The configuration of the hydrogen atoms in HfV2Hx compounds at low temperatures: A NMR study

The ⁵¹V nuclear electric quadrupole interaction and the shape of the proton resonance line in the low temperature phase of the HfV₂H_x (0≤x≤4) compounds has been studied by NMR. The data yield information about the hydrogen arrangement in the low temperature phase. They confirm the results of recent neutron diffraction studies that for x=4 and ordered super lattice of the hydrogen atoms exists, with the hydrogen atoms occupying only tetrahedral interstitial sites formed by two Hf and two V atoms. Our results show that in the lower concentration compound the hydrogen atoms most probably also occupy the same type of interstices, but without crystallographic order.     

Proton mobility hysteresis in 1T-HxTaS2, A 1H-n.m.r. study on the strong interaction host lattice-intercalant

¹H-n.m.r. spectra on 1T-H_0.33TaS₂ indicate a superstructure arrangement of hydrogen in tetrahedral sites of the van der Waals gap. Temperature dependent measurements reveal the proton mobility to be hysteretic. Location as well as dynamic behaviour of hydrogen is discussed in terms of a strong correlation between electronic features of the host lattice and the intercalant position.     

Proton magnetic resonance studies of TiCuH

The TiCuH structure can be described as alternating layers of TiH₂ and Cu atoms and, thus, corresponds to a two-dimensional analog of metal hydrides with CaF₂- type structures. Measurements of the proton relaxation times for TiCuH yield an activation energy of 0.79 eV for hydrogen diffusion. This result strongly suggests the H-atoms in TiCuH jump directly between the tetrahedral sites rather than through the vacant octahedral sites as proposed for H-atom jumps in f.c.c. TiH_x. In addition, the Korringa constant and hydrogen diffusion rate for TiCuH are compared with similar parameters for TiH_x and several ternary titanium hydrides.     

Hydrogen dynamics in Ce2Fe17H5: inelastic and quasielastic neutron scattering studies

The vibrational spectrum of hydrogen and the parameters of H jump motion in the rhombohedral Th(2)Zn(17)-type compound Ce(2)Fe(17)H(5) have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ce(2)Fe(2) sites participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The H jump rate τ(-1) of this localized motion is found to change from 3.9 × 10(9) s(-1) at T = 140 K to 4.9 × 10(11) s(-1) at T = 350 K, and the temperature dependence of τ(-1) in the range 140-350 K is well described by the Arrhenius law with the activation energy of 103±3 meV. Our results suggest that the hydrogen jump rate in Th(2)Zn(17)-type compounds strongly increases with decreasing nearest-neighbor distance between the tetrahedral sites within the hexagons. Since each such hexagon in Ce(2)Fe(17)H(5) is populated by two hydrogen atoms, the jump motions of H atoms on the same hexagon should be correlated.     

Temperature and hydration dependence of proton MAS NMR spectra in MCM-41: Model based on motion induced chemical shift averaging

The proton MAS NMR spectra in MCM-41 at low hydration levels (less than hydration amounting to one water molecule per surface hydroxyl group) show complex proton resonance peak structures, with hydroxyl proton resonances seen in dry MCM-41 disappearing as water is introduced into the pores and new peaks appearing, representing water and hydrated silanol groups. Surface hydroxyl group–water molecule chemical exchange and chemical shift averaging brought about by a water molecule visiting different surface hydrogen bonding sites have been proposed as possible causes for the observed spectral changes. In this report a simple model based on chemical shift averaging, due to the making and breaking of hydrogen bonds as water molecules move on the MCM-41 surface, is shown to fully reproduce the NMR spectra, both as a function of hydration and temperature. Surface proton–water proton chemical exchange is not required in this model at low hydration levels.     

Location of hydrogen in intercalation compounds of layered transition metal disulfides, A H NMR study

¹H NMR spectra of the hydrogen bronzes H_xNbS₂, H_xTaS₂ (2H modification), H_xTiS₂ and H_xVS₂ (1T-modif ication) have been measured in order to study location and mobility of hydrogen in layered chalcogenide intercalation compounds. In those materials which crystallize with 2H-modification the large majority of hydrogen atoms is immobile at ambient temperature and appears to be located in intralayer positions with symmetrical hydrogen metal four center bonding. In H_xTiS₂ and H_xVS₂ hydrogen occurs clustered at adjacent tetrahedral sites in the van der Waals gap and diffusion takes place between adjacent tetrahedral sites via S-H…S type hydrogen bonds.     

