Neutral Water Molecules Research Articles

A redox switch allows binding of Fe(II) and Fe(III) ions in the cyanobacterial iron-binding protein FutA from Prochlorococcus

The marine cyanobacterium Prochlorococcus is a main contributor to global photosynthesis, whilst being limited by iron availability. Cyanobacterial genomes generally encode two different types of FutA iron-binding proteins: periplasmic FutA2 ABC transporter subunits bind Fe(III), while cytosolic FutA1 binds Fe(II). Owing to their small size and their economized genome Prochlorococcus ecotypes typically possess a single futA gene. How the encoded FutA protein might bind different Fe oxidation states was previously unknown. Here, we use structural biology techniques at room temperature to probe the dynamic behavior of FutA. Neutron diffraction confirmed four negatively charged tyrosinates, that together with a neutral water molecule coordinate iron in trigonal bipyramidal geometry. Positioning of the positively charged Arg103 side chain in the second coordination shell yields an overall charge-neutral Fe(III) binding state in structures determined by neutron diffraction and serial femtosecond crystallography. Conventional rotation X-ray crystallography using a home source revealed X-ray-induced photoreduction of the iron center with observation of the Fe(II) binding state; here, an additional positioning of the Arg203 side chain in the second coordination shell maintained an overall charge neutral Fe(II) binding site. Dose series using serial synchrotron crystallography and an XFEL X-ray pump-probe approach capture the transition between Fe(III) and Fe(II) states, revealing how Arg203 operates as a switch to accommodate the different iron oxidation states. This switching ability of the Prochlorococcus FutA protein may reflect ecological adaptation by genome streamlining and loss of specialized FutA proteins.

Hydrogen-bonding interactions in 5-fluorocytosine-urea (2/1), 5-fluorocytosine-5-fluorocytosinium 3,5-dinitrosalicylate-water (2/1/1) and 2-amino-4-chloro-6-methylpyrimidine-6-chloronicotinic acid (1/1).

Three new compounds, namely, 5-fluorocytosine-urea (2/1), 2C4H4FN3O·CH4N2O, (I), 5-fluorocytosine-5-fluorocytosinium 3,5-dinitrosalicylate-water (2/1/1), 2C4H4FN3O·C4H5FN3O+·C7H2N2O7-·H2O, (II), and 2-amino-4-chloro-6-methylpyrimidine-6-chloronicotinic acid (1/1), C6H4ClNO2·C5H6ClN3, (III), have been synthesized and characterized by single-crystal X-ray diffraction. In compound (I), 5-fluorocytosine (5FC) molecules A and B form two different homosynthons [R22(8) ring motif], one formed via N-H...O hydrogen bonds and the second via N-H...N hydrogen bonds. In addition to this interaction, a sequence of fused-ring motifs [R21(6), R33(8), R22(8), R43(10) and R22(8)] are formed, generating a supramolecular ladder-like hydrogen-bonded pattern. In compound (II), 5FC and 5-fluorocytosinium are linked by triple hydrogen bonds, generating two fused-ring motifs [R22(8)]. The neutral 5FC and protonated 5-fluorocytosinum cation form a dimeric synthon [R22(8) ring motif] via N-H...O and N-H...N hydrogen bonds. On either side of the dimeric synthon, the neutral 5FC, 5-fluorocytosinium cation, 3,5-dinitrosalicylate anion and water molecule are hydrogen bonded through N-H...O, N-H...N, N-H...OW and OW-HW...O hydrogen bonds, forming a large ring motif [R1010(56)], leading to a three-dimensional supramolecular network. In compound (III), 2-amino-4-chloro-6-methylpyrimidine (ACP) interacts with the carboxylic acid group of 6-chloronicotinic acid via N-H...O and O-H...O hydrogen bonds, generating an R22(8) primary ring motif. Furthermore, the ACP molecules form a base pair via N-H...N hydrogen bonds. The primary motif and base pair combine to form tetrameric units, which are further connected by Cl...Cl interactions. In addition to this hydrogen-bonding interaction, compounds (I) and (III) are further enriched by π-π stacking interactions.

