Frequencies Of Stretching Modes Research Articles

Raman studies of technetium in borosilicate waste glass

The characterization of technetium (Tc) environments in borosilicate glasses is important with regard to the long-term isolation of Tc in nuclear wastes from the environment by waste vitrification. In this study, Raman spectroscopy was used to detect Tc in various borosilicate waste glass formulations in which Tc valence and average coordination environment were previously characterized by X-ray absorption spectroscopy. Raman spectral comparisons between chemically equivalent Tc-containing and Tc-free borosilicate glass pairs indicate Tc-dependent features near 320 and 910 cm−1 resulting from internal vibrational modes of pertechnetate tetrahedral (Tc7+ in TcO4 −), similar to those observed for pertechnetate aqueous solutions. Initial observations of the pertechnetate Tc–O symmetric stretch mode near 910cm−1 indicate a peak area dependence on pertechnetate concentration and a detection limit near 50 ppm Tc7+ in borosilicate glass. Pertechnetate Tc–O symmetric stretch mode frequency versus composition relationships for the borosilicate waste glasses suggest that pertechnetate tetrahedra are surrounded by network-modifying cations. However, Raman spectroscopy is insensitive to six-coordinated Tc4+ species in borosilicate glasses.

Enhanced ferroelectric properties of multilayer SrBi 2Ta 2O 9/SrBi 2Nb 2O 9 thin films for NVRAM applications

Ferroelectric SrBi 2Ta 2O 9/SrBi 2Nb 2O 9 (SBT/SBN) multilayer thin films with various stacking periodicity were deposited on Pt/TiO 2/SiO 2/Si substrate by pulsed laser deposition technique. The X-ray diffraction patterns indicated that the perovskite phase was fully formed with polycrystalline structure in all the films. The Raman spectra showed the frequency of the O–Ta–O stretching mode for multilayer and single layer SrBi 2(Ta 0.5Nb 0.5) 2O 9 (SBNT) samples was ∼827–829 cm −1, which was in between the stretching mode frequency in SBT (∼813 cm −1) and SBN (∼834 cm −1) thin films. The dielectric constant was increased from 300 (SBT) to ∼373 at 100 kHz in the double layer SBT/SBN sample with thickness of each layer being ∼200 nm. The remanent polarization (2 P r) for this film was obtained ∼41.7 μC/cm 2, which is much higher, compared to pure SBT film (∼19.2 μC/cm 2). The coercive field of this double layer film (67 kV/cm) was found to be lower than SBN film (98 kV/cm).

Tieftemperaturuntersuchung von Wasserstoffbrückenbindungen in Lithiumtetrahydroxoborat mit Raman‐Spektroskopie, Röntgen‐ und Neutronenbeugung (Li11B(OD)4)

AbstractLow Temperature Investigation of Hydrogen Bridge Bonds in Lithium Tetrahydroxoborate by Raman Spectroscopy, X‐Ray and Neutron Diffraction (Li11B(OD)4)Low temperature Raman spectroscopic measurements on isotopically diluted Li11B(OH)4 with 8 % D and Li11B(OD)4 with 8 % H reveal four crystallographically different hydrogen bridge bonds. With decreasing temperatures beginning at ∼50 K measured down to ∼10 K the stretching modes of the hydroxide ions shift to higher wave numbers. For the strongest bond O–D···O the frequency shift is 16 cm−1and for the weakest 7 cm−1. For O–H···O the maximum in the frequency shift is 22 cm−1. X‐ray single crystal (LiB(OH)4) and neutron powder diffraction (Li11B(OD)4) data result in bond lengths for the four hydroxide ions in the range of 0.943 (3) Å ≤ d(O–D) ≤ 0.974 (3) Å. The value of the effect of inversion of the stretching mode frequencies seems to correlate with the strength of the hydrogen bridge bonds and is found to be different for the two isotopes H and D in this compound.

