Bending Band Research Articles

A high pressure infrared spectroscopic study of PbCO3-cerussite: constraints on the structure of the post-aragonite phase

We have measured the infrared spectrum of aragonite-structured PbCO3-cerussite to 41 GPa at 300 K in the diamond anvil cell. We observed a phase transition from an orthorhombic to a trigonal structure beginning at ~15 GPa, manifested by a splitting of the ν2-out-of-plane bending vibration and a broadening and dramatic decrease in amplitude of the ν1-symmetric stretching vibration of the carbonate group. While the locations of the ν1-symmetric stretching and ν4-in-plane bending bands are similar between the low- and high-pressure phases, their mode shifts and peak shapes change markedly near the transition. In particular, the ν1 symmetric stretch has an essentially zero pressure shift in the high pressure phase, and its dramatically enhanced peak width indicates that it may be symmetry forbidden. The decreased mode shifts of the carbonate vibrations after the phase transition suggest that the carbonate group is less compressible in the new structure. The spectral changes observed are consistent with a small, trigonal unit cell, with space group $${P\bar{3}{1c}}$$ and two formula units, instead of a previously proposed orthorhombic cell with sixteen formula units. This structure is identical to that of the high-pressure phase of BaCO3, and likely CaCO3 as well. Our results thus indicate that the post-aragonite, high-pressure phase of divalent-cation carbonates may be a comparatively high-symmetry trigonal structure.

Surface phonons of clean and hydrogen terminated Si(1 1 0) surfaces

We report an investigation of the dynamical properties of clean and monohydride-terminated Si(1 1 0) surfaces, measured by means of high resolution electron energy loss spectroscopy (HREELS). On both surfaces a number of surface phonons is observed and successfully assigned to predicted modes. A (1 × 5) reconstruction is found to be the stable surface phase corresponding to monohydride coverage of Si(1 1 0). For the Si–H stretching and bending bands of this monohydride Si(1 1 0) surface we find a relatively strong dispersion along the [ 1 1 ¯ 0 ] direction. Apparently, this is related to the coupling of oscillators along the Si chains of the (1 1 0) surface. In addition, surface vibrations induced by adsorbed residual gases are measured and analysed.

Temperature Dependence of Water Vibrational Spectrum: A Molecular Dynamics Simulation Study

Vibrational spectroscopy studies show that the bulk water bending band becomes narrower with increasing temperature (Marechal, Y. J. Mol. Struct. 1994, 322, 105). Since this counterintuitive effect is not associated with the quantum nature of nuclear motion a molecular dynamics (MD) simulation is expected to reproduce it even in the classical limit. We have performed a classical MD simulation of the flexible simple point charge (SPC) and extended SPC (SPC/E) water models to determine the temperature dependence of the bulk water vibrational spectrum. The intramolecular water potential proposed by Toukan and Rahman, including a stretch−bend coupling term, was applied. We performed MD simulations at −4 and 80 °C to compare the calculated vibrational spectra, in particular, the band associated with the bending mode, with the experiment. The experimentally determined narrowing of the bending band with increasing temperature is not reproducible by MD simulation with the applied force field. However, the results...

Liquid and Ice Water and Glycerol/Water Glasses Compared by Infrared Spectroscopy from 295 to 12 K

Infrared absorption spectra for liquid and ice H2O and D2O and for protonated and deuterated glycerol/water glasses are compared in the spectral range of 1000−4000 cm-1 and the temperature range of 295−12 K. As temperature decreases, the absorbance bands of the OH and OD stretching modes of glycerol/water solution shift to lower frequency and the band intensifies and narrows. The center of the absorption band for OH stretch for glycerol/D2O/H2O solution (60/32/8 v/v) is at 3364 cm-1 at 295 K and at 3277 cm-1 at 25 K. The full width at half-maximum (fwhm) of the band is 259 cm-1 at 295 K and 186 cm-1 at 25 K. The width of the OH stretch band for D2O that is doped with H2O is 193 cm-1 in liquid (295 K) and 24 cm-1 in ice (12 K). The broader width for the OH stretch at 12 K for glycerol/water relative to that seen in ice is consistent with inhomogeneous broadening due to the glassy nature of glycerol/water solutions. The HOH and DOD bending (scissoring) bands for water in glycerol shift to higher frequency, ...