Ising-like Magnetism in Quasi-Two-Dimensional Co(NO3)2·2H2O.

The appearance of electrically neutral water molecules in the structure of cobalt dinitrate dihydrate, Co(NO3)2⋅2H2O, drastically changes its magnetic properties as compared to its waterless counterpart, Co(NO3)2. The title compound shows Ising-like behavior reflected in its thermodynamic properties. It experiences long-range antiferromagnetic order at TN = 20.5 K and metamagnetic transition at µ0HC = 0.76 T. First-principles calculations produce the values of leading exchange interactions J1 ~ 10 K and J2 ~ 0.5 K and single-ion anisotropy D ~ 1 K which allows us to consider Co(NO3)2⋅2H2O as a quasi-two-dimensional magnetic system.

Mechanism for the Efficient Homogeneous Nucleation of Ice in a Weakly Ionized, Ultracold Plasma

It is proposed that the rapid observed homogeneous nucleation of ice dust in a cold, weakly ionized plasma depends on the formation of hydroxide (OH−) by fast electrons impacting water molecules. These OH− ions attract neutral water molecules because of the high dipole moment of the water molecules and so hydrates of the form (OH)−(H2O) n are formed. The hydrates continuously grow in the cold environment to become macroscopic ice grains. These ice grains are negatively charged as a result of electron impact and so continue to attract water molecules. Because hydroxide is a negative ion, unlike positive ions, it does not suffer recombination loss from collision with plasma electrons. Recombination with positive ions is minimal because positive ions are few in number (weak ionization) and slow-moving as result of being in thermal equilibrium with the cold background gas.

Ионообменные свойства твердых растворов на основе гидратированных форм антимонат-вольфраматов одновалентных металлов

The research of tungsten-antimony crystalline acid (TACA) structural transformations in the condition of ion-exchange and thermolysis of its substituted M+, H+-forms (M+ – Li, Na, K, Ag) were conducted. The data of thermogravimetric and qualitative X-ray phase analyses made it possible to conclude that the thermolysis of TACA and its derivatives proceeds in a wide temperature range from 300 to 1150 K being accompanied by the removal of crystalline water molecules with the formation of phases mixture containing complex antimony oxides of the ( -,  - Sb2O4) modification and WO3. It was shown that compounds based on hydrated forms of monovalent metal antimonates-tungstates are stable up to 1023 K with a pyrochlore-type structure. For pyrochlore-like phases, a monotonic dependence of unit cell parameter a on ion-exchange degree α and the ionic radius of metals r was revealed indicating the formation of solid solutions Мх(Н3О)1–хWSbO6 nН2О (M+ – Li, Na, K, Ag; 0.0≤х<1.0; 0.0≤n<2.0) with a limited range of solubility from the crystal chemistry point of view. Within the framework of the Fd-3m space group, based on the data of X-ray diffraction analysis (Rietveld method), the structural characteristics of TACA and its substituted M+, H+-forms were refined, and a model for populating the corresponding metal ions by crystallographic positions of the pyrochlore-type structure was proposed. Using a complex of physicochemical methods (thermogravimetric, X-ray diffraction analyses and IR spectroscopy), a correlation between the composition of the obtained compounds, structural disorder, and ion-exchange properties were determined. According to the data of thermogravimetry and IR spectroscopy, it follows that the degree of compounds hydration analyzed depends on the nature of the alkaline ion. This allows to conclude that lithium and sodium ions are located in 16d– positions, dragging neutral water molecules into the structure occupying 8b-positions. In this case, potassium ions can partially occupy both 16d- and 8b-positions of the structure.