Anharmonicity and lattice coupling of bond-centered hydrogen and interstitial oxygen defects in monoisotopic silicon crystals

We discuss the vibrational dynamics of bond-centered protons $({\mathrm{H}}_{\mathrm{BC}}^{+})$ and deuterons $({\mathrm{D}}_{\mathrm{BC}}^{+})$ in monoisotopic ($^{28}\mathrm{Si}$, $^{29}\mathrm{Si}$, and $^{30}\mathrm{Si}$) silicon crystals, based on joint infrared absorption measurements and ab initio modeling studies. Protons and deuterons have been implanted at temperatures below $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and in situ--type infrared absorption measurements have subsequently been performed at $8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A major absorption line is observed at $1998\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ after proton implantation, which has previously been ascribed to a local mode of ${\mathrm{H}}_{\mathrm{BC}}^{+}$. We find that the ${\mathrm{H}}_{\mathrm{BC}}^{+}$ mode at $1998\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ displays an anomalous (positive) frequency shift when the Si isotope mass is increased, unlike the analogous ${\mathrm{D}}_{\mathrm{BC}}^{+}$ mode at $1448\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which shows a negative shift. This effect cannot be described with a purely harmonic model. We show that the mode frequencies are accurately accounted for with a simple model based on a linear $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}$ structure when anharmonicity, volumetric effects due to the host-isotope mass, and the coupling of the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}$ unit to the lattice are taken into account. Interstitial oxygen $({\mathrm{O}}_{i})$ atoms in silicon, also located at the bond-center site, are as well investigated in a parallel way. The relative contributions of the different terms of the vibrational model to the mode frequency of ${\mathrm{H}}_{\mathrm{BC}}^{+}$ and ${\mathrm{O}}_{i}$ are compared. The anomalous (positive) isotope shift of ${\mathrm{H}}_{\mathrm{BC}}^{+}$ results from mixing via anharmonicity of ${A}_{2u}$ and ${A}_{1g}$ modes of the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}$ unit, which shows that a reliable vibrational model has to take into consideration the local structure of the defect. The mode frequency of the ${\mathrm{O}}_{i}$ defect exhibits the normal (negative) isotope shift, because the relatively modest contribution of anharmonicity diminishes the importance of the mode mixing. The effect of the defect-lattice coupling on the stretch-mode frequencies of ${\mathrm{H}}_{\mathrm{BC}}^{+}$ and ${\mathrm{O}}_{i}$ is also discussed.

Binding of the N interstitial with neutral MgH inp-type GaN investigated with density functional theory

Density-functional theory and the generalized gradient approximation were utilized to investigate the interaction of N interstitials with neutral MgH centers in $p$-type GaN. Formation energies and the corresponding local-energy minimum configurations of the defect complex are reported along with the energy to dissociate the complex into isolated MgH and a N interstitial. The conclusion is that N interstitials will bind with MgH centers in $p$-type GaN(Mg,H), with binding energies of 0.71, 0.44, and $0.29\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the $+3$, $+2$, and $+1$ charge states, respectively. Hydrogen local-mode vibrational frequencies for the bound complex were estimated and compared to previous results for isolated MgH. The binding of a N interstitial to MgH perturbs the H stretch-mode frequency by $25\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. These results are discussed in the context of recent experiments at Sandia National Laboratories, which suggest that N interstitials, created by proton irradiation, become mobile during subsequent annealing and bind with MgH centers in $p$-type GaN(Mg,H).

Carbon substitutions inMgB2within the two-band Eliashberg theory

We study the effects of C substitutions in MgB_2 within the two-band model in the Eliashberg formulation. We use as input the B-B stretching-mode frequency and the partial densities of states N_{sigma}(EF) and N_{pi}(EF), recently calculated for Mg(B_{1-x}C_{x})_2 at various x values from first-principles density functional methods. We then take the prefactor in the Coulomb pseudopotential matrix, mu, and the interband scattering parameter, Gamma^{sigma pi}, as the only adjustable parameters. The dependence on the C content of Tc and of the gaps (Delta_{sigma} and Delta_{pi}) recently measured in Mg(B_{1-x}C_{x})_2 single crystals indicate an almost linear decrease of mu on increasing x, with an increase in interband scattering that makes the gaps merge at x=0.132. In polycrystals, instead, where the gap merging is not observed, no interband scattering is required to fit the experimental data.

