Simple models of adsorbed polymers: Vibrational properties

The vibrational properties of polymers adsorbed on surfaces are investigated within the frame of two simple models. First the surface is modelled as a one-dimensional infinite harmonic chain. Polymeric “ molecules ” are represented as harmonic chains of finite length. The coupling between the polymers and the surface is treated within the frame of the Interface Response Theory. Upon adsorption of polymers, resonant vibrational modes appear as well defined peaks in the variation of the density of states of the system. The effect of the interaction between adsorbed polymers via phonons is investigated and shown to lead to antiresonances in the spectral density of states and the formation of gaps in the density of states as the surface coverage is increased. A second more realistic model is introduced where the same finite harmonic chains (polymers) are grafted on a two dimensional [001] surface cut through a cubic harmonic crystal. Again the variation in density of states exhibits resonant modes between the polymers and the substrate. However, in this case, the interaction between polymers is short range. Adsorption of a film of polymers produces resonant modes which remain well-defined features within the substrate bulk band.     

Vibrational and electron paramagnetic resonance properties of free and MgO supported AuCO complexes

The bonding, spin density related properties, and vibrational frequency of CO bound to single Au atom in the gas-phase or supported on MgO surfaces have been investigated with a variety of computational methods and models: periodic plane waves calculations have been compared with molecular approaches based on atomic orbital basis sets; pseudopotential methods with all electron fully relativistic calculations; various density functional theory (DFT) exchange-correlation functionals with the unrestricted coupled-cluster singles and doubles with perturbative connected triples [CCSD(T)]. AuCO is a bent molecule but the potential for bending is very soft, and small changes in the bond angle result in large changes in the CO gas-phase vibrational frequency. At the equilibrium geometry the DFT calculated vibrational shift of CO with respect to the free molecule is about -150 cm(-1), whereas smaller values -60-70 cm(-1) are predicted by the more accurate CCSD(T) method. These relatively large differences are due to the weak and nonclassic bonding in this complex. Upon adsorption on MgO, the CO vibrational shift becomes much larger, about -290 cm(-1), due to charge transfer from the basic surface oxide anion to AuCO. This large redshift is predicted by all methods, and is fully consistent with that measured for MgOAuCO complexes. The strong influence of the support on the AuCO bonding is equally well described by all different approaches.     

Vibrational spectra of CO adsorbed on iridium alloys

The vibrational spectra of carbon monoxide chemisorbed on alloys of Ir-Rh, Ir-Pd, Ir-Pt and Ir-Cu of different composition have been investigated at temperatures from 30 to 400°C. It has been shown that the surface of an alloy does not correspond in its properties to a simple mixture of the two components concerned. In the case of CO chemisorbed on Ir-Rh, Ir-Pd and Ir-Cu, a lowering of the frequency of CO has been observed. This effect is probably due to electron transitions between the alloy components.     

Vibrational analysis of propene adsorbed in zeolites Y

Normal coordinate analysis was performed for propene adsorbed in faujasites NaY, CaY and MgY applying a valence force model. The force fields obtained were compared with that of the free molecule. Only small shifts of the fundamentals are observed due to the weak interaction with the zeolites. Although in general the results are comparable to those of propene adsorbed in different zeolites A, some distinct features such as a stronger influence on the CC stretching vibration, a smaller hindrance of the CH bending vibration and a higher reduction of the CH (CH₃) bond strength are detectable, which must be attributed to both the different composition and structure of the zeolite framework. The frequency shifts and the corresponding changes of the force constants are related to the geometry of the sorption complex of propene on the zeolite surface.     

Vibrational behavior of adsorbed CO2 on single-walled carbon nanotubes

We present theoretical and experimental evidence for CO(2) adsorption on different sites of single walled carbon nanotube (SWNT) bundles. We use local density approximation density functional theory (LDA-DFT) calculations to compute the adsorption energies and vibrational frequencies for CO(2) adsorbed on SWNT bundles. The LDA-DFT calculations give a range of shifts for the asymmetric stretching mode from about -6 to -20 cm(-1) for internally bound CO(2), and a range from -4 to -16 cm(-1) for externally bound CO(2) at low densities. The magnitude of the shift is larger for CO(2) adsorbed parallel to the SWNT surface; various perpendicular configurations yield much smaller theoretical shifts. The asymmetric stretching mode for CO(2) adsorbed in groove sites and interstitial sites exhibits calculated shifts of -22.2 and -23.8 cm(-1), respectively. The calculations show that vibrational mode softening is due to three effects: (1) dynamic image charges in the nanotube; (2) the confining effect of the adsorption potential; (3) dynamic dipole coupling with other adsorbate molecules. Infrared measurements indicate that two families of CO(2) adsorption sites are present. One family, exhibiting a shift of about -20 cm(-1) is assigned to internally bound CO(2) molecules in a parallel configuration. This type of CO(2) is readily displaced by Xe, a test for densely populated adsorbed species, which are expected to be present on the highest adsorption energy sites in the interior of the nanotubes. The second family exhibits a shift of about -7 cm(-1) and the site location and configuration for these species is ambiguous, based on comparison with the theoretical shifts. The population of the internally bound CO(2) may be enhanced by established etching procedures that open the entry ports for adsorption, namely, ozone oxidation followed by annealing in vacuum at 873 K. Xenon displacement experiments indicate that internally bound CO(2) is preferentially displaced relative to the -7 cm(-1) shifted species. The -7 cm(-1) shifted species is assigned to CO(2) adsorbed on the external surface based on results from etching and Xe displacement experiments.     