The Interplay Between Lead Vacancy and Water Rationalizes the Puzzle of Charge Carrier Lifetimes in CH3NH3PbI3: Time‐Domain Ab Initio Analysis

AbstractThe perovskite CH3NH3PbI3 excited‐state lifetimes exhibit conflicting experimental results under humid environments. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that the interplay between lead vacancy and water can rationalize the puzzle. The lead vacancy reduces NA coupling by localizing holes, slowing electron–hole recombination. By creating a deep electron trap state, the coexistence of a neutral lead vacancy and water molecules enhances NA coupling, accelerating charge recombination by a factor of over 3. By eliminating the mid‐gap state by accepting two photoexcited electrons, the negatively charged lead vacancy interacting with water molecules increases the carrier lifetime over 2 times longer than in the pristine system. The simulations rationalize the positive and negative effects of water on the solar cell performance exposure to humidity.

Size-dependent proton localization in hydrated uracil clusters: A joint experimental and theoretical study.

A collision-induced dissociation study of hydrated protonated uracil (H2O)n=1-15UH+ clusters is reported. The mass-selected clusters collide with water molecules and rare gases at a controlled center of mass collision energy. From these measurements, absolute fragmentation cross sections and branching ratios are extracted as a function of the uracil hydration. For small clusters, up to n = 4, we observe that only neutral water molecules are evaporated upon collisions, whereas, for larger clusters, neutral uracil is also evaporated: this transition in the nature of the evaporation products is interpreted considering the lowest-energy isomers of each species that are obtained from a combination of density-functional based tight-binding and MP2 calculations. The simulations show that in (H2O)1-4UH+ the proton is located on the uracil molecule or on a water molecule strongly bound to uracil whereas, in larger clusters, the proton is bound to water molecules far from uracil. This correlation between the structure of the low-energy isomers and the experimental fragmentation channel suggests that dissociation may occur in a very short time after collisions so that energy has not enough time to be redistributed among all degrees of freedom and the ground-state geometry of the parent cluster partly determines the nature of the favored fragmentation channels. Of course, thermal dissociations originating from long lived, thus thermalized, collision complexes cannot be ruled out but they are not expected to play the major role since the experimental results can be satisfactorily accounted for by assuming that the fragmentation processes are mainly impulsive.

Catalytic Mechanism of Amyloid-β Peptide Degradation by Insulin Degrading Enzyme: Insights from Quantum Mechanics and Molecular Mechanics Style Møller-Plesset Second Order Perturbation Theory Calculation.

Insulin degrading enzyme (IDE), a metalloprotease that degrades amyloid-β (Aβ) peptides and insulin, is associated with Alzheimer's disease and diabetes. The mechanism of IDE catalyzed degrading of Aβ peptides, which is of fundamental importance in the design of therapeutic methods for Alzheimer's disease, has not been fully understood. In this work, combined quantum mechanics and molecular mechanics (QM/MM) style Møller-Plesset second order perturbation theory (MP2) geometry optimization calculations are performed to investigate the catalytic mechanism of the Aβ40 Phe19-Phe20 peptide bond cleavage by human IDE. The analyses using QM/MM MP2 optimization suggest that a neutral water molecule is at the active site of the enzyme-substrate (ES) complex. The water molecule is in hydrogen bonding with the nearby anionic Glu111 of IDE but not directly bound to the catalytic Zn ion. This is confirmed by QM/MM DFTB3 molecular dynamics simulation. Our studies also reveal that the hydrolysis of the Aβ40 Phe19-Phe20 peptide bond by IDE consists of four key steps. The neutral water is first activated by moving toward and binding to the Zn ion. A gem-diol intermediate is then formed by the activated neutral water molecule attacking the C atom of the Phe19-Phe20 peptide bond. The next is the protonation of the N atom of Phe19-Phe20 peptide bond to form an intermediate with an elongated C-N bond. The final step is the breaking of the Phe19-Phe20 C-N bond. The final step is the rate-determining step with a calculated Gibbs free energy of activation of 17.34 kcal/mol, in good agreement with the experimental value 16.7 kcal/mol. This mechanism provides the basis for the design of biochemical methods to modulate the activity of IDE in humans.

