Mono- and Binuclear Complexes in a Centrifuge-Less Cloud-Point Extraction System for the Spectrophotometric Determination of Zinc(II).

The hydrophobic reagent 6-hexyl-4-(2-thiazolylazo)resorcinol (HTAR) was investigated as part of a cloud-point extraction (CPE) system for the spectrophotometric determination of Zn(II). In the system, complexes with different stoichiometries, including 1:1 and 2:2 (Zn:HTAR), are formed. Their ground-state equilibrium geometries were optimized at the B3LYP/6-31G level of theory. The obtained structures were then used to calculate vertical excitation energies in order to generate theoretical UV/Vis absorption spectra. The comparison between theoretical and experimental spectra demonstrated that, under optimal conditions, a binuclear complex containing oxygen-bridging atoms is the dominant species. The absorbance was found to be linearly dependent on the concentration of Zn(II) within the range of 15.7 to 209 ng mL-1 (R2 = 0.9996). The fraction extracted (%E), logarithm of the conditional extraction constant (log Kex), and molar absorption coefficient (ε) at λmax = 553 nm were calculated to be 98.3%, 15.9, and 4.47 × 105 L mol-1 cm-1, respectively. The method developed is characterized by simplicity, convenience, profitability, sensitivity, and ecological friendliness. It has been successfully applied to the analysis of pharmaceutical and industrial samples.

Experimental and theoretical study of the cytosine tautomerism through excited states.

CONTEXT: The irradiation of water solution of cytosine with UV light (λmax = 254 nm) shows oxo-hydroxy tautomerism with a rate constant of 6.297 × 10-3 min-1. The order of the reaction implies a tautomeric conversion. After removing the UV light source, we observed a dark reaction with a rate constant of 1.473 × 10-3 min-1 which leads to a restoration of the initial tautomer as before the irradiation. The mechanism of oxo-hydroxy tautomerism of cytosine in water solution was studied in the excited state. It was found that the transformations occur along the 1πσ* excited-state reaction paths which link the Franck-Condon geometries of the tautomers and the conical intersections S0/S1 connected with the H-detachment processes of the corresponding bonds. Furthermore, we established that the conical intersections S0/S1 are also mutually accessible along the 1πσ* excited-state reaction paths. METHODS: The ground-state equilibrium geometries were optimized at the B3LYP/aug-cc-pVDZ level of theory in water environment according to PCM as well as at the CC2/aug-cc-pVDZ level in the gas phase. The TD B3LYP and CC2 methods were applied for the study of the excited states. The tautomerization mechanisms were studied with the use of the linear interpolation in internal coordinates approach using the optimized geometries of tautomers minima and conical intersections S0/S1 at the CASSCF(6,6)/6-31G* level. All calculations were performed with the GAUSSIAN 16 commercial software.

Spectrophotometric Determination of Molybdenum(VI) as a Ternary Complex with 4-Nitrocatechol and Benzalkonium Chloride.

A new liquid-liquid extraction system for molybdenum(VI) was studied. It contains 4-nitrocatechol (4NC) as a complexing chromogenic reagent and benzalkonium chloride (BZC) as a source of heavy cations (BZ+), which are prone to form chloroform-extractable ion-association complexes. The optimum conditions for the determination of trace molybdenum(VI) were found: concentrations of 4NC and BZC (7.5 × 10-4 mol dm-3 and 1.9 × 10-4 mol dm-3, respectively), acidity (3.75 × 10-2 mol dm-3 H2SO4), extraction time (3 min), and wavelength (439 nm). The molar absorptivity, limit of detection, and linear working range were 5.5 × 104 dm3 mol-1 cm-1, 5.6 ng cm-3, and 18.6-3100 µg cm-3, respectively. The effect of foreign ions was examined, and the developed procedure was applied to the analysis of synthetic mixtures and real samples (potable waters and steels). The composition of the extracted complex was 1:1:2 (Mo:4NC:BZ). Three possible structures of its anionic part [MoVI(4NC)O2(OH)2]2- were discussed based on optimizations at the B3LYP/3-21G level of theory, and simulated UV/Vis absorption spectra were obtained with the TD Hamiltonian.

