IR characterization of surface hydroxy groups on haematite

The OH stretching absorptions (3670, 3640 and 3460 cm^–1) detected on haematite samples obtained by thermal decomposition of goethite, are assigned to monocoordinated and bridged free surface OH's and to hydrogen bonded OH's in surface micropores, respectively, on the basis of their behavior to successive adsorption of methanol, pyridine and D₂O and surface structural models.

---

OH (3670, 3640 3460 ^–1) , . OH, OH, , , , D₂O, .

     

Adsorption properties and surface chemistry of MgO

The adsorption properties of MgO, which is used as a sorbent and catalyst support, were studied using gas chromatography. The test absorbents used were n-alkanes (which show only nonspecific dispersion interactions when physisorbed on any adsorbent) and adsorbates whose molecules are capable of specific interactions with the surface reactive sites of MgO. Adsorption isotherms were measured for CHCl₃, CH₃NO₂, CH₃CN, (CH₃)₂CO, CH₃COOC₂H₅, and (C₂H₅)₂O on MgO at 50–100°C. Differential molar enthalpy changes (?ΔH), equal to molar heats of adsorption, were determined. For polar adsorbates, contributions from dispersive and specific interactions into ?ΔH were determined. The electron-acceptor and electron-donor abilities of the MgO surface were estimated.     

Determination of hydroxy and sulfhydryl groups on the surface of sulfided hydrorefining catalysts

Previously proposed methods for the determination of surface hydroxy groups of oxides, based on the reaction of dimethylzinc tetrahydrofuranate with active hydrogen atoms, and for the determination of surface sulfhydryl groups of sulfides, based on the reaction with silver ions, were applied to simultaneous determination of OH and SH groups present on the surface of a sulfided hydrorefining CoMo/Al₂O₃ catalyst.

---

, , , , OH SH, -CoMo/Al₂O₃.

     

Determination of the surface hydroxy groups on alumina and silica-alumina catalysts by dimethylzinc

The earlier proposed pulse flow method for the determination of surface OH groups based on the reaction of gaseous dimethylzinc tetrahydrofuranate with active hydrogen was applied to alumina and silica-alumina catalysts. The concentrations of OH groups found by this method are in a good agreement with the published data, obtained by deuterium exchange and gravimetry.

---

, , OH, , - . OH, , , .

     

Proton donor ability of surface hydroxy groups from the IR spectra of hydrogen-bonded complexes

The influence of the strength of N-bases on the shift of (OH) in hydrogenbonded complexes upon adsorption on the surface of a decationized zeolite, phosphates and aerosil differing in the proton donor ability of surface hydroxy groups has been investigated. It is shown that the acid-base properties of the adsorbent-adsorbate pair produce additive influence on the above frequency shifts. A method for the determination of the proton donor ability of surface hydroxy groups (in terms of pK_a) from the IR spectra of hydrogen-bonded complexes is suggested.

---

- (OH) , , . , - - . ( pK_a) .

     

Estimation of proton donor ability of surface hydroxy groups in terms of proton affinity

A method to estimate the proton donor ability of hydroxy groups in terms of proton affinity (PA) is suggested. The PA of hydroxy groups of aerosil and of decationated zeolite type Y have been determined.

---

(PA). PA .

     

Quantum-chemical study of the structure and thermochemical properties of nitropiperazines and nitrosopiperazines

We have used MNDO and PM3 approximations and the density functional method (6–31G*) for a quantum chemical study of the structure and some physicochemical properties of N-nitro and N-nitroso derivatives of piperazine, furazano[3,4-b]piperazine, and bisfurazano[3,4-b;3,4-e]piperazine. We have analyzed the structural, electronic, and thermochemical characteristics and the enthalpies of formation for the compounds in the gas phase and in the solid phase. We have found a correlation between the strength of the N-N bond and the N-N bond length, the pyramidality of the nitrogen atom of the piperazine ring, and the size of the energy gap between the frontier orbitals. Based on calculations by the density functional method, we have carried out a comparative analysis of the thermochemical stability of the compounds in homolytic reactions.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1817–1839, December, 2004.     

Influence of oxygen compound structural elements of stretching vibration frequency of surface hydroxy groups

Stretching vibration frequencies of OH groups ( OH) in acidic oxygen compounds are determined by the electrostatic characteristics (E_p) of the polyhedron containing this group. Empirical dependence of OH various hydroxy groups on the number and nature of their binding cations is suggested.

---

, OH- ( OH) (). .

     

Spectroscopic properties of trapped electrons on the surface of MgO nanoparticles.

To characterise electron-trapping sites on the surface of MgO nanoparticles, surface colour centres were generated using UV light in conjunction with selected hydrogen-based electron sources. Four different colour-centre species, including the characteristic (e-)(H+) or F(S)+(H) centre, were identified due to the distinct shape of the respective electron paramagnetic resonance (EPR) signals. The analysis of the EPR saturation behaviour down to microwave powers of 5 x 10(-3) mW reveals an enhanced spin-relaxation probability of the (e-)(H+) centre compared to all other F(S)+ centres that do not exhibit significant magnetic interactions with hydroxylic protons. Beside the dipolar magnetic interaction in the (e-)(H+) centre observed by EPR, the electronic interaction between the unpaired electron and the proton of a closely spaced OH group produces a redshift of the OH stretching band by about 70 to 170 cm(-1), as observed by infrared spectroscopy. EPR and IR spectroscopic data obtained after the selective address of individual reaction channels for surface colour-centre formation point to the fact that (e-)(H+) centres are formed by trapping electrons from H atoms. Consequently, the underlying surface defect does not belong to the sites of the MgO surface, which chemisorb hydrogen via a heterolytic splitting process.     