Influence of Exchangeable Cations on Water Adsorption by Soil Clays

The interaction of water with the clay fractions (<2 μm) from two midwestern soils was studied using Fourier transform infrared (FTIR) spectroscopy and gravimetric methods. The soil clay fractions were obtained from a Blount loam (fine, illitic, mesic Aeric Epiaqualfs) and a Fayette silty clay loam (fine‐silty, mixed, mesic, superactive Typic Hapludalfs). These clay fractions were exchanged with Mg2+, 50:50 Ca2+/Mg2+, Ca2+, Na+, and K+ to determine the influence of the exchangeable cation on their water sorption behavior. Water sorption isotherms and FTIR spectra of the clays were collected simultaneously using a gravimetric‐spectroscopic cell. Overall, the amount of water sorbed by the samples increased as the ionic potential of the exchangeable cation increased and was strongly correlated to the hydration energy of the cations (P > F < 0.0001). The position of the H–O–H bending band (ν2 mode) also increased with increasing ionic potential of the exchangeable cation indicating strengthening of water H bonds. In addition, it was observed that the position of this band decreased with increasing water content for the Mg‐exchanged clays compared with an overall increase for the Ca‐exchanged samples. X‐ray diffraction patterns indicated an expansion of the phyllosilicate clay minerals as the water activity increased; however, no differences were observed between the Ca‐ and Mg‐exchanged samples. This study shows that the molecular properties of water on Ca‐ and Mg‐exchanged soil clays are similar to that on specimen clays and provides new insight about the role of exchangeable cations in soils.

Adsorption and vibrational spectroscopy of ammonia at mordenite: ab initio study.

The adsorption of ammonia at various active centers at the outer and inner surfaces of mordenite, involving Brønsted acid (BA) sites, terminal silanol groups, and Lewis sites has been investigated using periodic ab initio density-functional theory. It is shown that ammonia forms an ammonium ion when adsorbed at strong BA sites. The calculated adsorption energies for different BA sites vary in the interval from 111.5 to 174.7 kJ/mol depending on the local environment of the adduct. The lowest adsorption energy is found for a monodentate complex in the main channel, the highest for a tetradentate configuration in the side pocket. At weak BA sites such as terminal silanol groups or a defect with a BA site in a two-membered ring ammonia is H bonded via the N atom. Additional weak H bonds are formed between H atoms of ammonia and O atoms of neighboring terminal silanol groups. The calculated adsorption energies for such adducts range between 61.7 and 70.9 kJ/mol. The interaction of ammonia with different Lewis sites is shown to range between weak (DeltaE(ads)=17.8 kJ/mol) and very strong (DeltaE(ads)=161.7 kJ/mol), the strongest Lewis site being a tricoordinated Al atom at the outer surface. Our results are in very good agreement with the distribution of desorption energies estimated from temperature-programmed desorption (TPD) and microcalorimetry experiments, the multipeaked structure of the TPD spectra is shown to arise from strong and weak Brønsted and Lewis sites. The vibrational properties of the adsorption complexes are investigated using a force-constant approach. The stretching and bending modes of NH(4) (+) adsorbed to the zeolite are strongly influenced by the local environment. The strongest redshift is calculated for the asymmetric stretching mode involving the NH group hydrogen bonded to the bridging O atom of the BA site, the shift is largest for a monodentate and smallest for a tetradentate adsorption complex. The reduced symmetry of the adsorbate also leads to a substantial splitting of the stretching and bending modes. In agreement with experiment we show that the main vibrational feature which differentiates coordinatively bonded ammonia from a hydrogen-bonded ammonium ion is the absence of bending modes above 1630 cm(-1) and in the region between 1260 and 1600 cm(-1), and a low-frequency bending band in the range from 1130 to 1260 cm(-1). The calculated distribution of vibrational frequencies agrees very well with the measured infrared adsorption spectra. From the comparison of the adsorption data and the vibrational spectra we conclude that due to the complex adsorption geometry the redshift of the asymmetric stretching is a better measure of the acidity of an active sites than the adsorption energy.