Thermal versus photochemical tautomerization of cytosine and guanine: a BLYP computational study along the IRC curves.

We performed a theoretical study, at the BLYP/aug-cc-pVDZ theoretical level, of the mechanisms of H-transfer between biologically relevant tautomers of cytosine and guanine. The analysis of the ground- and excited state energy barriers of the conversions showed that in some cases they are lower as compared to the ground state reactions. The inclusion of one water molecule as a catalyst in the oxo-amino form of cytosine and guanine drastically changes the mechanism of the reactions in the ground as well as in the excited state. For example, the ground state reaction path of cytosine reduces the energy barrier from 165 to 60 kJ.mol-1 and for guanine, this reduction is from 92 kJ.mol-1 to 24 kJ.mol-1. The results obtained are in support to the high photostability of the nucleic acid bases which guarantee the normal biological function of nucleic acids.

Photoinduced phenomena in water solution of melamine explaining the photostability of the compound.

Two tautomers of melamine (triamino and imino-diamino) were studied at the BLYP/aug-cc-pVDZ theoretical level. It was found that the two tautomers are bridged with the 1πσ* excited-state reaction path. The high photostability of melamine in water solution was explained with the mechanism of ring deformation which occurs along the 1ππ* excited-state reaction path. The two mechanisms are investigated at the TD BLYP level of theory using the linear interpolation in internal coordinates approach.

Complex Formation in a Liquid-Liquid Extraction System Containing Vanadium(IV/V), 2,3-Dihydroxynaphtahlene and Thiazolyl Blue.

Liquid-liquid extraction systems for VIV/V containing 2,3-dihydroxynaphtahlene (DN) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (thiazolyl blue, MTT) were studied. The optimum conditions for VIV and VV extraction were found. VIV is extracted in chloroform as a 1:2:2 complex (V:DN:MTT) with lmax = 570 nm and e570 = 2.9´104 dm3 mol-1 cm-1. However, this wavelength was found unsuitable for precise spectrophotometric measurements due to time dependent absorbance changes. VV forms predominantly a 1:1:1 complex with lmax = 335 nm. The calibration graph for this oxidation state is linear in the range of 0.06-1.5 mg cm-3. The molar absorptivity, Sandell's sensitivity and limit of detection were calculated to be 1.6 ´ 104 dm3 mol-1 cm-1, 3.2 ng cm-2 and 0.02 mg cm-3, respectively. The ground-state equilibrium geometries of the anionic parts of the extracted ion-associates, [VIVO(DN2-)2]2-and [VVO2(DN2-)]-, were optimized at the BLYP/6-31++G* level of theory.

Investigation of the mechanisms of photo-induced formation of cyclobutane dimers of cytosine and 2,4-diaminopyrimidine.

The mechanisms of the formation of cyclobutane dimers (CBD) of cytosine and 2,4-diaminopyrimidine were studied at the CC2 theoretical level and cc-pVDZ basis functions. Four orientations of the two monomers are explored: cys-syn, cis-anti, trans-syn, and trans-anti. The research revealed that in all cases the cyclobutane structures are formed along the (1)ππ* excited-state reaction paths of the stacked aggregates. We localized the S1/S0 conical intersections mediating those transformations. The results obtained agree well with the previously reported investigations on the cis-syn cyclodimer formations of other pyrimidines.

Solvent influence on the excited states of the oxo form of barbituric acid and the mechanisms of the out-of-plane non-radiative elongation of the N-H bond: a comparative theoretical and experimental study.

The solvent influence on the excited states, emission and absorption energies of the oxo for of barbituric acid was studied with experimental (UV and fluorescence spectra) and theoretical methods. The excited-state reaction paths of the out-pf-plane elongation of the N-H bond of the oxo form of barbituric acid were also investigated (TD DFT level) to the conical intersections mediating internal conversions to the ground state. The (1)nσ* excited state was found to be the driven electronic state. We found that the increase of the polarity of the solvent reduces the (1)nσ* excited state decay rate through a non-planar conical intersection and increases its energy. Thus, solvents with higher polarity disfavor the non-radiative decay through conical intersections.

