Neutral Molecular Form Research Articles

Mesoporous cerium oxide-anchored magnetic polyhedrons derived from MIL-100(Fe) for enhanced removal of arsenite from aqueous solution

Efficient elimination of As(III) from drinking water and wastewater has been a challenge because of its neutral molecular form. To address this problem, a novel nanocomposite, mesoporous cerium oxide-anchored magnetic polyhedrons derived from MIL-100(Fe) was fabricated via a strategy combining impregnation and calcination. The resultant products (denoted as Fe2O3/CeO2-t) exhibited a unique octahedral nanostructure decorated by mesoporous cerium oxide. Surface modification of CeO2 enhanced As(III) removal in comparison to unmodified Fe2O3. Particularly, Fe2O3/CeO2-4 h can reduce As(III) concentration from 180 to 10 µg/L within 20 min, which was almost 9 times faster than unmodified Fe2O3. The adsorption behavior conformed to the pseudo-second-order kinetic model (R2 = 0.9908) and the Freundlich isotherm model (R2 = 0.9943). The maximum adsorption capacity of As(III) by Fe2O3/CeO2-4 h was 68.25 mg/g, higher than those reported for similar adsorbents. Its enhanced removal mechanism can be attributed mainly to the mesoporous characteristics and oxidization ability of surface ceria. The composite can be separated from water by external magnets and easily regenerated. This study may offer a clue to the design of metal-organic framework-based composites as an alternative adsorbent for arsenite cleanup.

Combined Theoretical and Experimental Study of the Photophysics of Asulam

The photophysics of the neutral molecular form of the herbicide asulam has been described in a joint experimental and theoretical, at the CASPT2 level, study. The unique π → π* aromatic electronic transition (f, ca. 0.5) shows a weak red-shift as the polarity of the solvent is increased, whereas the fluorescence band undergoes larger red-shifts. Solvatochromic data point to higher dipole moment in the excited state than in the ground state (μ(g) < μ(e)). The observed increase in pKa in the excited state (pKa* - pKa, ca. 3) is consistent with the results of the Kamlet-Abboud-Taft and Catalán et al. multiparametric approaches. Fluorescence quantum yield varies with the solvent, higher in water (ϕ(f) = 0.16) and lower in methanol and 1-propanol (approx. 0.02). Room temperature fluorescence lifetime in aqueous solution is (1.0 ± 0.2) ns, whereas the phosphorescence lifetime in glassy EtOH at 77 K and the corresponding quantum yield are (1.1 ± 0.1) s and 0.36, respectively. The lack of mirror image symmetry between modified absorption and fluorescence spectra reflects different nuclear configurations in the absorbing and emitting states. The low value measured for the fluorescence quantum yield is justified by an efficient nonradiative decay channel, related with the presence of an easily accessible conical intersection between the initially populated singlet bright (1)(L(a) ππ*) state and the ground state (gs/ππ*)(CI). Along the main decay path of the (1)(L(a) ππ*) state the system undergoes an internal conversion process that switches part of the population from the bright (1)(L(a) ππ*) to the dark (1)(L(b) ππ*) state, which is responsible for the fluorescence. Additionally, singlet-triplet crossing regions have been found, a fact that can explain the phosphorescent emission detected. An intersystem crossing region between the phosphorescent state (3)(L(a) ππ*) and the ground state has been characterized, which contributes to the nonradiative deactivation of the excitation energy.

Fluorescent Properties of Hymecromone and Fluorimetric Analysis of Hymecromone in Compound Dantong Capsule

Fluorescence spectra of hymecromone (4MU) aqueous solutions are investigated at different pHs. Two fluorescent species of 4MU, neutral molecular form and anion form, are considered to be the main fluorescent forms. Quantum yields of the two forms are measured to be 0.74 at pH 5.98 and 0.95 at pH 9.75, respectively. The ionization constant of 7-hydroxyl proton of 4MU is determined to be by a pH-fluorescence method. Addition of methanol into 4MU aqueous solution leads to a blue shift of maximum emission wavelength from 445 nm to 380 nm, and a decrease in fluorescence intensity. 3D fluorescence spectra of Chinese patent drug Compound Dantong Capsule (CDC) and its four component herbal drugs are also investigated. Based on their fluorescent properties, a novel fluorimetric method is proposed for the selective determination of 4MU in CDC without preseparation. The new method is suitable for the routine quality evaluation of CDC.

FTIR, FT-Raman, SERS and DFT study on melamine

A joint experimental and theoretical study performed on neutral and protonated molecular forms of melamine is presented. FTIR, FT-Raman and surface-enhanced Raman scattering (SERS) in conjunction with density functional theory (DFT) calculations, were used for the characterization of the structural, vibrational and electronic properties of the neutral molecular form of melamine. The protonated molecular form was evidenced by SERS spectroscopy, and also, by the calculated Raman spectrum of the protonated endocyclic nitrogen of melamine molecule. The adsorption geometry of melamine on the silver surface was deduced based on the molecular electrostatic potential (MEP) map and SERS selection rules.

Two types of monoethyl alpha-anilinobenzylphosphonates: a zwitterion and a molecular compound.

