Modelling studies of water in crystalline nanoporous aluminosilicates

The paper presents a review of molecular modelling studies of hydrated nanoporous aluminosilicates (zeolites and clays) performed during the last decade. A special emphasis is set on the calculation of the dynamical quantities and collective properties of the confined water. Some new results concerning the behaviour of water molecules in the siliceous silicalite and zeolite beta structures are presented.     

IR spectral and quantum-chemical studies of binary extractants

The composition and ionic character of formation of binary extractants based on quaternary ammonium bases and organic acids were confirmed using spectral and quantum-chemical methods, the structures of the binary extractants were obtained. The energy of bond between cation and anion in molecules of the binary extractants was determined, it is by an order of magnitude higher than the energy of H bond and close to the energy of chemical (ionic) bond.     

Adsorption-complex isomerism and quantum-chemical studies in heterogeneous catalysis

The present quantum-chemical studies of the interaction of gases with solid surfaces indicate frequently the existence of several different structures of the model cluster complex with the adsorbed system. The usual approach to this adsorption-complex isomerism consists of selection of the potential energy-lowest member of the set of isomers. A weighting treatment is presented within which all the isomeric structures (or the part of them that is active in the respective experimental technique) can contribute to the values of observables. The treatment suggested requires information amount which corresponds to the usual capabilities of today's numerical quantum chemistry, i.e., concept of localized adsorption is employed. In this way correct theoretical equivalents of experimental data can be obtained. Two types of the isomerism are considered, viz., site-caused isomerism and adsorbate-caused isomerism. It is shown that the treatment in the latter case can, under acceptable assumptions, be reduced to the recently described technique for the site-caused isomerism. An illustrative example is presented based on the literature data about the water–silica interaction. The results are important for correct comparison of theoretical and observed data in the field of heterogeneous catalysis and sorption.     

Comparative quantum-chemical study of complexes of cyclic ether with ethylene glycol

The electronic structure and geometry of cyclic ethers??tetrahydrofuran and 1,4-dioxane??and their 1: 1 and 1: 2 complexes with ethylene glycol, as well as their complexes with ethylene glycol dimers, have been studied by density functional theory method at the B3LYP/6-31+G** level. It has been shown that moderate hydrogen bonds are mainly responsible for complex formation. A great number of conformations of the H-bonded complexes have been found.     

Infrared and quantum-chemical studies of the structure and vibrations of trisilylamine

New infrared spectra are reported for variously labelled trisilylamines. Quantum-chemical (QC) calculations of structure and force field have been made at HF, MP2 and B3LYP levels, each with the 6-31G* and 6-311G** basis sets. At each level, a minimum in the potential surface occurs at the C3h configuration. No evidence was found for a significant variation in SiH bond length with orientation. The appearance of two bands in the infrared spectrum of N(SiH3)3 in the 2nuSiH region is explained by local mode theory in terms of transitions to (200) and (110) levels. In the gas phase, signal averaging appears to occur in the nuSiH region in the infrared spectrum, but not in the Raman. In solid films, both IR and Raman spectra indicate the presence of a range of SiH bond strengths, corresponding to an absence of any site symmetry. Each complete QC calculated force field was fitted to the frequencies observed for N(SiH3)3 and N(SiD3)3, using nine independent scale factors. An interaction force constant between nu(as)NSi3 and delta(s)SiH3 motions required further adjustment. Unobserved frequencies in the d0 and d9 species are predicted. The out-of-plane skeletal bending mode is expected to lie between 170 and 200 cm(-1). Unscaled SiH3 torsional frequencies vary from 64 cm(-1) upwards. The effect of the presence of three internal rotors on the spectra throughout calls for theoretical study.     

Phototransformation studies of halo-substituted cinnamates by Raman spectrometry and quantum-chemical computations

The comprehension of the cinnamic derivative phototransformation mechanisms is particularly important when these molecules are used as addressable photosensitive layers. In this work we show that the halo-substituted cinnamate sensitivity to the phototransformation is a function of the excitation wavelength and the substituent nature and position. With this intention, we underline the existence of various isomers and rotatomers by Raman spectroscopy and we assign the observed vibrational modes with the help of quantum-chemical calculations. These various aspects of our work clarify the relative roles of the steric, inductive and mesomeric effects according to the considered substitution.     