Excited-state relaxation paths of oxo/hydroxy and N9H/N7H tautomers of guanine: a CC2 theoretical study.

We performed a theoretical investigation, at the CC2/aug-cc-pVDZ level, of the ring-deformation mechanisms of four guanine tautomers (oxo, hydroxy, N9H, and N7H). The study showed that the optimized conical intersections S0/S1 are accessible through the (1)ππ* excited states of tautomers. The optimized conical intersections S0/S1, which show deformation at the pyrimidine ring, have high energies. This means that the relaxations of the (1)ππ* excited states via internal conversion are disfavored. For two tautomers we found crossing points (1)ππ*/(1)πσ* of the excited-state reaction paths, revealing the possibility of a population of the (1)πσ* excited state by the (1)ππ* excited state.

Comparative study of the relaxation mechanisms of the excited states of cytosine and isocytosine.

An experimental and theoretical investigation was performed to study the photostability of cytosine and isocytosine. The experimental UV irradiation of acetonitrile solutions of the two compounds showed that the amino-oxo tautomer of cytosine is photostable while the amino-oxo tautomer of isocytosine tautomerizes to the amino-hydroxy form. The theoretical investigations were carried out at the CC2 level of theory. They were performed to explain the experimental observations. It was found that the (1)ππ(*) excited states of the ring deformation mechanisms of cytosine and isocytosine relax (internal conversion) to the ground states of the amino-oxo forms of the compounds. We propose a channel for the radiationless deactivation of the repulsive (1)πσ(*) excited state of the amino-oxo form of isocytosine to the ground state of the amino-hydroxy tautomer.

Photoinduced tautomerism of 2-thiobarbituric Acid studied by theoretical and experimental methods.

Combined, theoretical and experimental, investigation was performed to study the mechanism of the photoinduced tautomerism of 2-thiobarbituric acid (TBA). The irradiation of the solution of TBA in polar aprotic solvent with UV light (maximum at 366 nm) showed oxo-hydroxy photoisomerization of the triketo form of TBA to the hydroxy-imino tautomer. The studied mechanisms (TD DFT) of the photoinduced NH and OH dissociations in the keto and enol tautomer revealed that the proton detachment in the triketo tautomer occurs in the bright 1nSσ* excited state. In the hydroxy-imino tautomer this mechanism is driven by the repulsive 1πσ* excited state. The excited-state relaxation mechanisms occur by low-lying So-S1 conical intersections.

Comparison of the non-radiative decay mechanisms of 4-pyrimidinone and uracil: an ab initio study.

We performed a comparative theoretical study of the relaxation mechanisms of the excited states of uracil and 4-pyrimidinone with the CASSCF, CASPT2, and CC2 ab initio methods. The calculated vertical excitation energies agree with the experimental UV absorption maxima of the two compounds. Three low-lying conical intersections between the S(0) and S(1) states (one for uracil, two for 4-pyrimidinone) are established. They are accessible from the Franck-Condon region of the 1pipi* state through out-of-plane deformations related to C=C (for uracil) or C=N (for 4-pyrimidinone) torsions of the heterocyclic ring. These conical intersections mediate the radiationless deactivation of the compounds after excitation of the lowest 1pipi* state. The relaxation of the 1pipi* state of 4-pyrimidinone via C=C twisting is hindered by a barrier. The relaxed scan of the C=N double-bond twisting of 4-pyrimidinone indicates that the formation of the Dewar form may represent a photochemical channel in 4-pyrimidinone. This fact is detrimental for the photostability of 4-pyrimidinone, since the Dewar form is separated by a high potential-energy barrier from the canonical form of 4-pyrimidinone on the ground-state potential-energy surface, which prevents a thermal back-reaction. The investigation of the vertical excitation energies and the reaction paths shows that 4-pyrimidinone is less photostable than uracil.

Computational (DFT and TD DFT) study of the electron structure of the tautomers/conformers of uridine and deoxyuridine and the processes of intramolecular proton transfers.