Effects of ionization on the phase behavior of poly(N-isopropylacrylamide-co-acrylic acid) and poly(N,N-diethylacrylamide-co-acrylic acid) in water

Phase transitions of poly(N-isopropylacrylamide-co-acrylic acid) (PiPA-AA) and poly(N,N- diethylacrylamide-co-acrylic acid) (PdEA-AA) in water have been investigated by means of turbidimetry, Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The phase transition temperatures (T p) of these copolymers increase with the degree of ionization (α) of the acrylic acid (AA) units, which in turn is dependent on the pH of the solutions. Apparent values of pK a for the AA units, determined from the pH dependencies of T p, are 4.7 and 5.4 for PiPA-AA and PdEA-AA, respectively. Differences between T p for PiPA-AA and T p for PiPA homopolymer (ΔT p) are +1.5 and −0.2 °C/mol% of AA at α=1 and 0, respectively. The values of ΔT p for PdEA-AA are +2.6 (ionic) and −0.5 (nonionic)°C/mol%, indicating that the incorporated AA units have a larger effect on PdEA than on PiPA. DSC measurements performed with each of these copolymers at different pH values show a linear relationship between T p and the enthalpy of transition (ΔH). IR measurements of PiPA-AA show that the profiles of IR bands from both iPA and AA units exhibit critical changes at T p of the copolymer. Heating the solution above T p leads to shifts of the amide II, C–H stretch, and C–H bend bands from the iPA units toward lower wavenumbers, as well as a shift of the amide I band from the iPA units toward higher wavenumbers. A decrease in the intensity of the symmetric C=O stretch IR band from carboxylate anions (1560 cm−1), and an increase in the intensity of the C=O stretch band from COOH groups (1705 cm−1) suggest that a partial protonation of the carboxylate groups (COO−+H+→COOH) takes place upon the phase transition.

Band offset measurements of the GaN (0001)/HfO2 interface

Photoemission spectroscopy has been used to observe the interface electronic states as HfO2 was deposited on clean n-type Ga-face GaN (0001) surfaces. The HfO2 was formed by repeated deposition of several monolayers of Hf followed by remote plasma oxidation at 300 °C, and a 650 °C densification anneal. The 650 °C anneal resulted in a 0.6 and 0.4 eV change in band bending and valence band offset, respectively. The final annealed GaN/HfO2 interface exhibited a valence band offset of 0.3 eV and a conduction band offset of 2.1 eV. A 2.0 eV deviation was found from the electron affinity band offset model.

The vibration–rotation of H2O and its complexation with CO2 in solid argon revisited

Fourier-transform infrared spectroscopy in the frequency range 400–4000 cm−1 has been used to investigate the absorption of H2O and H2O:CO2 complex isolated in solid argon. Thanks to the lowest temperature reached in our experiment, temperature effects and nuclear spin conversion studies allow us to propose a new assignment of the rovibrational lines in the bending band ν2 for the quasi-freely rotating H2O. An additional wide structure observed in this band shows two maxima around 1657.4 and 1661.3 cm−1, with nuclear spin conversion of the high-frequency part into the low-frequency one. This structure is tentatively attributed to a rotation–translation coupling of the molecule in the cage. However, the equivalent effect is not observed in the vibrational stretching bands ν1 and ν3. Finally, in double doping experiments with CO2 important new structures appear, allowing us to unambiguously extract the frequencies of the lines of the H2O:CO2 complex.

Ab Initio Molecular Dynamics Computation of the Infrared Spectrum of Aqueous Uracil

Recent progress in the development of ab initio molecular dynamics methods for the computation of infrared absorption spectra in condensed molecular systems is reviewed and illustrated by a detailed account of an application to aqueous uracil. Similar to classical force field simulations, the spectrum is obtained as the Fourier transform of the polarization time autocorrelation function. The density functional methodology for the computation of electronic polarization in periodic supercells is briefly outlined, and also the effect of quantum corrections is discussed. The spectral patterns obtained for the model system in the 2000−1000 cm-1 domain are in good agreement with experiment. Comparing to the low-temperature vacuum spectrum computed by similar time-dependent methods, we found that the narrow amide bending band in a vacuum is spread out over a 500 cm-1 wide interval in solution with a substantially blue-shifted high-frequency end. The highest increase in frequency was found for N1−H1 bending. The ...

