NMR and IR study of fluorobenzene and hexafluorobenzene adsorbed on alumina

The adsorption of fluorobenzene (C6H5F) and hexafluorobenzene (C6F6) onto the surface of neutral alumina is investigated by reflectance IR spectroscopy, near-IR spectroscopy, and measurement of 19F NMR chemical shift values. Chemical shifts are dependent on surface coverage and reveal multiple peaks where different adsorption environments occur. C6H5F appears to be adsorbed through polar interactions with surface hydroxyls, whereas C6F6 shows separate resonances for the first layer of coverage and outer layers. Available surface areas are estimated, and reorientation of adsorbed hexafluorobenzene is proposed to account for chemical shift behavior and differences between calculated and measured monolayer coverage.     

NMR investigation of concentrated alumina and silica slurries

The longitudinal relaxation times (T₁) of water in concentrated silica and alumina slurries were measured as a function of solids content. It was shown that the results could be fit very well with a two-phase fast-exchange model between free and surface-bound water. As expected, values of T₁ for bound water were in the order of 20–2000 times lower than that for free water, indicating a higher effective viscosity of the surface-bound water. The strength of the interaction depended on the particular surface, and all of the aluminas examined interacted more strongly with water than the two silicas studied, which themselves differed considerably. The chemical mechanical polishing (CMP) removal rate of tantalum by silica slurries was shown to be directly correlated with the interaction parameters, derived from the NMR relation times rather than with total surface hydroxyl group concentration.     

Influence of V content on the nature and strength of acidic sites in VSibeta zeolite evidenced by IR spectroscopy

VSibeta zeolites prepared by a two-step postsynthesis method have been characterized by physical techniques. A significant reduction of intensity of the IR band near 3515 cm(-1) after impregnation of dealuminated beta zeolite with aqueous NH4VO3 indicates that V ions specifically react with hydrogen-bonded SiO-H groups of vacant T sites. IR bands at 3618 and 3645 cm(-1) are assigned to SiO-H groups interacting with V and to VO-H groups, respectively. In VSibeta, diffuse reflectance UV-visible data show that below 1.9 wt % V is present as lattice tetrahedral species and at higher content as extra-lattice octahedral species (mononuclear and polynuclear). VSibeta samples are EPR-silent at 298 or 77 K suggesting that there are no paramagnetic VIV ions. IR studies show that V-OH groups are less acidic than Si-OH-Al groups of parent HAlbeta zeolite. IR results of CO adsorption evidence three kinds of Lewis acidic sites, related to lattice mononuclear and extra-lattice mononuclear and polynuclear V species. Quantitative IR studies of ammonia and pyridine adsorption reveal that only about half of V introduced into zeolite is able to form either Br?nsted or Lewis acidic sites.     

Nature and structure of aluminum surface sites grafted on silica from a combination of high-field aluminum-27 solid-state NMR spectroscopy and first-principles calculations

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.     

Interface structure of silicon nanocrystals embedded in an amorphous silica matrix

We performed molecular dynamics simulations of the atomic structure of silicon nanocrystals embedded in a stoichiometric amorphous silica matrix. The atom–atom interactions are described by a combination of well-assessed potentials for bulk silicon and SiO₂, plus a mixing term to allow adjusting the charge transfer at the interface between Si and silica. For the free-standing Si nanocrystals, we find that the spherical structure is favoured with respect to the faceted one, up to at least a diameter of 6 nm. Correspondingly, the surface layer shows a higher diffusivity than the bulk. When embedded in the silica matrix, nanocrystals are under severe mechanical stress which is released by the combined formation of porosity at the interface and of bridging Si–O–Si bonds, whose density increases with the nanocrystal size. Vibrational frequencies specific to the interface bonding are identified and discussed.     

