Dechlorination of poly(vinylidene chloride) in NaOH/ethylene glycol as a function of NaOH concentration, temperature, and solvent

The wet dechlorination treatment of poly(vinylidene chloride) (PVDC) was evaluated at atmospheric pressure in a solution of NaOH in ethylene glycol (EG), as a function of NaOH concentration, temperature, and solvent. Hydroxide ion from NaOH was required for dechlorination with EG acting solely as a solvent. The wet treatment exhibited significantly enhanced dechlorination efficiency over traditional thermal techniques, with a reaction efficiency as high as 92.8% in 1.0 M NaOH at 190 °C. Dechlorination reactions of PVDC in both NaOH/EG and NaOH/H₂O were expressed by an apparent first-order reaction. At 190 °C, the apparent rate constant in 1.0 M NaOH/EG was approximately 1.4 times larger than in 1.0 M NaOH/H₂O, with an apparent activation energy of 82.8 kJ mol⁻¹, indicating that the reaction proceeded under chemical control. The degree of dechlorination increased with increasing reaction temperature, favouring the elimination of HCl over the hydroxyl substitution of chloride.     

Synthesis of [F]xenon difluoride as a radiolabeling reagent from [F]fluoride ion in a micro-reactor and at production scale

[¹⁸F]Xenon difluoride ([¹⁸F]XeF₂), was produced by treating xenon difluoride with cyclotron-produced [¹⁸F]fluoride ion to provide a potentially useful agent for labeling novel radiotracers with fluorine-18 (t_1/2 = 109.7 min) for imaging applications with positron emission tomography. Firstly, the effects of various reaction parameters, for example, vessel material, solvent, cation and base on this process were studied at room temperature. Glass vials facilitated the reaction more readily than polypropylene vials. The reaction was less efficient in acetonitrile than in dichloromethane. Cs⁺ or K⁺ with or without the cryptand, K 2.2.2, was acceptable as counter cation. The production of [¹⁸F]XeF₂ was retarded by K₂CO₃, suggesting that generation of hydrogen fluoride in the reaction milieu promoted the incorporation of fluorine-18 into xenon difluoride. Secondly, the effect of temperature was studied using a microfluidic platform in which [¹⁸F]XeF₂ was produced in acetonitrile at elevated temperature (≥85 °C) over 94 s. These results enabled us to develop a method for obtaining [¹⁸F]XeF₂ on a production scale (up to 25 mCi) through reaction of [¹⁸F]fluoride ion with xenon difluoride in acetonitrile at 90 °C for 10 min. [¹⁸F]XeF₂ was separated from the reaction mixture by distillation at 110 °C. Furthermore, [¹⁸F]XeF₂ was shown to be reactive towards substrates, such as 1-((trimethylsilyl)oxy)cyclohexene and fluorene.     

Fluoride-Promoted Ligand Exchange in Diaryliodonium Salts

Diaryliodonium salts are shown to undergo rapid, fluoride-promoted aryl exchange reactions at room temperature in acetonitrile. Aryl exchange is shown to be exquisitely sensitive to the concentration of fluoride ion in solution; fast exchange is observed as the fluoride concentration approaches a stoichiometric amount at 50 mM substrate concentration. The reaction is slowed, but not halted if benzene is the solvent, indicating that free fluoride ion or a four-coordinate anionic I(III) species may be responsible for the exchange. The fluoride-promoted aryl exchange reaction is general and allows direct measurement of the relative stabilities of diaryliodonium salts featuring different aryl substituents. The aryl exchange reaction may be of practical use for the preparation of hitherto inaccessible diaryliodonium salts, thus it also has implications for labeling radiotracers for molecular imaging with ¹⁸F-fluoride (t_1/2 = 109.7 min).     

ATRP of MMA initiated by 2-bromomethyl-4,5-diphenyloxazole at room temperature and study of fluorescent property

Atom transfer radical polymerization (ATRP) of MMA was conducted successfully at ambient temperature (25 °C) using 2-bromomethyl-4,5-diphenyloxazole as initiator, CuBr/2,2′-bipyridine (BPY) as catalyst, and 1,4-Dioxane as solvent. Factors such as the reaction temperature, mole ratio of monomer and catalyst to initiator, solvent and so on, which can affect the ATRP system, were discussed in the paper. Chain extension was conducted using polymer as the macro-initiator which was characterized via ¹H NMR. The optical property of initiator was well preserved in the obtained PMMA, and the end-functionalized PMMA exhibited obvious fluorescent emission at 367 nm.     

