Hydrogen Atom Of OH Group Research Articles

Complex Weblike Hydrogen Bonding in Large “Drain Pipe” Channels of Wightmanite Revealed by New X-Ray and Spectroscopic Measurements

ABSTRACT The wallpaper-type crystal structure of wightmanite, Mg5(BO3)O(OH)5·1–2H2O, has been reanalyzed in order to better understand the position and bonding of hydrogen atoms. Single-crystal structure refinement yielded the monoclinic I2/m unit cell a = 13.5165(18), b = 3.0981(3), c = 18.170(3)Å, ß = 91.441(6)°, and V = 760.65(17)Å3, Z = 4. Hydrogen atoms of OH groups pointing to the inside of the elliptical channels oriented parallel to [010] are arranged in the form of two overlying, a–c parallel planar pentagons. The two pentagons point in opposite directions. Hydrogen-bond analysis shows that the hydroxyl groups are linked by complex polyfurcated, intra-molecular hydrogen bonds forming a web-like network coating the walls of the channels. The longest distance between hydrogens (7.226 Å) is observed in the pentagonal planes of the channel. The anisotropically refined oxygen atoms of the zeolitic water show their strongest vibration parallel to the b axis and in the direction of the largest diameter of the elliptical channel and similarly form a complex inter-molecular hydrogen-bond system to the hydroxyl groups coating the channel walls. This complex bonding is expressed in the Raman spectrum by a broad band between 3100 and 3300 cm–1 that is assigned to the OH / H2O stretching mode and one strong band at 3661 cm–1 attributable to an OH-stretching mode. Infrared spectra also show a pronounced broad band between 3200 and 3700 cm–1 attributed to H2O and OH-stretching modes. The weak bands around 1600 cm–1 observed in the Raman and IR spectra are probably due to relatively weakly bound water in the channels.

Deuterium kinetic isotope effect (DKIE) in copper-induced LDL peroxidation: Interrelated effects of on inhibition and propagation

LDL peroxidation plays a major role in many physiological and pathophysiological processes. The mechanisms of LDL peroxidation induced by transition metal ions have therefore been studied intensively. It has been proposed that the mechanism involves free radical production that occurs via decomposition of hydroperoxides. This, in turn, requires the cleavage of O–H bonds. Cleavage of O–D bond is slower and comparison of the kinetics in D2O to the kinetics in H2O is therefore a straightforward way to test this aspect of the alleged mechanism. The kinetics of peroxidation exhibit marked deuterium kinetic isotope effects at all the stages of oxidation under all the studied conditions. We found that the rate of propagation of copper-induced peroxidation is a monotonically decreasing function of D2O fraction in D2O/H2O mixtures. The only elementary reaction that involves “exchangeable” hydrogen at this stage is copper-induced decomposition of conjugated hydroperoxides. Therefore, we conclude that the latter step is rate-limiting reaction including cleavage of oxygen-hydrogen bond of hydroperoxide. The lag preceding rapid peroxidation exhibits a biphasic dependence on the fraction of D2O. This may be understood on the basis of the effect of substituting hydrogen atoms by deuterium. Specifically, such substitution is expected to decrease both the rate of initiation of peroxidation and the potency of the antioxidant. We interpret our results in terms of the effects of isotopic substitution on the rates of the reactions that involve the abstraction of “exchangeable” hydrogen atoms of OH groups in tocopherol and hydroperoxides.

Effect of mono- and di-hydration on the stability and tautomerisms of different tautomers of creatinine: A thermodynamic and mechanistic study

Relative stabilities of nine tautomers of creatinine in gas and aqueous phases and hydrated forms were studied by M062X, MP2 and B3LYP computational methods. It was found that two tautomers (an amino and an imino) have significant stability and relative abundances among all. The imino tautomer is more stable in gas phase while the amino form is more stable in aqueous phase and in di-hydrated form. The stability and relative abundances of the both amino and imino tautomers in mono-hydrated form were approximately the same. Activation energies and Gibbs free energies of isomerizations in creatinine, including intramolecular proton transfers and internal OH rotations, were calculated. The energy barriers of rotations of hydrogen atom of OH group around C–O bond were smaller than 40kJ/mol. Two types of proton transfer reactions including imine↔amine and keto↔enol tautomerisms were considered. The activation energies of keto–enol tautomerisms, in which the proton is transferred between oxygen and carbon atoms, were in the range 270–300kJ/mol. Catalysis effect of one and two water molecules on the intramolecular proton transfers was studied. It was found that the activation energies are decreased by about 130 and 150kJ/mol in the presence of one and two water molecules, respectively.