Computational study of osmabenzyne: The solvent effects on the structure and spectroscopic properties (IR,NMR)

In this article, the structural, electronic, and spectroscopic properties of osmabenzyne Os{≡CC(SiH₃)=C(CH₃)C(SiH₃)=CH}Cl₂(PH₃)₂ are explored in the gas phase and five solvents. The effects of solvents on the structural parameters, frontier orbital energies, and spectroscopic parameters of the complex are elucidated using the polarizable continuum model. The wavenumbers of selected IR-active vibrations in different solvents are obtained and correlated with the Kirkwood–Bauer–Magat equation. In addition, thermodynamic parameters of solvation are calculated for the complex. ¹H and ¹³C NMR chemical shifts are estimated using the gauge-invariant atomic orbital method.     

Surface acidic sites of highly disperse magnesium chloride: IR and ESR spectroscopy studies

Experimental data obtained by means of some high performance techniques allow to identify and register quantitatively the Lewis acidic sites on the surface of highly disperse magnesium chloride. Carbon monoxide adsorption on magnesium chloride has been studied with the IR diffuse reflectance spectroscopy (IRS-DR). The interaction of some stable nitroxyl radicals with magnesium chloride has been investigated with the ESR technique.     

NMR and IR spectroscopic study of the structure of quaternary salts of 1-vinylpyrazoles

Conclusions IR and NMR spectral analysis indicated that quaternization of 1-vinylpyrazoles leads to a decrease in conjugation between the heterocycle and the vinyl group and to a significant leveling out of the charges on the atoms in the heterocycle and vinyl group.DeceasedTranslated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1007–1012, May, 1985.     

Conformational structure of N-methylpropionamide and N-methylisobutyroamede. NMR shift reagent and IR study

The structures of the stable conformers of N-methyjpropionamide and N-methyliso-butyroamide in CCl₄, solution were determined by a combination of IR spectroscopy and NMR spectroscopy with lanthanide shift reagents. N-methyl-propionamide was found to exist in the form of two rotational isomers, 1 and 2, with the ethyl group twisted out of the plane of the amide bond by the angles Ψ = 140 and 20°, respectively. For these two conformers, the enthalpy difference is ΔH = 2.13 ± 0.08 kcal mole^?1 and the entropy difference ΔS = 7.81 ± 0.55 cal mole^?1 grad^?1. N-methylisobutyroamide exists in a single form, with the two C-methyl group positions very close to those found in the two isomers of N-methylpropionamide.     

Pore structure of samples of amorphous silica of various origins by data on 1H NMR

Pore structure of amorphous silica obtained from biogenic and mineral raw materials was studied by the nuclear magnetic resonance (¹H NMR) at 200–298 K. As biogenic raw materials were used fruit shells, rice and oats straw, larch needles, horsetail pedicels, and diatomic algae. The pore size distribution in silica samples of various origins was determined.     

Adsorption mechanism of substituted pyridines on silica suspensions: an NMR study

Adsorption isotherms of 2,5-dimethylpyridine (2,5-DMP) on Aerosil 200 silica from water-2,5-DMP binary mixtures are known to exhibit special features indicative of surface phase transitions in the adsorbed layer. We have made similar observations on another substituted pyridine, 2,4,6-trimethylpyridine (2,4,6-TMP). By using 1D and 2D NMR spectroscopy, we investigated adsorbed layers on silica in suspension in water/substituted-pyridine mixtures and demonstrated the existence of adsorbed species specific signals. Comparison of signals with those displayed on NMR spectra of liquid binary mixtures under various pH conditions rules out adsorption via interaction of the surface silanol group and the lone electron pair of the nitrogen atom. A mechanism of adsorption through the interaction of surface siloxane oxygen and the aromatic pi-system is proposed; it is consistent with both thermodynamic measurements and stacking of substituted pyridines within the adsorbed layer.     