Salt-assisted liquid-liquid microextraction with water-miscible organic solvents for the determination of carbonyl compounds by high-performance liquid chromatography

A simple and rapid method has been reported for the determination of carbonyl compounds involving reaction with 2,4-dinitrophenylhydrazine and extraction of hydrazones with water-miscible organic solvent acetonitrile when the phase separation occurs by addition of ammonium sulphate, a process called salt-assisted liquid-liquid microextraction. The extract was analyzed by high-performance liquid chromatography with UV detection at 360 nm. The procedure has been optimized with respect to solvent suitable for extraction, salt for phase separation between water and organic solvent, reaction temperature and reaction time. The method has been validated when a linear dynamic range was obtained between the amount of analyte and peak area of hydrazones in the range 7 μg-15 mg L⁻¹, the correlation coefficient over 0.9964-0.9991, and the limit of detection in the range 0.58-3.2 μg L⁻¹. Spiked water samples have been analyzed with adequate accuracy, and application of the method has been demonstrated in the analysis of benzaldehyde formed as oxidation product in pharmaceutical preparation where benzyl alcohol is used as preservative, and for a keto drug dexketoprofen.     

Modification of the halogen end groups of polystyrene prepared by ATRP

The stabilization modification of the halogen end groups of polystyrene prepared by atom transfer radical polymerization (ATRP) has been attempted. The reaction mechanism adopted is radical chain transfer reaction, and iso-propylbenzene is employed as not only the chain transfer agent but also the solvent. Moreover, Cu⁰ is used as the acceptor of the transformed halogen atom in some experiments. As evidenced by ¹H NMR analysis of the modified products, the halogen end group can really be converted into the much more stable carbon-hydrogen structure. When Cu⁰ is not used, the conversion of the halogen end groups rises rapidly during the early stage and the increase rate slows down after about 8 h reaction. In view of the influence of reaction temperature on the modification, the conversion increases almost exponentially with temperature in the range of 80-100 °C, and the increase rate slows down at higher temperature. ¹H NMR and SEC analyses prove that the modification reaction does not destroy the polymer backbone and the molecular weights remain almost the same as those of the unmodified samples. When Cu⁰ is introduced, the modification reaction proceeds much rapidly, the conversion of the halogen end groups rises almost linearly at the early stage and the nearly complete (>95%) dehalogenation of the polymeric chains is observed after only 12 h reaction. However, the molecular weights rise and the polydispersities become wider after the modification, which implies that the modification is accompanied with the couple termination of the polystyrene radicals besides chain transfer reaction. Furthermore, the couple termination can be restrained at some lower catalyst concentration. Indeed, the modified polymers show improved thermal stability, the initial weight loss temperatures is increased from 196 °C to 378 °C for the linear polystyrene and from 203 °C to 261 °C for the hyperbranched polystyrene.     

Counterion and solvent effects on the anionic polymerization of β-butyrolactone initiated with acetic acid salts

The comparative studies on the anionic polymerization of β-butyrolactone (BL) initiated with various salts of acetic acid have revealed strong sensitivity of the reaction rate on solvent polarity (benzene, THF, DMSO) and size of counterion. It was found that the polymerization rate in THF depends on the size of counterion and the type of macrocyclic ligand; it decreases in the following order: K⁺/Kryptofix® 222 ≈TBA⁺ > K⁺/18C6 > Na⁺/18C6 > Na⁺/15C5 > K⁺. It was also shown that the anionic polymerization of BL initiated with carboxylic acid salts depends strongly on the solvent polarity. In the polymerization initiated by acetate anions with a large counterion, the high-polar solvent as DMSO affects unfavorably the reaction rate, however, when a small counterion is applied, the opposite tendency is observed.     