Site dependent hardening of the lanthanum metal lattice by hydrogen absorption

The compressibility of lanthanum (La) metal and its hydrides were measured at room temperature by high pressure synchrotron X-ray diffraction. La metal pressurized in a hydrogen medium forms a hydride with an fcc metal lattice, which likely contains hydrogen at a concentration close to 3.0 and persists over the measured pressure span up to 21 GPa. Equations of state have been determined by helium compression experiments for LaH₂ with tetrahedral interstitial sites fully occupied with hydrogen atoms and for LaH_2.46 with octahedral interstitial sites partially occupied with hydrogen atoms and tetrahedral sites fully occupied. Both hydrides possess fcc metal lattices. The bulk modulus values B₀ are 66.7 ± 1.2 GPa for LaH₂ and 68.4±1.0 GPa for LaH_2.46. These values are three times larger than that of La metal and are very close to each other despite the difference in hydrogen occupation states. The hardening of the metal lattice by hydrogenation is attributed predominantly to hydrogen-metal interactions at the tetrahedral sites and is most pronounced for La, which has the largest ionic radius among rare-earth metals.     

Chemical exchange in KHSeO4 and NH4HSeO4 studied by two-dimensional NMR

The microscopic mechanism of proton transport in partially deuterated potassium hydrogen selenate (KHSe) and in partially deuterated ammonium hydrogen selenate (AHSe) were studied by means of one-dimensional Fourier transform²H nuclear magnetic resonance (NMR), two-dimensional²H NMR and dielectric measurements over a wide temperature range. In both systems, KHSe and AHSe, the slow chemical exchange processes of deuterons between different hydrogen bridges occur. It was established that the rates of exchange between deuteron sites, which are involved in infinite chains of hydrogen bonds, are approximately the same for both crystals. The rates of exchange between these positions and the deuterons in the dimer groups of KHSe are approximately hundred times more slowly. On the basis of our findings, we discuss the models of the microscopic mechanism of hydrogen transport for both substances.     

质子化学位移各向异性的测量

测量质子化学位移各向异性（CSA）有助于表征分子结构与其动力学,但由于¹H-¹H同核偶极耦合相互作用很强及质子各向异性化学位移较小,测量质子化学位移各向异性仍具有巨大挑战,特别是对含有多种质子的生物大分子,如蛋白质.本文简要综述了测量质子化学位移各向异性的方法,包括同核去耦慢速魔角旋转方法、超快魔角旋转方法、对称重耦（RN_n^v）方法、xCSA方法以及量子化学计算方法.我们重点介绍了在高速魔角旋转条件下蛋白质氨基质子化学位移各向异性的测量及它们与氢键长度、蛋白质二级结构之间的关系.     

Neutron diffraction study of hydrogen in α titanium

It has been determined, using neutron diffraction, that hydrogen dissolved in the α (h.c.p.) phase of titanium is distributed randomly among the tetrahedral sites.     

Temperature dependence of hydrogen site occupancy in Yttrium dihydride

Neutron inelastic scattering and diffraction techniques were used to determine the temperature dependence of the octahedral site occupancy of hydrogen in YH_2.0 between 15 and 420 K. The results show an approximately constant fraction of the hydrogen atoms on octahedral sites up to 200 K followed by a monotonic decrease contrary to intuition and simple theory. This decrease in site disorder with increasing temperature is explained qualitatively by including the different effects of hydrogen at tetrahedral and octahedral sites on the metal lattice entropy and quantitatively by including hydrogen-hydrogen interactions.     

Dilatometric and electrochemical studies on hydrogen transport through RuO2 coated on palladium

Combined electrochemical and dilatometry measurements were used to characterize the transport of hydrogen through thin RuO₂ layers coated on palladium wire electrodes. Hydrogen dissolved in aqueous solutions penetrated through the oxide in a pH-dependent mechanism that combined diffusion of molecular hydrogen and pH-dependent proton hopping through redox sites within the oxide lattice. When cathodically charged, hydrogen was generated and absorbed at the oxide-solution interface only after Ru (IV) reduction occurs, and then, transported into the metal.     

Hydrogen and muonium in silicon

First principles calculations of the properties of hydrogen and muonium in silicon are presented. H⁺ and H⁻ are shown to have definite preferences for bond-centred and tetrahedral interstitial sites respectively whereas H⁰ (or a muon) is shown to be stable at two sites with almost equal energies, the bond-centred and antibonding sites. The structures of normal and isotropic muonium are discussed. In contrast to common belief the tetrahedral site is shown to be unstable with the muon moving spontaneously towards one of the neighbouring silicon atoms. The barrier to motion between equivalent antibonding sites is low suggesting that the normal muonium signal is isotropic because of motional averaging, not due to the symmetry of a well defined equilibrium site.     

Use of random numbers in computer simulation of magnetic resonance signals

The proton magnetic resonance spectra of water molecules diffusing over regular sites in natrolite are numerically simulated. Experimental data and the results of simulation are found to be in good agreement in a wide temperature interval including the transition range, where the magnetic resonance spectrum is a superposition of a wide unaveraged spectrum and a spectrum averaged by the motion of molecules. The simulation uses generators of pseudorandom-number Markovian sequences. For a binary Markovian process, it is shown, in particular, that the time within which a physical quantity remains constant is described by an exponentially distributed random number.     