Effect of vibrational pre-excitation on sub-femtosecond structural evolution of water cation in 2A1 state

We study the effects of vibrational excitation of neutral water molecule on sub-femtosecond nuclear dynamics of the 2A1 electronic state of cationic water and its isotopomers that are induced by high-harmonic generation (HHG) process. Both the photoelectron spectra and the autocorrelation functions of electronically excited states of M2O+ (M = H, D, and T) that are produced by Franck-Condon ionization of vibrationally pre-excited M2O to its 2A1 electronic state are calculated. HHG signals are also calculated from the square of the absolute value of autocorrelation functions. In addition, the ratio of the HHG signal of D2O+ to that of H2O+ and that between T2O+ and H2O+ are calculated with respect to time, for varying initially prepared vibrational state of the corresponding neutral molecule. Vibrational dynamics are notably strong on the 2A1 state of the cationic species, as the initial vibrational quantum number increases. The HHG signal of a heavier isotopomer is larger than that of water when the initial vibrational state is on its ground state. The expectation values of the bond length and bond angle show quasiperiodic oscillations with respect to time, which are ascribed to the observed overall rise in the HHG signals. Exceptionally strong vibrational dynamics observed on the 2A1 state potential energy surface of the cationic water molecule can be attributed to the excitation of water bending mode upon Franck-Condon ionization of the corresponding neutral molecule. Here, the observed peaks in the ratios of the HHG signals are theoretically explained in terms of time-evolving molecular structures at two different turning points of the 2A1 surface. We anticipate that the present computational method of solving time-dependent Schrödinger equation would be of use to provide explanations on both ultrafast and chiroptical HHG spectroscopy of polyatomic molecules with certain handedness.

Synthesis, crystal structure, vibrational spectra, optical and DFT study of N-(3-ammonium propyl)-1, 3 diaminopropane tris iodate dehydrate [(C6H20N3) (IO3)3·2H2O

The new inorganic-organic iodate hybrid compound N-(3-ammonium propyl)-1, 3 diaminopropane tris iodate dehydrate [(C6H20N3) (IO3)3·2H2O] denoted as BATID was synthesized and characterized by single X-ray diffraction. This iodate salt crystallizes in the orthorhombic system with acentric space group Aba2. The crystal structure consists of discrete ionic entities of (C6H20N3)3+cations, (IO3)- iodate anions and neutral water molecules linked via simple and bifurcated NH ⋯OI and O(W)H⋯OI hydrogen bonds. TG analysis agrees well with the XRD data showing that the crystal structure of BATID is hydrated. DSC measurement shows that BATID compound exhibits a single phase transition around 361 K. DFT calculation were performed to optimize the molecular geometry and to calculate harmonic vibrational frequencies, electric dipole moments (μ), first Hyperpolarizability (βTot), atomic Mulliken charges and HOMO-LUMO properties using B3LYP method with (6-311 + G*, LanLD2Z (ECP)) mixed basis sets.The calculated structural parameters (bond lengths and angles) are in good agreement with the experimental XRD data. An optical analysis was also undertaken using reflectance diffuse UV–visible and fluorescence spectroscopy showing that BATID has wide transparent windows as well as two strong blue–violet fluorescence emissions at room temperature.

Synthesis, characterization, crystal structure, thermal analysis of a new co-crystal supramolecular dinuclear zinc (II) complex containing chelidamate ligand

A new co-crystal supramolecular dinuclear compound of Zn(II) namely (dapH)2(hypydcH)2·[Zn2(hypydc)2(H2O)6]·2H2O (1), where [hypydcH2 = 4-hydroxypyridine-2,6-dicarboxylic acid (chelidamic acid), dap = 3,4-diaminopyridine] has been synthesized and characterized by elemental analysis, spectral (IR, UV–Vis), thermal (TG/DTG) analysis and single crystal X-ray diffraction. The zinc (II) cation sits on a center of inversion. In the crystal structure of 1, chelidamate dianions have been coordinated to ZnII ion in a three dentate fashion through two oxygen and one nitrogen atoms from (hypydc)2– ligands. Furthermore, dicarboxylic groups link two zinc (II) centers to each other via oxygen atoms of carboxylic groups, also three water molecules coordinated to metal centers. Consequently, each Zn(II) atom is seven-coordinate and demonstrate approximately pentagonal-bipyramidal ZnNO6 coordination geometry. The metal fragments are linked via the central four-membered Zn2O2 ring. Surprisingly, in 1 a proton transfer ion pair of chelidamic acid and 3,4-diaminpyridine crystallized accompanied with neutral dinuclear complex of Zn(II). Interestingly, the structural analysis illustrates that anionic and cationic fragment accompanied with neutral dinuclear complex and uncoordinated water molecules assemble the crystal structure of 1 into a fascinating supramolecular architecture.