Continuous determination of total flavonoids in Platycladus orientalis (L.) Franco by dynamic microwave-assisted extraction coupled with on-line derivatization and ultraviolet-visible detection

This paper describes a new method for the determination of total flavonoids in Platycladus orientalis (L.) Franco. The method was based on dynamic microwave-assisted extraction (DMAE) coupled with on-line derivatization and ultraviolet-visible (UV-vis) detection. The influence of the experimental conditions was tested. Maximum extraction yield was achieved using 80% aqueous methanol of extraction solvent; 80 W of microwave output power; 5 min of extraction time; 1.0 mL min⁻¹ of extraction solvent flow rate. The derivatization reaction between aluminium chloride and flavonoid is one of the most sensitive and selective reactions for total flavonoids determination. The optimized derivatization conditions are as follows: derivatization reagent 1.5% aluminium chloride methanol solution; reaction coil length 100 cm; derivatization reagent flow rate 1.5 mL min⁻¹. The detection and quantification limits obtained are 0.28 and 0.92 mg g⁻¹, respectively. The intra-day and inter-day precisions (R.S.D.) obtained are 1.5% and 4.6%, respectively. Mean recovery is 98.5%. This method was successfully applied to the determination of total flavonoids in P. orientalis (L.) Franco and compared with heat reflux extraction. The results showed that the higher extraction yield of total flavonoids was obtained by DMAE with shorter extraction time (5 min) and small quantity of extraction solvent (5 mL).     

Determination of aliphatic amines by gas chromatography–mass spectrometry after in-syringe derivatization with pentafluorobenzoyl chloride

A simple and highly sensitive gas chromatographic method has been developed for the determination of low molecular weight short-chain aliphatic amines (SCAAs) after their simultaneous extraction and in-syringe derivatization with pentafluorobenzoyl chloride (PFBOC). Derivatization of the low molecular weight aliphatic amines in bicarbonate buffer of pH 10.5 with PFBOC was followed by immersed solvent microextraction. Derivatization conditions, including reagent concentration, reaction pH, ionic concentration of buffer, reaction time, stirring rate, reaction temperature and extraction solvent, have been investigated for method optimization. Linearity was studied within range of 0.15 pg ml⁻¹–50 ng ml⁻¹. The correlation coefficients were between 0.9934 and 0.9999. Detection limit of derivatized amines proved to be in the range of 0.117–1.527 pg ml⁻¹, and the intraday and interday relative standard deviation (RSD) values were less than 8% with respect to peak area. The method was applied for analysis of lake, river and industrial waste water. The recoveries of extraction from lake, river and industrial waste water samples, which have been spiked with different levels of aliphatic amines, were in the range of 68–99%, 63–102% and 62–105%, respectively.     

Determination of bioavailable soluble arsenic and phosphates in mine tailings by spectrophotometric Sequential Injection Analysis

By using a simple Sequential Injection Analysis (SIA) manifold and in base to the kinetic reaction of the molybdenum with As(V) and P(V) was possible to determine As(III), As(V) and P(V) in simple, binary and ternary samples. The activation energies for the reaction between molybdenum and As(V) and P(V) were of 70.90 kJ mol⁻¹ and of 19.02 kJ mol⁻¹, respectively, therefore it was possible to determine both analytes in mixtures by using different reaction temperature. When the analyses were carried out at room temperature, only the P(V) supplied analytical signal; with increased temperature, the kinetics of reaction for As(V) also increased, and a signal was obtained, being 55 °C the optimum temperature. In order to determine As(III), it was oxidized into As(V) with KIO_3, and the reaction was carried out in the same way as for As(V). To resolve mixtures, an equations system from six calibration curves with different sequences of SIA at different temperature was performed. The lineal ranges were between 0.5 μg mL⁻¹ and 10 μg mL⁻¹ with a repeatability and reproducibility between 0.7% and 5.2% and detection limits between 0.36 μg mL⁻¹ and 0.58 μg mL⁻¹. In binary mixtures of P(V)/As(V) the recoveries were close to 100% for both analytes at ratios lesser than 10:1. For As(V)/As(III) ratios between 1:1 and 5:1 the recoveries were ranged between 85% and 95%. The method was applied in mine tailings and in arsenopyrite. The results showed that the soluble arsenic was found oxidized as As(V). These results were compared with those obtained by atomic absorption spectrometry and both proved to be very close.     