Two-Dimensional ATR−FTIR Spectroscopic Investigation on Water Diffusion in Polypropylene Film: Water Bending Vibration

Two-dimensional (2D) correlation ATR−FTIR spectroscopy was used to study the dynamic diffusion behavior and state of water in syndiotactic polypropylene (S-PP). The 2D asynchronous spectra of water bending band clearly reveal that there are three separate bands in the 1800−1500 cm-1 region. These three bands at 1676, 1645, and 1592 cm-1 are assigned, respectively, to the aggregated water with strong hydrogen bond, the aggregated water with moderate strong hydrogen bond, and the “free water”. On the basis of this approach, the following mechanism for the diffusion process of water in S-PP has been proposed: the aggregated water molecules first diffuse into the polymer during the diffusion process. The water molecules with strong hydrogen bond will diffuse slower than water with moderate strong hydrogen bond because of their large size. In the late stage of the diffusion process, some aggregated water molecules in the PP matrix are forced to dissolve into the “free water” form.

Conformational substates and dynamic properties of carbonmonoxy hemoglobin

Heme pocket dynamics of human carbonmonoxy hemoglobin (HbCO) is studied by Fourier transform infrared spectroscopy. The CO stretching band at various temperatures in the interval 300–10 K is analyzed in terms of three taxonomic A substates; however, in HbCO the band attributed to the A 1 taxonomic substate accounts for ≈90% of the total intensity in the pH range 8.8–4.5. Two different regimes as a function of temperature are observed: below 160 K, the peak frequency and the bandwidth of the A 1 band have constant values whereas, above this temperature, a linear temperature dependence is observed, suggesting the occurrence of transitions between statistical substates within the A 1 taxonomic substate in this protein. The relationship between the heme pocket dynamics (as monitored by the thermal behavior of the CO stretching band), the overall dynamic properties of the protein matrix (as monitored by the thermal behavior of Amide II and Amide I′ bands) and the glass transition of the solvent (as monitored by the thermal behavior of the bending band of water) is also investigated. From this analysis, we derive the picture of a very soft heme pocket of hemoglobin characterized by rather large anharmonic terms and strongly coupled to the dynamic properties of the solvent.

Raman-spectral depolarisation ratios of ions in concentrated aqueous solution. The next-to-negligible effect of highly asymmetric ion surroundings on the symmetry properties of polarisability changes during vibrations of symmetric ions.: Ammonium sulphate and tetramethylammonium bromide

Depolarisation ratios ρ have been measured for the Raman spectra of solutions of composition (NH 4) 2 SO 4 · 11H 2O and (CH 3) 4NBr · 29D 2O. Even though the former's vibration spectrum shows clear evidence of lowered ion symmetries (presence of ν 1 of SO 4 2− in the IR spectrum, IR versus R ν max shifts for ν 3 and ν 4 of SO 4 2− and ν 4 of NH 4 +) ν 1 of SO 4 2− has (apparent) ρ of only 0.014, while ν 2, ν 3 and ν 4 of SO 4 2− and ν 4 (probably also ν 2) of NH 4 + have ρ in the range 0.73–0.77; within the experimental error and base line uncertainty the latter are equal to 0.75, i.e. to ρ max with the geometry of the optics used. For (CH 3) 4NBr symmetric N +C stretching has ρ 0.012; all-in-phase CH stretching and four overtones in Fermi resonance with it have ρ in the range 0.02–0.035, but the deviation from zero here is in part due to underlying or overlapping depolarised bands. The sufficiently well isolated antisymmetric CH stretching and degenerate CH bending bands again have ρ in the range 0.74–0.76. These results show that the selection rules in respect of ρ, which apply strictly only to isolated molecules, are for practical purposes still valid for molecules in strongly symmetry-distorting external environments in the liquid phase. More specifically: (A) During vibrations in which quasi-spherical intramolecular symmetry is retained, the externally caused aspherical component of the polarizability ellipsoid does not change aspherically to a sufficient extent for an appreciably intense anisotropic Raman band to appear. (B) During intramolecularly anti-symmetric vibrations of symmetric molecules, the portion of the externally caused distortion of the polarizability ellipsoid that fails to cancel over a whole vibration period is not large enough to give rise to an appreciably intense isotropic component of the Raman band. This means in practice ρ for these Raman bands is still ρ max, even for concentrated aqueous solutions.