Vibrational spectroscopy of adsorbed species on nickel by neutron inelastic scattering

In a test of the utility of neutron inelastic spectroscopy (NIS) for studying molecular vibrations in surface reactions, we have examined the adsorption, co-adsorption, and reaction of hydrogen and carbon monoxide on Raney nickel catalysts.     

Vibrational frequencies and normal modes of Co adsorbed on Ni(001)

The vibrational modes of CO adsorbed on a Ni(001) metal surface have been obtained using two different cluster models to represent the on-top and bridge-bonding situations. New modes, not previously available in the literature, are obtained for the bridge cluster.     

Rheology of polystyrene latexes with adsorbed and free gelatin

The rheology of monodisperse polystyrene latex particles of two different particle radii (26 and 67 nm) has been studied with a range of concentrations of the polyampholyte gelatin. Gelatin contributes to the rheology by adsorption to the particles and by thickening the continuous phase. High viscosities and strong shear thinning are measured for low volume fractions of latex. A procedure is presented to deconvolute the effects of free and bound gelatin by applying simple hard-sphere models. This procedure allows us to estimate the effective size of the gelatin-covered particles as well as the continuous-phase gelatin concentration and viscosity. The layer thicknesses from rheology agree well with those from PCS. The effect of varying particle volume fraction, ionic strength, pH and gelatin and surfactant concentration on the rheology of these suspensions is presented. For the smaller latex, the adsorbed layer occupies a greater fraction of the effective volume. Increasing free polymer concentration reduces the adsorbed-layer thickness. The reduced critical shear stress increases with the suspension viscosity for suspensions of the 26 nm latex but is constant for the 67 nm latex. At very high shear (>2000 s⁻¹), the suspensions show excess shear thinning over that expected from a hard-sphere model. This excess thinning is attributed to deformation of the adsorbed gelatin layer under high shear stress and interpreted in terms of an empirical interparticle potential.     

Vibrational properties of sodium microclusters

The structural and vibrational properties of Na_n (n=2–8) microclusters at very low temperature (T≈0.1 K) are analyzed by tight-binding molecular dynamics simulations. By using the Fourier transform of the velocity autocorrelation function from the trajectories of tight-binding molecular dynamics, the present model is successful in predicting some of the experimental, far infrared spectra and agrees very well with a standard finite difference harmonic method and some other more sophisticated calculations.     

Preparation of adsorbed lipase and its properties

Summary 1. Under certain conditions, lipase immobilized on cellulose-based ion-exchange materials can be obtained. The best adsorbent is DEAE-cellulose.2. The optimum conditions for the immobilization of lipase on DEAE-cellulose are pH 8.0, time of contact with the support 15–20 min, temperature 10° C.3. The immobilized lipase (DEAE-cellulose-lipase) retains the main functional properties of the native enzyme.Department of Microbiology, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 267–271, March–April, 1977.     

Vibrational analysis of free nine-atomic and bound five-atomic tetrahedral molecules

A theoretical investigation of the vibrational motion of a free nine-atomic tetrahedral molecule is given using Wilson's. FG-matrix method. By going to the limit of infinite mass of the outermost atoms, the vibrational frequences of a bound five-atomic tetrahedral molecule are obtained. This limiting case is also alternatively treated by the explicit consideration of the hindered translational and rotational motions. Such a model would probably approximate the state of affairs for amorphous substances with a tetrahedral basic unit.

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Zusammenfassung Die Schwingungen eines freien, neunatomigen, tetraedrischen Moleküls werden analysiert. Indem man die Massen der äußersten Atome unendlich werden läßt, erhält man die Schwingungsfrequenzen eines gebundenen, fünfatomigen tetraedrischen Moleküls. Dieser Fall wurde auch direkt unter Berücksichtigung der Behinderung der Bewegungen behandelt. Dieses Modell könnte zur Beschreibung amorpher Substanzen aus tetraedrischen Grundeinheiten dienen.

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Résumé Etude théorique du mouvement vibratoire d'une molécule libre tétraédrique à neuf atomes au moyen de la méthode de la matrice FG de Wilson. En passant à la limite de masse infinie des atomes périphériques, les fréquences vibrationnelles d'une molécule liée tétraédrique à cinq atomes sont obtenues. Ce cas limite est aussi traité par la considération explicité des mouvements de rotation et de translation non libres. Un tel modéle constitue probablement une approximation pour les substances amorphes à unité de base tétraédrique.