Simple hollow fiber renewal liquid membrane extraction method for pre-concentration of Cd(II) in environmental samples and detection by Flame Atomic Absorption Spectrometry

A hollow fiber renewal liquid membrane (HFRLM) extraction method to determine cadmium (II) in water samples using Flame Atomic Absorption Spectrometry (FAAS) was developed. Ammonium O,O-diethyl dithiophosphate (DDTP) was used to complex cadmium (II) in an acid medium to obtain a neutral hydrophobic complex (ML₂). The organic solvent introduced to the sample extracts this complex from the aqueous solution and carries it over the poly(dimethylsiloxane) (PDMS) membrane, that had their walls previously filled with the same organic solvent. The organic solvent is solubilized inside the PDMS membrane, leading to a homogeneous phase. The complex strips the lumen of the membrane where, at higher pH, the complex Cd-DDTP is broken down and cadmium (II) is released into the stripping phase. EDTA was used to complex the cadmium (II), helping to trap the analyte in the stripping phase. A multivariate procedure was used to optimize the studied variables. The optimized variables were: sample (donor phase) pH 3.25, DDTP concentration 0.05% (m/v), stripping (acceptor phase) pH 8.75, EDTA concentration 1.5 × 10⁻² mol L⁻¹, extraction temperature 40 °C, extraction time 40 min, a solvent mixture N-butyl acetate and hexane (60/40%, v/v) with a volume of 100 μL, and addition of ammonium sulfate to saturate the sample. The sample volume used was 20 mL and the stripping volume was 165 μL. The analyte enrichment factor was 120, limit of detection (LOD) 1.3 μg L⁻¹, relative standard deviation (RSD) 5.5% and the working linear range 2-30 μg L⁻¹.     

Dynamic headspace time-extended helix liquid-phase microextraction

Liquid-phase microextraction (LPME) has been proved to be a fast, inexpensive and effective sample pre-treatment technique for the analyses of pesticides and many other compounds. In this investigation, a new headspace microextraction technique, dynamic headspace time-extended helix liquid-phase microextraction (DHS-TEH-LPME), is presented. In this work, use of a solvent cooling system, permits the temperature of the extraction solvent to be lowered. Lowering the temperature of the extraction solvent not only reduces solvent loss but also extends the feasible extraction time, thereby improving extraction efficiency. Use of a larger volume of the solvent not only extends the feasible extraction time but also, after extraction, leaves a larger volume to be directly injected into the gas chromatography (GC) to increase extraction efficiency and instrument signal. The DHS-TEH-LPME technique was used to extract six organochlorine pesticides (OCPs) from 110 ml water samples that had been spiked with the analytes at ng/l levels, and stirred for 60 min. The proposed method attained enrichments up to 2121 fold. The effects of extraction solvent identity, sample agitation, extraction time, extraction temperature, and salt concentration on extraction performance were also investigated. The method detection limits (MDLs) varied from 0.2 to 25 ng/l. The calibration curves were linear for at least 2 orders of magnitude with R² ≧ 0.996. Relative recoveries in river water were more than 86%.     

An on-line method for pressurized hot water extraction and enzymatic hydrolysis of quercetin glucosides from onions

A novel environmentally sound continuous-flow hot water extraction and enzymatic hydrolysis method for determination of quercetin in onion raw materials was successfully constructed using a stepwise optimization approach. In the first step, enzymatic hydrolysis of quercetin-3,4′-diglucoside to quercetin was optimized using a three level central composite design considering temperature (75–95 °C), pH (3–6) and volume concentration of ethanol (5–15%). The enzyme used was a thermostable β-glucosidase variant (termed TnBgl1A_N221S/P342L) covalently immobilized on either of two acrylic support-materials (Eupergit^® C 250L or monolithic cryogel). Optimal reaction conditions were irrespective of support 84 °C, 5% ethanol and pH 5.5, and at these conditions, no significant loss of enzyme activity was observed during 72 h of use. In a second step, hot water extractions from chopped yellow onions, run at the optimal temperature for hydrolysis, were optimized in a two level design with respect to pH (2.6 and 5.5), ethanol concentration (0 and 5%) and flow rate (1 and 3 mL min⁻¹) Obtained results showed that the total quercetin extraction yield was 1.7 times higher using a flow rate of 3 mL min⁻¹ (extraction time 90 min), compared to a flow rate of 1 mL min⁻¹ (extraction time 240 min). Presence of 5% ethanol was favorable for the extraction yield, while a further decrease in pH was not, not even for the extraction step alone. Finally, the complete continuous flow method (84 °C, 5% ethanol, pH 5.5, 3 mL min⁻¹) was used to extract quercetin from yellow, red and shallot onions and resulted in higher or similar yield (e.g. 8.4 ± 0.7 μmol g⁻¹ fresh weight yellow onion) compared to a conventional batch extraction method using methanol as extraction solvent.     