Photoreduction of oxoisoaporphine dyes by amines: transient-absorption and semiempirical quantum-chemical studies

Photoreduction by amines of oxoisoaporphine dyes occurs via a stepwise mechanism of electron-proton-electron transfer that leads to the metastable N-hydrogen oxoisoaporphine anion. During photoreduction that occurs from the triplet manifold of the oxoisoaporphine, a radical ion A(-)(*), a neutral-hydrogenated radical A-NH(*), and the metastable ion A-NH(-) of the oxoisoaporphine are formed. We present time-resolved spectroscopic data and quantum mechanical semiempirical PM3 and ZINDO/S results for the transient species formed during the flash photolysis of oxoisoaporphines in the presence of amines. These calculations reproduce adequately the experimental spectra of the triplet-triplet absorption near 450 nm, and that of neutral hydrogenated radical of the studied oxoisoaporphines centered at 390 nm. A transient absorption observed near 490 nm, for all of the studied systems, was explained by considering the formation of radical ion pair between the radical anion of the oxoisoaporphine, A(-)(*), and the radical cation of the amine, whose ZINDO/S calculated spectra generate the strongest transition near the experimentally observed absorption maximum at 490 nm, supporting the formation of a radical ion pair complex as the first step of the photoreduction.     

Electron diffraction and quantum-chemical studies of the molecular structure of 2-methylbenzenesulfochloride

A combined electron diffraction and quantum-chemical (MP2/6-31G**) study of the molecular structure of 2-methylbenzenesulfochloride at 336(5) K was carried out. It was found that the gas phase contained only one conformer, C ₁. The following structural parameters were obtained: r _h1(C-H)_av = 1.095(8) Å, r _h1(C-C)_Ph = 1.402(4) Å, r _h1(C_Ph-C_meth) = 1.507(13) Å, r _h1(C_Ph-S) = 1.763(6) Å, r _h1(S=O) = 1.418(4) Å, r _h1(S-Cl) = 2.048(5) Å, ∠(H-C-H)_meth/av = 107.3(96)°, ∠(Cl-S-O)_av = 106.4(3)°, ∠C_Ph-S-Cl = 100.8(9), ∠O=S=O = 120.8(10)°. The C_C-C_S-S-Cl torsion angle that defines the position of the S-Cl bond relative to the plane of the benzene ring is 75.6(20)°. The B3LYP/6-311+G** calculated barriers of internal rotation of the methyl and sulfochloride groups are 1.2 kcal/mol and V ₀₁ = 10.2 (V ₀₂ = 4.1) kcal/mol, respectively.     

Lewis acid sites of bivalent silicon and quantum-chemical studies of interaction with CO

According to the results of quantum-chemical ab initio studies taking into account electron correlation, the structure of the adduct of the Lewis acid site R₂Si with CO where CO bond activation takes place, is suggested.

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R₂Si , CO.

     

Quantum-chemical studies of acidic centers in aluminophosphates

In terms of the MINDO/3 method in the cluster approximation, the probable Lewis and Brönsted acidic centers of aluminophosphates have been studied.

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MINDO/3 .

     

Physical properties of sol-gel aluminosilicates

The control of pore diameter distributions of sol-gel aluminosilicates by manipulation of preparation conditions is described. The influence of water, aluminium content, solvent concentration, type, acidity and temperature are evaluated. Pore diameter distributions are derived using predominantly α_s plots and t plots, but supportive data is provided by using a range of probe adsorbates and pore structural models. The influence of preparation conditions on surface chemistry is investigated by using Fourier transform infrared (FTIR) spectroscopy and measurements of surface charge and gas diffusion.     

Experimental and quantum-chemical studies of histamine complexes with copper(II) ion

Histamine-copper(II) complexes have been studied using experimental methods and density functional theory. Preferred coordination centres and possible structures of aqua complexes have been determined. On the basis of equilibrium and spectroscopic studies the endocyclic nitrogen atom has been confirmed to as a coordinating centre in the CuH(Hist), Cu(Hist) and Cu(Hist)(OH) complexes. The involvement of the amino group linked to the aliphatic chain in the Cu(II) coordination has been additionally proven by the detection of the Cu(Hist) and Cu(Hist)(OH) complexes. The computed stabilisation energies demonstrate that the Cu(H₂O)₄(Hist) and Cu(OH)(Hist)(H₂O)₃ chelates as well as the CuH(H₂O)₅(Hist) compounds are the most energetically stable in the media studied. The most stable conformers of the neutral form of the histamine molecule are in the Cu(Hist) and Cu(Hist)(OH) species. These complexes have a gauche structure stabilized by an intramolecular hydrogen bond. The electronic Jahn-Teller effect is mainly responsible for the tetragonal distortion of the octahedral MHL-(H₂O)₅ complex. Strong electrostatic interactions and polarisation effects contribute to the enhanced stability for all of the complexes studied. The results of the computations confirm that histamine is effective in coordinating to the Cu(II) ions in biological systems. The theoretical results fully confirm the coordination modes proposed in the experiment and predict the most reliable geometry and energetic stability of the aqua complexes.     

