed the expected greater stability of the gauche conformation of compound 1 compared to its anti conformation. Contrary to compound 1, the anti conformations of compounds 2–4 are more stable than their gauche conformation. The stability of the anti conformation compared to the gauche conformation increases from compound 1 to compound 4. The NBO analysis of donor–acceptor (σ → σ*) interactions showed that the generalized anomeric effect (GAE) is in favor of the gauche conformation of compound 1. Contrary to compound 1, GAE is in favor of the anti conformations of compounds 2–4. The GAE values calculated (i.e., GAE_anti − GAE_gauche) increase from compound 1 to compound 4. On the other hand, the calculated dipole moment values for the gauche conformations decrease from compound 1 to compound 4. In the conflict between the GAE and dipole moments, the former succeeded in accounting for the increase of the anti conformation stability from compound 1 to compound 4. There is a direct correlation between the calculated GAE, ∆[r _c–c(G) − r _c–c(A)] and ∆[r _c–x(A) − r _c–x(G)] parameters. The correlations between the GAE, bond orders, total steric exchange energies (TSEEs), ΔG _{Anti–Gauche}, ΔG ^‡(Gauche → Gauche′, C _2v), ΔG ^‡(Anti → Gauche, C ₂), dipole–dipole interactions, structural parameters, and conformational behaviors of compounds 1–4 have been investigated.     

Computational modelling of de novo synthesis of Dibenzofuran: oxidative pathways of Pyrene and Benzodibenzofuran

The dominating route to polychlorinated Dibenzo-p-dioxin and Dibenzofuran formation in the “cold zones” of flue gas cleaning systems of municipal solid waste incinerators is the so-called de novo synthesis, that is, carbonaceous matrix burnoff with simultaneous oxidation and chlorination reactions. Pyrene (1) and Benzodibenzofuran (2) were chosen as the model compounds of carbonaceous material present in fly ash. Possible routes of Dibenzofuran formation by oxidative pathways of compounds (1) and (2) were investigated by theoretical calculations at the density functional theory level. The key intermediate peroxy radical, formed by reaction with molecular oxygen, can follow three main paths leading to Dibenzofuran. In the kinetically favourite path, the highest energetic barriers (25–30 kcal mol⁻¹) are encountered in the steps where CO molecules are released from ketene-like structures. These findings agree with previously reported temperature-programmed desorption results on CO desorption. Moreover, along this path, phenanthrene and biphenyl intermediates are formed, in agreement with the detection of these products in previously reported experimental Pyrene oxidation. Along the preferred path, different steric constraints in compounds (1) and (2) play a role in determining the relative stability of the intermediates, while they have less influence on the energetic barriers. As a consequence, compounds (1) and (2) should present similar kinetic behaviour as they present similar energetic barriers.     

Solute-Solvent Interaction Effects on Protonation Equilibrium of Some Water-Insoluble Flavonoids

The protonation constants of three different flavonoids (naringenin, chrysin, and daidzein) were determined in water–DMSO mixed solvents using a potentiometric method at 25.0 (±0.1) °C and 1.00 mol⋅dm⁻³ tetra-n-butylammonium chloride as supporting electrolyte. The protonation constants were characterized and were analyzed in various solvent media in terms of the Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the protonation constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity) or π ^∗ (dipolarity/polarizability) are poor for all compounds, but the dual-parameter α and π ^∗ correlation presents significant improvement with regard to the single- and multi-parameter models. A linear correlation is observed when the logarithm of the experimental protonation constants is plotted versus the calculated ones when the KAT parameters are considered. The protonation constants of the flavonoids in water were also calculated by the Yasuda-Shedlovsky extrapolation method at zero percent organic solvent. Finally, the results are discussed in terms of the effect of solvent composition on the protonation constants.     