FTIR studies of chitosan acetate based polymer electrolytes

Chitosan is the product when partially deacetylated chitin dissolves in dilute acetic acid. As such, depending on the degree of deacetylation, the carbonyl, CONHR band can be observed at ∼1670 cm −1 and the amine, NH 2 band at 1590 cm −1. When lithium triflate is added to chitosan to form a film of chitosan acetate–salt complex, the bands assigned to chitosan in the complex and the spectrum as a whole shift to lower wavenumbers. The carbonyl band is observed to shift to as low as 1645 cm −1 and the amine band to as low as 1560 cm −1. These indicate chitosan–salt interactions. Also present are the bands due to lithium triflate i.e. ∼761, 1033, 1182 and 1263 cm −1. When chitosan and ethylene carbonate (EC) are dissolved in acetic acid to form a film of plasticized chitosan acetate, the bands in the infrared spectrum of the films do not show any significant shift indicating that EC does not interact with chitosan. EC–LiCF 3SO 3 interactions are indicated by the shifting of the CO bending band from 718 cm −1 in the spectrum of EC to 725 cm −1 in the EC–salt spectrum. The Li +–EC is also evident in the ring breathing region at 893 cm −1 in the pure EC spectrum. This band has shifted to 898 cm −1 in the EC–salt spectrum. CO stretching in the doublet observed at 1774 and 1803 cm −1 in the spectrum of pure EC has shifted to 1777 and 1808 cm −1 in the EC–salt spectrum.

Infrared and Raman studies of water molecule normal vibrations in crystalline hydrates which form the chain structures

Infrared and Raman spectra of aqua-pentachloro-complexes (M 2 I[FeCl 5(H 2O)] (M I=K +, Rb +), M 2 I[InCl 5(H 2O)] (M I=K +, Rb +, Cs +)) and their partially deuterated analogues have been studied in the range 100–4000 cm −1, at room (RT) and liquid-nitrogen (LNT) temperature. In the vibrational spectra of these compounds, the following spectral effects were observed: (a) low HOH bending frequencies, (b) simultaneous shift of OH stretching and HOH bending bands towards lower frequencies at LNT, (c) mixing of libration modes. Moreover a strong influence of central cations (M III) and alkali cations (M I) on the frequency of water molecule normal vibrations was noticed. A detailed analysis of structural and spectral data suggests that above mentioned effects may be correlated with the chain structure of studied monohydrates.

Collisional parameters of H 2O lines: effects of vibration

A complex semiclassical model for the calculation of line widths and shifts of H 2O broadened by N 2, derived from the Robert and Bonamy approach, is tested by comparisons with measurements for selected transitions in various vibrational bands. The lines retained, which involve rotational states with Kc= J or J−1 have been chosen for two reasons. The first is that they show large variations of the widths with J and thus enable a severe test of the model. The second is that, as explained in this paper, they are well-suited for the study of the effects of vibration on the collisional parameters. The measured values have been extracted from an updated version of a database built years ago (JQSRT 52 (1994) 481) that contains all available measurements. Comparisons between experimental and calculated widths and shifts at room temperature illustrate the quality of the model and clearly demonstrate, for the first time, that there is a vibrational dependence of the broadening. Values of collisional parameters are first studied in fundamental bands. This shows that the theoretical approach accounts for most of the dependence of broadening and shifting on rotational quantum numbers: the variations of γ, which reach a factor of nearly 20 from low to high J values, are correctly accounted for by the model as are some specific features of the shifts δ. Analysis confirms that the bending and stretching vibrations have significantly different effects on δ, due to the vibrational dependence of the intermolecular potential. On the other hand, differences on the widths are rather small with slightly smaller broadening for lines of the bending band. Calculations show that there is a spectroscopic effect, due to the larger rotational constant A in the v 2=1 state. Calculations made for overtone bands involving numerous quanta of the stretching vibration are then presented. They predict that a significant dependence of the width should be observed for high J lines due to the effect of vibration on the interaction potential. This is confirmed by comparisons with measurements for lines involving a change of three and four quanta of stretching vibration.