Micromechanism of proton conduction in a KHSeO4 crystal

The proton exchange in a potassium hydrogen selenate crystal is investigated in detail. Partial deuteration of the crystal (approximately 80% of the protons are replaced by deuterons) makes it possible to use the method of nuclear magnetic resonance (NMR) on quadrupole nuclei to perform research into proton (deuteron) transport. In addition to conventional Fourier-transform NMR spectroscopy, elementary processes of deuteron chemical exchange are studied by two-dimensional NMR spectroscopy, which provides unique information regarding these processes. Slow exchange between protons of hydrogen bond chains and dimers consisting of two SeO₄ groups is revealed. It is established that this process is responsible for the proton conduction in the potassium hydrogen selenate. The NMR data are compared with the results of dielectric measurements carried out at a frequency of 1 kHz.     

Wide-line proton magnetic resonance in the solid solution phase of the yttrium-hydrogen system

The proton magnetic resonance and its temperature dependence have been studied in the solid solution phase of the yttriumhydrogen system, yielding information on hydrogen locations and diffusion in yttrium metal and on the phase boundaries at 300 K of the two-phase, yttrium + yttrium dihydride, region.     

A solid-state V NMR characterization of vanadium sites in LiCoxNi1−xVO4

LiCo_xNi_1−xVO₄ compounds (x=0, 0.2, 0.5, 0.8, 1.0) have been prepared for lithium ion batteries by different preparation methods, namely, wet-chemistry (WC) and solid-state (SS) routes. ⁵¹V nuclear magnetic resonance (NMR) has been employed to accurately assess the vanadium environments in these materials and results show that vanadium atoms are distributed in octahedral and tetrahedral sites. The fractions of vanadium atoms in tetrahedral/octahedral sites depends on composition and sample preparation conditions. The tetrahedral site, which is particularly dominant in samples produced via the WC route, is more affected by paramagnetic interactions and its ⁵¹V NMR spectrum is essentially Gaussian in shape. The octahedral site, on the other hand, exhibits first-order quadrupolar broadened satellites. Even though tetrahedral sites for a given x are comparable for WC and SS materials, there are some differences amongst octahedral environments, namely in the distributions of quadrupolar broadened satellites. The larger satellite distribution observed for WC materials is indicative of a larger distribution of structural defects for these as compared to SS materials.     

Mössbauer effect and electron spin resonance study of CdxMg1−xFe2O4 system

This work deals with the application of Mossbauer Effect (ME) in studying the crystal electric field and the cation distribution among tetrahedral (A) and the octahedral (B) sites of the spinal structure in the ferrite system Cd_x Mg_1–x Fe₂O₄ (x=0, 0.2,...1). The electron spin resonance technique (ESR) was also applied for studying this ferrite system. It was possible to characterize the ESR spectra of ferrite through the combination with the ME spectra.The ESR spectra of magnesium ferrite showed two resonance positions of Fe³⁺ ions and indicated that a strong exchange interaction is dominant in the pure Mg-ferrite. For high Cd concentration ferrites only single resonance line was observed. These results could be interpretted on the basis of the ME results where it indicated that Cd²⁺ ions prefer tetrahedral positions, forcing the Fe³⁺ ions from these positions to join those in the octahedral sites. The complete site occupation with different types of cations was successfully achieved from the ME spectra. The values of the quadrupole splitting indicated that for each ferrite in the system there exists an electric field gradient surrounding the Fe³⁺ ions in each of the octahedral and tetrahedral sites. The increase in the Mg-concentration increases the symmetry of the electric field at these sites.     

Calculation of the correlation time for motional narrowing of the 181Ta Mössbauer line due to hydrogen diffusion

The correlation time for motional narrowing of a Mössbauer resonance line (¹⁸¹Ta) by hydrogen diffusion has been calculated by applying the random walk theory. The correlation times τ_c are 12.546 541τ and 3.756 711τ for tetrahedral and octahedral occupancies, respectively, where τ is the mean residence time of a hydrogen atom.     

Spectroscopic Studies of Proton Transfer Equilibria In Hydrogen Bonded Complexes

Abstract

Recent studies on proton transfer equilibria for many hydrogen bonded complexes are discussed. These studies employ various spectroscopic techniques as infrared, ultraviolet, ¹H NMR, ¹³C NMR, ¹⁵N NMR and nuclear quadrupole resonance (NQR) spectroscopy. Special attention has been paid to Fourier transform infrared (FTIR), and it forms the main focus of this review, in particular for the study of proton transfer equilibria in proton sponges hydrogen bonded complexes. The influence of proton transfer equilibria on the physical, chemical and biological properties of hydrogen bonded complexes is shown. Some applications of proton transfer equilibria are also discussed.