Conformational analysis and interpretation of ν(OH) band in the IR spectrum of o-vinylphenol: a DFT study

Two-dimensional cyclic potential energy surface for internal rotation of vinyl and hydroxyl substituents in o-vinylphenol molecule was constructed by the B3LYP/6-311G(d) method. It was shown that o-vinylphenol molecule exists in the gas phase as a mixture of seven rotamers denoted as A (A′), B (B′), C (C′) and D. The B3LYP/cc-pVTZ calculated percentage of the rotamers A and A′ in which OH…π intramolecular interaction occurs, is at most 24%. The height of barriers t interconversions between o-vinylphenol rotamers varies from 0.1 to 5.2 kcal mol⁻¹. According to B3LYP/cc-pVTZ calculations, the inclusion of solvent effect in the framework of the polarizable continuum model for a solution of o-vinylphenol in CCl₄ leads to a decrease in theoretical values of ν(OH) frequencies by about 4–9 cm⁻¹ and to an increase in the percentage of the rotamers without intramolecular hydrogen bond by about 4.3% compared to the corresponding gas-phase values. The simulated IR spectral contours of ν(OH) bands are in good agreement with experimental one Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 499–508, March, 2008.     

Chemical effects on K and L shell production cross sections and transfer probabilities in Nb compounds

In this study, the chemical effects on σ_Ki (i = α, β), σ_Lα cross sections, K_β/K_α X-ray intensity ratios and vacancy transfer probabilities from K to L (η _KL) for pure Nb and Nb compounds were investigated. The samples were excited by 59.5 keV γ-rays from ²⁴¹Am and 5.96 keV photon energy from a ⁵⁵Fe annular radioactive sources. K and L X-rays emitted by samples were counted by an Ultra-LEGe detector with a resolution of 150 eV at 5.9 keV. While it was observed that the chemical bonding had an effect on the σ_Kβ, σ_Lα cross sections and K_β/K_α X-ray intensity ratios for compounds, it was almost negligible for σ_Kα cross section because K_α transitions (2P_3/2,1/2→1S_1/2) occurred in inner shells. It is well known that interactions between central element atom and ligands come into existence in valence state, so outer energy levels are sensitive to the chemical environment. The experimental values of σ_Kα cross section and η _KL are in good agreement with theoretically calculated and other experimental values of pure niobium, but the experimental values of the σ_Kβ, σ_Lα cross sections and K_β/K_α X-ray intensity ratios have differences for some compounds because valence electrons have different bond distances and binding energies in different compounds.     

Ab initio studies of NMR chemical shifts for calix[4]arene and its derivatives

Ab initio calculations are performed for the calix[4]arene (1) and its derivatives (2 and 3), in this study. ¹H and ¹³C NMR measured spectral data given in our previous work are used to elucidate the structures of the prepared calix[4]arenes (1–3). The molecular geometry and chemical shift are calculated by using ab initio calculations based on the Hartree-Fock (HF) and the density functional theory (DFT) in the ground state. The results obtained from both methods are in agreement with the experimental results. The results of molecular geometry and chemical shifts show that DFT approach is closer to the experimental data than HF method.     

Three forms of squaric acid with pyrazine and pyridine derivatives: an experimental and theoretical study

Three new squarate salts were synthesized and combined with experimental and theoretical study on molecular, vibrational, and electronical properties. Squaric acid was crystallized as HSQ⁻ [SQ: squarate] monoanion in [(C₁₃H₁₂NO₂)(HC₄O₄)] (I), as uncharged H₂SQ in [(C₅H₅N₃O)(H₂C₄O₄)] (II), and as SQ²⁻ dianion form in [C₆H₉N₂)₂(C₄O₄)] (III). They crystallize in the triclinic and monoclinic crystal system with space group P−1, P2₁/c, and P2₁/c, respectively. Crystal structure analysis reveals that, far from forming discrete ionic species in (I) and (II), it is likely that there is large degree of proton sharing between the two hydrogen squarate anions in (I) and between the neutral moieties in (II), with the H atom lying almost symmetrically between the donor and acceptor sites, as evidenced by the long O–H and N–H bonds and short H···O distances. Ab initio calculations have been carried out for three compounds by using DFT/B3LYP and HF methods at 6-31++G(d,p) basis set. Although the supramolecular interactions have some influences on the molecular geometry in solid state phase, calculated data show that the predicted geometries can reproduce the structural parameters. The results of the optimized molecular structure for three compounds obtained on the basis of two models are presented and compared with the experimental X-ray data. Calculated vibrational frequencies are consistent with each other and experimental IR data. The theoretical electronic absorption spectra have been calculated by both TD–DFT and HF–CIS methods. Molecular orbital coefficients analysis suggest that the electronic transitions are mainly assigned to n → π* and π → π* electronic transitions.     