Gas Phase Synthesis of Protonated Glycine by Chemical Dynamics Simulations.

In the present work, we investigated the reaction dynamics that will possibly lead to the formation of protonated glycine by an ion-molecule collision. In particular, two analogous reactions were studied: NH3OH+ + CH3COOH and NH2OH2+ + CH3COOH that were suggested by previous experiments to be able to form protonated glycine loosing a neutral water molecule. Chemical dynamics simulations show that both reactants can form a molecule with the mass of the protonated glycine but with different structures, if some translational energy is given to the system. The reaction mechanisms for the most relevant product isomers are discussed as well as the role of collision energy in determining reaction products. Finally, in comparing collision dynamics at room and at very low initial internal temperature of the reactants, the same behavior was obtained for forming the protonated glycine isomers products. This supports the use of standard gas phase ion-chemistry setups to study collision-induced reactivity as a model for astrophysical cold conditions, when some relative translation energy is given to the system.

Sequential evaporation of water molecules from protonated water clusters: measurement of the velocity distributions of the evaporated molecules and statistical analysis.

Velocity distributions of neutral water molecules evaporated after collision induced dissociation of protonated water clusters H+(H2O)n≤10 were measured using the combined correlated ion and neutral fragment time-of-flight (COINTOF) and velocity map imaging (VMI) techniques. As observed previously, all measured velocity distributions exhibit two contributions, with a low velocity part identified by statistical molecular dynamics (SMD) simulations as events obeying the Maxwell-Boltzmann statistics and a high velocity contribution corresponding to non-ergodic events in which energy redistribution is incomplete. In contrast to earlier studies, where the evaporation of a single molecule was probed, the present study is concerned with events involving the evaporation of up to five water molecules. In particular, we discuss here in detail the cases of two and three evaporated molecules. Evaporation of several water molecules after CID can be interpreted in general as a sequential evaporation process. In addition to the SMD calculations, a Monte Carlo (MC) based simulation was developed allowing the reconstruction of the velocity distribution produced by the evaporation of m molecules from H+(H2O)n≤10 cluster ions using the measured velocity distributions for singly evaporated molecules as the input. The observed broadening of the low-velocity part of the distributions for the evaporation of two and three molecules as compared to the width for the evaporation of a single molecule results from the cumulative recoil velocity of the successive ion residues as well as the intrinsically broader distributions for decreasingly smaller parent clusters. Further MC simulations were carried out assuming that a certain proportion of non-ergodic events is responsible for the first evaporation in such a sequential evaporation series, thereby allowing to model the entire velocity distribution.

Solid-state supramolecular architectures formed by co-crystallization of melamine and 2-, 3- and 4-fluorophenylacetic acids

A family of supramolecular complexes of melamine with fluorophenylacetic acid isomers using solvent-assisted and evaporation-based techniques has been prepared. Crystallization of melamine with 2-fluorophenylacetic acid yield hydrated ionic supramolecular complex (1), whereas crystallization of melamine with 3- and 4-fluorophenylacetic acids leads to formation of neutral supramolecular complexes (2, 3), all with base to acid ratio of 1:2. The supramolecular assembly is driven by the noncovalent interactions, most commonly by the hydrogen bonds. The components of the crystal 1 interact via NH⋯O and OH⋯N hydrogen bonds with R22(8) and R32(10) graphs forming ionic supramolecular complex, whereas the components in the crystals 2 and 3 interact with a graph of R22(8) forming neutral supramolecular complexes. The singly protonated melamin-1-ium residues in 1 interact each other via a pair of NH⋯N hydrogen bonds forming one dimensional chains along [−110] that interact via NH⋯O with deprotonated and neutral 2-fluorophenylacetic acid units and water molecules forming ribbon. In 2 and 3 co-crystals the melamine interacts with 3- and 4-fluorophenylacetic acids via a pair of NH⋯O hydrogen bonds forming pseudo one-dimensional supramolecular chains along [010] direction. Hirshfeld surface and analysis of 2D fingerprint plots have been analysed both quantitatively and qualitatively interactions that governing the supramolecular organisation. The IR and Raman vibrational characterization of the supramolecular complexes 1–3 was supported by the spectra of their deuterated analogues.