Structural features of two novel alluaudite-like arsenates Cd1.16Zn2.34(AsO4)1.5(HAsO4)(H2AsO4)0.5 and Cd0.74Mg2.76(AsO4)1.5(HAsO4)(H2AsO4)0.5

Two new compounds, Cd1.16Zn2.34(AsO4)1.5(HAsO4)(H2AsO4)0.5 (1) and Cd0.74Mg2.76(AsO4)1.5(HAsO4)(H2AsO4)0.5 (2), have been prepared hydrothermally. Their crystal structures consist of chains of edge-sharing M1O4(OH0.5)2, M1aO4(OH0.5)2, M2O5(OH0.5), and M2aO5(OH0.5) octahedra (M1, M1a=Zn, Cd; M2, M2a=Zn for 1, and M1, M1a=Mg, Cd; M2, M2a=Mg for 2) that are stacked parallel to (101) and are connected by the [(AsO4)0.5(AsO3(OH))0.5]2.5− and [(AsO4)0.5(AsO2(OH)2)0.5]2− tetrahedra. These chains produce two types of channels parallel to the c-axis. Cd atoms are located in channels 2, while in channels 1 are situated hydrogen atoms of OH groups. The infrared spectra clearly show the presence of broad OH stretching and bending vibrations centred at 3236, 2392 1575 and 1396cm−1 in (1), and 3210, 2379 1602 and 1310cm−1 in (2). The OH stretching frequency is in good agreement with O⋯O distances. Furthermore, structural characteristics of compounds with similar alluaudite-like structures were discussed.

Balance, uniformity, and asymmetry of the structure of volborthite Cu3(OH)2(V2O7)·2H2O

Four structural models of volborthite Cu3(OH)2(V2O7)·2H2O (a = 10.646(2) A, b = 5.867(1) A, c = 14.432(2) A, β = 95.19(1)°, V = 897.7(5) A3, Z = 4, R/R w = 0.038/0.046) calculated in the space groups determined from the systematic absences are compared. Based on the structure balance and the similarity of constituting polyhedra, values of the R factor, and isotropic thermal parameters, the space group Ia is found to be preferable, which is the only possible asymmetric and uniform variant. Hydrogen atoms of OH-groups, oxygen atoms and, partially, hydrogen atoms of water are localized.

The role of environment in the nonradiative relaxation of the triplet state of naphthalene derivatives in solid solutions at 77 K

The nonradiative relaxation of the triplet states of oxy-and amino-derivatives of naphthalene and conjugated ions is studied by luminescent and kinetic methods (measurements of the phosphorescence decay and of phosphorescence excitation and phosphorescence spectra) in solid solutions of ethanol-h 6, ethanol-d 6, and mixtures of toluene-h 8 with piperidine-h 11 at 77 K. It is found that, along with intramolecular factors, a microsolvate surrounding a molecule or an ion substantially affects the nonradiative relaxation. The contribution of this factor in ethanol increases in the series cation-polar molecule-anion and in mixtures of toluene with piperidine—with increasing piperidine concentration. The results are interpreted assuming the inductive-resonance dipole-dipole transfer of the triplet energy to the dipole acceptors of intramolecular bonds and bonds with molecules of the environment. The relative arrangement of hydrogen atoms of OH groups of ethanol molecules in microsolvates of cations and anions estimated using the inductive-resonance model agrees with the difference in the structure of solvates of oppositely charged ions, which is caused by the electrostatic charge-dipole interaction and the distribution of the electron density in the ground state of the corresponding emitting center. The inductive-resonance model was used for studying the features of solvation processes involving polar molecules. It is shown that the difference between the structures of microsolvates of 2-oxy-naphthalene molecules in solvents with close dipole moments (ethanol and piperidine) is mainly caused by the different ability of these solvent molecules to form associates. The structure of microsolvates of oxy-derivatives of naphthalene in the associated amphoteric solvent is found to depend on the number and position of substituents. In oxy-derivatives of naphthalene with spatially separated OH groups in ethano-d 6, deuteroexchange occurs in both substituents, whereas in the naphthalene derivative with adjacent OH groups this occurs only in one of the groups. Comparison of the phosphorescence spectra of hydrogen-bond complexes and proton-transfer complexes in nonpolar solvents at 77 K revealed the existence of molecular naphthol entities in the triplet state that were formed from ionized entities in the singlet state.