Hydrogen Donors in ZnO

AbstractZinc oxide (ZnO) has shown great promise as a wide-bandgap semiconductor with a range of optical, electronic, and mechanical applications. The presence of compensating donors, however, is a major roadblock to achieving p-type conductivity. Recent first-principles calculations and experimental studies have shown that hydrogen acts as a shallow donor in ZnO, in contrast to hydrogen's usual role as a passivating impurity. Given the omnipresence of hydrogen during growth and processing, it is important to determine the structure and stability of hydrogen donors in ZnO.To address these issues, we performed vibrational spectroscopy on bulk, single-crystal ZnO samples annealed in hydrogen (H2) or deuterium (D2) gas. Using infrared (IR) spectroscopy, we observed O-H and O-D stretch modes at 3326.3 cm-1 and 2470.3 cm-1 respectively, at a sample temperature of 10 K. These frequencies indicate that hydrogen forms a bond with a host oxygen atom, consistent with either an antibonding or bond-centered model. In the antibonding configuration, hydrogen attaches to a host oxygen and points away from the Zn-O bond. In the bond-centered configuration, hydrogen sits between the Zn and O. To discriminate between these two models, we measured the shift of the stretch-mode frequency as a function of hydrostatic pressure. By comparing with first-principles calculations, we conclude that the antibonding model is the correct one.Surprisingly, we found that the O-H complex is unstable at room temperature. After a few weeks, the peak intensity decreases substantially. It is possible that the hydrogen forms H2 molecules, which have essentially no IR signature. Electrical measurements show a corresponding decrease in electron concentration, which is consistent with the formation of neutral H2 molecules. The correlation between the electrical and spectroscopic measurements, however, is not perfect. We therefore speculate that there may be a second “hidden” hydrogen donor. One candidate for such a donor is a hydrogen-decorated oxygen vacancy.

Computational Study of the Reactivity of N-Phenylacetamides in the Alkaline Hydrolysis Reaction

A number of molecular quantities were tested as reactivity indexes for the alkaline hydrolysis reaction of a series of 15 N-phenylacetamides para substituted in the aromatic ring. The Hammett σ constants and the shifts of carbonyl stretching mode frequencies correlate satisfactorily with the predicted Hartree−Frock (HF)/6-31+G(d) and density functional theory B3LYP/6-31+G(d,p) energy changes for the rate-determining stage of the reaction, namely, the addition of a hydroxide ion and formation of a tetrahedral intermediate. Natural bond orbital atomic charges at the carbonyl carbon atom, the site of the nucleophilic attack, also offer a satisfactory basis for discussing the reactivity of the carbonyl compounds studied. The electrophilicity index ω describes well the overall tendency of the changing reactivity of the studied compounds. A quantitative description of the reactivity of the studied amides in the reaction considered is provided by the theoretically estimated electrostatic potential at the carbony...

Conformation of some biologically active aromatic ureas

Experimental IR spectroscopic data for the N–H stretching mode frequencies for several types of tri-substituted ureas containing benzyl and/or phenyl substituents as well as theoretical results from B3LYP/6–31G(d,p) computations on selected compounds provide sufficient evidence to determine the conformational state of these molecules. Two types of N–H bands may be found the spectra: (a) A type band due to a classical trans conformation (t rans I) of the CO NH structure; (b) B type band arising from an alternative trans form ( trans II), in which the N–H band is involved in a hydrogen bond like interaction with the aromatic ring at the neighbouring nitrogen atom (benzyl or phenyl substituents). The N–H band of trans I CO NH structure is observed at frequencies higher than 3460 cm −1, the actual position depending on weather the non-substituted N–H group is linked to aryl or alkyl substituents. The N–H band of the trans II rotameric structure is observed at 3430–3420 cm −1.

Structural Dependence of NH Stretch Mode Frequency Shifts in Amide and Peptide Systems

The NH stretch (s) frequency, ν(NH), in peptides and proteins, as in many amide group hydrogen bond (HB) systems, is used as an indicator of the presence and strength of the bond. This is based on the concept that ν(NH) is associated with an essentially localized mode whose frequency is influenced only by the HB interaction. We examine this assumption through ab initio studies of N-methylacetamide (NMA) and of 78 conformers of the alanine dipeptide (ADP). Based on our recent studies of nitrogen pyramidalization in NMA (J. Phys. Chem. A 2003, 107, 1825), we find that such nonplanarity, whether deriving from a nonplanar peptide group or only from a steric constraint, results in an increase in the NH bond length, r(NH), and thus in a decrease in ν(NH). This is accompanied by changes in other internal coordinates that make small contributions to this mode, emphasizing that although NH s is a highly localized mode it is not completely so. It is not surprising, therefore, to find that changes in the environment of the NH group, such as exist in the ADP conformers, lead to variations in ν(NH) even in the absence of traditional HB configurations. The ADP results permit an analysis of these effects and also illustrate the important role, as we recently emphasized (J. Phys. Chem. A 2002, 106, 11663), of electrical interactions in affecting r(NH) and thus ν(NH). We show how the increased strength of this interaction in ADP conformers with “standard” HBs leads, through the much larger induced dipole derivative, to the large increase in infrared intensity of NH s bands in such structures.

The degree of charge transfer in ground and charge-separated states revealed by ultrafast visible pump/mid-IR probe spectroscopy.