On the structure of amorphous calcium carbonate--a detailed study by solid-state NMR spectroscopy

The calcium carbonate phases calcite, aragonite, vaterite, monohydrocalcite (calcium carbonate monohydrate), and ikaite (calcium carbonate hexahydrate) were studied by solid-state NMR spectroscopy ( (1)H and (13)C). Further model compounds were sodium hydrogencarbonate, potassium hydrogencarbonate, and calcium hydroxide. With the help of these data, the structure of synthetically prepared additive-free amorphous calcium carbonate (ACC) was analyzed. ACC contains molecular water (as H 2O), a small amount of mobile hydroxide, and no hydrogencarbonate. This supports the concept of ACC as a transient precursor in the formation of calcium carbonate biominerals.     

Revisiting structure of silica gels from water glass: an 1H and 29Si MAS and CP-MAS NMR study

The silica gels, derived from water glass solution with pH adjusted at 3.0 and 9.9, were revisited to investigate their constitution, although water glass has been studied for last tens of decades on gelation. Solid-state nuclear magnetic resonance spectroscopy was applied to the nuclei ¹H and ²⁹Si, by the use of magic angle spinning (MAS), ¹H → ²⁹Si CP-MAS (CP: cross-polarization), and modern techniques such as 2D HETCOR (two dimensional heteronuclear correlation), and variable-contact time CP techniques. Gelation time (t_gel) showed U-letter shape dependence on pH. All gels consisted of Qⁿ groups (n: 2, 3, and 4), where Qⁿ stands for a silicate unite [(O_1/2)_nSi (–O^?)_4?n] (n: 0–4). The analysis of the ¹H → ²⁹Si CP kinetics and ¹H-²⁹Si HETCOR spectra elucidated the presence of four kinds of ¹H nuclei, i.e., those giving a peak at 6.9 ppm in chemical shift δ: ¹H–OSi hydrogen bonded to H₂O molecules; one at 4.3 ppm: ¹H of adsorbed water molecules, hydrogen-bonded to the silanol groups; one at 1.7 ppm: ¹H–OSi confined in the gel lattice, including that forming aggregations like Si–O^H/_NaO–Si; and one at 4.2 ppm: ¹H of water molecules on the outermost hydration layer.     

Manganese alkane complexes: an IR and NMR spectroscopic investigation

Manganese propane and manganese butane complexes derived from CpMn(CO)(3) were generated photochemically at 130-136 K with the alkane as solvent and characterized by FTIR spectroscopy and by (1)H NMR spectroscopy with in situ laser photolysis. Time-resolved IR spectroscopic measurements were performed at room temperature with the same laser wavelength. The ν(CO) bands in the IR spectra of the photoproducts in propane are shifted to low frequency with respect to CpMn(CO)(3), consistent with formation of CpMn(CO)(2)(propane). The (1)H NMR spectra conform to the criteria for alkane complexes: a high-field resonance for the η(2)-CH protons that shifts substantially on partial deuteration of the alkane and exhibits a coupling constant J(C-H) on (13)C-labeling of ca. 120 Hz. The NMR spectrum of each system exhibits two diagnostic product resonances in the high-field region for the η(2)-CH protons, corresponding to CpMn(CO)(2)(η(2)-C1-H-alkane) and CpMn(CO)(2)(η(2)-C2-H-alkane) isomers. Partial deuteration of the alkane at C1 results in characteristic strong isotopic perturbation of equilibrium of the η(2)-CH resonance of CpMn(CO)(2)(η(2)-C1-H-alkane). With propane-(13)C(1), the η(2)-CH resonance of CpMn(CO)(2)(η(2)-C1-H-alkane) isomer exhibits (13)C satellites with J(C-H) = 119 Hz. The corresponding resonance of CpMn(CO)(2)(η(2)-C2-H-alkane) is identified by use of propane-2,2-d(2). The lifetimes of the (η(2)-C1-H-alkane) isomers of the manganese complexes were determined by NMR spectroscopy as 22 ± 2 min at 134 K (propane) and 5.5 min at 136 K (butane). The corresponding spectra and lifetimes of the CpRe(CO)(2)(alkane) complexes were measured for reference (CpRe(CO)(2)(propane) lifetime ca. 60 min at 161 K; CpRe(CO)(2)(butane) 13 min at 171 K). The lifetimes determined by IR spectroscopy were similar to those determined by NMR spectroscopy, thereby supporting the assignments. These measurements extend the range of alkane complexes characterized by NMR spectroscopy from rhenium and rhodium derivatives to include less stable manganese derivatives.     