Colorimetric Microdetermination of Mercury(II) with Some Quinoxaline Azo Compounds in Presence of an Anionic Surfactant

Summary.  A new simple, rapid, sensitive, and selective method is proposed for the microdetermination of mercury. Mercury(II) forms insoluble complexes with 2,3-dichloro-6-(2-hydroxy-3,5-dinitrophenylazo)-quinoxaline (1), 2,3-dichloro-6-(5-amino-3-carboxy-2-hydroxy-phenylazo)-quinoxaline (2), 2,3-dichloro-6-(2,7-dihydroxynaphth-1-ylazo)-quinoxaline (3), and 2,3-dichloro-6-(3-carboxy-2-hydroxy-naphth-1-ylazo)-quinoxaline (4) in aqueous acidic medium; the complexes can be made soluble by the action of an anionic surfactant. The solution of the pink coloured compounds is stable for at least 24 h. Beer’s law is obeyed over the concentration range from 0.1 to 2.8 μg · cm⁻³ of mercury. For a more accurate analysis, Ringbom optimum concentration ranges were found to be 0.25–2.5 μg · cm⁻³. The molar absorpitivity, Sandell sensitivity, and relative standard deviations were also calculated. A slight interference from Pd²⁺ and Cd²⁺ is exhibited by the first three ligands, whereas the last one is only negligibly affected by these metal ions. Strong interference from Ag(I) is evident for all ligands, whereas alkali, alkaline earth, and other transition metals tested posed negligible interference. 15 μg · cm⁻³ of Cd²⁺ and Pd²⁺ or 10 μg · cm⁻³ of Ag⁺ can be tolerated if 1.0 mg of potassium bromide and 2.0 mg of citrate as masking agents are added for the determination of 1.5 μg · cm⁻³ of mercury(II). The method was applied to the determination of methyl- and ethylmercury chloride and the analysis of environmental water samples. Received August 7, 2000. Accepted (revised) October 18, 2000     

Modeling Ar and Kr matrix effect on the ν s (Cl–H) and ν l (Cl–H) of Cl–H···NH3 by the IEF-PCM method

Ar and Kr matrix effect on the geometry and Cl–H stretching (ν _s (Cl–H)) and librational (ν _l (Cl–H)) frequencies of the hydrogen-bonded complex Cl–H···NH₃ are simulated within the framework of polarizable continuum model with integral equation formalism (IEF-PCM) at B3LYP and MP2 levels of theory with the basis set 6-311++G(2df,2pd). Within the framework of B3LYP and IEF-PCM, the simulated gas phase, Ar, and Kr matrix ν _s (Cl–H) of the complex are 2140, 1684, and 1550 cm⁻¹, respectively, which deviate from the experimental values (~2200, 1371, and 1218 cm⁻¹) by −60, 313, and 332 cm⁻¹. Within the framework of MP2 and IEF-PCM, the gas phase, Ar, and Kr matrix ν _s (Cl–H) are calculated as 2366, 2037, and 1957 cm⁻¹ by the harmonic approximation, and as 2177, 1876, and 1665 cm⁻¹ by the full-dimensional anharmonic correction. The matrix effect modeling is of greater importance than the anharmonic correction in accounting for the large experimental gas phase to Ar or Kr matrix shift of the ν _s (Cl–H) (−829 or −982 cm⁻¹). Our calculations do not support the assignment of the 733.8 and 736.9 cm⁻¹ bands to the Ar and Kr matrix ν _l (Cl–H).     

Hammett constants and NMR shifts of α-acyloxy carboxamide derivatives

Geometrical structures, Hammett constants, ¹H and ¹³C chemical shift values, molecular electrostatic potential maps, and several thermodynamic parameters of α-acyloxy carboxamide derivatives (4a–o) were calculated using HF and DFT/B3LYP methods with 6-31G(d) basis set. The optimized structures were compared with analogous compound. The ¹H and ¹³C NMR shielding tensors were computed with the Gauge-Independent Atomic Orbital (GIAO) method. Comparison of the experimental ¹H and ¹³C NMR chemical shifts of 4a–o molecules with the theoretical data indicates good agreement.