The crystal structures of the potential antitumour agents monoethyl (alpha-anilinobenzyl)phosphonate, C(15)H(18)NO(3)P, (I), and its 4-azobenzene-substituted derivative monoethyl [alpha-[4-(phenyldiazenyl)anilino]benzyl]phosphonate, C(21)H(22)N(3)O(3)P, (II), are described. A zwitterionic form of (I) and a neutral molecular form of (II) are observed, which is fully in accordance with previously reported spectroscopic studies. In both structures, hydrogen bonding induces the formation of zigzag head-to-head double layers parallel to the crystallographic b axis.

Study of the reactivity of tris(pyrazolyl)amine and bis(pyrazolyl)amine ligands toward Rh(I). Crystal structure of [Rh 3Cl 3(cod) 3tdma] · CH 3CN ( tdma=tris[(3,5-dimethyl-1-pyrazolyl)methyl]amine), a C 3-symmetric compound

Rh(I) compounds [Rh 2Cl 2(cod) 2 L] ( L=tris[(1-pyrazolyl)methyl]amine ( tpma) ( 1), tris[2-(1-pyrazolyl)ethyl]amine ( tpea) ( 2), tris[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine ( tdea) ( 3), bis[2-(1-pyrazolyl)ethyl]amine ( bpea) ( 4) and bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine ( bdea) ( 5)), and [Rh 3Cl 3(cod) 3 tdma] ( tdma=tris[3,5-dimethyl-1-pyrazolyl)methyl]amine ( 6), have been prepared, and characterised by elemental analyses, conductivity, IR, 1H and 13C NMR spectroscopy and liquid mass spectrometry (with electrospray and APCI interfaces). The 1H NMR spectra and molar conductance of [Rh 2Cl 2(cod) 2 L] complexes suggested the presence of 1:1 electrolyte species [Rh(cod) L] + [RhCl 2(cod)] − in solution. A combined electrospray and APCI liquid mass spectrometry study confirmed the presence of both [Rh(cod) L] + and [RhCl 2(cod)] − ions in solution, but the neutral molecular form of complexes in solution was not found. The crystal structure of complex [Rh 3Cl 3(cod) 3 tdma] was resolved by X-ray diffraction. The molecular structure is unusual and consists of a ligand tdma coordinated to three “RhCl(cod)” units with a C 3-symmetry.

Organometallic rhodium (I) complexes with 1-alkylaminopyrazole ligands

New bidentate NN′ and tridentate NN′ N 1-alkylaminopyrazoles were synthesized and characterized by elemental analyses and spectroscopic methods. The reaction of [RhCl(cod) 2] (cod=cycloocta-1,5-diene) with one equivalent of L 1-alkylaminopyrazoles afforded Rh 2Cl 2(L)(cod) 2 complexes (L= NN′ and NN′ N). These rhodium (I) compounds were studied by IR, 1H- and 13C-NMR and liquid mass (with electrospray and APCI interfaces) spectrometries. The 1H-NMR spectra and molar conductances of these complexes suggested the presence of 1:1 electrolyte species, [Rh(L)cod] + [RhCl 2(cod)] −, in solution. A combined electrospray and APCI liquid mass spectroscopy study confirmed the presence of both [Rh(L)cod] + and [RhCl 2(cod)] − species in solution but the existence of a neutral molecular form of complexes in solution could not be demonstrated.

Determination of pK a values from solubility data

At constant ionic strength the pH-dependent solubility of an ionogenic compound can be calculated by means of a method described by Krebs and Speakman (Krebs, Speakman, 1945) if the pK a value of the substance and the solubility of the neutral molecular form are known. In the following we derive a method which allows for the calculation of both the pK a value and the solubility of the neutral molecular form of a substance from its solubility as a function of the pH of the medium.

The interconversion of the zwitterion and the uncharged form of γ-aminobutyric acid (GABA) at low temperatures

The infrared spectra of γ-aminobutyric acid (GABA) in the solid state and inert gas matrices at several temperatures and concentrations have been analyzed. Evidence of a neutral molecular form of GABA is shown for the first time; this compound was known until now only in its zwitterionic or cationic forms. The intermolecular mechanism is also established for the formation of the GABA zwitterion. This final step is shown to be preceeded by hydrogen-bonded intermediates.

Proposed partition mechanism of tetracycline.

The partition mechanism of tetracycline in an n-octanol/H2O system was studied by mathematical treatment of the changes in the apparent partition coefficient determined by Colaizzi and Klink (J. Pharm. Sci., 58, 1184 (1969)) and by us, and by measuring the spectral properties of tetracycline in organic solvents. Tetracycline exists in a neutral molecular form in n-octanol, so it could be concluded that transfer of tetracycline from an aqueous to an organic phase is governed not by the amount of the zwitterionic form, but by the amount of the neutral molecular form in the aqueous phase. Results showed that the tetracycline molecule itself is hydrophobic, but that the concentration of the neutral molecular species in the aqueous phase is very small. The relationship between the apparent partition coefficient and the three macroscopic ionization constants of tetracycline at a certain pH was also clarified. From this the macroscopic ionization constants were evaluated. The values agreed very well with those determined by potentiometric titration.

Note on stability of p‐nitrophenol in aqueous solutions

AbstractIt has been shown that p‐nitrophenol (PNP) solution is stable only at pH ≥ 9 when PNP is in the completely ionised form. In benzene, PNP exists in a neutral molecular form. These considerations are important in the estimation of surface areas of solids by adsorption of PNP from solution.