Unraveling protein folding mechanism by analyzing the hierarchy of models with increasing level of detail.

Taking protein G with 56 residues for a case study, we investigate the mechanism of protein folding. In addition to its native structure possessing α-helix and β-sheet contents of 27% and 39%, respectively, we construct a number of misfolded decoys with a wide variety of α-helix and β-sheet contents. We then consider a hierarchy of 8 different models with increasing level of detail in terms of the number of entropic and energetic physical factors incorporated. The polyatomic structure is always taken into account, but the side chains are removed in half of the models. The solvent is formed by either neutral hard spheres or water molecules. Protein intramolecular hydrogen bonds (H-bonds) and protein-solvent H-bonds (the latter is present only in water) are accounted for or not, depending on the model considered. We then apply a physics-based free-energy function (FEF) corresponding to each model and investigate which structures are most stabilized. This special approach taken on a step-by-step basis enables us to clarify the role of each physical factor in contributing to the structural stability and separately elucidate its effect. Depending on the model employed, significantly different structures such as very compact configurations with no secondary structures and configurations of associated α-helices are optimally stabilized. The native structure can be identified as that with lowest FEF only when the most detailed model is employed. This result is significant for at least the two reasons: The most detailed model considered here is able to capture the fundamental aspects of protein folding notwithstanding its simplicity; and it is shown that the native structure is stabilized by a complex interplay of minimal multiple factors that must be all included in the description. In the absence of even a single of these factors, the protein is likely to be driven towards a different, more stable state.

Thermochemical Investigations of Hydrated Nickel Dication Complexes by Threshold Collision-Induced Dissociation and Theory.

The experimental bond energies of Ni2+(H2O)x complexes, where x = 4-11, are determined by threshold collision-induced dissociation using a guided ion beam tandem mass spectrometer with an electrospray ionization source. The electrospray ionization source produces a distribution of Ni2+(H2O)x complexes, where an in-source fragmentation technique is employed to access the x = 4-6 complexes and control the population of excited isomers. The kinetic energy-dependent cross sections are modeled to yield 0 K bond energies for sequential loss of neutral water molecules, which are converted to 298 K binding energies. Analysis of the primary and secondary water losses from the Ni2+(H2O)x reactant ion complexes, x = 4-11, provide accurate thermochemistry for the hydration energies of Ni2+ and yield the first experimental values for x = 4 and 5 binding energies. Speculative thermochemistry for excited isomers of the x = 4-6 complexes is also obtained. Quantum chemical calculations explore the relative energies of possible geometries. Theoretical bond energies for ground structures are used for direct comparison with experimental values. Our experimental results agree well with previously calculated and experimentally obtained binding enthalpies as well as with the more extensive quantum chemical calculations performed here.

Crystal structure, vibrational spectra, optical and DFT studies of bis (3-azaniumylpropyl) azanium pentachloroantimonate (III) chloride monohydrate (C6H20N3)SbCl5·Cl·H2O