Headspace liquid-phase microextraction of trihalomethanes in drinking water and their gas chromatographic determination

In the present work, a novel method for the determination of trihalomethanes (THMs) such as chloroform, dichlorobromomethane, chlorodibromomethane and bromoform in drinking water has been described. It is based on coupling headspace liquid-phase microextraction (HS-LPME) with gas chromatography-electron capture detector (GC-ECD). A microdrop of organic solvent at the tip of a commercial microsyringe was used to extract analytes from aqueous samples. Three organic solvents—xylene, ethylene glycol and 1-octanol—were compared and 1-octanol was the most sensitive solvent for the analytes. Extraction conditions such as headspace volume, extraction time, stirring rate, content of NaCl and extraction temperature were found to have significant influence on extraction efficiency. The optimized conditions were 15 ml headspace volume in a 40 ml vial, 10 min extraction time and 800 rpm stirring rate at 20 °C with 0.3 g ml⁻¹ NaCl. The linear range was 1-100 μg l⁻¹ for THMs. The limits of detection (LODs) ranged from 0.15 μg l⁻¹ (for dichlorobromomethane and chlorodibromomethane) to 0.4 μg l⁻¹ (for chloroform); and relative standard deviations (RSD) for most of THMs at the 10 μg l⁻¹ level were below 10%. Real samples collected from tap water and well water were successfully analyzed using the proposed method. The recovery of spiked water samples was from 101 to 112%.     

Dispersive liquid–liquid microextraction method based on solidification of floating organic drop for extraction of organochlorine pesticides in water samples

A new simple and rapid dispersive liquid–liquid microextraction method has been developed for the extraction and analysis of organochlorine pesticides (OCPs) in water samples. The method is based on the solidification of a floating organic drop (DLLME-SFO) and is combined with gas chromatography/electron capture detection (GC/ECD). Very little solvent is required in this method. The disperser solvent (200 μL acetonitrile) containing 10 μL hexadecane (HEX) is rapidly injected by a syringe into the 5.0 mL water sample. After centrifugation, the fine HEX droplets (6 ± 0.5 μL) float at the top of the screw-cap test tube. The test tube is then cooled in an ice bath. After 5 min, the HEX solvent solidifies and is then transferred into a conical vial, where it melts quickly at room temperature, and 1 μL of it is injected into a gas chromatograph for analysis. Under optimum conditions, the enrichment factors and extraction recoveries are high and range between 37–872 and 82.9–102.5%, respectively. The linear range is wide (0.025–20 μg L⁻¹), and the limits of detection are between 0.011 and 0.11 μg L⁻¹ for most of the analytes. The relative standard deviation (RSD) for 1 μg L⁻¹ of OCPs in water was in the range of 5.8–8.8%. The performance of the method was gauged by analyzing samples of lake and tap water.     

Synthesis of non-aggregating chlorins and isobacteriochlorins from meso-tetrakis(pentafluorophenyl)porphyrin: a study using 1,3-dipolar cycloadditions under mild conditions

The 1,3-dipolar cycloaddition of meso-tetrakis(pentafluorophenyl)porphyrin and its nickel complex, with the bulky azomethine ylide dipole was studied under mild conditions, and yielded chlorin and isobacteriochlorin derivatives self-prevented from aggregation. The reactions were performed at room temperature or 0 °C, and we were able to establish a set of reaction conditions to obtain only the chlorin or the isobacteriochlorin. These compounds were evaluated in solution, and no aggregation was observed at less than 25 mM (∼30 mg mL⁻¹) using ¹H NMR experiments.     