Six uridine and six deoxyuridine isomers were studied at the B3LYP and TD B3LYP theoretical level and 6-31+G(d) basis function. The stability and the excited states of the isomers were studied in order to clarify some known experimental data. It was established that the rotation of the oxo uracil ring in uridine is energetically more likely to occur in the excited state than in the ground state, driven by the bright (1) pipi* state and the dark charge transfer (1)npi* state. Very high energy barriers (on the S(o)) were found for thermal intramolecular proton transfer processes.

Ground state intermolecular proton transfer in the supersystems thymine-(H2O)n and thymine-(CH3OH)n, n = 1,2: a theoretical study.

Twelve binary and eight ternary supersystems between thymine and methanol, and water were investigated in the ground state at the B3LYP and MP2 levels of theory using B3LYP/6-311 + + G(d,p) basis functions. The thermodynamics of complex formations and the mechanisms of intermolecular proton transfers were clarified in order to find out the most stable H-boned system. It was established that the energy barriers of the water/methanol-assisted proton transfers are several times lower than those of the intramolecular proton transfers in the DNA/RNA bases. The X-ray powder spectra of thymine, and this precrystallized from water and methanol showed that water molecules are incorporated in the crystal lattice of thymine forming H-bridges between thymine molecules.

Investigation of the intermolecular proton transfer in the supersystems adenine-methanol/ethanol/i-propanol: MP2 and DFT levels study.

Twelve H-bonded supersystems constructed between the adenine tautomers and methanol, ethanol, and i-propanol were studied at the B3LYP and MP2 levels of theory using 6-311G(d,p) and 6-311++G(d,p) basis functions. The thermodynamic parameters of the complex formations were calculated in order to estimate the exact stability of the supersystems. It was proven that the calculated energy barriers of the alcohol-assisted proton transfers are about 60% lower than those of the intramolecular proton transfers in adenine found earlier (Gu and Leszczynski in J Phys Chem A 103:2744-2750, 1999).

Theoretical study of the intermolecular H-bonding and intermolecular proton transfer between isocytosine tautomeric forms and R,S-lactic acid.

Eight H-bonded complexes between isocytosine (isoC) tautomeric forms and R/S-lactic acid (LA) have been studied at the B3LYP and HF levels of theory using 6-31+G(d) basis set. The energy barriers of the intermolecular proton transfers were also estimated as the results showed that they are several times lower than those of the intramolecular proton transfers of isoC in the gas phase. Furthermore, the energy barriers of the tautomerizations in which the carboxylic H-atom takes part are several times lower than those in which the LA OH group assists the proton transfer.

DFT study of the gas phase proton transfer in guanine assisted by water, methanol, and hydrogen peroxide.

A computational study of hydrogen-bonded complexes between the oxo-/hydroxo-amino N7/9H tautomers of guanine and water, methanol, and hydrogen peroxide has been performed at the B3LYP/6-31 + G(d) level of theory. The mechanisms of the water-, methanol-, and hydrogen peroxide-assisted proton transfers in guanine were studied and compared with the intramolecular proton transfer in guanine in the gas phase. It was found that the assisted proton transfers pass through about three times lower energy barriers than those found for isolated guanine tautomers.

Theoretical study of the hydrogen-bonded complexes serotonin-water/hydrogen peroxide.

Eight H-bonded complexes between serotonin (5-hydroxy-tryptamine) and water/hydrogen peroxide were studied at the B3LYP and HF levels of theory, using the 6-31+G(d) basis set. A thermodynamic analysis was performed in order to find the most stable complex. The calculated bonding parameters showed that the most stable H-bonded complex is formed between serotonin and hydrogen peroxide by means of the intermolecular H-bond -H2N...H-OOH. Theoretical study of the hydrogen-bonded supersystems serotonin-water/hydrogen peroxide. [Figure: see text].

H-bonded complexes between acetylacetone and two molecules of methanol: HF and DFT level study.

Five stable H-bonded complexes (supersystems) between acetylacetone and two methanol molecules were investigated at the B3LYP and HF levels of theory using the 6-311G** and 6-11++G** basis sets. The most stable complex was found as the one with the highest relative bonding and interaction energies. All vibrational frequencies resulting from calculations with the 6-311++G** basis set were compared with the recorded IR spectrum of acetylacetone/methanol mixture in a molar ratio 1:2.