The crystal structure of (C6H20N3)SbCl5·Cl·H2O is built up of [NH3(CH2)3NH2(CH2)3NH3]3+ cations, [SbCl5]2− anions, free Cl− anions and neutral water molecules connected together by NH⋯Cl, NH⋯O and OH⋯Cl hydrogen bonds. The optical band gap determined by diffuse reflection spectroscopy (DRS) is 3.78eV for a direct allowed transition. Optimized molecular geometry, atomic Mulliken charges, harmonic vibrational frequencies, HOMO–LUMO and related molecular properties of the (C6H20N3)SbCl5·Cl·H2O compound were calculated by Density functional theory (DFT) using B3LYP method with GenECP sets. The calculated structural parameters (bond lengths and angles) are in good agreement with the experimental XRD data. The vibrational unscaled wavenumbers were calculated and scaled by a proper scaling factor of 0.984. Acceptable consistency was observed between calculated and experimental results. The assignments of wavenumbers were made on the basis of potential energy distribution (PED) using Vibrational Energy Distribution Analysis (VEDA) software. The HOMO–LUMO study was extended to calculate various molecular parameters like ionization potential, electron affinity, global hardness, electro-chemical potential, electronegativity and global electrophilicity of the given molecule.

Coordination-Dependent Spectroscopic Signatures of Divalent Metal Ion Binding to Carboxylate Head Groups: H2- and He-Tagged Vibrational Spectra of M2+·RCO2¯ (M = Mg and Ca, R = -CD3, -CD2CD3) Complexes.

We explore the intramolecular distortions present in divalent metal ion-carboxylate ion pairs using vibrational spectroscopy of the cryogenically cooled, mass-selected species isolated in the gas phase. The spectral signatures of the C-O stretching modes are identified using the perdeutero isotopologues of the acetate and propionate anions to avoid congestion arising from the CH2 fundamentals. Both Ca2+ and Mg2+ are observed to bind in a symmetrical, so-called "bidentate" arrangement to the -CO2¯ group. The very strong deformations of the head groups displayed by the binary complexes dramatically relax when either neutral water molecules or counterions are attached to the Mg2+RCO2¯ cation. These results emphasize the critical role that local coordination plays when using the RCO2¯ bands to deduce the metal ion complexation motif in condensed media.

Wetting-induced layer contraction in illite and mica-family relatives

When dry illite is wetted, its layer structure contracts along the c-axis by up to 0.2Å. This behavior contrasts with that of smectite which shows interlayer swelling on wetting. Layer contraction has also been found to occur in glauconite and celadonite that are structurally more disordered than illite, as well as with phengite and artificially degraded hydromuscovite. In contrast, muscovite proper does not show contraction. The contraction is ascribed to the deprotonation of hydronium ions (H3O+→H2O+H+) occupying interlayer K+ positions. The hydronium ion is approximately 5% larger than the neutral water molecule. This difference in size is proportional to the magnitude of contraction. The change in molecular volume and interlayer separation of illite particles may explain the anomalous decrease in density after dehydration under pressure. This paper reports on wetting-induced contraction in illite and related layer silicates, using humidity-controlled X-ray diffraction.

Structural characterization, vibrational studies and optical properties of a new luminescent organic-inorganic material [C6H20N3]BiI6·H2O

The (C6H20N3)BiI6·H2O compound is a new crystal belonging to the large hybrid organic-inorganic compounds family, which has been synthesized by the solvent evaporation method and characterized by single crystal X-ray diffraction at room. It crystallizes in the zero-dimensional (0-D) triclinic P1¯ space group. The cell dimensions are: a = 8.572 (5) Å, b = 11.838 (5) Å, c = 12.707 (5) Å, α = 74.317 (5)°, β = 71.043 (5)° and γ = 71.419 (5)°. The crystal structure consists of a discrete ionic layer of bis(3-azaniumylpropyl)azanium cations, hexa-iodobismuthate anions and neutral water molecules linked via simple and bifurcated NH⋯I(OW) and O(W)H⋯I hydrogen bonds. The structure of the prepared compound has been investigated using the group theory and IR and Raman spectroscopic techniques. The detailed interpretation of the vibrational modes was carried out. The optical absorption measurements on a (C6H20N3)BiI6·H2O pellet have revealed two absorption bands at 2.27 and 2.78 eV. In the end, two room temperature photoluminescence emissions attributed to the excitonic transition in the [BiI6]3− anions, were observed in the red spectral range at energy values of 1.87 and 2.05 eV.