Electron diffraction and quantum-chemical studies of the conformational properties of the 1,3-benzenedisulfochloride molecule

A combined electron diffraction (T = 394(5) K) and quantum-chemical (MP2/6-31G**) study has been performed to investigate the molecular structure of 1,3-benzenedisulfochloride (1,3-BDSC). The 1,3-BDSC molecule was found to exist as the trans (I) and cis (II) stable conformers where the planes containing S-Cl bonds are perpendicular to the plane of the benzene ring. The energy of conformer I is 0.13 kJ/mol lower than that of conformer II. The mutual effect of the sulfochloride groups was found to be absent, which is evident from the coincident bond lengths and angles in the two conformers. The main structural parameters of the conformers are r _h1(C-H)_av = 1.103(4) Å, r _h1(C-C)_av = 1.401(3) Å, r _h1(C-S) = 1.767(4) Å, r _h1(S=O) = 1.422(3) Å, r _h1(S-Cl) = 2.048(4) Å, ∠Cl-S-O = 106.6(2)°, ∠C-S-Cl = 100.4(5)°, ∠ O-S-O = 123.2(5)°.     

Nonempirical quantum-chemical studies on Cu+ carbonyl and isocarbonyl complexes

Nonempirical quantum-chemical calculations with the double-zeta basis set have been performed for the Cu⁺ complexes with one, two, and four CO molecules having carbonyl, isocarbonyl and -bridged structures.

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Cu⁺ , CO. , - .

     

Stereospecific ion-molecule reactions of anabolic-type steroid tertiary alcohols with proton-donating cations

Isobutane and methane chemical ionization (CI) mass spectra of C-17a-epimeric, 17a-substituted 3-methoxyestra-1,3,5(10),8-tetraen-17a-ols and at C-17-epimeric 17-substituted 3-methoxyestra-1,3,5(10)-trien-17-ols, as well as of some their derivatives, have been studied. In each epimeric pair, the peak intensity ratio [MH-H(2)O](+) / [MH](+) is greater for stereoisomers having an axial (or quasi-axial) hydroxyl group. The same regularity in the peak intensity ratio [MH-CH(3)COOH](+) / [MH](+) is valid for acetates in the D-homo series. The observed quantitative differences in CI mass spectra of epimers are explained by steric hindrance of the attack of the proton-donating cation caused by the angular 18-methyl group. No differences in the ease of elimination of the silanol molecule were observed in CI mass spectra of epimeric silyl ethers.     

Experimental and quantum-chemical studies on photoionization and dissociative photoionization of CH2Br2

The dissociative photoionization of CH2Br2 in a region approximately 10-24 eV was investigated with photoionization mass spectroscopy using a synchrotron radiation source. An adiabatic ionization energy of 10.25 eV determined for CH2Br2 agrees satisfactorily with predictions of 10.26 and 10.25 eV with G2 and G3 methods, respectively. Observed major fragment ions CH2Br+, CHBr+, and CBr+ show appearance energies at 11.22, 12.59, and 15.42 eV, respectively; minor fragment ions CHBr2+, Br+, and CH2+ appear at 12.64, 15.31, and 16.80 eV, respectively. Energies for formation of observed fragment ions and their neutral counterparts upon ionization of CH2Br2 are computed with G2 and G3 methods. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. An upper limit of DeltaH0f,298(CHBr+) < or = 300.7 +/- 1.5 kcal mol(-1) is derived experimentally; the adiabatic ionization energy of CHBr is thus derived to be < or = 9.17 +/- 0.23 eV. Literature values for DeltaH0f,298(CBr+) = 362.5 kcal mol(-1) and ionization energy of 10.43 eV for CBr are revised to be less than 332 kcal mol(-1) and 9.11 eV, respectively. Also based on a new experimental ionization energy, DeltaH0f,298(CH2Br2+) is revised to be 236.4 +/- 1.5 kcal mol(-1).