We demonstrate a new femtosecond visible pump/mid-IR probe spectroscopic approach to assess directly the ground- and excited-state degrees of charge transfer (CT) in donor-spacer-acceptor (D-Sp-A) structures. Two classes of (porphinato)zinc(II) (PZn)-based D-Sp-A compounds with either quinonyl (Q) or N-(N'-octyl)pyromellitic diimide (PI) electron acceptors were interrogated. Carbonyl antisymmetric stretching mode frequency domain transient-IR spectra of these species were recorded and analyzed for the Q/PI moieties. These data show that the acceptor mode frequency shift, DeltanuA, determined by this method provides a more accurate measure of the degree of CT in ground and charge-separated states relative to other techniques which rely on the ground-state frequency shift alone. This approach enables determination of new experimental benchmarks to test the power of complimentary computational methods and provides a means to probe the degree of CT in transitions that either overlap strongly with other bands or possess low oscillator strength.

Anisotropic thermal expansion of layeredMoO3crystals

A structural study of orthorhombic molybdenum trioxide ${\mathrm{MoO}}_{3}$ with a layered structure has been performed using synchrotron-radiation powder diffraction from 110 to 1000 K. ${\mathrm{MoO}}_{3}$ exhibits an anisotropic thermal expansion. With increasing temperature, the lattice expands remarkably along the a axis of the stacking direction, while it expands a little along the c axis but contracts slightly along the b axis. Based on the quasiharmonic approximation, the temperature variations in the lattice constants and thermal expansion coefficient are analyzed and they are reproduced quantitatively using three stretching mode frequencies for $\mathrm{Mo}---\mathrm{O}$ bonds, 1000, 820, and 670 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, as well as two low frequencies, 250 and 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The large thermal expansion along the a axis is attributed to the coexistence of high frequency vibration due to the covalent $\mathrm{Mo}---\mathrm{O}$ bond and high compressibility due to the van der Waals gaps in ${\mathrm{MoO}}_{3}.$

FT‐IR studies of sickle hemoglobin interaction with phosphatidylserine

The interaction of dilauroylphosphatidylserine (DLPS) vesicles with both normal human hemoglobin (HbA) and hemoglobin from patients with sickle cell disease (HbS) has been investigated with FTIR spectroscopy. Changes in the conformational order of the phospholipid chains were directly monitored via the acyl chain CH2 symmetric stretching mode frequencies. The hemoglobin oxygenation state was simultaneously monitored via the S−H stretching contour. The current study reveals that under oxygenated conditions, DLPS shows little interaction with either HbA or HbS. In contrast, deoxyHbS interacted with DLPS by abolishing the gel−liquid crystal phase transition.

Configurations, energies, and thermodynamics of the neutral MgH complex in GaN

Atomic configurations corresponding to local-energy minima for the neutral MgH complex in wurtzite GaN are identified using density-functional theory and the generalized-gradient approximation for exchange and correlation. MgH binding energies, H local-mode vibration frequencies, and configurational degeneracies for the six lowest-energy configurations are used, along with corresponding results for isolated H+, to compute equilibrium H state populations in Mg-doped GaN as a function of temperature. For a Mg concentration of 1×1019/cm3 and a H/Mg concentration ratio of 0.99, MgH is found to be the majority H species at room temperature with isolated H+ becoming the majority species at T≈550 °C. Among the MgH states, one is found to dominate at all temperatures. The dominant configuration consists of H at an antibonding site of a N neighbor of the substitutional Mg, with the Mg–N and N–H bonds nearly aligned and the N–H bond oriented at an angle of ∼109° with the c axis. The H stretch-mode frequency of the dominant state is consistent with the peak observed in Fourier-transform infrared reflection spectra from Mg-doped GaN samples.

Dihydrogen Bonding under High Pressure: A Raman Study of BH3NH3 Molecular Crystal

The effects of high pressures (up to 40 kbar) on the dihydrogen-bonded BH3NH3 molecular crystal were investigated using Raman spectroscopy in diamond and moissanite anvil cells. The stretching mode frequencies of the NH3 proton donor groups exhibited moderate red shifts with increasing pressures, as found in many conventional hydrogen-bonded systems of weak to medium strength. The stretching modes corresponding to the BH3 proton acceptor group, on the other hand, showed large blue shifts with increasing pressure, which, however, are not related to the changes in the N−H···H−B interactions. The BH3NH3 crystals undergo a pressure-induced disorder−order phase transition around 8 kbar, which is facilitated by the presence of dihydrogen bonds.