Optical surface properties and morphology of MgO and CaO nanocrystals

Optical absorption and photoluminescence emission properties of dehydroxylated MgO and CaO nanocrystals are discussed with respect to particle morphology and size. On MgO nanocubes with pronounced corner and edge features two emission bands at 3.4 and 3.3 eV result from the excitation of 4-coordinated surface O(4C)(2-) anions in edges at 5.4 eV and of regular oxygen-terminated corners at 4.6 eV, respectively. Morphologically ill-defined CaO particles are a factor of 5 larger, do not display regular corner features, and show only one photoluminescence emission band at 3.0 eV. The associated excitation spectrum indicates electronic excitations above the energy required to excite regular oxygen-terminated CaO corners. It is concluded that in the case of morphologically well-defined MgO nanocubes variations in the next coordination of oxygen-terminated corners can effectively be probed by photoluminescence spectroscopy and thus allows for discrimination between 3-coordinated surface O(2-) in regular corner sites and kinks.     

Quantum chemical study of the physical characteristics of the hydroxy groups of zeolites, III

The CNDO/2 method was applied to calculate the electronic structure of HY zeolite using cluster models Si₃AlO₁₂H ₈ ^– and Si₃AlO₁₂H₉. It has been shown that the probability of occurrence of H on a particular type of oxygen as well as the other physical characteristics of zeolite depend on both the type of oxygen and the position of the Al atom.

---

/2 , Si₃AlO₁₂H₈ Si₃AlO₁₂H₉. , , , Al.

     

Quantum chemical study of the physical characteristics of the hydroxy groups of zeolites, V

The CNDO/2 method is employed in the study of the acid strength of O–H groups of the HY and HX zeolites using their cluster models of the Si₄Al₂O₆(OH) ₁₂ ^2– and Si₃Al₃O₆(OH) ₁₂ ^3– type. It is shown that the O–H groups of the HY zeolites are of more acidic character than those of the HX zeolites. The reasons of the heterogeneity of O–H groups are shortly discussed on the basis of the theoretical results.

---

/2 OH HY HX, Si₄Al₂O₆(OH) ₁₂ ^3– Si₃Al₃O₆(OH) ₁₂ ^3– . , OH HY , OH HX . OH.

---

---

Part IV, cf. Ref. /1/     

Quantum-chemical study of interaction of gases with molecular fragment of carbon surface

The atom-atom potential method has been applied in ranking industrial gases in their affinity for carbon surfaces. From an analysis of the Coulombic induction, and dispersion contributions to the total energy, and also an analysis of the repulsion potentials, it has been found that the energy of dispersion interaction is a factor with a substantial influence on the affinity of the molecules for the matrix.Sorption and Fine Inorganic Synthesis Branch, Institute of General and Inorganic Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 488–490, July–August, 1991. Original article submitted December 17, 1990.     

Quantum-chemical study of allylstannanes

It has been shown by the MNDO method that in allyl compounds of tin, the atomic orbitals of the heteroatom interact with the -orbital through the bridge group, but interact with the ^*-orbital mainly through space. The position and intensity of the long-wave electronic transitions for methylvinylstannane should not depend on the conformation, with the ,- and , pseudo--conjugation effects being approximately identical, whereas for allylstannane, the ,-conjugation is considerably stronger. In molecules containing several allyl fragments bonded to heavy atoms, ,-conjugation is far weaker than ,-conjugation in polyenes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2071–2076, September, 1991.     

Local ordering of hydroxy groups in hydroxyapatite

Density functional theory calculations of hydroxyapatite identified the oxygen and hydrogen positions of the hydroxy groups in the crystal structure to be well defined, alternating in a column in the c-direction, and we predict that the experimentally found oxygen and hydrogen disorder is due to the presence in the crystal of differently oriented locally ordered domains, a finding which is important for studies of crystal growth and surface reactivity.     

Quantum-chemical study on the mechanism of formation of geminal hydroxy thiols by reaction of 1,3-dihalopropan-2-ones with hydrogen sulfide

The reaction mechanism of 1,3-dihalopropan-2-ones with hydrogen sulfide in the presence of hydrogen chloride was studied in terms of the density functional theory. Unlike 1-halopropan-2-ones which give rise to 1-halo-2-sulfanylpropan-2-ols via preliminary enolization, 1,3-dihalopropan-2-ones preferentially undergo direct nucleophilic attack on the carbonyl group by hydrogen sulfide. The potential energy surface for rotational isomerism of 1,3-dihalopropane-2-thiones and 1,3-dihalo-2-sulfanylpropan-2-ols was analyzed, and their most stable rotamers were identified.