Headspace in-drop derivatization of carbonyl compounds for their analysis by high-performance liquid chromatography-diode array detection

A simple and rapid method has been reported for the determination of carbonyl compounds involving sample preparation by headspace single drop microextraction using 1-butanol as extraction solvent containing 2,4-dinitrophenylhydrazine for hydrazone formation, and direct transfer of the drop into the injector for high-performance liquid chromatography with diode array detection. An angle-cut polytetrafluoroethylene sleeve, 3 mm × 0.5 mm, was fixed at the tip of the syringe needle and this allowed the use of 7 μL drop of solvent drop for extraction and derivatization. The procedure has been optimized with respect to suitable solvent for headspace drop formation, drop volume, concentration of reagent, sample temperature, reaction time, and headspace-to-sample volume ratio. The method has been validated when rectilinear relationship was obtained between the amount of analyte and peak area ratio of hydrazones in the range 0.01-15 mg L⁻¹, the correlation coefficient over 0.996-0.999, and the limit of detection in the range 1.7-24.1 μg L⁻¹. Spiked real samples have been analyzed with adequate accuracy, and application has been demonstrated of the method for analysis of carbonyl compounds formed as oxidation products.     

Interfacial synthesis of bisphenol A tetrachlorocyclotriphosphazene from bisphenol A and hexachlorocyclotriphosphazene

The effect of solvent purity on the synthesis and yield of bisphenol A tetrachlorocyclotriphosphazene (BATCCP) has not been described in the literature. The purpose of this research was to synthesize BATCCP hybrid monomers and to evaluate the effect of solvent purity on the BATCCP production. BATCCP monomers were prepared by an interfacial procedure in a water/toluene system as a function of time with the assistance of a phase transfer catalyst, tetraoctylammonium bromide. ¹H and ³¹P NMR confirmed the production of BATCCP monomer by the appearance of chemical shifts at 7.18 and 5.35 ppm in the ¹H NMR and 23.4 and 13.9 ppm in the ³¹P NMR, respectively. Distillation of the toluene, not suggested in previous reports of HCCP hybrid synthesis, resulted in an improvement of actual % yield to 40% and stability of the product throughout the 1440 min reaction as confirmed by MALDI, compared with an 11% actual yield at 15 min, decaying to 2% over a 1440 min reaction when the synthesis was performed with ‘anhydrous toluene’ as provided commercially without further distillation.     

Syntheses,characterization and crystal structures of mixed-ligand Cu(II), Ni(II) and Mn(II) complexes of N-(1-phenyl-3-methyl-4-propenylidene-5-pyrazolone)-salicylidene hydrazide containing ethanol or pyridine as a co-ligand

Five new mixed-ligand complexes [CuL(EtOH)] (1), [NiL(EtOH)₃] (2), [Mn₂L₂(μ₂-EtOH)₂(EtOH)₂] (3), [CuL(Py)] · MeOH (4) and [NiL(Py)₃] (5) (L²⁻ = N-(1-phenyl-3-methyl-4-propenylidene-5-pyrazolone)-salicylidene hydrazide anion, Py = pyridine) have been synthesized and characterized by elemental analyses, IR spectra, thermal analyses and single crystal X-ray diffraction. The crystallographic structural analyses of these complexes reveal that the ligand (H₂L) itself undergoes isomerization from the keto form to the enol form in the reaction, then loses two protons and acts as a double negatively charged tridentate chelating agent coordinated to the metal ion in the solution. The final results show that when a co-ligand was present in the synthetic reaction, other coordination sites around the metal ions Cu²⁺ and Ni²⁺ were completed either by the ethanol or pyridine molecules under the common solvent reaction or solvothermal syntheses conditions, respectively. In the case of the Mn²⁺ ion, it was still coordinated with the solvent molecules regardless of whether it was synthesized under the common solvent reaction or solvothermal syntheses with pyridine. The reason for this difference might be attributed to the fact that the coordination modes and bonding effect of the co-ligand with the metal ions are different, the final complexes tend to form the most stable compound.     

A DFT study on the effect of hydrogen bonding on the reaction of a μ-benzoquinone diruthenium complex with acetylene

A DFT study was carried out to investigate the reaction mechanisms of a model μ-benzoquinone diruthenium complex {CpRu(μ-H)}₂(μ-η²:η²-C₆H₄O₂), derived from the experimental compound {Cp*Ru(μ-H)}₂(μ-η²:η²-C₆H₃RO₂) (R = H or R = Me, Cp* = η⁵-C₅Me₅), with acetylene both in aprotic and protic solvents. Results of calculations show that the influence of the solvent methanol on the reaction is mainly on the step of acetylene coordination. Enhanced hydrogen bonding is the reason for acceleration of the reaction in protic solvent, which is supported by NBO charge analysis.