Surface-Enhanced Raman Scattering Spectra of Thiourea Adsorbed at an Iron Electrode in NaClO4 Solution

Surface-enhanced Raman scattering (SERS) spectra of thiourea at an electrochemically activated iron electrode have been investigated as a function of applied potential. The marked downshift of the CS stretching and upshift of the NCN symmetric stretching by comparison of the SERS with the normal Raman spectrum of thiourea are clearly observed and interpreted by coordination of thiourea with the iron surface through sulfur atom. The appearance of the low-frequency mode at ca. 283 cm-1 assigned to the Fe−S vibration supports the assumption of S-coordination of thiourea. The orientation of thiourea molecules is assumed to be slightly inclined to the iron surface at an intermediate angle due to the observation of the SCNN out-of-plane bending band. Potential dependence of the SERS spectra shows that thiourea may interact more strongly with the iron surface at relatively positive potentials. Coadsorption of the supporting electrolyte anion ClO4- with thiourea has also been confirmed by the SERS spectra.

Mixed trehalose/sucrose glasses used for protein incorporation as studied by infrared and optical spectroscopy

Evaporation of water from a 1/1 mixture of trehalose and sucrose gives rise to optically clear glasses that are transparent in the UV and visible ranges and do not crystallize when they are prepared at ambient temperatures. Two proteins, liver alcohol dehydrogenase and parvalbumin, and the tryptophan derivative N-acetyl-tryptophanamide were incorporated into the glasses. Infrared spectroscopy of the amide I band reveals that the proteins retain secondary structure in the glass over a temperature range of 20–300 K. The amide II band of the protein and the HOH bending band of residual water in the glass shift with temperature changes, consistent with increased H-bonding strength as temperature is lowered. Phosphorescence of tryptophan can be seen from the proteins at room temperature, which shows the immobilization of the protein by the glass and the curbing of oxygen diffusion. It is suggested that using mixed sugars to form glasses is a way to immobilize proteins over a wide temperature range without distortions from solvent crystals.

The effect of short chain branching on local chain organisation in isotopically labelled blends of polyethylene

The complementary techniques of small angle neutron scattering (SANS) and infra-red spectroscopy have been used to determine features of molecular trajectory for isotopic blends incorporating linear polyethylene and copolymers containing butyl or hexyl branches. SANS data show both a more compact conformation for a copolymer guest molecule than for a linear guest and also a smaller molecular expansion with increasing molecular weight when both guest and host are copolymers. These blends also show the smallest proportion of [lcub ]110[rcub ] isolated guest stems, while wholly linear blends show the largest proportion, on the evidence of the infra-red CD 2 bending band profiles. Estimates are made of the sizes of ‘groups’ of adjacent stems, and these also show a corresponding dependence on the sample type, indicating a higher proportion of adjacent re-entry for copolymer blends than for linear blends.

The role of water in synthesised hydrotalcites of formula Mg xZn 6− xCr 2(OH) 16(CO 3) · 4H 2O and Ni xCo 6− xCr 2(OH) 16(CO 3) · 4H 2O—an infrared spectroscopic study

Infrared spectroscopy has been used to characterise synthesised hydrotalcites of formula Mg x Zn 6− x Cr 2(OH) 16(CO 3) · 4H 2O and Ni x Co 6− x Cr 2(OH) 16(CO 3) · 4H 2O. The infrared spectra are conveniently subdivided into spectral features based (a) upon the carbonate anion (b) the hydroxyl units (c) water units. Three carbonate antisymmetric stretching vibrations are observed at around 1358, 1387 and 1482 cm −1. The 1482 cm −1 band is attributed to the CO stretching band of carbonate hydrogen bonded to water. Variation of the intensity ratio of the 1358 and 1387 cm −1 modes is linear and cation dependent. By using the water bending band profile at 1630 cm −1 four types of water are identified (a) water hydrogen bonded to the interlayer carbonate ion (b) water hydrogen bonded to the hydrotalcite hydroxyl surface (c) coordinated water and (d) interlamellar water. It is proposed that the water is highly structured in the hydrotalcite interlayer as it is hydrogen bonded to both the carbonate anion, adjacent water molecules and the hydroxyl surface.