Raman spectra of birnessite manganese dioxides

Structural features of layered manganese dioxides of the birnessite family are studied using Raman scattering spectroscopy. This local probe is capable of analysing directly the near-neighbour environment of oxygen coordination around manganese and lithium cations. Four types of sol–gel birnessite (SGB) are considered: lithium birnessite (Li-Bir), sodium birnessite (Na-Bir), sol–gel birnessite (SG-Bir), and sol–gel Co-doped birnessite (SGCo-Bir). Thus, in a first approach, we consider the overall spectral features of birnessites such as the superposition of the spectra of local structures, while the lattice modes are discussed in the spectroscopic symmetry. Results show the specific spectroscopic fingerprints of SG-Bir single phases, the site occupancy of Co ions in the substituted SGCo-Bir compound, and vibrations due to lithium ions with their oxygen neighbours in Li-Bir, Li 0.32MnO 2·0.6H 2O. A correlation between the interlayer d-spacing and the stretching mode frequencies of birnessite oxides has been established.

Explanation of the structural changes in the Si : H thin films by monohydride cluster formation

We have studied the structural changes in Si : H thin films under conditions of increasing SiH4 depletion for three different sets of SiH4–Ar mixtures in the ratios 10 : 90, 5 : 95, 1 : 99, respectively by varying the power density in plasma enhanced chemical vapour deposition. The structural properties of the deposited Si : H materials have been studied by Fourier transform infrared spectroscopy, small angle x-ray scattering and transmission electron microscopy. It was observed that the shift in the SiH stretching mode frequency has a one to one correspondence with the changes in the microstructures in the films. This shift has been explained on the basis of the changes in the electronegativity of the matrix due to monohydride cluster formation in the neighbourhood of a SiH bond. It has been observed that the onset of microcrystallites formation coincides with the maximum shift of the SiH stretching mode frequency. The role of Ar* in mediating the structural changes with dilution and power density have been discussed.

Theoretical study of the OH/OD stretching regions of the vibrational spectra of ice Ih

In this paper we present results of our theoretical studies of the vibrational spectra of ice Ih in the stretching mode frequency range. They include simulation of the infrared and Raman spectra for the OH and OD stretching regions, the effects of a full range of isotopic dilution on the spectra (including polarized Raman spectra) and computational modeling of the observed influence of each dilution step on the properties of vibrationally excited states and on the infrared and Raman spectra. In our calculations we included effects of frequency lowering due to hydrogen bonding, and of intramolecular and intermolecular coupling between bonds. In the crystalline isotopomers the properties of the vibrationally excited states and of the spectra are determined by a complex interplay between the size distributions of the embedded clusters and the intermolecular and intramolecular couplings.

Theory supplemented by experiment. Electronic effects on the rotational stability of the amide group in p-substituted acetanilides.

The electronic effect of polar substituents on the barrier of internal rotation around the amide carbon-nitrogen bond in a series of 10 p-substituted acetanilides is studied by applying density functional theory at the B3LYP/6-31G(d,p) level. The theoretical results are supplemented by experimental data on the amide C=O and N-H stretching mode frequency shifts. It is shown that computations at the theoretical level employed provide a valuable approach in studying the factors determining the conformational stability of the studied series of compounds. It is found that an excellent linear dependence between the barriers of rotation and frequency shifts exists. It is concluded that the variations of the amide C=O stretching mode frequency can be used for quantitative characterization of the amide group conformational flexibility in the studied series of acetanilides.

He I photoelectron spectroscopy of formic acid isotopomers HCOOH and DCOOD

He I photoelectron spectra (PES) of HCOOH and DCOOD have been measured with an electron kinetic energy resolution of 15 meV. Analysis of observed vibronic structure in the two PES bands in the 11.3–13.5 eV energy region provided values for several vibrational modes of the X 2 A ′ and A ̃ 2 A ″ electronic states of HCOOH + and DCOOD +. This analysis is based on the implications of structural changes expected to occur on ionization, using literature values for calculated geometries of these states. The vibronic assignments of both PES bands were consistent with isotopic effects on vibrational mode frequencies and were helped by considerations on relations between carbon–oxygen bond lengths and stretch mode frequencies. The analysis of the second PES band indicates that the calculated structure of the A ̃ 2 A ″ state of the ion requires more refined theoretical treatment. The PES first ionization energy of HCOOH agrees well with that obtained from Rydberg series in absorption spectra, and values more precise than hitherto were obtained for the first and second ionization energies of DCOOD, 11.335±0.002 eV and 12.380±0.002 eV respectively, and the second ionization energy of HCOOH, 12.378±0.002 eV.