Nature of the Lewis acid sites on molybdenum and ruthenium sulfides: an electrostatic potential study

The energy of formation and the Lewis acid strength of sulfur vacancies or coordinative unsaturated sites on the MoS2 edges were studied using density functional theory for periodic systems and an electrostatic potential-based methodology. The results suggest that the more energetically favorable sites are located on the sulfur edges; however, their Lewis acid strength is considerably smaller than the site acidity at the molybdenum edges. The acid strength for the reported most hydrodesulfurization active site of RuS2 was also determined. In general, the Lewis acid for the site on RuS2 is 100% smaller than the sites on the Mo edges and around 20% larger than the most favorable site on the S edges of MoS2. Binding of the pyridine molecule in the eta1 adsorption configuration on the considered sites has corroborated the trend of Lewis acidity suggested by the electrostatic potential methodology.     

IR spectroscopic study of the basicity of aminated silica gels

The aminated silica gels SiO₂/SOCl₂/NH₃ (I), SiO₂/SiCl₄/NH₃ (II), SiO₂/BCl₃/NH₃ (III), and SiO₂/γ-aminopropyltriethoxysilane (SiO₂/APTES, IV) have been synthesized. According to DRIFT spectroscopy and chemical analysis data, the surface amino groups of I–III are “free,” while those of IV interact with the surface OH groups of the silica gel and with one another. The strength of basic sites has been measured on the proton affinity (PA) scale as the shift of the ν(CD) band of adsorbed deuterochloroform. The basicity of an aminated silica gel depends on its chemical composition. Silica gel IV (PA = 938 kJ/mol) is a stronger base than I–III (PA = 829 kJ/mol). As the basicity of the NH₂ group decreases, the N-H stretching band shifts to higher frequencies.     

Interaction of hydronium ion with dibenzo-18-crown-6: NMR, IR, and theoretical study

Interaction of dibenzo-18-crown-6 (DBC) with H 3O (+) (HP) in nitrobenzene- d 5 and dichloromethane- d 2 was studied by using (1)H and (13)C NMR spectra and relaxations, FTIR spectra, and quantum chemical DFT calculations. NMR shows that the DBC*HP complex is in a dynamic equilibrium with the reactants, the equilibrium constant K being 0.66 x 10 (3), 1.16 x 10 (4), and 1.03 x 10 (4) L x mol (-1) in CD 2Cl 2, nitrobenzene, and acetonitrile, respectively. The complex appears to have a C 2 v symmetry in NMR, but FTIR combined with DFT normal mode calculations suggest that such high symmetry is only apparent and due to exchange averaging of the structure. FTIR spectra as well as energy-optimized DFT calculations show that the most stable state of the complex in solution is that with three linear hydrogen bonds of HP with one CH 2-O-CH 2 and two Ar-O-Ar oxygen atoms. The structure is similar to that found in solid state but adopts a somewhat different conformation in solution. The dynamics of exchange between bound and free DBC was studied by NMR transverse relaxation. It was found to be too fast to give reproducible results when measured with the ordinary CPMG sequence or its variant DIFTRE removing residual static dipolar interaction, but it could be established by rotating-frame measurements with high intensity of the spin-lock field. The correlation time of exchange was found to be 5.6 x 10 (-6) and 3.8 x 10 (-6) s in dichloromethane and nitrobenzene, respectively. Such fast exchange can be explained by cooperative assistance of present water molecules.     

Electron acceptor properties of aluminosilicate, alumina and silica

A solution of diphenylamine in benzene is shown to produce cation-radicals on the surface of aluminosilicate, alumina and silica, whose electron acceptor properties for diphenylamine decrease in the sequence: aluminosilicata > alumina > silica.

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