     

Vibrational spectroscopy and study of the state and properties of catalysts

Conclusions It can be seen from the list of publications that the method of vibrational spectroscopy has a very broad application in catalysis. A distinctive characteristic of the present situation in this field is the transition to quantitative characterization of the properties of heterogeneous catalysts and to a wider use of theoretical calculations of normal vibrations in very diverse problems, and to a classifying by these means the properties of the substances studied. In the very near future, we can expect a considerable intensification of these trends, and development of more precise methods for the investigation of the properties of surface of solid bodies based on Raman spectroscopy.Institute of Catalysis, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 28, No. 3, pp. 133–144, May–June, 1987.     

Vibrational and correlation effects on properties of water

Large basis set, ab initio potential energy and property surfaces of water have been used with quantum Monte Carlo vibrational analysis in the evaluation of the molecule's rotational constants, zero-point energy, and dipole moment. While there are clearly differences in vibrational state parameters due to including correlation effects, the vibrational averaging effect on rotational constants is very nearly additive with the correlation effect. This has implications for evaluation and estimation of properties of molecules in specific vibrational states. Received: 9 February 1999 / Accepted: 11 February 1999 / Published online: 5 May 1999     

Quinoline, quinazoline and acridone alkaloids

This review covers the isolation, structure determination, synthesis and biological activity of quinoline, quinazoline and acridone alkaloids from plant, microbial and animal sources. The literature from July 2001 to June 2002 is reviewed, and 125 references are cited.     

Vibrational analysis of free and hydrogen bonded complexes of nicotinamide and picolinamide

Harmonic and anharmonic vibrations of free nicotinamide (NIA) and picolinamide (PIA) molecules together with their hydrogen bonded complexes H₂O–NIA and H₂O–PIA have been studied by means of density functional method. The calculation results of the vibrational spectra of free molecules have been investigated and are compared to the available experimental spectra. The vibrational wavenumbers of both molecules have also been calculated by polarizable continuum model (PCM) that represents the solvent as a polarizable continuum and places the solute in a cavity within the solvent (water is chosen as the solvent in this study). The results of PCM calculations and the H₂O–NIA, H₂O–PIA complexes, are used to investigate the H-bonding interactions of both molecules with the water molecule. The harmonic wavenumbers have been scaled by proper factors obtained by comparing the observed versus calculated wavenumbers and it is shown that anharmonic corrections on the vibrational spectra provided a better agreement between the observed and calculated wavenumbers compared to the results obtained by scaling factor method.     

Vibrational frequencies of CO adsorbed on silica supported Mo atoms from density functional calculations

The interaction of CO with silica supported molybdenum atoms has been studied by means of density functional calculations and cluster models. Experimentally two bands in the IR spectra of adsorbed CO have been observed at 2170 and 1990 cm⁻¹ with vibrational shifts of +27 and −153 cm⁻¹, respectively, with respect to the gas-phase molecule, the peak at +27 cm⁻¹ has been related to the presence of neutral Mo atoms anchored to two oxygen atoms of the SiO₂ substrate. Possible reactive sites at the Mo/SiO₂ interface have been explored as candidates for CO adsorption. Mo atoms in various formal oxidation states, from +II to +VI, have been considered. Both molecular and cluster models of the Mo/SiO₂ interface have been employed. The analysis shows that a neutral Mo(II) atom, proposed to be responsible for the blue-shift of ν(CO), is not likely to be the origin of the IR band at 2170 cm⁻¹. Only Mo atoms in high oxidation states or Mo cations carrying a real positive charge can account for the positive shifts in the CO frequency.     

Investigation of the alumina properties with adsorbed polyvinyl alcohol

The influence of solution pH on the structure of polyvinyl alcohol adsorption layer on the alumina surface was investigated. The spectrophotometry, viscosimetry, thermogravimetry, potentiometric titration and microelectrophoresis were applied in experiments. These methods enable determination of the following parameters: adsorbed amount of PVA, stability of suspension without and with polymer, thickness of its adsorption layers, changes in thermal characteristics of Al₂O₃ surface with the adsorbed polymer, surface charge density and zeta potential of solid particles in the presence and absence of PVA, respectively. All measurements were carried out in the pH range 3–9. The obtained results indicate that pH has a great influence on the conformation of PVA chains adsorbed on the alumina surface. It is due to incomplete hydrolysis of acetate groups of polyvinyl alcohol macromolecules (degree of hydrolysis 97.5%), which dissociate with the increasing pH. Moreover, the polymer adsorption on the alumina surface causes changes in the course of thermogravimetric curves. The effect of weight loss for Al₂O₃–PVA systems is smaller than that of Al₂O₃ without polymer. It is due to elimination of water molecules from the solid surface by adsorbed polymer.     

Design,synthesis and investigation of the interaction behavior between two acridone derivatives, 8-chloro acridone and nitrile cyanide acridone with calf thymus DNA,by different spectroscopic techniques

Journal of the Iranian Chemical Society - Evaluating the binding interaction between biomacromolecules and various chemical compounds is one of the most biologically researched topics. The present...     

Quinoline, quinazoline and acridone alkaloids

This review covers the isolation, structure determination, synthesis and biological activity of quinoline, quinazoline and acridone alkaloids from plant, microbial and animal sources. The literature from July 2002 to June 2003 is reviewed, and 119 references are cited.